How many times will lizards become "snakes"?

Nature Talk
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The scientific consensus is that snakes evolved from lizards – and that they had hindlimbs for millions of years during the transition. But the reason snakes have long been considered a separate kind of reptiles is because of other features they have which lizards do not. This raises the question, how much difference does it take to be considered something other than a legless lizard?

Amphisbaenians are another snake-like lineage descended from lizards. One key difference is that whereas snakes have a reduced left lung, amphisbaenians have a reduced right lung. For a long time they were considered a third squamate group, alongside the lizards and snakes, since they seem so different from both.

Then there are the legless lizards, many of which move in very snake-like ways, but are not (yet) different enough from the other lizards to be considered another “kind.” All of the Pygopodidae, all of the Dibamidae, the monogeneric Anniellidae; several genera of skinks from three different subfamilies, two of which comprise mostly skinks with limbs; one of the three subfamilies of Anguidae. yet unlike the snakes and amphisbaenians, these were never classified as different “kinds” of reptiles, but were regognized as types of lizards.

Even stranger is that the snakes are phylogenetically closer to agamids and iguanids than they are to the amphisbaenians and the scinciforms, and further still from the Gekkota, in which the Pygopodidae are. The Dibamidae seem to be basal to all the others, therefore the group most eligible to be a separate “kind.” So evolving into “snakes” appears to be a general “lizard thing” to do, occurring independently in different lineages, rather than a characteristic of a specific lineage.

I have heard of the term “carcinization,” which refers to the evolutionary tendency of various marine crustaceans to evolve into crab-like body shapes. I suppose there could be a similar term for this tendency of lizards to evolve into snake-like forms. “Ophidization”?

Nowadays we use DNA to draw out evolutionary trees, and so the snakes and amphisbaenians are nested in the lizard tree. Still, given a few more million years of evolution, one could conceive of, say, the glass lizards or the legless skinks becoming as diverse and divergent as the snakes are today, enough that a Linnean-type taxonomist might classify them as separate orders. How different would they have to be from mainstream lizards for this to happen?

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Interesting question. I have often wondered what would be the evolutionary advantage of a lizard losing all it’s limbs and devising an entirely new mode of movement

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It shows that nature doesn’t fit into the neat boxes mankind likes to make. Birds are bona fide maniraptoran theropod dinosaurs, and as far as I know, we still classify them as a separate “class” from the rest of Dinosauria.

We ourselves, also don’t fall neatly into labels or boxes: we are hominins, we are apes, we are upsized Old World monkeys that lost their tails, we in turn are haplorhines alongside tarsiers, we are primates, we are mammals, and so on and so forth.

Animals are an excellent example of Dollo’s law: a creature identified by certain features will never evolve back to a state before those features had evolved. To put it in a simplified way: once a lizard, always a lizard.

What we call snakes are a highly evolved group of specialised lizards with consistent features that are differentiated enough, and go back far enough in time, for us humans to see and think of them as something separate to what we recognise as “lizard”.

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It’s entirely at the whim of one person to make the proposal and a few others to follow along–and for others down the road to change as they see fit based on the predominant philosophical naming convention of the time. Birds are a lineage of feathered dinosaurs. Snakes are a lineage of legless lizards. Humans (and other tetrapods) are a lineage of legged fish. The relationships are based on data, the naming of the groups is just a point of view.

When it was realized that birds were a type of dinosaur, and since dinosaurs were considered reptiles birds were then reptiles, people flipped out and we’ve been debating it ever since. Since some people didn’t like the idea of birds being reptiles (for the same reason they don’t like mammals being called reptiles), some people’s solution was to kick some former reptiles out of the club.

So snakes aren’t called legless lizards for the same reason that birds and mammals aren’t called reptiles or fish (they have the body plans that define those groups). But how different does something have to be to be given a equivalent taxonomic rank? It’s all based on comfort and intuition, not anything quantifiable.

Also, you probably already know, but we have two classifications systems to chose from. One which can be (and often is) based solely on appearance and one which can only be based on relationships (the latter came on the scene in 1988 and has been the subject of debate among scientists ever since). The ultimate source of the naming problem is that nature, via evolution, gives us a continuum and one can always and forever argue over how to divide a continuum into segments.

Your question of how many times will snake-like lizards evolve–well, no one can predict the future.

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Modern legless lizards have difference in eye and ear structure that separate them from real snakes, as well as feeding strategies, will they became new snakes? Likely they will become something of their own, that’s like modern otters can become seal-like if niches will be opened, but they still will have slightly different body.

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And sure, other snake-like lizards could evolve in the future. But it’s not going to be a “snake” in the same sense. In theory, millions of years from now we could have Squamata and another order of legless lizard. Maybe at some point in the future, there will indeed be different Orders of legless lizards. Maybe then, in theory, the definition of snake would change into more of a common name? Interesting thoughts

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Good responses already provided here. Lizards as a group seem to be predisposed to head down the path of limb reduction if not full limblessness. Not all of course, but certain clades such as the skinks. If you’ve ever watched one of the skink species that has limbs make an escape, they look like they are already half-way to being snakes in their body movements. If something works, it will likely be selected for, and maybe limbs are just not all that useful for certain lifestyles. I wonder how many genes are actually involved in controlling limb development and how “easy” it is to select for reduced (or no) limb development in these groups.

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I’m not a reductionist sort of person - I believe that what happens in the form of non-human life is a product of both chance and selection. Amphibian groups have lost legs, partly because they have that genetic ‘aspect’ to them, and for some reason it persisted. Snakes, a group that arose a long time ago, have succeeded in their role but have characteristics not found in amphibians.
It’s similar to mammals - I doubt there would ever be a truly legless/armless mammal. It’s not an option available in our genes for some reason. Even mammals with reduced limbs - whales, seals - still retain the mammalian 'basic plan. Fins are flippers (same bones), and the hind limbs may be reduced, but are derived from the basic plan. A lot of whales have vestigial pelvises and hind limbs. The body shape suits their lifestyle, but the basic plan remains. Chance and selection. Whales, though, will never become fish.

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In mammals, an obligate aquatic lifestyle seems to be necessary for radical limb reduction.

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True. Selective pressure I assume. But they will never evolve into fish. That’s all I mean!

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I think given the right selective pressure over enough time they could. Correct me if I’m wrong, but go far back enough along the evolutionary chain and there were no mammals at all. You might not get back to an exact genetic configuration that existed before, but you can certainly change from one class to another (or even one kingdom to another. Go back far enough and there were no plants or animals, only simple organisms like bacteria). I don’t think there’s any rule that states evolution must always go from simple to complex.

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We all are fish anyway, but it’s unlikely to get same mutations, just because mutations are so unpredictable, it’s a small chance to get the same thing twice, and fish evolved from organisms that had a certain set of signs, now when mammals evolve, they go from different point, e.g. we have cranium, lower jaws, ear bones, mammals are unlikely to get back to same gastro-system and skin likely will not get scales, but maybe something similar from hairs, but those won’t be fish. Something lancelet-like will have easier time evolving into fish than a mammal, because it’s simpler and will evolve to complexity, now there’s a complex organism becoming a different comple organims and probably with downfalls that you’d want to avoid doing the same thing twice.

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Not sure the term is still used, but in evolutionary biology there is (was) the term “canalization” which suggests that there are some pathways that an organism is unable to take evolutionarily due to genetic constraints. Basically, once you start down a given path morphologically there are limits on where you can go from there. Could a lineage of cetaceans ever recolonize land as a terrestrial animal? Maybe, maybe not.

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Adjusting the question a bit so that it reads “How many times have squamates lost limbs?” the answer is at least 25 (including the snake lineage).

I’m reading a paper about loss of genes that control limb development in snakes. It turns out that some of the genes that facilitate limb development also facilitate penis development. “For instance, mutations in several different genes result in human syndromes characterized by defects of the limbs and phallus”. And they’ve found that in snakes, several genes that facilitate limb development are still functional–being conserved because they were needed for phallus development.

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Looks like this paper is as much as we knew back in 2017.

“… the picture that is emerging from these studies is that early truncation of snake hind limb development was caused by deletions in the ZPS [gene]”.

“In an elegant test of the functional consequences of the python ZRS deletions in vivo, Kvon et al. (2016) used CRISPR-Cas9 to replace the mouse ZRS sequence with the python sequence containing the deletions. The resulting mice developed dramatic distal limb truncations that resembled the python adult limb morphology, demonstrating that the deletions in the python ZRS are sufficient to disrupt transcription of Shh and development of limbs (Kvon et al., 2016). When they restored a 17-bp deletion in an Ets1 and Hoxd13 binding site, Kvonet al. (2016) were able to rescue normal limb development in the engineered mice, thereby demonstrating how microdeletions in critical regions of the ZRS can have dramatic effects on limb development.Whether reduction of hind limbs during the evolution of snakes was caused solely by ZRS mutations will require further studies, including additional comparisons of snake genomes”

And there’s fossil evidence that a snake with limbs re-acquired limbs from an ancestor that was limbless (such a feat would have required that very few mutational changes caused the original limblessness) because the chance of many back mutations occurring is slim to none.

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That may still hold true, but in this case it’s not the pathway that matters, just the end state. To use a physics analogy, there may be many “forbidden” pathways under the normal approximations, but as long as there is a pathway, the state will occur given a long enough time scale. What states are allowed is determined only by the effective potential, or in this analogy those evolutionary pressures.

The emergence of habitable land was a global change in evolutionary pressure that produced a new favorable state (land-dwelling air-breathers). Realistically, of course, evolutionary pressures on Earth generally vary on smaller scales in time and location, which may be why we haven’t seen real-world examples of something like a mammal evolving back into something like a fish, since then. To observe a large change like that would require an equally large global change in evolutionary pressure.

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Another interesting thing to think about would be to consider what conditions would be necessary to produce an organism that doesn’t fit in any of our currently established categories. Are there really just a handful of kingdoms that are possible?

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I imagine shallow waters with abundance of fish and no predators could make something lazier out of dolphins, if water levels would drop lower, it could drive them on land, but they need to transition to harder skind sometime before that and second pair of legs should emerge in water too, maybe they could be adaptive at that hunting strategy?
@screedius by current taxonomy we already have too many kingdoms, hah, but seriously, in one-(or oligo-)celled organisms it’s easier to just become a new kingdom and they did it many times.

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As long as they don’t do the weird pseudo-legs like searobins. I learned what those were for the first time a couple week ago in a coastal ecology course, and the way they crawl around on them creeps me out a little. :grimacing:

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Sounds like they’d take the manatee niche there.

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