Serious Questions on Evolution

[Chou Toshio]

UNDERSTAND: The following thread is not meant to be a place to attack the theory of evolution or fuel any sort of Creationist v. Darwinist debate. Please check your religious questions at the door. This thread is meant to
answer earnest, honest, and scientifically-minded questions about Evolution. I have some honest questions, and I would like people with more scientific background to point me in the direction of research, findings,
or theories to help my understanding.

So with that out of the way, I'd like to post my question, the first of the thread:

QUESTION 1: Problems with Fish

So by the layman's understanding of evolution, about 400 million years ago, the first fish pulled themselves out of the water, and began their evolutionary journey to eventually become all the forms of land vertebrates that would conquer the world above water. Though they were not alone--
this was a world already colonized by lichens, mosses, and the first
primitive plants, which sustained a world dominated by the ancestors of millipedes, Scorpions, and other invertebrates. Fish were about to bring their speed-enabling vertebrates, keen eyes, and more developed brains to the land-- once they could get adapted. Then, once they did, amphibians,
reptiles, birds, and mammals--all descended from the same fish ancestor--would spread across the land, evolving into all the shapes we know today.

This is the story that's largely been told to us and largely accepted, but I have a bit of a problem wrapping my head around this story, and the
problem is in converging evolution.

Recently in the news, there was an article about how bichir have been raised on land, and proven themselves able to become better breathers and walkers based on their surrounding conditions-- this being a demonstration of how fish could evolve into land vertebrates. In fact there are several of species of fish with different degrees of amphibious abilities today; even with the land as dominated as it is by reptiles, birds, and mammals,
there are still niches for semi-amphibious fish such that they have convergently evolved in several instances from totally different lines.

Convergent evolution happens all the time, and we constantly observe it-- where totally different groups evolve similar attributes independently,boften due to similar environmental pressures or similar niches. The point is that when something works, nature tends to make it more than once. This can be seen in how octopus and fish evolved similar occular eye systems, or similarities in body structure between canid wolves, and the extinct marsupial wolves. Or how sharks and mammals both have penises.

So where was convergent evolution 400 million years ago when fish colonized the land? Why are all modern land vertebrates descended from the same line?

Why aren't there more groups of vertebrates with completely independent lineages, evolved from different fish? How could only 1 line, only 1 species succeed in producing all the lines of vertebrates that would dominate the world?

In the world where the first fish moved onto land, there must have been countless open niches and opportunities for this colonization to happen. Fish exist in all the worlds oceans, and after the Cambrian Explosion all the world's waters were full of life. So with all those oceans full of fish, and all the millions of rivers, streams, bays, inlets, beaches, and
countless land/water environments, there would have been endless opportunities for different fish in different places to make the move onto land. There should have been a huge number of lineages of amphibious fish,
and what's more, these lineages should have been given very prime opportunity to evolve independently of each other.

We're talking about fish just barely able to breath, minimum functionality for water retention, and locomotive abilities that would have been... terrible my modern standards. Even today's most successful modern animals
can take millions of years to colonize big expanses of the globe.

You can probably see what I'm getting at-- the world should have been ripe for fish to evolve into land organisms many, many times; and each of these lineages, separated by thousands of miles and great geographic barriers,
should have been given ample time to evolve independently. Water-tight skin and scales, muscles, lungs, speed, and power-- all the best adaptations of vertebrate animals should have had ample time to convergently evolve in completely different groups of vertebrates evolved from totally different fish in totally different parts of the world.

But even if they evolved into similar forms, it would have been impossible for them to interbreed-- just like it would be impossible for a marsupial wolf to breed with a canid wolf, coming from totally different lines of ancestry. It would not have been possible for their legacies to re-combine.

At the same time, life is so adaptable and evolution so powerful, it seems impossible to me that only one fish's lineage would survive, only 1 lineage
would persevere, and perhaps compete the others to extinction. Extinction just doesn't happen that easily, and colonization and intermingling of species don't drive entire evolutionary branches under. Just like Zebra (which evolved in North America) and Gazelles/Ampala co-exist in Africa today, you'd expect some species of both lines to survive and adapt around each other.

By the time the fully land adapted descendants of two different fish lineages met, they both should have sufficiently evolved and diversified such that some
species of both lines would survive.

But that's not what happened.

If that were to have happened, we'd see a far greater diversity of groups, with completely different genetic backgrounds. The reality is that you can count the main branches on 1 hand-- amphibians, reptiles, birds, and mammals-- with all extinct vertebrates we know also being descendants of
one of these lines; and scientists believing that ALL these lines came from one origin line-- one species of fish.

To me, that doesn't make sense. Convergent evolution happens so easily. Fish take on semi-aquatic forms in so many places and with so much ease, even in a world with fully terrestrial vertebrate competitors. The opportunity for different lines to form back then should have been far greater.

How is it possible that we didn't come up with a far greater number of vertebrate groups from completely different ancestries?

Whoever has the answer or can point out a reasonable scientific
explanation, please chime in.


Note: The question is NOT to challenge the view of a single fish ancestor-- the genetic testing probably supports it. The problem for me is that it seems wildly improbable for ONLY one fish lineage to have succeeded in colonization of the land. How did it happen that way?

 

[Ender]

I'm going to give my best conjecture for this, and I'm sure that there will be a lot of things here that might be inaccurate or illogical so please correct me if you see anything like that.

The first premise we have to accept is that evolution necessarily promotes optimization, but does not necessarily promote the ideal solution. The difference between these concepts can be illustrated by the mathematical concepts of local maxima and absolute maxima. Evolution will (usually) drive fitness (which is the ability for an organism to survive to reproductive age and bear fertile offspring) toward the closest local maxima and then keep adjusting fitness so it stays as close that that local maxima as possible. This local maxima will change as the environment around the organism changes. However, this local maxima may be far lower in fitness than the absolute maxima (which would be the ideal form of the organism). Unfortunately, for an organism to reach the absolute maxima from a different local maxima, it would have to journey through a valley of low fitness, which is not a viable evolutionary strategy. This ultimately means that evolution takes the easiest possible path to "acceptable" in most cases.

Next, we have to understand that evolution, in its essence, is completely limited by preexisting genetic material and mutation/mutation rate. Though this is a crude example, a fish cannot spontaneously generate wings just because there are flying insects in its habitat that it could increase its fitness by preying upon. If the genetic material necessary to generate wings came into being and it was favorable for the fish to have this trait, it's possible (not even likely, just possible - we'll get to that in a second) that this trait would be selected for and eventually become common in this species of fish. But that's a huge if.

We must now understand that natural selection is not the only mechanism by which evolution occurs. In the case of convergent evolution (as well as countless others), genetic drift plays a large role as well. Genetic drift is the change in the frequency of a trait (more specifically an allele) over time that's not due to selective factors or changes in the gene pool. This is especially important in populations where a mutation has introduced a new allele to a single member. Even though this trait might be selectively favorable, there is a high chance it will just be lost due to genetic drift, especially since the allele frequency at n=1 is likely to be extremely extremely low. If that one copy of the allele is lost, then the trait is lost until mutation or something else re-introduces it to the gene pool.

When we talk about convergent evolution with respect from changing from an aquatic to terrestrial organism, we have to take into account the huge number of changes that need to occur for the organism to have some chance of surviving on land. It needs to be able to subsist on air, it needs to be able to move, it needs to be able to retain moisture, it needs to be able to balance salt content in its body, it needs to be able to not die from direct sun exposure, it needs to be able to see properly (through whatever means that may be), and it needs to have the gastrointestinal biochemistry to metabolize food it finds on land. If we look at all these changes together, that's an awful lot of mutations that need to occur together in order to give an organism even a passable chance at moving from sea to land. Going back to our first point, if the development of functional systems that were conducive to these processes required the lineage to go through a period of low fitness between maxima, it doesn't make sense that evolution would force it to take that path. Convergent evolution makes more sense the less complicated something is and the more of an advantage it provides. Something like an "eye" would be a much more likely candidate for independent formation because photoreceptor cells are relatively simple genetically (at least compared to switching from sea to land) and they offer an additional and widely useful sensory advantage that would be heavily selected for and increase fitness within the organism's current environment.

It also might be true that even if an organism had access to all these traits, it still might choose to stay in the water, as even though it had traits that allowed it to survive on land, it might still have better fitness in the water, and then due to natural selection, these traits might be phased out since they don't offer any competitive advantage in the water. Remember that the organism had to have evolved enough of these traits to even survive on land while still living in their original aquatic environment, so the traits they developed had to be at least selectively neutral in water for them to have stayed long enough to make it to the transition to land.

The transition to land itself was most likely induced by a buildup of selectively neutral evolutionary changes that didn't really affect fitness in water, but because these organisms were being pushed out of their niche by other species that were perhaps encroaching upon their aquatic niches, they had to find another niche - land. However, if we take a step back, we see how unlikely it is that any of this would happen, let alone happen together. I'm actually astounded that life left the oceans at all. There are so many steps that had to happen perfectly for vertebrates to make the transition to land that the odds of having it happen more than once are astronomically low.

Even if multiple lines of fish did make this miraculous evolutionary transition to land, it's unlikely that any of them would have been well-adapted for survival. They would probably have struggled to survive and reproduce a lot. If this did happen, chances are a favorable mutation happened in one lineage only once they actually got on land, and this mutation gave set that lineage on the road to terrestrial survival. For example, perhaps of six fish species that managed to survive on land for x generations, only species #4 developed a way to keep their body moist and not desiccate in the sun. This would give this species a continual chance at survival, and convergent evolution wouldn't necessarily occur for the other ones because the's no guarantee that they would ever get the genetic material needed to have that trait.

Evolution does the best it can with what it has where it is. It isn't responsible for introducing new traits into a population, just responsible for how those traits spread or if they die out. Convergent evolution occurs when the same trait changes spontaneously appear independently in populations where it would increase fitness. There's nothing that is forcing evolution to converge independently just because it's a good idea. There is still that high degree of chance involved.

I hope that this helped to clarify a little bit. I'm no expert by any means, just a college biology student who likes playing around with cells and organs more than evolution, but I've always enjoyed studying evolution and I'm glad that such an engaging thread about it now exists here.

 

[Cresselia~~]

Last time I did research (for a paper), I see that there was an argument over which fish group was the ancestor of amphibians.
Whether it was the lungfish or coelecanth.

I think the current majority is with lungfish, but there's a group from Japan that opposes it via tests for certain proteins.

Oh, maybe it's better if I post the paper here: (no academic access required, it's free for all)
http://www.nature.com/nature/journal/v496/n7445/full/nature12027.html

But I'm still with the majority that believe it's the Lungfish who is the ancestor of amphibians.
My points:
Lungfish has a HUGE genome. Way greater than the Coelecanth.
It will explain your question about how so many vertebrates arise from one single ancestor-- because the Lungfish genome is huge in the first place. The abundance of transposons also aided new genes and gene combination. Lungfish itself carries a great variety of genes-- it can become anything, like Ditto :p

 

[Chou Toshio]

Ender thanks for your intelligent response, and I know that you have me the best answer you could without being an expert-- and a lot of important obstacles you brought up.

However-- I was aware of the great degree of challenge in a move to land and it still doesn't make sense to me, given these factor:

Note: Let's call our line-- the line that actually exists-- Line A, with any hypothetical second line of fish lineage which could have arisen as line B.

-huge geographic space: we're talking about ALL the coastal land and inlet (fresh and salt) environments in the entire world. Literally millions of open environments with only also slow-moving invertebrates as competition. Countless environments where even spending brief times out of water would have been beneficial. Each of those habitats is a new potential chance for a line B.

Also consider that animals don't just move when their current niche is under pressure, but when opportunity to colonize exists-- and new colonizeable space is a very strong selective pressure.

-long given time period: The first reptiles appear 310 million years ago, and just because you have reptiles (all assumedly from one line, one geographic starting point) doesn't mean you have a colonized planet. Sluggish amphibians hugging water finally give rise to the first fully terrestrial vertebrates 90 million years after the line first leaves the water, and it'll be millions and millions of years more before it can be direct competition to anything clear across the globe-- so we're looking at a window of about 150 million years+ (or more than a third of the time of all vertebrate history on land) for any potential line B to evolve independently of line A.

-perceived willingness for fish to evolve into amphibious forms: There are many, many unrelated fish even today with some forms of amphibious adaptations developed to various degrees. Even from simple Bettas and other groups with air-breathing swim bladders, or fish like the jumping tetra that steal moments above water for various strategies, but also more developed examples like the bichirs, the lungfish, and mudskippers/gobies. There's also the possibility for the same broader line to give rise to multiple branches. Talking about the lungfish and it's wealth of genetic material as an ancestral line-- lungfish exist in Africa, South America, and Australia; in the primitive world of 400 million years ago, it's easy to imagine the same lungfish line evolving onto land in largely divided different locations to give rise to a line B or C for example.


To me, given all this opportunity from time and space, the tree of vertebrate evolution seems too pretty and organized--

While Darwin's 1 family tree makes sense for ALL of life, when it comes to on-land vertebrates, I really would expected a grove of several trees instead of one unified one.

 

[Ender]

Chou Toshio

One thing that's possible is that multiple incidences did appear, perhaps frequently, but that they appeared at different periods of time, and perhaps only the first group to actually make the transition flourished and the other groups died out. This has precedent in invertebrate evolution (I think Stephen J Gould elaborates on this in his work on the Burgess Shale in Wonderful Life, but I haven't read that book in at least 5 years so I could definitely be mistaken) where we expect to see a huge multitude of phylogenies but instead we see a few number of discrete trees. Of course, Gould has recieved a plethora of criticism for his work and I'm not sure I entirely agree with all of it myself (though again I'm nowhere near an expert), but I think this particular idea has merit and could be translated as an explanation for the phenomenon we're currently discussing. Not sure if it's right, but it's just an idea to perhaps look into.

Without any data on this, it's really hard to say (and I have really spotty wifi on my phone right now), but do you know if there's any evidence in the fossil record that these multiple incidences occurred and their lineages just died out? If this is the case, the question changes from "why didn't this happen" to "why didn't these lineages survive".

If this isn't the case, one other argument might be that just because time and opportunity exists doesn't mean the most favorable (or even a favorable) outcome will occur. Natural selection and it's derivatives are truly a very small part of the evolutionary process in the grand scheme of things so it's possible that these opportunities were passed by for one reason or another. Again just conjecture and food for thought. I personally think that the "it happened multiple times with only one presently surviving lineage" is more plausible than this idea.

 

[Woodchuck]

This thread's question is fascinating in that it is very similar to the Fermi paradox: with countless other star systems with the similar elements that make up our habitat on Earth, and with more and more discoveries of planets in their stars' habitable zones, why hasn't humanity come into contact with aliens?

While most of the proposed solutions to the Fermi paradox are not relevant to this situation (WE'RE LIVING INSIDE A COMPUTER SIMULATION! ONLY ONE VERTEBRATE ANCESTRAL LINE HAS BEEN PROGRAMMED IN!), one that is relevant is the concept of the "great filter". The idea is that somewhere along the way to becoming an interstellar civilization, there is some tremendous developmental step that a species or civilization must cross before it can reach a state of being able to make contact with other life. It's possible that the step from living underwater to living on land is one such step. Perhaps being able to successfully transition, despite all the potential benefits, is so unlikely due to all the obstacles that once a single species managed to cross onto land, it quickly spread and became so dominant on land that no other species would be able to get a foothold (as it would presumably be quickly eaten or outcompeted to extinction by better-adapted land species already there.)

I guess this only superficially reminded me of aliens, and I just found an excuse to bring that up, but I'm okay with that. The point is that it is totally possible that the evolutionary "events" for individual species to cross onto land are so far apart that the first species could gain dominance on land before a successor species could have any hope of competing for niches. A significant component of evolution is the proliferation of species to fill available niches; one vertebrate species arriving on land could branch into new species (relatively) quickly and occupy enough of the niches to make it impossible for other species to survive on land.


e: if you are disappointed by the lack of a pun in this post, my only response can be: is the concept of a woodchuck post without a pun so alien to you?

 

[~Eon~]

"You can probably see what I'm getting at-- the world should have been ripe for fish to evolve into land organisms many, many times; and each of these lineages, separated by thousands of miles and great geographic barriers,
should have been given ample time to evolve independently. Water-tight skin and scales, muscles, lungs, speed, and power-- all the best adaptations of vertebrate animals should have had ample time to convergently evolve in completely different groups of vertebrates evolved from totally different fish in totally different parts of the world."


I think the misconception you have is that only the fish that led to the current amphibians, mammals, etc. evolved. Every organism is capable of evolving its just that not all of the life forms made managed to survive. The Precambrian (or was it Cambrian explosion) had the most diversity in life forms that the world had ever seen and I am sure many of them did make it to land but as my biology book states, there are more extinct species than there are alive species. It is difficult to study the evolutionary origins of species that already went extinct that long ago because the only thing you can study is morphology which is difficult given the diversity.

You make it sound like there were thousands of different species of fish and it sounds strange that only one species led to all the vertebrates today. You should try thinking of it as thousands of different types of fish led to less than a thousand types of new species of which many made it but eventually only our ancestors survived.

"But even if they evolved into similar forms, it would have been impossible for them to interbreed-- just like it would be impossible for a marsupial wolf to breed with a canid wolf, coming from totally different lines of ancestry. It would not have been possible for their legacies to re-combine."

This is not true there are several factors that affect interbreeding such as morphological, temporal, and hormonal, and even genetic reasons. This is what makes the classification of species sort of difficult because it can be hard to differentiate two types of organisms because despite all of these factors some species can still breed. For example did you know domestic dogs can mate with wolves and even have fertile offspring.

In my opinion it sounds more likely that interbreeding occurred because its unlikely that the same species developed new trait after new trait. It's more likely that one species of fish developing jaw bones mates with a fish developing amniotic egg sac. If the progeny has both traits it will be more successful on land than the species with a bone jaw that mated with a species without an amniotic egg sac. If you see what I'm getting at here.

 

[Imanalt]

Uh im tired so I'm not going to write as much as everyone else here, but my understanding was that basically a broad group developed the ability to go on land well before other fish, which seems pretty realistic. Then it was millions of years before any other groups of fish were capable of it. By this point, there had been significant optimization of the creatures that came on land, as well as filling the majority of niches. This leads to the newly on land creatures having a significant disadvantage, as they're now competing directly against animals which have had much more time to adapt to their environment.

Basically the flaw in your understanding is that the amount of time it takes to saturate and adapt to an environment once there is much shorter than the amount of time it takes for a new group to be able to come onto land, and so the newcomers are at a disadvantage.

 

[shade]

the main mistake you make with you understanding of evolution is that you give off the idea that evolution is there to solve problems. evolution is not necessarily directional, as in just because some fish had developed adaptations for a semi-terrestrial lifestyle does not mean that other fish would respond. also, just because it would be advantageous for it to happen does not mean that it is ever going to happen. for example, having an eye in the back of my head would be really cool, but just because it would be good to happen does not mean that it ever will (even though eyes are really easy to make evolutionary wise). furthermore, it probably was not an evolutionary advantage to develop such adaptations for 99% of the fish species at the time anyway. the main candidates are lobe-finned fish that lived in shallow ponds, who sometimes might need to travel from pond to pond to avoid dessication, find new food sources or simply just keep moving on. as you can see this is a rather niche market in which adaptations for a semi-terrestrial lifestyle would be even remotely advantageous. for a marine fish, these traits developing would be an evolutionary dead-end and probably decrease fitness.

 

[whistle]

disclaimer, I suck at evolution, no formal business here just intuition/some logic

your post makes sense. three ideas I want to brain vomit with poor punctuation (the first two are not very relevant):

1. (my only [minor] disagreement) usually it is assumed that the same trait does NOT evolve more than once; examples of convergent evolution are perhaps the exceptions that prove the rule. maximum parsimony is basically evolution's version of occam's razor; from a statistical perspective, if you are constructing a tree for two species with trait X it is better to assume that they both came from something with trait X rather than something with trait Y. this still allows for convergent evolution, but you might be giving its frequency too much weight.

2. fossils = putting together a puzzle without knowing what the final picture should be or even if all the pieces you are given are in the right puzzle. I read this review on tetrapod evolution: http://www.annualreviews.org/doi/full/10.1146/annurev.ecolsys.38.091206.095546. (it mostly covers specific changes in skeletal/developmental biology accompanied by references to specific fossil finds but still has some sections that are of more interest to this discussion, some paragraphs at end of post). the field apparently flipped views multiple times in the last few decades (between the same theories) regarding the closest fish relative. it also has many mentions of gaps in the fossil record, collection of samples biased towards locations in north america/europe/asia, etc. this is entirely a tangential point and does not directly address your confusion but I think the whole discussion becomes more palatable if you put it in a frame of scientific uncertainty.

3. tracing your what-if scenario. suppose a bunch of lobe-finned fish (or whatever the "consensus" is these days) around the world are living in conditions that are right for tetrapod evolution. if they can't all intermingle then certainly a large subset will be able to intermingle and these will exchange genetic information through breeding/horizontal gene transfer. then at a certain point you are dealing with all of them as their own class or clade or family or whatever the proper term is, but no longer are they distinct. the leftovers who were isolated (if it happens) might die out, since the main group is constantly dying out as well but is being "refreshed" by replacements (of species, not individuals). shit, if you have some leftovers left, who says they evolve into modern tetrapods? maybe those dudes became some obscure fossil that we think is a dinosaur, or went back into the water. or maybe (going back to the idea of scientific uncertainty) they are still around, we just haven't sampled them and realized that they are "tetrapods" but not from the same line. for that matter, why are we assuming that all of our hypothetical lines make a quick/simultaneous shift to land and then decide to radically evolve as if they are in a race? if there was such a large niche to be filled in moving to land, and in so many radically different ways so as to guarantee no line intermingling, why didn't any of it happen before the designated start line?

according to the review, fish -> tetrapod occurred in the upper devonian (400 mya), then there was a huge extinction event (375 mya) where 70-82% of marine species died, then modern tetrapods diversified in the carboniferous (350 mya). to adapt to the new atmosphere/climate there were many different ways for the tetrapod line to evolve, which explains why diversification occurs now rather than earlier. the bottleneck also seems like it would "prefer" that organisms with similar traits (ones that lived in similar environments, or had similar food requirements...) survive and go on to form the modern tetrapod line. so even if there were multiple separate lines before, it's possible that the common ancestor was still just a single "line". whatever "line" means - I am not sure whether the term in the context of this field means a species, but I am somewhat skeptical because of the low resolution of their data. maybe it means a species in the abstract (in the sense that mitochondrial eve is an individual or that LUCA is a single organism), or maybe they are more broad and simply mean that a certain group of organisms became modern tetrapods (and the differences between members of this group are smoothed over by time or even masked by modern differences in tetrapods). some of the lines in the review about the diversity of devonian tetrapods may lead to this line of thinking, in which case I suspect most of your confusion disappears and it's just an issue of semantics/philosophy in terms of what a "common ancestor" can be.

this is all not explicitly taking into account ender/shade's points about agnostic evolution and the specificity/difficulty of adaptation to land; especially when those are taken into account, it just seems like the multiple line scenario has to jump through too many statistical hoops.

selected paragraphs of review

[cut a bunch of paragraphs and sections on fossils and limb development in the middle because no one gonna read all that]

ABSTRACT

The traditional notion of a gap between fishes and amphibians has been closed by a wealth of fish-like fossil tetrapods, many discovered since the mid 1980s. This review summarizes these discoveries and explores their significance relative to changing ideas about early tetrapod phylogeny, biogeography, and ecology. Research emphasis can now shift to broader-based questions, including the whole of the early tetrapod radiation, from the divergence from other lobed-finned fishes to the origins of modern amphibians and amniotes. The fish-to-tetrapod morphological transition occurred within the Upper Devonian; the divergence of modern tetrapod groups is an Early Carboniferous event. Modern tetrapods emerged in the aftermath of one of the five major extinction episodes in the fossil record, but the earlier Devonian tetrapod radiation is not well understood. Tetrapod limbs, paired fins, and comparative developmental data are reviewed; again, research emphasis needs to change to explore the origins of tetrapod diversity.

INTRODUCTION

Archegosaurus (Goldfuss 1847) was the original missing link. Seized by evolutionists after Richard Owen (1859, in Desmond 1982) declared that this “old Carboniferous reptile” conducted the march of development from fish to primitive amphibian, the treatment of Archegosaurus foreshadowed portrayals of Ichthyostega, Acanthostega, Tiktaalik, and others besides: each depicted at pond- or swamp-side with tail trailing (significantly) in the water (Milner et al. 1986). Evolutionary trees of tetrapod ancestry have long since branched and filled to accommodate earlier and more thoroughly transitional forms (Clack 2002), but the vignette of beached missing links has persisted. Unfortunately, this paleo-cliché reduces the exploration of tetrapod origins to the discovery of substitute candidates for this brief episode in vertebrate history. However, questions about the origin of tetrapods now concern a much wider range of paleobiological issues. The origin of tetrapods includes the whole of the tetrapod stem (see sidebar, Defining a Tetrapod), with many groups of fish-like (i.e., finned) taxa only recently being incorporated into this wider framework (Ahlberg & Johanson 1998, Coates et al. 2002, Jeffery 2002, Johanson et al. 2003). It is now possible to ask how the origins of the tetrapod total and crown groups relate to morphological changes and the emergence of a conventional tetrapod body plan. The fin-to-limb transition is an exceptionally rich area of integrative research and debate (e.g., Zákány et al. 1997, Coates et al. 2002, Davis et al. 2004a,b, Friedman et al. 2007). The origin of tetrapods and the water-to-land transition are not synonymous, but both events are associated with global climatic, atmospheric, and tectonic changes, as well as with serial extinctions at the end of the Devonian (Algeo et al. 2001, Berner et al. 2007, Blieck et al. 2007, Clack 2007). Taxon and character sets are now large enough to be mined for large-scale evolutionary trends (Ruta et al. 2006, Wagner et al. 2006). This review gathers reports and articles on this topic published in the past few years—some of which have gained exceptionally widespread attention—and places them in context and suggests agendas for future research.

THE POSITION OF TETRAPODS WITHIN VERTEBRATE PHYLOGENY

The first question about tetrapod origin concerns the identity of the closest relatives of land vertebrates. This issue emerged within the nineteenth century (Desmond 1982) as discoveries of lungfishes confounded diagnoses of living tetrapods as a natural group, and after fossil “rhipidistian” fishes were recognized as belonging within the same group as limb-bearing tetrapods. Widespread acceptance of evolutionary theory redirected systematic research to discover the particular rhipidistian ancestors of tetrapods. By the early twentieth century, phylogenetic hypotheses had multiplied considerably, and ranged from polyphyletic origins of limbed vertebrates from several rhipidistian groups (Jarvik 1980; Säve-Söderbergh 1932, and references therein) to a monophyletic origin from “osteolepiforms” (Watson 1920; also reviewed by Panchen & Smithson 1987). Rare objections to this rhipidistian hegemony (e.g., Kesteven 1950) were sidelined. With tetrapod ancestry anchored to specific fossils, such as the osteolepiform Eusthenopteron, research focused on the adaptive circumstances surrounding the invasion of dry land.

However, by the late 1970s, this passive acceptance of osteolepiforms as the closest fish relatives of terrestrial vertebrates provoked a reexamination of the status quo. A withering critique of research on tetrapod origins (Rosen et al. 1981) concluded that most characters claimed to link Eusthenopteron to tetrapods were either primitive or spurious. Emphasis was placed on the choana, a palatal nostril framed by a diagnostic bone arrangement, present in Eusthenopteron and tetrapods. Lungfishes also possess a palatal nostril, homologized by several nineteenth-century anatomists with the tetrapod choana (Desmond 1982), but dismissed by most twentieth-century paleontologists as convergent. However, newly prepared material of a Late Devonian lungfish (Griphognathus) revealed a bone-surrounded palatal nostril that Rosen et al. (1981) offered as evidence that a true choana was, in fact, present in primitive lungfishes.

The ensuing controversy spurred paleontologists to frame explicitly cladistic hypotheses to refute the arguments of Rosen et al. (1981) and reinstate osteolepiforms as the closest relatives of limbed tetrapods (e.g., Panchen & Smithson 1987, Schultze 1991). Further evidence emerged from the discovery of the Early Devonian Diabolepis (Chang 1995, and references therein), combining lungfish specializations with generalized sarcopterygian conditions, including possession of two external nostrils. As the sister group of lungfishes (Chang 1995), Diabolepis indicates that lungfish palatal nostrils are convergent with the choanae of limbed vertebrates and osteolepiforms. Two decades later, another Chinese Devonian fish, the primitive osteolepiform Kenichthys, added a postscript to the choana debate: its posterior nostril penetrates the skull exterior close to the upper jaw rim, presenting a possible incipient condition for the choana (Zhu & Ahlberg 2004). A summary of the present consensus on sarcopterygian interrelationships is shown in Figure 1 (for further discussion see Friedman 2007).

DEVONIAN TETRAPOD DIVERSITY

Four major Devonian groups belong to the tetrapod stem lineage: rhizodonts, osteolepidids, tristichopterids, and “elpistostegalids” plus limbed tetrapods (Figures 1 and 2a). Most stem tetrapods are osteolepiforms, a grade of fin-bearing groups including rhizodonts, osteolepidids, and tristichopterids, but excluding elpistostegalids. Devonian examples are known from every continent and their diversity totals approximately 40 genera. Devonian tetrapods for which limbs have been discovered or implied total approximately a dozen genera, although digit-bearing limbs are known in only three: Acanthostega, Ichthyostega, and Tulerpeton. Approximately seven more unnamed forms are reported from fragments (Clack 2005). Elpistostegalids are the most informative taxa for understanding anatomical changes associated with the fish-to-tetrapod transition. They are known from a handful of genera, all exclusive to the Northern Hemisphere (Daeschler et al. 2006).

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HABITATS AND PALEOBIOGEOGRAPHY

Thus far, the late Givetian to early Frasnian elpistostegalids are confined to the fringe of Euramerica (Daeschler et al. 2006) (Figure 5). The Euramerican fringe also includes the Viséan midland valley of Scotland, source of the earliest crown-group tetrapods. Sandwiched between these last two groups, the earliest limbed (and less-certainly limbed) genera have been collected from a wide area, including the Frasnian-Famennian of Euramerica, North China, and easternmost Gondwana (Blieck et al. 2007). The paleoenvironments of these Late Devonian tetrapods range from proximal, near-shore marine localities to continental, freshwater lakes and rivers (Blieck et al. 2007, Lebedev 2004). Paleocontinental reconstruction (Averbuch et al. 2005) indicates that all 10 noted localities for what might be the earliest limbed tetrapods lie within 30° of the estimated equator, consistent with macroevolutionary ideas about cradles of diversity (Goldberg et al. 2005). However, it remains unclear whether this distribution is the result of collection bias. Marginal deposits of Late Devonian age have not been fully exploited in Africa, South America, or Antarctica.

THE END DEVONIAN EXTINCTION AND RECOVERY

The Late Devonian extinction, marked by an estimated loss of between 70% and 82% of marine species (McGhee 2001), extended from the latest Frasnian and into the Famennian. This drawn-out biotic crisis has been correlated with global cooling (Joachimski & Buggisch 2002, Streel et al. 2000), atmospheric change (Berner et al. 2007, Scott & Glasspool 2006, Algeo et al. 2001), and the radiation of terrestrial plants leading to aquatic eutrophication and anoxia on an intercontinental scale (Algeo et al. 2001). Furthermore, the Late Devonian was a period of intense tectonic activity, with incipient collisions of continental crustal blocks including Laurussia, Gondwana, Kazakhstan, and Siberia (Averbuch et al. 2005). These tectonic events closed entire oceanic domains (Figure 5), had a widespread influence on other marine environments, and probably contributed to global cooling (Averbuch et al. 2005, Blieck et al. 2007).

Tetrapods (the total group) originated prior to this episode of massive change, and by the end of it, most of the group seems to have perished. The greening (aquatic and terrestrial) of continents, from the late Silurian through to the Middle Devonian, was a late phase in a vast sequence of continental invasions (Labandeira 2005). But those processes that provided the structural and trophic complexity necessary for terrestrial vertebrate life might also have been those that devastated the tetrapod clade (Algeo et al. 2001).

The ∼15 million year post-Devonian trough in the record of limbed tetrapods (Clack 2002, Ruta et al. 2003) is also apparent in the fossil history of terrestrial arthropods (Ward et al. 2006). Absence of both groups throughout most of the Tournaisian has been attributed (Ward et al. 2006) to an estimated trough in atmospheric oxygen levels (Berner et al. 2007), constraining both groups to aquatic habitats. Physiological arguments have some bearing on this scenario, but the sudden diversity of Viséan limbed tetrapods implies that this gap might equally reflect unevenness of the fossil record (Clack 2007). Tetrapod phylogeny clearly underwent multiple branching events and encompassed considerable morphological diversification during this interval, the results of which include aïstopods, adelogyriniids, temnospondyls, and Westlothiana (and we have correspondingly little idea about any hidden diversity of as yet unknown post-Devonian elpistostegalids, “acanthostegids,” and ichthyostegids). If there is any signature in the tetrapod record that might be more safely attributed to early Carboniferous atmospheric conditions, then it is the reduced size of these crown tetrapods and their close relatives (Figure 2), compared with the larger dimensions of earlier and more basally branching clades (Clack 2007).

CONCLUSIONS AND FUTURE DIRECTIONS

Early tetrapod distribution is clumpy at any scale, from detailed features of anatomy (note the almost bimodal array paired fin and limb skeletons; Figure 4) up to the patchy distribution of body shapes and higher taxonomic categories. These patterns should be investigated; it seems unlikely that they result wholly from extinctions editing chunks from evenly spread morphological continuity (cf. Erwin 2007). The use of nontraditional node-based rather than character-based group definitions is disputed (Blieck et al. 2007, Clack 2007), but it permits a better perspective of early tetrapod evolution, and provides an explicit means of framing questions about group origins and change. The “vast structural gaps” (Milner et al. 1986) separating Ichthyostega from osteolepiforms and Carboniferous tetrapods have effectively closed: The research program started by Owen approximately 150 years ago is largely completed. Narratives of morphological change from fish to tetrapod can be refined, but there are other issues to address. The turnover in clade composition across the Devono-Carboniferous boundary is dramatic, and we note that it yields two groups that radiate significantly within the post-Devonian Paleozoic: limbed tetrapods and rhizodontids. If research explores only the limbed subset of the tetrapod total group, much of the evolutionary signal will be missed (as if research on mammal evolution ignored noneutherians). A detailed phylogenetic analysis of the whole of the tetrapod stem is needed. Similarly, Devonian tetrapod-containing biotas need to be subjected to the level of study applied to Carboniferous localities such as East Kirkton (Clarkson et al. 1994). Paleoecological understanding of the earliest tetrapods would also be assisted by substantial biomechanical analyses of structures such as lobed fins, and vertebrae retaining a large notochordal component. Finally, developmental analyses of differences between fins and limbs, rather than searches for general, and perhaps primitive, conditions, are more likely to improve our understanding of present and past morphological diversity.

 

[Chou Toshio]

the main mistake you make with you understanding of evolution is that you give off the idea that evolution is there to solve problems. evolution is not necessarily directional, as in just because some fish had developed adaptations for a semi-terrestrial lifestyle does not mean that other fish would respond. also, just because it would be advantageous for it to happen does not mean that it is ever going to happen. for example, having an eye in the back of my head would be really cool, but just because it would be good to happen does not mean that it ever will (even though eyes are really easy to make evolutionary wise). furthermore, it probably was not an evolutionary advantage to develop such adaptations for 99% of the fish species at the time anyway. the main candidates are lobe-finned fish that lived in shallow ponds, who sometimes might need to travel from pond to pond to avoid dessication, find new food sources or simply just keep moving on. as you can see this is a rather niche market in which adaptations for a semi-terrestrial lifestyle would be even remotely advantageous. for a marine fish, these traits developing would be an evolutionary dead-end and probably decrease fitness.

I am not at all assuming that Evolution is there to solve a problem-- only confused due to my understanding of:
-time opportunity
-geographic/environmental barriers
-strength of tendency of organisms to migrate to environments, especially with less competition
-perceivable opportunity (even hopping out of water is a good trick when the only land predators are primitive centipedes and scorpions)

eon-- dogs and wolves are a terrible example; they share a very close common ancestor and are even classified as the same species by some. Even coyote/wolf hybrids are reproductively viable. I am proposing lines separated by thousands of miles and tens of millions of years of evolution.

So far, the best (actually answers the question as opposed to side-skirting by highlighting odds) possible explanations in this thread (from my view):
-Several lines perhaps did evolve, but some:
a) have already died out and are unrecognizeable as separate lines just based on fossil record
b) mass extiction (which could fuel A)
c) Lines that do form are too closely related, such that they can interbreed OR are unrecognizeable as different lines (same "Clade"), which to me seems especially possible given the time for evolution between now and then-- things vastly different by that era's standard all look the same to us today, looking through fossils.
d) actually are seperate lines, but we just haven't noticed them (not as important given the number of mass extinctions having taken place between now and then.
e) all of the above

So yeah, maybe my mind is stubborn but I'm much more willing to believe in explanations that point out ways that several lines could be obscured or unknown to us based on available data, rather than explanations that rule out multiple lines "just because arbitrary odds". I'm more willing to bet on explanations that argue potential for information insufficiency on this subject.

Whistles ideas make a lot of sense to me.


Looking at the broader picture of life moving onto land, the reality is that even in invertebrates (who are much more diverse than vertebrates), the number of main groups is relatively small.

Arthropoda are far more diverse than fish for instance, but it's pretty much just insects/centi-milipedes and Arachnids that dominate the land. Sure there are several fully terrestrial crusteceans (pill bugs, coconut crabs, land crabs) as well, but for all the diversity of post-cambrian arthropods, the vast majority of history's terrestrial arthropods are insects and arachnids.

In mollusks, it's only slugs/snails that made the move into land in the long haul.