It’s a surprisingly hard question to answer, but here are some things evolutionary biology is clear on:
- legs do not necessarily convey an advantage.
- leglessness is not a disadvantage.
- swimming and entering burrows are two good reasons to not have legs, but it’s very possible that this is trying to invent retroactive unicorns to justify the presence of hoofprints.
- sometimes pressures are not easy to identify, and traits become fixed by incremental advantage, or brought along as an incidental effect of a different benefit.
- many species from distant regions of the tree of life have similar body shapes to snakes, some also being legless - this means they evolved convergently - which implies that it’s more advantageous than you’d think. Limbless lizards like slow-worms, which are not closely related to snakes but are also legless reptiles, indicate that there are potentially many general factors that make it advantageous to lose legs. (This is not always true, but it’s a reasonable mindset to start with: characteristics that you find in multiple unrelated places across the globe and across the tree of life tend to be “general” advantages that convey lots of benefits in different situations. And the reverse is generally true; weird singular specialisations restricted to one or two animals in one or two places in all of history = a really niche combination of circumstances that probably isn’t an advantage as soon as the circumstances change. limbless elongated shapes are fairly common in nature, and found around the world, in both aquatic and terrestrial environments - that’s a selling point on Planet Earth.)
As a question, it’s a good one, but unlikely to have a single satisfactory factor we can put our fingers on and recite, like, “it was a bright sunny morning one day and snakes completely forgot to put on their fucking legs.”
Or, like, “there’s two ways to get legless. The first way involves forgetting your legs, and the second involves drinking a lot, which might lead to the first one, but in most cases if you want to get legless you still have to work out what you’re going to do with your genes, ha.”
What is of tremendous interest and genetic significance, however, is:
Copy-pasting ribs is not a normal thing to do, and yet on the journey to snakes becoming totally legless , they did the rib thing first and lost their legs afterwards. It wasn’t a case of a chubby short lizard suddenly losing its limbs, so it had to hustle and get really long to make its proportions more aesthetic. Snakes had already proceeded down an entire genetic plotline involving copy-pasting ribs, like this huge investment had already been made while they still had their legs nailed on. Not only do the ribs go past the lungs and around all of the organs, only stopping when the tail starts, but the vertebra themselves are replicated. Sure, you may know that snake have more spine-bones and rib-bones than you do, but have you really sat down with that? Most vertebrate animals have broadly similar amounts of spine and ribs. Humans have 12. Cats have 13. Snakes have “between 200 and 400.” That’s the weird bit!! That’s the weird bit!!!
And that’s what they did first. FIRST they got long and ribbed (and ribbed and ribbed and ribbed and ribbed). Early snake fossils are definitely clear about the long/ribbed choices. Then they gradually lost their legs, which possibly wasn’t particularly important, or lessened in importance over time. After all, as bio people will leap to tell you, some pythons still have vestigial leg bones - etc etc., left in the back pocket and forgotten about essentially - but there aren’t any snakes running about legless who forgot to get long.
Interestingly, studies of early snakes like Najash rionegrina show that other keystone skeletal characteristics of snakes, like their wide mouths and jaw hinges, were already developing while Najash still had legs. Najash was a robust, wide-mouthed, long creature, with a stressful amount of ribs, like an extended Komodo dragon. Evidence like this makes me think, personally, that legs and their presence/absence only seem like a huge deal and a huge question to us. We feel like legs shouldn’t be forgettable or trivial, we feel like their loss should be an Event and a Choice that defines a family. But in fact, legs are trivial in comparison to tiny little internal tweaks, like a particular specification of skull hinge, or a flick of paired genes on-and-off to add another unit of spine. The journey towards snakiness certainly suggests so. Perhaps legs can, in fact, just be left in a back pocket - or shrugged off forgettably, like an outgrown pair of genes.
