It’s been 25 years since Gould’s Wonderful Life: The Burgess Shale and the Nature of History
Gould’s answer was no — that the role of chance was too great, and because the forms of life do not represent optimum ideals built by perfectly plastic forms, but rather are kludges built atop limiting and enabling prior adaptations. Some people, like Simon Conway Morris, argue otherwise that there are ideal forms (including us bipedal anthropoids) upon which life will tend to converge. I favor Gould’s view rather strongly — it’s absurd to talk about evolution without an appreciation that all organisms are the product of history, and that what traits we have now are almost entirely contingent on what traits our ancestors had.
My favorite example of the error of limited thinking about adaptive ideal forms is a comparison of bony fish and squid. Both are fast swimming, active predators that are torpedo shaped in motion (physics constrains that!), but in detail everything is completely different. If the pre-Cambrian ancestor of all chordates were accidentally squashed, there is no reason to assume that the pre-Cambrian ancestor of all molluscs would then evolve a line of torpedo-shaped predators with brainy skull at the front and an undulating muscular body for propulsion. Why should they? They have a demonstrated capacity in our universe to evolve into torpedo-shaped predators with tentacles at the front and propulsion via a jet of water squirted out of a muscular mantle cavity.
The body plan of the ancestor dictates what capacities the descendant will have. There is no reason to assume that the world as we see it now was inevitable — that’s simply a failure of imagination and reason.
An article by Zach Zorich explores other examples of evolutionary contingency. Much of it is dedicated to that fascinatingly concrete example of actually being able to roll back the tape of life, the Lenski experiments, in which populations can be frozen and restarted at any time. In that case we seen on a molecular level that the evolution of specific biochemical problems is not inevitable at all, but depends entirely on the presence of prior mutations. History, and your parents, matter!
But I also like this example.
“Not everything is possible,” no matter the process, Wake explains. “Organisms evolve within the framework of their inherited traits.” Organisms can’t pass on mutations that kill them or prevent them from reproducing. In the case of Hydromantes salamanders, their ancestors had to overcome a serious limitation: To acquire their ballistic tongues they had to lose their lungs. That’s because their tongue partly derives from muscles that their predecessors instead used to pump air into the lungs. Now, that formerly small and weak muscle is much larger and stronger. It wraps like a spring around a tapered bone at the back of the mouth, and when the muscle squeezes, the bone generates the force that fires the tongue along with its bones out of the mouth. So, Hydromantes’ ancestor did not simply acquire a mutation and evolve a fast ballistic tongue. Instead, the adaptation followed a series of mutations that first enabled the creature to overcome its reliance on lungs for oxygen and buoyancy control. Each change was contingent on the one before it.
Chameleons, on the other hand, retain their lungs. Instead of re-tooling their lung anatomy, they have evolved a piece of collagen that allows them to catapult their tongues at prey. On the surface, salamander and chameleon tongues converge, but not upon closer inspection. It takes a chameleon 20 milliseconds to shoot its tongue at its prey, a positively glacial pace when compared to the Hydromantes’ five-millisecond firing time. Why are chameleons stuck hunting with such slow tongues? The answer is that they have encountered a kind of obstacle to convergent evolution. The chameleon’s tongue is fast enough to ensure their survival, but they lack the “framework of inherited traits” to evolve the salamanders’ deadlier ballistic anatomy. The chameleons have reached what biologists call an “adaptive peak.”
Before anyone says that ballistic tongues represent an example of convergence, too, I’ll agree…but with the caveat that this is an example of modifying extant traits in the tetrapod toolkit. Two vertebrates evolved ballistic tongues, because they share the trait of having tongues. Squid also have a high speed prey capture mechanism, only lacking tongues, they use a pair of arms.
As D’Arcy Wentworth Thompson put it, “Everything is the way it is because of how it got that way.” You can’t appreciate evolution unless you also recognize the path it takes…it’s all about the trajectory.
So the main argument in favor of the ideal forms theory is statistic probability, right? When evolving a complex trait requires many mutations of many generations, there is little room for chance, just as throwing one million coins will yield around 500k heads. If we assumed that all possible mutations happen eventually, then it would require a lot of bad luck for the best possible mutation not to survive.
But there are other ways chance influences evolution. Even a proponent of ideal forms would have to agree that live would look different today if no big rock had fallen on Mexico 65M years ago (or, if all chordates were accidentally squashed).
So what I’m wondering about is to which extend evolution of a species is guided by external accidents vs genetic accidents. Are there any opinions or insights on that matter?
I don’t think it’s completely a matter of chance that complex, multi-cellular organisms have evolved, any more than it was completely a matter of chance that stellar evolution gave rise to heavier-than-iron elements.
@2: Well, it did take a billion years or so between the appearance of single-celled organisms and multicellular organisms. So, one could argue that it took a lot of evolutionary “effort” (metaphorically speaking) to cross that barrier.
Sexual reproduction is another barrier that took a lot of “effort”. I would find it completely plausible to have a world full of multicellular organisms that don’t reproduce sexually.
If you’re saying that evolution behaves deterministically, then you’re left with the question as to why the Vulcans or other Star Trekian humanoids haven’t already visited Earth.
@3:
If you’re going with stochastic processes and contingency, then over time some chain of events will lead to multi-cellular life, sexual reproduction, and yes, potentially Vulcans. Somewhere. It could well be that the current solitude we’re experiencing is because someone had to be first.
I agree with everything in PZ’s post, but I still think something similar to tetrapods would become the dominant land animal form. Having an internal skeleton and most of your neurons (which are deeply involved in sensory organs) pointing towards your dominant direction of travel are huge benefits. A land biome dominated by slugs and snails would be easy pickings for something that could lift itself off the ground and run or even trundle towards its prey with eyes and ears and teeth and claws (or whatever toolkit evolution provided) pointing forward.
Aww shucks, I guess that means Star Trek isn’t plausible after all.
Thanks for that link, that was a nice article. From that article:
Seems to me that Conway Morris is the one forgetting time scales. Who says that a niche will be around long enough for organisms to evolve into it? Especially since life itself can create and destroy niches.
As I understand it, the circumstances that favor a trait were there. That part is not a matter of chance. But there is more than one way to build a trait and which one of those becomes the blueprint for the population is a matter of chance.
So if we replay the tape, the same circumstances will be there, the same traits will have an advantage, but the blueprint will be different. The blueprint is what is passed on to offspring. So I’d say we end up with organisms that have the same capabilities (maybe that’s a better word than ‘traits’) as today, but with completely different foundations.
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Another thing.
‘The tape’ would be a record of the history of earth’s environment, a record of the selection pressures were present at a given time, that’s the mental image I have. Is that correct? Because that ignores the fact that animals change their own environment, and that makes it impossible to record the history of the environment in which life evolved, without also recording evolution itself.
@moarscienceplz in #5: arthropods have been the dominant land animals for much longer than tetrapods – and by some metrics, they still are (about 25% of known species are beetles, about 10-25% of animal biomass are ants, etc).
One question I’ve always wondered about this is how much are the metazoan body plans themselves contingent or constrained?
Are their historical/ancestral constraints on the pathway from single-celled ancestor to multicellular animal with a body plan, or are the environmental constraints more likely?
The traits that one would presume a single-celled ancestor would need to start along the path to multicellularity – things like signalling molecules, cell-cell adhesion, programmed cell death, etc do not seem to be the kind of traits that would put a very strong set of contingent constraints to the macroscopic body plan as the organism increased in size and cell number. It would seem that these kinds of traits would allow for a very side range of flexibility in the evolution of developmental pathways leading to multicellular body plans.
Is it possible that there are only so many ways to make a multicellular body from a single-celled origin that works in an aquatic environment, and if so, could earth-life have in fact explored/attempted most or all of them?
If the above is true then one could expect that if the tape of life were replayed, or life found on another planet (assuming aqueous environment) then analogs of all the major animal body plans would re-evolve, and then convergent evolution based on the shared characteristics and potentials of those body plans could produce more analogs for extant terrestrial organisms.
This is still contingent on the chordate lineage surviving into the Devonian. They weren’t particularly successful or dominant earlier on, and could easily have gone extinct.
Would any of the other major groups re-evolve an internal skeleton? The cephalopods perhaps, with their cuttlebones?
You’re blithely ignoring the fact that time to evolve behaviourally complex organisms is limited. Life seems to have arisen pretty much as soon as the planet was physically able to support it – at a minimum, it’s 3.5 billion years old. Eukaryotes (read Nick Lane to find out why prokaryotes would never evolve much anatomical or behavioural complexity) evolved – probably by a fluky symbiosis – maybe 1.5, just possibly 2 billion years ago. Motile multicellular organisms evolved about 600 million years ago. If it took 3 billion years to get from LUCA to the Cambrian explosion, it could take 5 or 10 billion. In somewhere around 500 million to 1 billion years, the sun will be producing so much heat that earth will suffer a runaway greenhouse effect – unless our successors put up a sunshade. Red dwarf stars have a much more stable heat output, but we have no idea if they could have life-supporting planets.
“about 600 million2 should be “less than 600 million”@12.
…and “”million2” should be “”million”” @13!
OK, I give up!
Idealised forms are certainly nonsense, and I am sure that rewinding the tape of life would not produce bipedal apes again. But I still tend to lean more towards the side of convergence. Perhaps it is just a matter of how precisely one wants to see something replicated before being satisfied that it is “the same”.
The point that is perhaps underappreciated here is that most of everything dies, and so we never get to see the full potential. The ancestors of the extant lineages could have evolved into many more different forms than they actually did if they had been released from the competition of other lineages that got there first. That is, if the first chordate got quashed, I would give other organisms a good chance of evolving an internal skeleton instead.
For example, one could make the same argument that all is contingent by pointing at the land plants for evidence: “they are monophyletic, the colonisation of dry land by photosynthetic organisms happened only once. If we replay the tape of life, the land might stay empty.” But surely right there we must realise that that is nonsense. If there weren’t any green plants on land then red or brown algae would be free to colonise it, and they would home in on the same solutions as the green plants did, because there is a limited number of body plans that work under those circumstances.
Had anaerobic bacteria not evolved into photosynthetic cyanobacteria, all the rest of evolution’s pathway would have been very much different. No oxygen, no eukaryotes.
How inevitable or not was the development of photosynthesis?
My first thought is, what constitutes an ideal form? Does biology have some intrinsic way to define ideal? Or is this some form of biological Platonism? I’m biology science challenged, so I look up Simon Conway Morris and find that Wikipedia says he “is a Christian…most popularly known for his theistic views of biological evolution.” If this is accurate, then he’s essentially like the Christian and Jewish Biblical archeologists who always seem to find evidence of the Bible stories. Confirmation bias is such an amazing aspect of humans. In fact, I probably experienced it when I discovered that an advocate for idealized forms happens to be a Christian.
Zorich gets one thing wrong in his salamander example, and I suspect he misread Wake. My understanding is that plethodontid salamanders lost their lungs early in their evolutionary history, while the ballistic tongue in Hydromantes (and a couple other genera) is a more recent innovation. (See Vieities et al, “A multigenic perspective on phylogenetic relationships in the largest family of salamanders, the Plethodontidae,” Molecular Phylogenetics and Evolution, 59 [2011]). So you could say the lungless condition allowed room for the expansion of the tongue apparatus. But that didn’t happen concurrently with the loss of the lungs.
As mentioned above, you have managed to overlook the arthropods.
Sure, the size of arthropods is somewhat limited; when they get to comicbook sizes, their limbs become vulnerable to buckling… assuming they wouldn’t happen to evolve some kind of de-facto endoskeleton, as insects and myriapods have done in their heads. But they give you more options to think about. For instance, why precisely four limbs? Why not six like insects or eight like tardigrades or a whole lot like myriapods? Why a long body with the limb pairs far apart, and not a short one like harvestmen or tilted cephalopods? And so on and so forth.
If you read closely, Zorich actually says that the lungs were lost first* and the ballistic apparatus evolved later; he just doesn’t express how much later and ends up giving a false impression.
* Probably the first plethodontids lived in mountain streams, where lungs are 1) unnecessary, because cold streams contain so much oxygen that breathing through the skin is enough and because metabolism at such temperatures is slow anyway, and 2) actively dangerous, because if you’re buoyant in such a stream, you’re just swept away. Secondarily terrestrial salamanders – making up two thirds of all salamander species. Isn’t that awesome. :-)
You can only get humanoids if you’ve got vertebrates, and only if the vertebrates have four limbs. But even having gotten vertebrates, how likely are four limbs? I can imagine more, but is that is some way less efficient? Arthropods and cephalopods can more or less have any number, but vertebrate limbs are attached, at least in real organisms, in more fundamental, invasive ways. Could a ribbed thorax support another pair of limbs? Could a pelvis? Could anatomy support more that one pelvic-like structure, or would the vertebrate system naturally converge on four as the most efficient use of that sort of anatomy? If the first limbed fish precursor had and continued to have only two, what effect would that have had on entering the terrestrial environment? Could early land dipods have been successful, or occurred at all?