Information

Why do living fossils like crocodiles remain so constant and not evolve?

Why do living fossils like crocodiles remain so constant and not evolve?



We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Crocodiles have supposedly remained unchanged for millions of years, and several other species are considered as "living fossils". How do such species remain so constant over time given that they will have had so much time to accumulate new mutations?


Evolution is a process of change by four mechanisms; mutation, migration, drift, and selection.

You are correct in thinking that, because crocodiles have been around for a long time, they could have accumulated many new mutations in that time, relative to other more recent species. However, mutation is only one of the important mechanisms underlying evolution.

How different are ancestral and modern crocodiles?

It seems the appearance of crocodiles has been fairly unchanged since their occurrence ~85 million years ago (mya).

How can they remain so unchanged?

Genetic variation may have been low in the ancestral population, this would reduce the potential for evolutionary change, as most change would have to occur through new mutations. It seems the populations of ancestral crocodiles were quite small, such that a genetic bottleneck may have occurred (which would reduce genetic variation). Mutation rates seem to be relatively low in crocodiles (also see here) which would reduce the rate at which novel mutation occurs, reducing the potential for evolution.

Note that many mutations will be neutral in their effect (or "synonymous") so won't have an obvious phenotypic effect, so there may be substantial evolution at the genetic level despite the phenotypic similarity.

Low rates of evolutionary change could suggests some other things may have also played a factor. Given that the populations have been through multiple genetic bottlenecks, genetic drift could have slowed down rates of evolution eroding genetic variance, removing rare mutations from the population.

If selection has been fairly constant over time then there is less chance that changes will occur. If selection were to change and favour new adaptations then these are likely to spread, but if selection remains fairly constant over time then it will continue to favour the same mutations. After a long time of consistent selection it is likely that most mutations will be deleterious (have a negative effect) and be removed from the population by selection. Darwin suggested that living fossils could occur because the environment they are in has remained fairly constant (from this link).


Crocodylomorpha were actually once a lot more varied than they are today, so their group isn't immune to change or evolution.

The tongue in cheek answer is to say, there were a lot more forms of crocodile in the past so chances are the single form we see today would look like one of them!

A better answer is: Species often evolve to fit a niche, they become specialised in their form both externally and internally. As long as this niche stays the same and neighbouring niches remain filled, the species will only become more suited to its niche. Over time you'd expect new mutations which made an individual fitter to become rarer, or have a smaller impact on survival so be selected for less strongly, thus the longer a niche exists, the more stable the form of animals living in it will become.


I think in the case of crocodiles in particular, this is a question of body type. Crocodiles have fixed jaws, meaning that they have lost the mechanism used to move their lower jaw from side to side. This simplification means that they can exert a huge amount of power, making it a great adaptation for their particular strategy of hiding in water and ambushing large land mammals that drink from it.

However, it also means that it's very hard for them to adapt to any other niche. Anything that would require eating smaller prey, or eating on land, would be untenable, because crocodiles can't chew. (Instead they close their mouth and rotate their whole body in the water to rip chunks of flesh off.) They can't easily re-evolve the ability to chew, because the mechanism needed to do that is pretty complicated, so their fixed jaw is more or less "locked in" evolutionarily.

So crocodiles have remained constant because they are very well adapted to a particular niche; because they've lost features that would allow them to adapt to other niches; and because the niche they occupy has been around for a very long time.

Personally I think these are the primary reasons. The low mutation rate mentioned in the other answers seems to me more likely to be effect than cause. If you are very tightly adapted to a particular niche then there is less advantage in genetically exploring other possibilities, so the descendants of individuals with lower mutation rates would have an advantage over those with higher mutation rates. This would provide an evolutionary pressure for a low mutation rate, which might explain the observation. (But this last paragraph is speculation on my part. I work on modelling evolution and I know that this kind of selection on mutation rates can occur, but I have no evidence at all about whether it happens in crocodiles.)


I think rg255 and Troyseph pretty much nailed it, but another thing to consider is the crocodilian's habitat. All surviving forms are aquatic, with at least one species - the marine crocodile - at home in the sea. In addition, most, if not all, crocodilians live in tropical or subtropical regions.

In fact, many "living fossils" are associated with tropical forests or the sea. The marine coelacanth is one of the most celebrated living fossils, for example. The most primitive living fishes also include the lungfishes, which are generally freshwater tropical species.

The tropical monotremes are considered the most primitive living mammals (and the platypus is semi-aquatic to boot).

Their niche and "biology" also make it relatively difficult for crocodiles to exploit other niches. Even if they were't tied to water, one could hardly imagine a crocodile climbing over a high, cold mountain pass to reach a lush, tropical forest on the other side.

It's worth noting that all the terrestrial crocodilians have died out, leaving only their aquatic or semi-aquatic relatives.

Here's one interesting source -- 12 of the most astounding 'living fossils' known to science.

Most of the species listed (including the coelacanth) are marine organisms. It also lists crocodiles, and I was surprised to learn that crocs evolved from marine organisms themselves. (See This handsome sea creature is where crocodiles came from).

Similar lists include tropical forest creatures, like the African okapi and Australasian cassowary. Note that rhinoceroses and tapirs now occur only in the tropics (and perhaps subtropical regions for African rhinos), now that the more recently evolved woolly rhinoceros is extinct.


Well, two things.

First the assumption that creatures that are living fossils are strong as a species isn't strictly true. Evolution; especially in relatively long-lived animals like crocodiles and sharks (compared to dogs, houseflies, j-walkers, etc.) is such a long process. It isn't factually verifiable that they are doing well as a species, only that they are in a stable food chain.

Second; and User23715 raises all these point, animals that aren't exposed to evolutionary motivators (mutagens, predation, and habitant change) won't have as much genetic divergence. A crocodile won't benefit from a Darwinian advantage if it doesn't provide enough of a survival advantage to eventually fuel a divergent evolution. Any dominant trait needs to be aggressive enough to stay dominant. In a simple dominant-recessive model it would need to be twice as potent for hundreds of generations (close to 2000 years for crocodilian life spans and birth rates).


A reptilian anachronism: American alligator older than we thought

From climate to the peninsula’s very shape, not much in Florida has stayed the same over the last 8 million years.

Except, it turns out, alligators.

While many of today’s top predators are more recent products of evolution, the modern American alligator is a reptile quite literally from another time. New University of Florida research shows these prehistoric-looking creatures have remained virtually untouched by major evolutionary change for at least 8 million years, and may be up to 6 million years older than previously thought. Besides some sharks and a handful of others, very few living vertebrate species have such a long duration in the fossil record with so little change.

“If we could step back in time 8 million years, you’d basically see the same animal crawling around then as you would see today in the Southeast. Even 30 million years ago, they didn’t look much different,” said Evan Whiting, a former UF undergraduate and the lead author of two studies published during summer 2016 in the Journal of Herpetology and Palaeogeography, Palaeoclimatology, Palaeoecology that document the alligator’s evolution – or lack thereof. "We were surprised to find fossil alligators from this deep in time that actually belong to the living species, rather than an extinct one."

Whiting, now a doctoral student at the University of Minnesota, describes the alligator as a survivor, withstanding sea-level fluctuations and extreme changes in climate that would have caused some less-adaptive animals to rapidly change or go extinct. Whiting also discovered that early American alligators likely shared the Florida coastline with a 25-foot now-extinct giant crocodile.

In modern times, however, he said alligators face a threat that could hinder the scaly reptiles’ ability to thrive like nothing in their past — humans.

Despite their resilience and adaptability, alligators were nearly hunted to extinction in the early 20th century. The Endangered Species Act has significantly improved the number of alligators in the wild, but there are still ongoing encounters between humans and alligators that are not desirable for either species and, in many places, alligator habitats are being destroyed or humans are moving into them, Whiting said.

“The same traits that allowed alligators to remain virtually the same through numerous environmental changes over millions of years can become a bit of a problem when they try to adapt to humans,” Whiting said. “Their adaptive nature is why we have alligators in swimming pools or crawling around golf courses.”

Whiting hopes his research findings serve to inform the public that the alligator was here first, and we should act accordingly by preserving the animal’s wild populations and its environment. By providing a more complete evolutionary history of the alligator, his research provides the groundwork for conserving habitats where alligators have dominated for millions of years.

“If we know from the fossil record that alligators have thrived in certain types of habitats since deep in time, we know which habitats to focus conservation and management efforts on today,” Whiting said.

Study authors began re-thinking the alligator’s evolutionary history after Whiting examined an ancient alligator skull, originally thought to be an extinct species, unearthed in Marion County, Florida, and found it to be virtually identical to the iconic modern species. He compared the ancient skull with dozens of other fossils and modern skeletons to look at the whole genus and trace major changes, or the lack thereof, in alligator morphology.

Whiting also studied the carbon and oxygen compositions of the teeth of both ancient alligators and the 20- to 25-foot extinct crocodile Gavialosuchus americanus that once dominated the Florida coastline and died out about 5 million years ago for unknown reasons. The presence of alligator and Gavialosuchus fossils at several localities in north Florida suggest the two species may have coexisted in places near the coast, he said.

Analysis of the teeth suggests, however, that the giant croc was a marine reptile, which sought its prey in ocean waters, while alligators tended to hunt in freshwater and on land. That doesn’t mean alligators weren’t occasionally eaten by the monster crocs, though.

“Evan’s research shows alligators didn’t evolve in a vacuum with no other crocodilians around,” said co-author David Steadman, ornithology curator at the Florida Museum of Natural History at the University of Florida. “The gators we see today do not really compete with anything, but millions of years ago it was not only competing with another type of crocodilian, it was competing with a much larger one.”

Steadman said the presence of the ancient crocodile in Florida may have helped keep the alligators in freshwater habitats, though it appears alligators have always been most comfortable in freshwater.

While modern alligators do look prehistoric as they bake on sandbars along the Suwannee River or stroll down sidewalks on the UF campus, study authors said they are not somehow immune to evolution. On the contrary, they are the result of an incredibly ancient evolutionary line. The group they belong to, Crocodylia, has been around for at least 84 million years and has diverse ancestors dating as far back as the Triassic, more than 200 million years ago.

Other study co-authors were John Krigbaum with UF’s anthropology department and Kent Vliet with UF’s biology department.


Many modern animals in dinosaur rock!

I asked Carl just how many modern types of animals he had found in the dinosaur rock layers.

&ldquoWe found fossilized examples from every major invertebrate animal phylum living today including: arthropods (insects, crustaceans etc.), shellfish, echinoderms (starfish, crinoids, brittle stars, etc.), corals, sponges, and segmented worms (earthworms, marine worms).

&ldquoThe vertebrates&mdashanimals with backbones such as fish, amphibians, reptiles, birds and mammals&mdashshow this same pattern.&rdquo


Squamates are the most diverse of all the reptile groups, with approximately 7,400 living species. Squamates include lizards, snakes, and worm-lizards. Squamates first appeared in the fossil record during the mid-Jurassic and probably existed before that time. The fossil record for squamates is rather sparse. Modern squamates arose about 160 million years ago, during the late Jurassic Period. The earliest lizard fossils are between 185 and 165 million years old.


Warm or Cold? Dinosaurs Had 'In-Between' Blood

Dinosaurs may not have been cold-blooded like modern reptiles or warm-blooded like mammals and birds — instead, they may have dominated the planet for 135 million years with blood that ran neither hot nor cold, but was a kind of in-between that's rare nowadays, researchers say.

Modern reptiles such as lizards, snakes and turtles are cold-blooded or ectothermic, meaning their body temperatures depend on their environments. Birds and mammals, on the other hand, are warm-blooded, meaning they control their own body temperatures, attempting to keep them at a safe constant — in the case of humans, at about 98.6 degrees Fahrenheit (37 degrees Celsius).

Dinosaurs are classified as reptiles, and so for many years scientists thought the beasts were cold-blooded, with slow metabolisms that forced them to lumber across the landscape. However, birds are modern-day dinosaurs and warm-blooded, with fast metabolic rates that give them active lifestyles, raising the question of whether or not their extinct dinosaur relatives were also warm-blooded. [Avian Ancestors: Dinosaurs That Learned to Fly (Images)]

Animal metabolism

To help solve this decades-old mystery, researchers developed a new method for analyzing the metabolism of extinct animals. They found "dinosaurs do not fit comfortably into either the cold-blooded or warm-blooded camp — they genuinely explored a middle way," said lead study author John Grady, a theoretical ecologist at the University of New Mexico.

Scientists often seek to deduce the metabolisms of extinct animals by looking at the rates at which their bones grow. The method resembles cutting into a tree and looking at the thickness of the rings of wood within, which can reveal how well or poorly that tree grew any given year. Similarly, looking at the way bone is deposited in layers in fossils reveals how quickly or slowly that animal might have grown.

Grady and his colleagues not only looked at growth rings in fossils, but also sought to estimate their metabolic rates by looking at changes in body size as animals grew from birth to adults. The researchers looked at a broad spectrum of animals encompassing both extinct and living species, including cold- and warm-blooded creatures, as well as dinosaurs.

The scientists found growth rate to be a good indicator of metabolic rates in living animals, ranging from sharks to birds. In general, warm-blooded mammals that grow about 10 times faster than cold-blooded reptiles also metabolize about 10 times faster.

When the researchers examined how fast dinosaurs grew, they found that the animals resembled neither mammals nor modern reptiles, and were neither ectotherms nor endotherms. Instead, dinosaurs occupied a middle ground, making them so-called "mesotherms."

Modern mesotherms

Today, such energetically intermediate animals are uncommon, but they do exist. For instance, the great white shark, tuna and leatherback sea turtle are mesotherms, as is the echidna, an egg-laying mammal from Australia. Like mammals, mesotherms generate enough heat to keep their blood warmer than their environment, but like modern reptiles, they do not maintain a constant body temperature. [See Photos of Echidna and Other Bizarre Monotremes]

"For instance, tuna body temperature declines when they dive into deep, colder waters, but it always stays above the surrounding water," Grady told Live Science.

Body size may play a role in mesothermy, because larger animals can conserve heat more easily. "For instance, leatherback sea turtles are mesotherms, but smaller green sea turtles are not," Grady said. However, mesothermy does not depend just on large size. "Mako sharks are mesotherms, but whale sharks are regular ectotherms," Grady said.

Endotherms can boost their metabolisms to warm up — "for instance, we shiver when cold, which generates heat," Grady said. "Mesotherms have adaptations to conserve heat, but they do not burn fat or shiver to warm up. Unlike us, they don't boost their metabolic rate to stay warm."

Some animals are what are known as gigantotherms, meaning they are just so massive that they maintain heat even though they do not actively control their body temperature.

"Gigantotherms like crocodiles rely on basking to heat up, so they are not mesotherms," Grady said. "Gigantotherms are slower to heat up and cool down, but if they rely on external heat sources like the sun, then they are not mesotherms. In general, mesotherms produce more heat than gigantotherms and have different mechanisms for conserving it."

Advantages of being a mesotherm

Mesothermy would have permitted dinosaurs to move, grow and reproduce faster than their cold-blooded reptilian relatives, making the dinosaurs more dangerous predators and more elusive prey. This may explain why dinosaurs dominated the world until their extinction about 65 million years ago, Grady suggested.

At the same time, dinosaurs' lower metabolic rates compared to mammals allowed them to get by on less food. This may have permitted the enormous bulk that many dinosaur species attained. "For instance, it is doubtful that a lion the size of T. rex would be able to eat enough wildebeests or elephants without starving to death," Grady said. "With their lower food demands, however, a real T. rex was able to get by just fine."

All in all, Grady suspected that where direct competition occurs, warm-blooded endotherms suppress mesotherms, mesotherms suppress active but cold-blooded ectotherms, and active ectotherms suppress more lethergic sit-and-wait ectotherms

Although mesothermy appears widespread among dinosaurs, not every dinosaur was necessarily a mesotherm, Grady said. "Dinosaurs were a big and diverse bunch, and some may have been endotherms or ectotherms," he said. "In particular, feathered dinosaurs are a bit of a mystery. What do you call a metabolically intermediate animal covered in feathers? Is it like the mesothermic echidna? Or just a low-power endotherm?"

The first bird, Archaeopteryx, "was more like a regular dinosaur than any living bird," Grady said. "It grew to maturity in about two years. In contrast, a similarly sized hawk grows in about six weeks, almost 20 times faster. Despite feathers and the ability to take flight, the first birds were not the active, hot-blooded fliers their descendants came to be."

These findings could help shed light on how warm-blooded animals such as humans evolved.

"The origins of endothermy in mammals and birds are unclear," Grady said. Studying the growth rates of the ancestors of birds and mammals "will shed light on these mysterious creatures."

The scientists detailed their findings in the June 13 issue of the journal Science.


In defense of living fossils

Lately there has been a wave of criticism of the concept of living fossils. First, recent research has challenged the status of paradigmatic living fossil taxa, such as coelacanths, cycads, and tuataras. Critics have also complained that the living fossil concept is vague and/or ambiguous, and that it is responsible for misconceptions about evolution. This paper defends a particular phylogenetic conception of living fossils, or taxa that (a) exhibit deep prehistoric morphological stability (b) contain few extant species and (c) make a high contribution to phylogenetic diversity. The paper shows how this conception of living fossils can make sense of recent research on contested cases. The phylogenetic living fossil concept has both theoretical and practical importance: theoretical, because it picks out an important explanatory target for evolutionary theory and practical, because it picks out taxa that we might wish to prioritize for conservation. The best way to defend the concept of living fossils is to get clearer about the reasons for defending living fossil taxa.

This is a preview of subscription content, access via your institution.


What You Will Learn

Evolutionists hold up the fossil record as evidence that evolution has taken place over the billions of years of earth history. Evolution is based on the presupposition that those billions of years have occurred. Using radiometric dating and other dating methods, evolutionists claim that life began on earth about 3.5 billion years ago. Creationists, using the Bible as a starting point, claim that the earth is only about 6,000 years old—and there is abundant evidence that is consistent with this claim. The huge difference is because evolutionists accept uniformitarian (slow and steady) assumptions while creationists believe the worldwide presence of rock layers containing fossils can best be explained by catastrophic processes (rapid and abrupt). Observational science has shown that fossils and rocks can form rapidly. The idea of a young earth is not compatible with evolution .

The sequence of fossils and the rock layers are supposed to strengthen the case for evolution . Two problems with this idea are that many of the fossils occur in the wrong layer sequence and that the rock layers are often bent to an amazing degree—both are evidence for a recent catastrophe. The geologic record should also be riddled with thousands of transitional forms that show a slow and gradual progression, as well as the many dead ends in the evolutionary story. The absence of these transitional forms and the abrupt appearance of many complex life forms is evidence that these groups were created by God and then later buried in the global Flood of Noah’s day. The recent discoveries of dinosaur bones with “fresh” tissue and fossilized amphibian bones with intact bone marrow clearly point to the fact that dinosaurs lived recently. In fact, they shared the earth with humans beginning on Day 6 of creation, just thousands of years ago. That history is accurately recorded in the Bible . Starting with the wrong presuppositions has led people to believe in the wrong history of the earth. Evolution fails to provide a consistent account of earth history, while the biblical creation model puts the evidence into a consistent framework.


Boundaries

English geologist John Phillips, the first person to create the global geologic timescale, first coined the term Mesozoic in the 1800s. Phillips found ways to correlate sediments found around the world to specific time periods, said Paul Olsen, a geoscientist at the Lamont-Doherty Earth Observatory at Columbia University in New York.

The Permian-Triassic boundary, at the start of the Mesozoic, is defined relative to a particular section of sediment in Meishan, China, where a type of extinct, eel-like creature known as a conodont first appeared, according to the International Commission on Stratigraphy.

The end boundary for the Mesozoic era, the Cretaceous-Paleogene boundary, is defined by a 20-inch (50 centimeters) thick sliver of rock in El Kef, Tunisia, which contains well-preserved fossils and traces of iridium and other elements from the asteroid impact that wiped out the dinosaurs. The Mesozoic era is divided up into the Triassic, Jurassic, and Cretaceous periods.


The Institute for Creation Research

The evolutionary story is one of constant change. It proposes that simpler life forms evolved into complicated organisms whose offspring branched out in ever more diverse directions. But the modern forms of some creatures are so similar to their ancestors&rsquo fossils that it is clear they haven&rsquot changed much at all. If some species diversified, why didn&rsquot others?

In a recent study, Michael Alfaro and colleagues took a close look at groups of animals that &ldquodiverged&rdquo over the course of &ldquogeologic time,&rdquo compared to those animals that stayed the same. His team analyzed diversity among animal groups through the fossil record, publishing their research online in the Proceedings of the National Academy of Sciences. 1

Some Cretaceous and especially Cenozoic strata contain an abrupt increase in the number of different mammal species, an event sometimes called the mammalian explosion. Alfaro began his investigation under the assumption that this profusion of mammal fossils represented their evolutionary &ldquodivergence&rdquo into various forms. Then, assuming the standard deep timescales assigned to rock layers, his team put numbers to the presumed evolutionary acceleration, concluding that mammals evolved seven times faster than expected during this time. But in the same period, other animals like tuataras (a lizard-like reptile) did not evolve at all.

Why did some animals have so much change so quickly, while others had almost none? In a University of California, Los Angeles press release, Alfaro stated:

That is one of the big mysteries about biodiversity&hellip.Why these evolutionary losers are still around is a very hard thing to explain. They have been drawing inside straights for hundreds of millions of years. It&rsquos a real mystery to biologists how there can be any tuataras, given their low rate of speciation. 2

After a few hundred million years of evolutionary existence, some kind of changes should be found. In fact, after all that time, there ought to be a record of dramatic, sequential changes to the tuatara form if natural selection of beneficial mutations were actually responsible for generating the diversity of life observed on earth.

The persistence of the unchanged tuatara form in the fossil record indicates that the massive timescales attached to the various rock strata must be in error. But rather than allow this evidence to challenge his assumption of deep time, Alfaro suggested that there must be some unknown naturalistic mechanism of preservation that counteracted the Darwinian naturalistic mechanism of change. Thus, for some unknown reason, tuataras, alligators, and crocodiles&mdashbut not mammals or birds&mdashwere almost miraculously preserved.

The research also contradicted the standard reasons given for the superior numbers of mammals, birds, and fish. Alfaro stated that &ldquothe timing of the rate increases [from this study] does not correspond to the appearance of key characteristics that have been invoked to explain the evolutionary success of these groups, such as hair on mammals or mammals&rsquo well-coordinated chewing ability or feathers on birds.&rdquo In other words, the animals did not diversify because of their supposed evolutionary advantages. Alfaro concluded, &ldquoWe need to look for new explanations.&rdquo 2

The accepted evolutionary scenario for biodiversity is unraveling. One alternative explanation without such contradictory baggage is that macroevolution was not, in fact, responsible for generating new animal kinds. Rather, each kind of animal was intentionally created with the potential for limited variation in response to environmental pressures.

Tuataras look the same today as their fossilized predecessors, not because some unknown natural magic force preserved their body form for hundreds of millions years, but because they were created a few thousand years ago as representatives of a distinct kind.

  1. Alfaro, M. E. et al. Nine exceptional radiations plus high turnover explain species diversity in jawed vertebrates. Proceedings of the National Academy of Sciences. Published online before print July 24, 2009.
  2. Wolper, S. Naming evolution&rsquos winners and losers: Mammals, birds show rich species diversity alligators not so much. UCLA press release, July 28, 2009.

* Mr. Thomas is Science Writer at the Institute for Creation Research.


Exploring the evolution of plants from water to land

Toward the end of the Neoproterozoic, producers were so abundant in aquatic environments that the low availability of resources limited population growth. In deep-water habitats there was insufficient light, and in shallower water there was a severe lack of inorganic nutrients (primarily nitrogen and phosphorous). While nutrients were more abundant out of the water, they remained inaccessible for most aquatic life.

Transition from aquatic to terrestrial environments required overcoming seemingly insurmountable obstacles: severe desiccation, large temperature fluctuations, intense solar radiation, and the effects of gravity, all of which rendered the terrestrial environment deadly for most aquatic life forms.

At the same time, there was strong selection to overcome these impediments as the ability to tolerate exposure to dry air afforded access to ample light and more abundant nutrients. The first algal lineages that ultimately persisted and thrived out of water sparked the diversification of numerous terrestrial groups.

The emergence of green life from the water was inevitable — the more abundant resources available on land were not likely to remain unexploited for long. The ancestors of land plants — the charophyte algae — were probably dependent on precipitation and runoff from dry land as the primary source of inorganic nutrients. With nutrient availability as a primary limitation to plant growth in the water, it was just a matter of time before the appropriate innovations appeared to allow colonization of terrestrial habitats. Survival on land required overcoming severe drying and exposure to sunlight strong selection gradients existed at the water’s edge where periodic exposure favored desiccation resistance. Under these circumstances any adaptations that improved tolerance to drying or the extraction of water and nutrients from the substrate would have spread, allowing early colonizers to incrementally invade drier habitats.

These first stages of transition to terrestrial habitats remain entirely unknown. There are no living plants that retain the morphological characteristics of the earliest land plants, and we do not have any fossils that can definitively be associated with transitional forms. While plants in the Bryophyta (mosses, liverworts, and hornworts) are often referred to as representatives of the earliest land plants, they actually are quite divergent and possess a number of complex traits that make them much more similar to other land plants than to streptophyte algae.

The characteristics shared among bryophyte groups include a multicellular sporophyte, parenchymous (i.e., undifferentiated cell) growth of the gametophyte, apical growth, and complex reproductive structures. The commonality of these traits among bryophyte lineages suggests that the common ancestor they share with other land plants (vascular plants, the Tracheophyta) probably possessed the same set of traits.

On the other hand, Bryophytes differ from other vascular plants by having a dominant gametophyte stage, a dependent sporophyte stage, and the lack of true vascular tissue (Tracheids). Sporophytes and gametophytes of some mosses possess conducting cells (Hydroids) that serve as vascular tissue, but since most bryophyte lineages do not have this feature, it is more likely that hydroids are independently derived and not homologous to the vascular tissue of the tracheophytes. This scenario is supported by the observation that conducting cells in moss sporophytes and liverwort gametophytes lack the cell wall thickenings and lignification present in the tracheids of vascular plants.

By considering the distribution of shared traits among extant lineages of embryophytes and streptophyte algae, we can make some logical guesses about the probable characteristics of the first land plants.

We can infer that the first fully terrestrial lineages were small plants that had a dominant gametophyte stage, and a diploid stage that was either a unicellular zygote or a simple multicellular sporophyte that was dependent on the gametophyte (traits shared with bryophytes and streptophyte algae). The gametophyte stage was probably a Thallus (a flat plant body consisting of undifferentiated cells, lacking specialized tissues and organs) that had apical growth and unicellular Rhizoids (hair- like extensions of cells that serve the same function as roots) — traits shared with the gametophytes of hornworts, liverworts, lycopods, and ferns.

These plants were restricted to areas of constant moisture, as they possessed little capacity to maintain their internal water status.

The picture that emerges for the first terrestrial plants is one of small, thalloid gametophytes with limited ability to maintain their internal water status and so remaining closely appressed to a moist substrate. These plants had a dominant gametophyte stage and probably produced motile sperm, so they required water for successful reproduction. The diploid stage would have been a unicellular zygote (similar to charophyte algae) or a reduced multicellular sporophyte (similar to liverworts). These first terrestrial plants may have been limited to locations with consistent moisture availability and some shade until adaptations appeared that allowed them to survive in more exposed sites.

As terrestrial lineages spread and became more abundant, competition would have ensued as habitat space with sufficient moisture became limiting to growth. Selection on terrestrial populations would have favored traits that contributed to their ability to colonize new habitat and to compete with other members of the plant community.

Mitchell B. Cruzan is Professor of Biology at Portland State University.