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What determined the evolution of different faces in humans?

What determined the evolution of different faces in humans?


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The most distinctive characteristic of a human is it's face - it is unique among each individual (with the exception of identical twins). It is uncertain to me if whether we best identify other humans by their face due to special brain parts developed for this (there are people with disease that cannot recognize faces) or for actual very distinctive physical face forms; maybe it is a combination of this.

Anyway, the question is about evolution. Why would distinct faces (or ability to recognize distinct faces) be something useful that would be retained by evolution? How is this better at survival?


Given the prevalance of myopia in humans I would have thought that facial recognition was secondary to other indicators such as gait recognition which is also unique to individuals though can be copied to an extent. Voice and shape are also used to identify people.

Being able to recognise individuals would allow the formation of groups, allow for structures of trust, reciprocal altruism to develop as well as punishing transgressors. Repeat interactions with other people require some kind of identification, but I dunno if facial recognition is the only/primary one.


Explain the development of the theory of evolution

  • Fossils serve to highlight the differences and similarities between current and extinct species, showing the evolution of form over time.
  • Similar anatomy across different species highlights their common origin and can be seen in homologous and vestigial structures.
  • Embryology provides evidence for evolution since the embryonic forms of divergent groups are extremely similar.
  • The natural distribution of species across different continents supports evolution species that evolved before the breakup of the supercontinent are distributed worldwide, whereas species that evolved more recently are more localized.
  • Molecular biology indicates that the molecular basis for life evolved very early and has been maintained with little variation across all life on the planet.

New technique reveals genes underlying human evolution

One of the best ways to study human evolution is by comparing us with nonhuman species that, evolutionarily speaking, are closely related to us. That closeness can help scientists narrow down precisely what makes us human, but that scope is so narrow it can also be extremely hard to define. To address this complication, researchers from Stanford University have developed a new technique for comparing genetic differences.

Through two separate sets of experiments with this technique, the researchers discovered new genetic differences between humans and chimpanzees. They found a significant disparity in the expression of the gene SSTR2 - which modulates the activity of neurons in the cerebral cortex and has been linked, in humans, to certain neuropsychiatric diseases such as Alzheimer's dementia and schizophrenia - and the gene EVC2, which is related to facial shape. The results were published March 17 in Nature and Nature Genetics, respectively.

"It's important to study human evolution, not only to understand where we came from, but also why humans get so many diseases that aren't seen in other species," said Rachel Agoglia, a recent Stanford genetics graduate student who is lead author of the Nature paper.

The Nature paper details the new technique, which involves fusing human and chimpanzee skin cells that had been modified to act like stem cells - highly malleable cells that can be prodded to transform into a variety of other cell types (albeit not a full organism).

"These cells serve a very important specific purpose in this type of study by allowing us to precisely compare human and chimpanzee genes and their activities side-by-side," said Hunter Fraser, associate professor of biology at Stanford's School of Humanities and Sciences. Fraser is senior author of the Nature Genetics paper and co-senior author of the Nature paper with Sergiu Pa?ca, associate professor of psychiatry and behavioral sciences in the Stanford School of Medicine.

Close comparisons

The Fraser lab is particularly interested in how the genetics of humans and other primates compare at the level of cis-regulatory elements, which affect the expression of nearby genes (located on the same DNA molecule, or chromosome). The alternative - called trans-regulatory factors - can regulate the expression of distant genes on other chromosomes elsewhere in the genome. Due to their broad effects, trans-regulatory factors (such as proteins) are less likely to differ among closely related species than cis-regulatory elements.

But even when scientists have access to similar cells from humans and chimpanzees, there is a risk of confounding factors. For example, differences in the timing of development between species is a significant hurdle in studying brain development, explained Pa?ca. This is because human brains and chimpanzee brains develop at very different rates and there is no exact way to directly compare them. By housing human and chimpanzee DNA within the same cellular nucleus, scientists can exclude most confounding factors.

For the initial experiments using these cells, Agoglia coaxed the cells into forming so-called cortical spheroids or organoids - a bundle of brain cells that closely mimics a developing mammalian cerebral cortex. The Pa?ca lab has been at the forefront of developing brain organoids and assembloids for the purpose of researching how the human brain is assembled and how this process goes awry in disease.

"The human brain is essentially inaccessible at the molecular and cellular level for most of its development, so we introduced cortical spheroids to help us gain access to these important processes," said Pa?ca, who is also the Bonnie Uytengsu and Family Director of Stanford Brain Organogenesis.

As the 3D clusters of brain cells develop and mature in a dish, their genetic activity mimics what happens in early neurodevelopment in each species. Because the human and chimpanzee DNA are bound together in the same cellular environment, they are exposed to the same conditions and mature in parallel. Therefore, any observed differences in the genetic activity of the two can reasonably be attributed to actual genetic differences between our two species.

Through studying brain organoids derived from the fused cells that were grown for 200 days, the researchers found thousands of genes that showed cis-regulatory differences between species. They decided to further investigate one of these genes - SSTR2 - which was more strongly expressed in human neurons and functions as a receptor for a neurotransmitter called somatostatin. In subsequent comparisons between human and chimpanzee cells, the researchers confirmed this elevated protein expression of SSTR2 in human cortical cells. Further, when the researchers exposed the chimpanzee cells and human cells to a small molecule drug that binds to SSTR2, they found that human neurons responded much more to the drug than the chimpanzee cells.

This suggests a way by which the activity of human neurons in cortical circuits can be modified by neurotransmitters. Interestingly, this neuromodulatory activity may also be related to disease since SSTR2 has been shown to be involved in brain disease.

"Evolution of the primate brain may have involved adding sophisticated neuromodulatory features to neural circuits, which under certain conditions can be perturbed and increase susceptibility to neuropsychiatric disease," said Pa?ca.

Fraser said these results are essentially "a proof of concept that the activity we're seeing in these fused cells is actually relevant for cellular physiology."

Investigating extreme differences

For the experiments published in Nature Genetics, the team coaxed their fused cells into cranial neural crest cells, which give rise to bones and cartilage in the skull and face, and determine facial appearance.

"We were interested in these types of cells because facial differences are considered some of the most extreme anatomical differences between humans and chimps - and these differences actually affect other aspects of our behavior and evolution, like feeding, our senses, brain expansion and speech," said David Gokhman, a postdoctoral scholar in the Fraser lab and lead author of the Nature Genetics paper. "Also, the most common congenital diseases in humans are related to facial structure."

In the fused cells, the researchers identified a gene expression pathway that is much more active in the chimpanzee genes of the cells than in the human genes - with one specific gene, called EVC2, appearing to be six times more active in chimpanzees. Existing research has shown that people who have inactive EVC2 genes have flatter faces than others, suggesting that this gene could explain why humans have flatter faces than other primates.

What's more, the researchers determined that 25 observable facial features associated with inactive EVC2 are noticeably different between humans and chimpanzees - and 23 of those are different in the direction the researchers would have predicted, given lower EVC2 activity in humans. In follow-up experiments, where the researchers reduced the activity of EVC2 in mice, the rodents, too, developed flatter faces.

Another tool in the toolbox

This new experimental platform is not intended to replace existing cell comparison studies, but the researchers hope it will support many new findings about human evolution, and evolution in general.

"Human development and the human genome have been very well studied," said Fraser. "My lab is very interested in human evolution, but, because we can build on such a wealth of knowledge, this work can also reveal new insights into the process of evolution more broadly."

Looking forward, the Fraser lab is working on differentiating the fused cells into other cell types, such as muscle cells, other types of neurons, skin cells and cartilage to expand their studies of uniquely human traits. The Pa?ca lab, meanwhile, is interested in investigating genetic dissimilarities related to astrocytes - large, multi-functional cells in the central nervous system often overlooked by scientists in favor of the flashier neurons.

"While people often think about how neurons have evolved, we should not underestimate how astrocytes have changed during evolution. The size difference alone, between human astrocytes and astrocytes in other primates, is massive," said Pa?ca. "My mentor, the late Ben Barres, called astrocytes 'the basis of humanity' and we absolutely think he was onto something."

Additional Stanford co-authors for the Nature paper are former research assistant Danqiong Sun, postdoctoral scholar Fikri Birey, senior research scientist Se-Jin Yoon, postdoctoral scholar Yuki Miura and former research associate Karen Sabatini.

This work was funded by a Stanford Bio-X Interdisciplinary Initiatives Seed Grant, the National Institutes of Health, the Department of Defense, the Stanford Center for Computational, Evolutionary and Human Genomics, the Stanford Medicine's Dean's Fellowship, MCHRI, the American Epilepsy Society, the Stanford Wu Tsai Neurosciences Institute's Big Idea Grants on Brain Rejuvenation and Human Brain Organogenesis, the Kwan Research Fund, the New York Stem Cell Robertson Investigator Award, and the Chan Zuckerberg Ben Barres Investigator Award.

Additional Stanford co-authors for the Nature Genetics paper are graduate student Maia Kinnebrew former undergraduate Wei Gordon former technician Danqiong Sun postdoctoral research fellows Vivek Bajpai and Sahin Naqvi Dmitri Petrov, the Michelle and Kevin Douglas Professor in the School of Humanities and Sciences Joanna Wysocka, the Lorry Lokey Professor and professor of developmental biology and Rajat Rohatgi, associate professor of biochemistry and of medicine. Researchers from University of California, San Francisco University of Michigan, Ann Arbor Yerkes National Primate Research Center Emory University School of Medicine and University of Pennsylvania are also co-authors.

This work was funded by the Human Frontier, Rothschild and Zuckerman fellowships, and the National Institutes of Health.

Fraser is a member of Stanford Bio-X, the Maternal & Child Health Research Institute (MCHRI), and the Stanford Cancer Institute. Pa?ca is a member of Stanford Bio-X, MCHRI and the Wu Tsai Neurosciences Institute, and a faculty fellow of Stanford ChEM-H.

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.


The Science Of Race, Revisited

T here's no doubt that different groups of people can look very different from one another. But to contemporary anthropologists and sociologists, the notion that there are distinct "races" of human beings, each with its own specific attributes, doesn't make much sense.

The same goes for biologists like Stanford University's Dr. Marcus Feldman, who has done pioneering research on the differences between human populations.

Recently, HuffPost Science posed several questions about race and racism to Feldman. Here, lightly edited, are his answers.

Does the concept of race have any scientific validity? Or have biologists discarded the term?

Many biologists have replaced the term “race” with "continental ancestry." This is because such a large fraction of the world has ancestry in more than one continent. The result is hyphenated nomenclature, which attempts to specify which continents are represented in one's ancestry.

For example, our president is as European in his ancestry as he is African. It is arbitrary which of these an observer chooses to emphasize. Obama’s opponents overtly and by implication denigrate him because of his African ancestry. But he is equally European.

How did the concept of race originate?

Probably from Aristotle’s predilection with classification. But more recently with [German physician Johann Friedrich] Blumenbach’s classification in 1775 of the five human races.

How do biologists today view race, and how has that view changed in recent years?

Biologists generally agree that with enough data on DNA it is possible to say that someone’s ancestry is more likely to include representation from a given set of continents. Genes contributing to phenotypes that are recognized through our senses (for example, sight or touch) as defining differences between people from different continents (commonly referred to as races) or from different populations comprise a small proportion of the human genome, perhaps 10% to 15%. This is the meaning of [biologist Richard] Lewontin’s 1972 paper and subsequent analyses of worldwide molecular genetic variation. How do biologists explain the differences between different populations of humans?

It depends what differences are referred to. Skin color differences, for example, may be the result of the action of 40 genes. Height might involve several hundred genes.

On one hand, some differences may be due to differences in the founding size of a population (for example, the relatively high frequency of some genetic diseases in Ashkenazi Jews could reflect the small original populations in Eastern Europe). Other differences could be due to natural selection--for example, tolerance of low oxygen pressure in Tibetans and Andean populations. Other differences are obviously cultural--for example, the preference of some Middle Eastern and South Asian populations to marry their cousins results in higher rates of genetic disorders in those populations than in other populations.

So why did we evolve to look so different from one another?

Some genes are involved in phenotypic differences that are detectable by the naked eye, and some are involved in musculature-related phenomena. Many people focus on these, ignoring the vast majority of genes whose differences are insignificant.

(Story continues below image.) Engraving by the British artist John Emslie (1839-1913) showing facial portraits from different parts of the world, and demonstrating racial and regional differences. The portraits are grouped under the headings: Asiatics, Australians, Europeans, Polynesians, Africans and Americans, and illustrate European perceptions of people indigenous to each area. How much does DNA differ from one population to another?

Eighty percent to 90 percent of genetic variation is within populations, so the fraction between populations is very small. As a result of the genes they carry, different populations can face different vulnerabilities—for example, their risk of suffering from certain diseases. Is there any evidence that certain populations have specific physical or intellectual attributes?

As I mentioned above, some populations do show higher frequencies of some disorders. These differences may be due to increases in the frequencies of genes that occurred by chance due to the small size or constitution of their founders. Other diseases can be due to cultural choices or societal constraints, such as dietary preferences or poverty. The latter are not genetically determined. Do any non-human animals exhibit races?

Biologists use the term "race" to describe variants of a species that exhibit phenotypic differences over geographical ranges. The term gets confused with sub-species and other names. [Evolutionary biologist Theodosius] Dobzhansky referred to fruit fly races, and others use the term for populations that have chromosomal differences but can still mate successfully. It is not clear what the exact criteria for such races are. Are humans “hard wired” to be suspicious of those who look different from ourselves?

"Hard-wired" is generally a synonym for genetically determined. Four-leggedness in dogs as opposed to two-leggedness in humans is probably genetic, but there is no evidence that I would accept xenophobia as genetic. Are humans starting to look more like one another?

As migration increases around the world, features that might previously have allowed our eyes to classify people will undoubtedly become blurred. Then the small fraction of DNA differences that differ between populations will become even smaller.

So from a biological standpoint, it doesn't seem to make much sense to use the term "race." Should we stop talking about race and racism in everyday life?

I think race is outdated and often pejorative, but racism is alive and (unfortunately) not decreasing. I think we must remain on the alert for racism and have ready responses to it when it rears its ugly head.

Next up in HuffPost Science’s four-part series on race & racism:


The Institute for Creation Research

Some people today, especially those of anti-Christian opinions, have the mistaken notion that the Bible prescribes permanent racial divisions among men and is, therefore, the cause of modern racial hatreds. As a matter of fact, the Bible says nothing whatever about race. Neither the word nor the concept of different "races" is found in the Bible at all. As far as one can learn from a study of Scripture, the writers of the Bible did not even know there were distinct races of men, in the sense of black and yellow and white races, or Caucasian and Mongol and Negroid races, or any other such divisions.

The Biblical divisions among men are those of "tongues, families, nations, and lands" (Genesis 10:5,20,31) rather than races. The vision of the redeemed saints in heaven (Revelation 7:9) is one of "all nations, and kindreds, and people, and tongues", but no mention is made of "races". The formation of the original divisions, after the Flood, was based on different languages (Genesis 11:6-9), supernaturally imposed by God, but nothing is said about any other physical differences.

Some have interpreted the Noahic prophecy concerning his three sons (Genesis 9:25-27) to refer to three races, Hamitic, Semitic and Japhetic, but such a meaning is in no way evident from the words of this passage. The prophecy applies to the descendants of Noah's sons, and the various nations to be formed from them, but nothing is said about three races. Modern anthropologists and historians employ a much-different terminology than this simple trifurcation for what they consider to be the various races among men.

Therefore, the origin of the concept of "race" must be sought elsewhere than in the Bible. If certain Christian writers have interpreted the Bible in a racist framework, the error is in the interpretation, not in the Bible itself. In the Bible, there is only one race&mdashthe human race! "(God) hath made of one, all nations of men" (Acts 17:26).

What Is a Race?

In modern terminology, a race of men may involve quite a large number of individual national and language groups. It is, therefore, a much broader generic concept than any of the Biblical divisions. In the terminology of biological taxonomy, it is roughly the same as a "variety", or a "sub-species". Biologists, of course, use the term to apply to sub-species of animals, as well as men.

For example, Charles Darwin selected as the subtitle for his book Origin of Species the phrase "The Preservation of Favoured Races in the Struggle for Life". It is clear from the context that he had races of animals primarily in mind, but at the same time it is also clear, as we shall see, that he thought of races of men in the same way.

That this concept is still held today is evident from the following words of leading modern evolutionist George Gaylord Simpson:

It is clear, therefore, that a race is not a Biblical category, but rather is a category of evolutionary biology. Each race is a sub-species, with a long evolutionary history of its own, in the process of evolving gradually into a distinct species.

As applied to man, this concept, of course, suggests that each of the various races of men is very different, though still inter-fertile, from all of the others. If they continue to be segregated, each will continue to compete as best it can with the other races in the struggle for existence and finally the fittest will survive. Or else, perhaps, they will gradually become so different from each other as to assume the character of separate species altogether (just as apes and men supposedly diverged from a common ancestor early in the so-called Tertiary Period).

Most modern biologists today would express these concepts somewhat differently than as above, and they undoubtedly would disavow the racist connotations. Nevertheless, this was certainly the point-of-view of the 19th century evolutionists, and it is difficult to interpret modern evolutionary theory, the so-called neo-Darwinian synthesis, much differently.

Nineteenth-Century Evolutionary Racism

The rise of modern evolutionary theory took place mostly in Europe, especially in England and Germany. Europeans, along with their American cousins, were then leading the world in industrial and military expansion, and were, therefore, inclined to think of themselves as somehow superior to the other nations of the world. This opinion was tremendously encouraged by the concurrent rise of Darwinian evolutionism and its simplistic approach to the idea of struggle between natural races, with the strongest surviving and thus contributing to the advance of evolution.

As the 19th century scientists were converted to evolution, they were thus also convinced of racism. They were certain that the white race was superior to other races, and the reason for this superiority was to be found in Darwinian theory. The white race had advanced farther up the evolutionary ladder and, therefore, was destined either to eliminate the other races in the struggle for existence or else to have to assume the "white man's burden" and to care for those inferior races that were incompetent to survive otherwise.

Charles Darwin himself, though strongly opposed to slavery on moral grounds, was convinced of white racial superiority. He wrote on one occasion as follows:

The man more responsible than any other for the widespread acceptance of evolution in the 19th century was Thomas Huxley. Soon after the American Civil War, in which the negro slaves were freed, he wrote as follows:

Racist sentiments such as these were held by all the 19th century evolutionists. A recent book 4 has documented this fact beyond any question. In a review of this book, a recent writer says:

A reviewer in another scientific journal says:

The Modern Harvest

In a day and age which practically worshipped at the shrine of scientific progress, as was true especially during the century from 1860 to 1960, such universal scientific racism was bound to have repercussions in the political and social realms. The seeds of evolutionary racism came to fullest fruition in the form of National Socialism in Germany. The philosopher Friedrich Nietzsche, a contemporary of Charles Darwin and an ardent evolutionist, popularized in Germany his concept of the superman, and then the master race. The ultimate outcome was Hitler, who elevated this philosophy to the status of a national policy.

However one may react morally against Hitler, he was certainly a consistent evolutionist. Sir Arthur Keith, one of the leading evolutionary anthropologists of our century, said:

With respect to the question of race struggle, as exemplified especially in Germany, Sir Arthur also observed:

In recent decades, the cause of racial liberation has made racism unpopular with intellectuals and only a few evolutionary scientists still openly espouse the idea of a long-term polyphyletic origin of the different races. 10 On the other hand, in very recent years, the pendulum has swung, and now we have highly vocal advocates of "black power" and "red power" and "yellow power", and these advocates are all doctrinaire evolutionists, who believe their own respective "races" are the fittest to survive in man&rsquos continuing struggle for existence.

The Creationist Position

According to the Biblical record of history, the Creator&rsquos divisions among men are linguistic and national divisions, not racial. Each nation has a distinct purpose and function in the corporate life of mankind, in the divine Plan (as, for that matter, does each individual).

No one nation is "better" than another, except in the sense of the blessings it has received from the Creator, perhaps in measure of its obedience to His Word and fulfillment of its calling. Such blessings are not an occasion for pride, but for gratitude.

References

* Dr. Henry M. Morris (1918-2006) was Founder and President Emeritus of ICR.

Cite this article: Morris, H. 1973. Evolution and Modern Racism. Acts & Facts. 2 (7).


Human Evolution: History, Timeline, and Future Predictions

The human evolutionary tree is a complex structure, branching and re-branching at several points along the timeline. Though a complete study of human evolution is beyond the scope of one article, it endeavors to highlight the main stages, and also tries to makes predictions about the next step in the ongoing process of human evolution.

The human evolutionary tree is a complex structure, branching and re-branching at several points along the timeline. Though a complete study of human evolution is beyond the scope of one article, it endeavors to highlight the main stages, and also tries to makes predictions about the next step in the ongoing process of human evolution.

Did you know?

All males amongst modern-day human beings possess a Y chromosome inherited from a male that lived in Africa about 140,000 years ago.

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The story of human evolution isn’t just a single tale. It is actually a collection of several short stories, each connected to the other like the links in a chain. The human tribe, or the hominini, has evolved over millions of years, from creatures you would scarcely imagine to have anything in common with. We are the result of the adaptive evolution of several different species. Thus, in essence, within each of us, you can find the ghosts and spirits of many ancient animals of the past.

But we humans are basically primates, and therefore, our history must have more to do with that of the monkeys or the chimpanzees, than say, for example, a fish. We have the same five fingers, the same front-facing eyes, and even have similar behavior and habits as them. But believe it or not, humans, monkeys, chimpanzees, and indeed all the animals that we see around us today were all fish once living in the oceans. Therefore, in order to learn about our past, we have to learn about not just the monkey, but also the fish inside us.

So let’s take a journey through the sands of time and let the tale unfold. This is the fascinating story of our bodies, and why we are built the way we are. This is the story of human evolution.

The Story of the Fish That Walked 400 MYA – 350 MYA

If I were to tell you that your earliest ancestor was a fish, would you believe me? Of course you won’t! A monkey maybe but a fish, no way! However, in all probability, that theory is true, and the evidence for it is present right there in your hands.

It is a widely accepted fact that life on earth began in the oceans. Nearly 3.6 billion years ago, the first living things in the form of simple celled organisms first appeared in water. These simple cells later combined to form multi-cellular life forms almost 1 billion years ago, and soon the oceans were swimming with all kinds of living things, including various fish, aquatic plants, etc.

Then, close to 365 million years ago, some ancient fish used their fins to crawl out of the oceans onto the lands. In order to move there, their fins evolved into feet and claws of reptiles, which later evolved into the paws of mammals with short fingers all pointing the same way. As these mammals spread over the lands and began living in various habitats, they evolved further, and finally their claws became hands, as these primitive mammals evolved into the first primates we are all the descendants of.

The Tale of Notharctus Tenebrosus 54 MYA – 38 MYA

Notharctus lived 54 to 38 million years ago. Though from its first fossil discovered in 1870, it was thought to be a member of an obsolete order of the mammalian family, the later discovery of an almost complete skeleton firmly established it as being a primate. It lived high up in the canopy of large ancient trees.

Notharctus is linked with humans, because it shared with us a unique characteristic – the opposable thumb. Life in the trees caused the lengthening of its fingers and the addition of an opposable thumb in order to allow it to reach the edible flowers and fruits growing at the ends of thin branches.

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Northarctus Tenebrosus had large hind limbs, and a tail which helped it to balance itself on the branches of trees. It must have had a body weight of around 10 lb, and from head to tail would have been nearly 40 cm long.

Notharctus and the following species which lived amongst the foliage provided us with another important characteristic – our color vision. Early primates saw only a limited range of colors, but then one group developed full red green and blue (RGB) vision, to distinguish the ripe fruits from the green unripe ones. Thus, we not only owe our ability to grasp (opposable thumbs) but also our full color vision to the lives led by Notharctus and our early ancestors high up on the trees.

The Tale of the Elusive Ancient Primate 8 MYA

According to studies, modern-day humans share 98% of their DNA composition with chimpanzees. This fact indicates that our race and that of the chimpanzees must have had a common point of origin. In fact, many theories suggest that the human tribe, or the hominini, and all the other species and subspecies within our genus, including the modern-day apes such as gorillas and chimpanzees must have all descended from the same ancient primate.

However, fossil-based evidence hasn’t been able to substantiate this theory, and perhaps it never will, because of the complexity involved in the classification and establishment of the relationship between the fossils of the numerous ape-like creatures of the time.

This elusive ancient primate, our common ancestor, must have evolved from the Notharctus species, and is believed to have existed 8 million years ago.

The Tale of Ardipithecus Ramidus – The Bipedal 5.6 MYA – 4.4 MYA

For several years, anthropologists had believed that bipedalism (the human ability to walk on two legs) was developed in response to changes in the habitat of our ancestors from woodlands to grasslands. However, the discovery of a new species – the Ardipithecus Ramidus in 1994, turned that theory right over its head.

Ardipithecus Ramidus lived mainly on trees, as is evidenced from its bone structure. However, the discovery of its skeleton showed that it had a hip bone structure which is remarkably similar to ours. Studies have concluded that such a hip structure must have enabled it to walk upright, though the reasons for it to do so are still unknown. One theory suggests that it must have stood up to have a larger field of vision while on ground where it must have felt less safer than up on the trees.

The Ardipithecus lived in the African continent, was 4 ft tall, and a good climber. It used all four limbs while on trees, but stood upright to walk on the ground. The members of this species also had smaller canines, which is another indicative factor towards our common lineages. However, another species was found that lived about the same time as Ardipithecus Ramidus, and which too was bipedal,. This discovery has since had many anthropologists debating about our relationship to it, with some even questioning whether Ardipithecus was even a hominini at all!

The Tale of ‘Lucy’ in the Ground with Earthworms 3.2 MYA – 1.7 MYA

In 1974, the fossilized remains of a female primate was found in Ethiopia. She was named Lucy, after the Beatles’ song Lucy in the Sky with Diamonds. Dating methods indicated that Lucy lived 3.2 million years ago. Another excavation in 1978 showed distinct tracks of human-like footprints made by members of the same species, walking on two legs (bipedal). This species was named Australopithecus aferensis, after the people and the land of Africa.

Thus, it is now believed that the single line of evolution from fish to reptiles to mammals and finally to primates must have branched off into two different lineages, nearly 6 million years ago. One ultimately leading to the gorillas and the chimpanzees of present day, while the other to modern-day human beings.

Many believe that the first in the human line of evolution was, not the Ardipithecus, but the Australopithecines, which lived in the continent of Africa nearly 3 million years ago. Just like the Ardipithecus, the members of this species displayed several distinct characteristics linking them to modern-day human beings, the most significant of which was the development of bipedalism, or the ability to walk on two legs.

The Australopithecines show skeletal features distinct from other primates and closer to that of modern human beings. They had a greater forearm-to-upper arm ratio as compared to other hominids of the time, and exhibited increased sexual dimorphism. Fossils reveal that the average height of adults was up to 1.5 meters (4.9 ft), and their weight was nearly 120 lb. Skull fossils indicate that their brains had a volume of about 600 cc.

Australopithecines later subdivided into various subspecies, including Australopithecus anamensis, A. sediba, A. africanus, and A. afarensis, with each showing subtle differences compared to the other. All these sub-subspecies thrived in various parts of the continent of Africa, till they finally became extinct almost 2 million years ago.

The Complex Story of the Homo 2.58 MYA – Present

Whether it was the Ardipithecus Ramidus or the Australopithecines, it is still unknown. But one thing is for sure, it was one of those two species that evolved into the final link in our chain of evolution – the genus Homo.

The long and complex homo lineage first began around 2.4 million years ago. Tracing the entire line along with all its branches down to us and establishing a relationship between each link is an almost impossible task. There are many species and subspecies within the genus homo which could or could not be related to us. The scientific opinion too, in this matter, is divided into many different camps, leaving us with the single option of detailing only the most significant members of this genus.

Among the earliest members of the homo genus were the homo habilis. These primates were very resemblant to the Australopiths in a number of ways. For instance, they too had long hanging arms. However, unlike the Australopiths, they had smaller teeth and much more human-like arms and feet. Their faces too were less protruding. They were short in stature, and had a brain size of 510 cc, which is roughly less than half the size of ours. Many homo habilis findings are accompanied with stone weapons, which points towards the development of mental capacity and intelligence. They lived in Africa nearly 2.33 million years ago.

Homo ergaster, or the African homo erectus, succeeded H. habilis. Their physical constitution evolved to be more nearer to modern human beings, but their brains, though larger than the H. habilis, were still smaller than ours. They are estimated to have been over 6 ft in height, with a less protrusive forehead, and smaller jaws and teeth. They had longer noses with downward-facing nostrils. They also had significantly larger brains compared to H. habilis, with skull findings pointing towards a cranial capacity of nearly 900 cc.

The H. erectus species thrived and existed for almost 1.5 million years. During this large span of time, they migrated out of the African continent and spread to other continents, including Europe and Asia, as is evidenced by their fossils found there. Homo erectus were also the first primates to use fire and hunt with weapons.

Lastly, in the final stages of human evolution, the neanderthals and the homo sapiens came into existence about 200,000 years ago. Both these species developed complex brain structures, and gave birth to language and culture, and their later members began to wear clothes.

Homo neanderthalensis, or the neanderthals, were very similar to modern-day human beings, with their DNA differing from ours by just 0.12%. They had larger brains than ours (1,600 cc), but at the same time had a larger bodily structure as well. The last of the neanderthals died out in Europe close to 40,000 years ago.

Early homo sapiens had nearly the same brain size as that of ours (1,350 cc), but had characteristic thick skulls and a prominent forehead. The rest of their anatomy was nearly similar to that of ours. Homo sapiens is the last surviving species of the genus homo, and modern-day human beings, or the homo sapiens sapiens are its subspecies.

The Untold Story of the Man From the Future In the Future

Human kind has come a long way. We were fish once, and now we eat fish for dinner! Our evolution has been phenomenal. So what does the future have in store for us? What evolutionary changes will modern-day human beings undergo? What will they look like in the future?

These certainly are interesting question, albeit, not very easy to answer. Making random predictions about the future can be dangerous. The great fictional detective Mr. Sherlock Holmes says, “I never guess. It is a shocking habit, destructive to the logical faculty”. However, when on a difficult case, often, he too would dare to predict, provided that there were enough facts to support his assumptions. The future of human evolution is one such challenging mystery. So, following the lead of Mr. Sherlock Holmes, in the following few lines, we too shall dare to predict the future evolution of present-day human beings, based on as many facts as we can gather.

Loss of Muscle Mass: Present-day humans hardly get any exercise. The majority of us live very sedentary lives. Thus, it is quite possible that future humans will have significantly less muscle mass, and will rely on machines to do all the physical work.

Increased Myopia (Nearsightedness): Our ancestors lived in the wild. They required a large range of vision to scour the landscape. But, with most of us migrating to the cities, and with our cities getting more and more congested, we are hardly ever required to see more than 20 ft. away. This problem is more likely to escalate in the future. Thus, it is safe to assume that in the future, our myopia will increase and our hyperopia (farsightedness) will decrease.

Increased Skull Size: The muscle that works the most is the muscle that grows. This fact foretells that, considering that modern-day humans use their brains a lot more than older generations, it is quite possible that future humans will have increased skull size to accommodate their gigantic brains.

Lower Immunity: Advanced medicines of the future will enable us to fight off diseases much more efficiently than our natural immune system. Therefore, in the future, we will no longer need to have an immune system to protect us.

Racial Uniformity: Our ancestors originated in Africa, and later moved to various other continents, including Asia, Europe, Australia, and America. The different environmental and climatic conditions of these different regions caused them to undergo further physical changes, and this gave rise to different races and ethnicity. But thanks to the advances in the modes of transportation and communication, the world is becoming one again. With modern human society becoming a mixture of people of different ethnicity intermingling and living together, it is quite in the books that in the future, the lines of distinction between various races shall blur, and future human beings will most likely go back to being one uniform race again.

Lesser Teeth and Smaller Toes: Fossils of feet show that our ancestors had wider feet with longer and spread toes. But since hardly any of us goes about barefoot anymore, it is likely that in the future, our feet will be smaller with shorter toes. Also, since we ingest only cooked food, which is softer and hardly requires any chewing, possibly in the future, humans will have very small teeth.

Taller and Balder: Humans today are better fed and protected against the elements of nature. Thus, our descendants are most likely to grow taller as well as have lesser hair on their bodies.

Increased Life Expectancy: “And when he shall be immortal, he shall become a God.” ―Anonymous. Though immortality might still be a distant dream, humans of the future will definitely live longer and better lives, just like we do when compared to our ancestors.

Selective Evolution-playing God: With all the terrific advances in science, especially in the field of genetic engineering, parents in the future will literally be able to ‘manufacture’ their children based on their personal preferences.

So, there you have it, a collection of short stories, which when combined together, form the single great tale of human evolution. We have come a long way, and the journey until now has been more than just exciting. From a single cell floating in the ocean waters, we have managed to become the multi-cellular wonders of nature that we are today. Where we will go from here, predictions apart, only time can tell.

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Phenotypic Plasticity in Nematodes

The organism studied by Babayan et al., Litomosoides sigmodontis, is a nematode used as a model of human filarial diseases. The complex life cycle of the parasite is shown in Figure 1. For the purpose of this discussion, infection of a rodent begins with inoculation of larvae by an arthropod vector. The larvae then mature to adults that produce the transmission (microfilariae, Mf) stage (additional stages in the vector can be ignored for the present). We focus on the endemic situation where fitness is proportional to the total transmission (R0) rather than the rate of transmission. We assume that the rate of transmission at a given time is proportional to the number of Mf present at that time in this case the fitness of the parasite will be proportional to the area under the curve of a plot of the density of Mf over the course of infection.


Key challenge: forecasting eco-evolutionary dynamics

Detecting microevolution is only a starting point—we need tools and approaches for monitoring the rate of change [63] and predicting the impact of human-induced microevolution [80]. So far, most eco-evolutionary analyses have retrospectively explained what has already happened. Some branches of science can successfully predict the future. Astronomy, for example, can precisely predict when a comet will pass close to the Earth. Evolutionary ecology, however, has yet to build a predictive framework [92]. Given that it is already difficult to document and predict contemporary evolution, as heritable traits under apparently strong directional selection often fail to show the expected evolutionary response [49], it will be very challenging to achieve this. Improved evolutionary predictability, particularly as it relates to human stressors, is an area of major concern in urgent need of resolution [54]. However, we can try to apply models that will tell us under what circumstances inherited or plastic trait changes are likely to occur, and develop more robust models integrating the genetic basis of traits that can help forecast the consequences of trait changes on population processes [80]. Although ecological models, such as integral projection models which can predict how changes in phenotype affect population growth over the short and long term, offer powerful opportunities to link genotype and phenotype to individual and population performance, the proper integration of quantitative genetics is still needed [67, 68]. An important step forward in predicting adaptation to human-induced challenges will be to fully harness genomic data and ecological information in an integrative framework such as the evolutionary response architecture proposed by Bay et al. [80]. This concept proposes that evolutionary prediction should be based on the integration of knowledge of genetic architecture, spatial heterogeneity, phenotypic plasticity, and population dynamics.

Given increasing evidence of trait changes driven by human activity, it is critical to document the consequences of trait changes (plastic or genetic) on population dynamics, community, and ecosystem function in wild species, especially those under human threats. For example, documenting eco-evolutionary dynamics, such as whether evolutionary rescue occurs frequently in nature, is central to understanding how species may cope with large-scale human-driven environmental changes characteristic of the Anthropocene. A recent review by Mimura et al. underlined the importance of understanding and monitoring the effect of human-driven changes on intraspecific variation—a major component of biodiversity [88]. However, we see that documenting microevolution in nature is not a simple task, and few studies have met the burden of proof required. Thus, the importance of microevolution to population persistence is still unknown and is likely to fluctuate [100].

A potentially good starting point is to investigate the consequence of phenotypic changes at higher levels of biological organization and, in parallel, try to evaluate whether those trait changes are driven by genetic changes or plasticity. Indeed, several researchers have argued that trait changes caused by human activity may be shaping ecological dynamics on a global scale [14]. Since traits change in response to human activities approximately twice as fast as they respond to other drivers [11], the first step might be to use trait changes in forecasting models to tackle two questions: 1) under what circumstances are trait changes more likely to happen and 2) what are the ecological consequences of those changes? Given that it is the overall phenotypic change that is likely to feed back on ecological dynamics [14, 101], generalization of the circumstances favoring trait changes and their consequences will be very useful. Having said that, we ought not to focus solely on phenotypic variation because plastic and genetic changes occur on different time scales and the latter may be more difficult to reverse [34]. This interplay will impact our predictions for population persistence on the short and medium time-scale. Evolutionary demographic approaches that integrate both traits and demographic information could be useful tools to tackle these questions and make predictions on population parameters in the presence or absence of evolution [102], as long as genetic transmission is correctly integrated [68].

Another useful avenue may be to use intraspecific genetic and phenotypic changes to monitor human impacts on wild populations [88]. Indeed, trait changes could be integrated as early warning signs of population collapse [103]. Clements and Ozgul [103] showed that including phenotypic information on body size in composite early warning indices can more accurately predict critical transitions in population dynamics than using abundance time-series alone. This framework could easily be expanded to include intraspecific data on fitness-related genetic variation. So far, however, composite early warning signals have only been applied in controlled environments and it is yet to be determined whether they can be used to detect bifurcations in population dynamics (major increase or collapse) in the wild.


156 The Evolution of Primates

By the end of this section, you will be able to do the following:

  • Describe the derived features that distinguish primates from other animals
  • Describe the defining features of the major groups of primates
  • Identify the major hominin precursors to modern humans
  • Explain why scientists are having difficulty determining the true lines of descent in hominids

Order Primates of class Mammalia includes lemurs, tarsiers, monkeys, apes, and humans. Non-human primates live primarily in the tropical or subtropical regions of South America, Africa, and Asia. They range in size from the mouse lemur at 30 grams (1 ounce) to the mountain gorilla at 200 kilograms (441 pounds). The characteristics and evolution of primates are of particular interest to us as they allow us to understand the evolution of our own species.

Characteristics of Primates

All primate species possess adaptations for climbing trees, as they all descended from tree-dwellers. This arboreal heritage of primates has resulted in hands and feet that are adapted for climbing, or brachiation (swinging through trees using the arms). These adaptations include, but are not limited to: 1) a rotating shoulder joint, 2) a big toe that is widely separated from the other toes (except humans) and thumbs sufficiently separated from fingers to allow for gripping branches, and 3) stereoscopic vision , two overlapping fields of vision from the eyes, which allows for the perception of depth and gauging distance. Other characteristics of primates are brains that are larger than those of most other mammals, claws that have been modified into flattened nails, typically only one offspring per pregnancy, and a trend toward holding the body upright.

Order Primates is divided into two groups: Strepsirrhini (“turned-nosed”) and Haplorhini (“simple-nosed”) primates. Strepsirrhines, also called the wet-nosed primates, include prosimians like the bush babies and pottos of Africa, the lemurs of Madagascar, and the lorises of Southeast Asia. Haplorhines, or dry-nosed primates, include tarsiers ((Figure)) and simians (New World monkeys, Old World monkeys, apes, and humans). In general, strepsirrhines tend to be nocturnal, have larger olfactory centers in the brain, and exhibit a smaller size and smaller brain than anthropoids. Haplorhines, with a few exceptions, are diurnal, and depend more on their vision. Another interesting difference between the strepsirrhines and haplorhines is that strepsirrhines have the enzymes for making vitamin C, while haplorhines have to get it from their food.


Evolution of Primates

The first primate-like mammals are referred to as proto-primates. They were roughly similar to squirrels and tree shrews in size and appearance. The existing fossil evidence (mostly from North Africa) is very fragmented. These proto-primates remain largely mysterious creatures until more fossil evidence becomes available. Although genetic evidence suggests that primates diverged from other mammals about 85 MYA, the oldest known primate-like mammals with a relatively robust fossil record date to about 65 MYA. Fossils like the proto-primate Plesiadapis (although some researchers do not agree that Plesiadapis was a proto-primate) had some features of the teeth and skeleton in common with true primates. They were found in North America and Europe in the Cenozoic and went extinct by the end of the Eocene.

The first true primates date to about 55 MYA in the Eocene epoch. They were found in North America, Europe, Asia, and Africa. These early primates resembled present-day prosimians such as lemurs. Evolutionary changes continued in these early primates, with larger brains and eyes, and smaller muzzles being the trend. By the end of the Eocene epoch, many of the early prosimian species went extinct due either to cooler temperatures or competition from the first monkeys.

Anthropoid monkeys evolved from prosimians during the Oligocene epoch. By 40 million years ago, evidence indicates that monkeys were present in the New World (South America) and the Old World (Africa and Asia). New World monkeys are also called Platyrrhini—a reference to their broad noses ((Figure)). Old World monkeys are called Catarrhini—a reference to their narrow, downward-pointed noses. There is still quite a bit of uncertainty about the origins of the New World monkeys. At the time the platyrrhines arose, the continents of South American and Africa had drifted apart. Therefore, it is thought that monkeys arose in the Old World and reached the New World either by drifting on log rafts or by crossing land bridges. Due to this reproductive isolation, New World monkeys and Old World monkeys underwent separate adaptive radiations over millions of years. The New World monkeys are all arboreal, whereas Old World monkeys include both arboreal and ground-dwelling species. The arboreal habits of the New World monkeys are reflected in the possession of prehensile or grasping tails by most species. The tails of Old World monkeys are never prehensile and are often reduced, and some species have ischial callosities—thickened patches of skin on their seats.


Apes evolved from the catarrhines in Africa midway through the Cenozoic, approximately 25 million years ago. Apes are generally larger than monkeys and they do not possess a tail. All apes are capable of moving through trees, although many species spend most their time on the ground. When walking quadrupedally, monkeys walk on their palms, while apes support the upper body on their knuckles. Apes are more intelligent than monkeys, and they have larger brains relative to body size. The apes are divided into two groups. The lesser apes comprise the family Hylobatidae , including gibbons and siamangs. The great apes include the genera Pan (chimpanzees and bonobos) Gorilla (gorillas), Pongo (orangutans), and Homo (humans) ((Figure)).


The very arboreal gibbons are smaller than the great apes they have low sexual dimorphism (that is, the sexes are not markedly different in size), although in some species, the sexes differ in color and they have relatively longer arms used for swinging through trees ((Figure)a). Two species of orangutan are native to different islands in Indonesia: Borneo (P. pygmaeus) and Sumatra (P. abelii). A third orangutan species, Pongo tapanuliensis, was reported in 2017 from the Batang Toru forest in Sumatra. Orangutans are arboreal and solitary. Males are much larger than females and have cheek and throat pouches when mature. Gorillas all live in Central Africa. The eastern and western populations are recognized as separate species, G. berengei and G. gorilla. Gorillas are strongly sexually dimorphic, with males about twice the size of females. In older males, called silverbacks, the hair on the back turns white or gray. Chimpanzees ((Figure)b) are the species considered to be most closely related to humans. However, the species most closely related to the chimpanzee is the bonobo. Genetic evidence suggests that chimpanzee and human lineages separated 5 to 7 MYA, while chimpanzee (Pan troglodytes) and bonobo (Pan paniscus) lineages separated about 2 MYA. Chimpanzees and bonobos both live in Central Africa, but the two species are separated by the Congo River, a significant geographic barrier. Bonobos are slighter than chimpanzees, but have longer legs and more hair on their heads. In chimpanzees, white tail tufts identify juveniles, while bonobos keep their white tail tufts for life. Bonobos also have higher-pitched voices than chimpanzees. Chimpanzees are more aggressive and sometimes kill animals from other groups, while bonobos are not known to do so. Both chimpanzees and bonobos are omnivorous. Orangutan and gorilla diets also include foods from multiple sources, although the predominant food items are fruits for orangutans and foliage for gorillas.


Human Evolution

The family Hominidae of order Primates includes the hominoids: the great apes and humans ((Figure)). Evidence from the fossil record and from a comparison of human and chimpanzee DNA suggests that humans and chimpanzees diverged from a common hominoid ancestor approximately six million years ago. Several species evolved from the evolutionary branch that includes humans, although our species is the only surviving member. The term hominin is used to refer to those species that evolved after this split of the primate line, thereby designating species that are more closely related to humans than to chimpanzees. A number of marker features differentiate humans from the other hominoids, including bipedalism or upright posture, increase in the size of the brain, and a fully opposable thumb that can touch the little finger. Bipedal hominins include several groups that were probably part of the modern human lineage—Australopithecus, Homo habilis, and Homo erectus—and several non-ancestral groups that can be considered “cousins” of modern humans, such as Neanderthals and Denisovans.

Determining the true lines of descent in hominins is difficult. In years past, when relatively few hominin fossils had been recovered, some scientists believed that considering them in order, from oldest to youngest, would demonstrate the course of evolution from early hominins to modern humans. In the past several years, however, many new fossils have been found, and it is clear that there was often more than one species alive at any one time and that many of the fossils found (and species named) represent hominin species that died out and are not ancestral to modern humans.


Very Early Hominins

Three species of very early hominids have made news in the late 20th and early 21st centuries: Ardipithecus, Sahelanthropus, and Orrorin. The youngest of the three species, Ardipithecus, was discovered in the 1990s, and dates to about 4.4 MYA. Although the bipedality of the early specimens was uncertain, several more specimens of Ardipithecus were discovered in the intervening years and demonstrated that the organism was bipedal. Two different species of Ardipithecus have been identified, A. ramidus and A. kadabba, whose specimens are older, dating to 5.6 MYA. However, the status of this genus as a human ancestor is uncertain.

The oldest of the three, Sahelanthropus tchadensis, was discovered in 2001-2002 and has been dated to nearly seven million years ago. There is a single specimen of this genus, a skull that was a surface find in Chad. The fossil, informally called “Toumai,” is a mosaic of primitive and evolved characteristics, and it is unclear how this fossil fits with the picture given by molecular data, namely that the line leading to modern humans and modern chimpanzees apparently bifurcated about six million years ago. It is not thought at this time that this species was an ancestor of modern humans.

A younger (c. 6 MYA) species, Orrorin tugenensis, is also a relatively recent discovery, found in 2000. There are several specimens of Orrorin. Some features of Orrorin are more similar to those of modern humans than are the australopithicenes, although Orrorin is much older. If Orrorin is a human ancestor, then the australopithicenes may not be in the direct human lineage. Additional specimens of these species may help to clarify their role.

Early Hominins: Genus Australopithecus

Australopithecus (“southern ape”) is a genus of hominin that evolved in eastern Africa approximately four million years ago and went extinct about two million years ago. This genus is of particular interest to us as it is thought that our genus, genus Homo, evolved from a common ancestor shared with Australopithecus about two million years ago (after likely passing through some transitional states). Australopithecus had a number of characteristics that were more similar to the great apes than to modern humans. For example, sexual dimorphism was more exaggerated than in modern humans. Males were up to 50 percent larger than females, a ratio that is similar to that seen in modern gorillas and orangutans. In contrast, modern human males are approximately 15 to 20 percent larger than females. The brain size of Australopithecus relative to its body mass was also smaller than in modern humans and more similar to that seen in the great apes. A key feature that Australopithecus had in common with modern humans was bipedalism, although it is likely that Australopithecus also spent time in trees. Hominin footprints, similar to those of modern humans, were found in Laetoli, Tanzania and dated to 3.6 million years ago. They showed that hominins at the time of Australopithecus were walking upright.

There were a number of Australopithecus species, which are often referred to as australopiths. Australopithecus anamensis lived about 4.2 million years ago. More is known about another early species, Australopithecus afarensis, which lived between 3.9 and 2.9 million years ago. This species demonstrates a trend in human evolution: the reduction of the dentition and jaw in size. A. afarensis ((Figure)a) had smaller canines and molars compared to apes, but these were larger than those of modern humans. Its brain size was 380 to 450 cubic centimeters, approximately the size of a modern chimpanzee brain. It also had prognathic jaws, which is a relatively longer jaw than that of modern humans. In the mid-1970s, the fossil of an adult female A. afarensis was found in the Afar region of Ethiopia and dated to 3.24 million years ago ((Figure)). The fossil, which is informally called “Lucy,” is significant because it was the most complete australopith fossil found, with 40 percent of the skeleton recovered.



Australopithecus africanus lived between two and three million years ago. It had a slender build and was bipedal, but had robust arm bones and, like other early hominids, may have spent significant time in trees. Its brain was larger than that of A. afarensis at 500 cubic centimeters, which is slightly less than one-third the size of modern human brains. Two other species, Australopithecus bahrelghazali and Australopithecus garhi, have been added to the roster of australopiths in recent years. A. bahrelghazali is unusual in being the only australopith found in Central Africa.

A Dead End: Genus Paranthropus

The australopiths had a relatively slender build and teeth that were suited for soft food. In the past several years, fossils of hominids of a different body type have been found and dated to approximately 2.5 million years ago. These hominids, of the genus Paranthropus, were muscular, stood 1.3 to 1.4 meters tall, and had large grinding teeth. Their molars showed heavy wear, suggesting that they had a coarse and fibrous vegetarian diet as opposed to the partially carnivorous diet of the australopiths. Paranthropus includes Paranthropus robustus of South Africa, and Paranthropus aethiopicus and Paranthropus boisei of East Africa. The hominids in this genus went extinct more than one million years ago and are not thought to be ancestral to modern humans, but rather members of an evolutionary branch on the hominin tree that left no descendants.

Early Hominins: Genus Homo

The human genus, Homo, first appeared between 2.5 and three million years ago. For many years, fossils of a species called H. habilis were the oldest examples in the genus Homo, but in 2010, a new species called Homo gautengensis was discovered and may be older. Compared to A. africanus, H. habilis had a number of features more similar to modern humans. H. habilis had a jaw that was less prognathic than the australopiths and a larger brain, at 600 to 750 cubic centimeters. However, H. habilis retained some features of older hominin species, such as long arms. The name H. habilis means “handy man,” which is a reference to the stone tools that have been found with its remains.

Watch this video about Smithsonian paleontologist Briana Pobiner explaining the link between hominin eating of meat and evolutionary trends.

H. erectus appeared approximately 1.8 million years ago ((Figure)). It is believed to have originated in East Africa and was the first hominin species to migrate out of Africa. Fossils of H. erectus have been found in India, China, Java, and Europe, and were known in the past as “Java Man” or “Peking Man.” H. erectus had a number of features that were more similar to modern humans than those of H. habilis. H. erectus was larger in size than earlier hominins, reaching heights up to 1.85 meters and weighing up to 65 kilograms, which are sizes similar to those of modern humans. Its degree of sexual dimorphism was less than in earlier species, with males being 20 to 30 percent larger than females, which is close to the size difference seen in our own species. H. erectus had a larger brain than earlier species at 775 to 1,100 cubic centimeters, which compares to the 1,130 to 1,260 cubic centimeters seen in modern human brains. H. erectus also had a nose with downward-facing nostrils similar to modern humans, rather than the forward-facing nostrils found in other primates. Longer, downward-facing nostrils allow for the warming of cold air before it enters the lungs and may have been an adaptation to colder climates. Artifacts found with fossils of H. erectus suggest that it was the first hominin to use fire, hunt, and have a home base. H. erectus is generally thought to have lived until about 50,000 years ago.


Humans: Homo sapiens

A number of species, sometimes called archaic Homo sapiens, apparently evolved from H. erectus starting about 500,000 years ago. These species include Homo heidelbergensis, Homo rhodesiensis, and Homo neanderthalensis. These archaic H. sapiens had a brain size similar to that of modern humans, averaging 1,200 to 1,400 cubic centimeters. They differed from modern humans by having a thick skull, a prominent brow ridge, and a receding chin. Some of these species survived until 30,000 to 10,000 years ago, overlapping with modern humans ((Figure)).


There is considerable debate about the origins of anatomically modern humans or Homo sapiens sapiens . As discussed earlier, H. erectus migrated out of Africa and into Asia and Europe in the first major wave of migration about 1.5 million years ago. It is thought that modern humans arose in Africa from H. erectus and migrated out of Africa about 100,000 years ago in a second major migration wave. Then, modern humans replaced H. erectus species that had migrated into Asia and Europe in the first wave.

This evolutionary timeline is supported by molecular evidence. One approach to studying the origins of modern humans is to examine mitochondrial DNA (mtDNA) from populations around the world. Because a fetus develops from an egg containing its mother’s mitochondria (which have their own, non-nuclear DNA), mtDNA is passed entirely through the maternal line. Mutations in mtDNA can now be used to estimate the timeline of genetic divergence. The resulting evidence suggests that all modern humans have mtDNA inherited from a common ancestor that lived in Africa about 160,000 years ago. Another approach to the molecular understanding of human evolution is to examine the Y chromosome, which is passed from father to son. This evidence suggests that all men today inherited a Y chromosome from a male that lived in Africa about 140,000 years ago.

The study of mitochondrial DNA led to the identification of another human species or subspecies, the Denisovans. DNA from teeth and finger bones suggested two things. First, the mitochondrial DNA was different from that of both modern humans and Neanderthals. Second, the genomic DNA suggested that the Denisovans shared a common ancestor with the Neanderthals. Genes from both Neanderthals and Denisovans have been identified in modern human populations, indicating that interbreeding among the three groups occurred over part of their range.

Section Summary

All primate species possess adaptations for climbing trees and probably descended from arboreal ancestors, although not all living species are arboreal. Other characteristics of primates are brains that are larger, relative to body size, than those of other mammals, claws that have been modified into flattened nails, typically only one young per pregnancy, stereoscopic vision, and a trend toward holding the body upright. Primates are divided into two groups: strepsirrhines, which include most prosimians, and haplorhines, which include simians. Monkeys evolved from prosimians during the Oligocene epoch. The simian line includes both platyrrhine and catarrhine branches. Apes evolved from catarrhines in Africa during the Miocene epoch. Apes are divided into the lesser apes and the greater apes. Hominins include those groups that gave rise to our own species, such as Australopithecus and H. erectus, and those groups that can be considered “cousins” of humans, such as Neanderthals and Denisovans. Fossil evidence shows that hominins at the time of Australopithecus were walking upright, the first evidence of bipedal hominins. A number of species, sometimes called archaic H. sapiens, evolved from H. erectus approximately 500,000 years ago. There is considerable debate about the origins of anatomically modern humans or H. sapiens sapiens, and the discussion will continue, as new evidence from fossil finds and genetic analysis emerges.


DISCUSSION

Contrary to our expectations, striking with a clenched fist appears to provide little or no performance advantage in terms of the force applied to the target. The force and force impulse of both forward and overhead strikes were not different when the subjects struck with a fist or an open palm. Our subjects did demonstrate a 15% increase in force impulse in side strikes with a fist, but the peak force of side strikes with a fist were not significantly greater than side slaps. Additionally, the maximum rate of change acceleration (i.e. jerk), which has been implicated in both traumatic brain and musculoskeletal injury (Ivancevic, 2009a Ivancevic, 2009b), was not different when the subjects struck with a fist or an open palm.

Although the forcefulness of a strike can be important to the outcome of a fight (e.g. accelerating the body, knocking an opponent off their feet or accelerating the head, causing unconsciousness or a concussion), local tissue damage, such as bone fracture and contusion, is produced not by force but by stress (force per area) (Farlow et al., 2000). Given that our subjects produced similar peak forces and force impulses when striking with fists and palms, striking with a fist increases the peak stress imposed on the target. We do not know the degree to which the applied pressure was uniform under the hand however, because the bag was deformable we can be confident that pressure was applied by the whole ventral surface of the hand, including both the palm and the fingers. The striking surface area of a fist is less than one-third the area of the whole hand and

60% of the area of the palm. This means that if the total force applied in a strike is the same, then the stress in the targeted tissue will be 1.7 to 3.0 times greater in a fist strike than in a palm strike. Thus, although striking with a fist appears not to result in more forceful strikes, fists increase the peak stresses that are imposed on the target and, therefore, the potential for injury.

Possibly the most significant result of this study is the finding that the structure of the human fist provides protective buttressing of the metacarpals, MCP joints and phalanges. Compared with the unbuttressed fist posture, stiffness of the second MCP joint doubled when the distal phalangeal pads were buttressed against the central palm and the palmar pads of the proximal phalanges. Stiffness of this joint doubled again when the thumb and the thenar eminence were rotated to firmly grip the dorsal surface of the distal phalanges of digits 2 and 3. Presumably, this fourfold increase in stiffness of the second MCP joint reflects significant increases in the stiffness of the MCP joints of digits 3–5. Increased MCP joint stiffness protects the MCP joints from extreme hyperflexion and likely reduces resulting bending moments on the metacarpals when the fist strikes a target. The subjects were able to support 79% of their upper body weight on their proximal phalanges, rather than their metacarpals, when the fist was fully buttressed. In contrast, when the fist was unbuttressed and the wrist was stabilized, the subjects were able to support only 32% of upper body weight on the proximal phalanges. This illustrates the extent to which force from digits 2 and 3 can be transferred through the thumb and thenar eminence to the wrist. Thus, the buttressing of the hand that is intrinsic to a formed fist: (1) protects the MCP joints from hyperflexion (2) secures the individual digits in a tight configuration that prevents potentially harmful strain at the interphalangeal joints (3) presumably helps to keep the metacarpals loaded in long-axis compression rather than bending and (4) makes possible a transfer of energy from digits 2 and 3, through the thenar eminence, to the wrist, unloading the metacarpals. This protective buttressing requires an integration of the proportions of the skeletal elements of the hand and may represent the primary advantage in striking with a fist.

Precision pad-to-pad grip could have evolved for manual manipulation in ways that are not compatible with a buttressed fist. A precision grip could have evolved through: (1) a shortening of the metacarpals and fingers and a lengthening of the thumb, as occurred in the hominin lineage (2) a substantial lengthening of the thumb ray only or (3) a predominant shortening of either the finger rays only or metacarpals 2–5 only. Importantly, these alternatives do not require strict coordination of the relative lengths of the phalanges or coordination of the length of the metacarpals (2–5) with the length of the first metacarpal.

In contrast to the geometry necessary for precision grip, a buttressed fist requires specific proportions among the skeletal elements of the hand. The fist of humans is characterized by buttressing of the tips of the distal phalanges against the palm and the pads of the distal phalanges against the palmar skin over the proximal phalanges. This dual contact requires integration of the relative lengths of the three phalanges and an integration of the lengths of the fingers with their diameter. If the phalanges of digits 2–5 were longer, as in members of the genus Pan, the tips of the distal phalanges could abut the palm, but the primary phalangeal pads would not abut the palmar pads of proximal phalanges, leaving a destabilizing space between the proximal and distal phalanges. If the proximal phalanges were too long relative to the distal phalanges, the tips of the fingers would not reach the palm. If the middle phalanges were too long, the primary phalangeal pads would also not abut the palmar pads of the proximal phalanges. Additionally, significant increases or decreases in the length of the distal phalanges would compromise the 90 deg angle between the metacarpals and proximal phalanges that forms the striking surface of the fist. As stated before, the length of the first metacarpal in relation to the lengths of metacarpals 2 and 3 is necessary for the precise integration that allows buttressing with the thenar eminence. Thus, the geometry of a fully buttressed fist provides a clear explanation for the specific skeletal proportions of the human hand.

Specialization of the hand for punching during the evolution of early hominins is consistent with proposed anatomical specialization for physical aggression (Carrier, 2004 Carrier, 2007 Carrier, 2011) and the apparent patterns of sexual dimorphism in these fossil species. Most species of early hominins (Australopithecus and Paranthropus) appear to have had pronounced sexual dimorphism in body size, with males being bigger than females (McHenry, 1996 Gordon et al., 2008 but for an alternative view see Reno et al., 2010). Among mammals, species in which males are larger than females tend to have polygynous mating systems and males compete physically for reproductive access to females (Clutton-Brock et al., 1982 Jarman, 1983 Parker, 1983 Alexander et al., 1979 Andersson, 1994). Specifically among primates, there is a positive correlation between size sexual dimorphism and the number of adult females per adult male in breeding groups (Clutton-Brock et al., 1977). Analyses of anthropoid primates show that size sexual dimorphism is strongly associated with both male–male competition levels and the ratio of mature males to females that are ready to mate (Plavcan and van Schaik, 1997a Plavcan and van Schaik, 1997b Plavcan, 1999 Plavcan, 2004). Thus, the evidence for size sexual dimorphism in early hominins suggests the presence of polygynous mating systems with high levels of male–male competition.

The forelimbs of great apes exhibit relatively high levels of sexual dimorphism. In Australopithecus afarensis, for example, the difference between large and small ulnae, radii and capitates is as great or greater than that between male and female means of the most dimorphic extant apes (McHenry, 1986 McHenry, 1991 McHenry, 1996). Forelimbs also appear to have been relatively dimorphic in both A. africanus and Paranthropus boisei (McHenry, 1996). In lowland gorillas, the greatest sexual dimorphism is in the weight of the forelimbs, the forelimb trunk binding muscles and the epaxial muscles (Zihlman and McFarland, 2000). In addition, in humans, the arms and upper body are more sexually dimorphic than the legs (Price et al., 2011) and the greatest dimorphism in size appears to be in the forearm and hand (Lindegard, 1953). Additionally, as would be expected if human hand proportions evolved as a result of sexual selection, there is also dimorphism in the shape of the hand. The ratio between the lengths of the second and fourth digits is lower in males than in females (Manning et al., 1998). This ratio is negatively correlated with levels of prenatal and adult testosterone (Manning et al., 1998), performance and success in football (soccer) (Manning and Taylor, 2001), and perceived male dominance (Neave et al., 2003). Importantly, among mammals, sexual dimorphism is often greatest in those characters that enhance a male's capacity to dominate other males (Parker, 1983 Andersson, 1994 Clutton-Brock and Harvey, 1977). Thus, the relatively high levels of sexual dimorphism in the arm and hand are consistent with the hypothesis that the proportions of the human hand have been influenced by sexual selection.

The skeletal proportions that make the buttressed fist of modern humans possible appear to have evolved at approximately the same time as habitual bipedalism. The earliest habitual biped Orrorin tugenensis, dating from 6 million years ago (Pickford et al., 2002 Richmond and Jungers, 2008), had a thumb anatomy that is more human-like than that of australopiths and displayed typical human-like features related to precision grasping (Almécija et al., 2010). In contrast, the hands of the 4.4-million-year-old hominin, Ardipithecus ramidus, are suggested to have been adapted for climbing and possibly foraging in distal branches, and are more similar in proportion to those of monkeys than to those of modern great apes and humans (Lovejoy et al., 2009 Crompton et al., 2010). Nevertheless, the earliest undisputed hominins, the australopiths, had manual proportions very similar to those of modern humans. Recent analysis of A. afarensis from locality AL 333/333w (Hadar, Ethiopia) indicates that this species possessed overall manual proportions, including an increased thumb/hand relationship, that ‘… is fully human and would have permitted pad-to-pad human-like precision grip capability’ (Alba et al., 2003). Based on the relative proportions of metacarpals 1–4, Australopithecus africanus also appears to have had human-like hand proportions (Green and Gordon, 2008). Well-preserved pollical metacarpal and distal phalangeal bones from the australopith Paranthropus robustus indicate that this 1.8-million-year-old contemporary of Homo also had hands that were adapted for precision grasping (Susman, 1994 Susman, 1988). A nearly complete hand of Australopithecus sediba (1.98 million years ago) demonstrates that this species had short fingers, a long thumb with a human-like palmar pad and a mobile proximal pulp, and a strong flexor pollicis longus muscle, all features that have been associated with a powerful, precision grip (Kivell et al., 2011). Thus, the evolution of human-like manual proportions were largely coincident with the evolution of habitual bipedalism. This is likely the result of selection for increased manual dexterity being released from the constraining influence of selection for performance in an arboreal environment (Alba et al., 2003). Alternatively, it has been suggested that the hand proportions of hominins may partially, or largely, be a pleiotrophic result of selection on the foot for terrestrial locomotion (Alba et al., 2003 Rolian et al., 2010). A third reason that human-like manual proportions appear in the fossil record coincident with evidence of habitual bipedalism is that sexual selection for improved fighting performance may have contributed to the evolution of both (Carrier, 2011 present study).

There appears to be a paradox in the evolution of the human hand. It is arguably our most important anatomical weapon, used to threaten, beat and sometimes kill to resolve conflict. Yet it is also the part of our musculoskeletal system that crafts and uses delicate tools, plays musical instruments, produces art, conveys complex intentions and emotions, and nurtures. Starting with the hand of an arboreal great ape ancestor, it is possible to imagine a number of evolutionary transformations that would have resulted in a club-like structure adapted for fighting. Similarly, as suggested above, there are a number of alternative hand proportions that are compatible with enhanced manual dexterity. There may, however, be only one set of skeletal proportions that allows the hand to function both as a mechanism for precise manipulation and as a club for striking. More than any other part of our anatomy, the hand represents the identity of Homo sapiens. Ultimately, the evolutionary significance of the human hand may lie in its remarkable ability to serve two seemingly incompatible, but intrinsically human, functions.


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Comments:

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