Reasons Why We Have Bipedal Apes, But Not Bipedal Lions, According To My Biological Anthropology Supervisor:
You know when creationists talk about how an eye couldn’t possibly evolve gradually, because half an eye is useless and a waste of resources and worse than no eye at all?
They’re wrong about eyes; a single photoreceptor cell (usually just an evolutionary ‘tweak’ away from a regular epidermal cell with biochemistry that happened to be photosensitive) is actually useful and great, and more is better. If you imagine breaking a modern wing in half and attaching it to a bird, “half a wing is useless” sounds true, but it stops sounding true when you realise that halfway to a wing doesn’t look like a modern bird wing but broken in half, it looks like a slightly enlarged membrane between a limb and your body that gives you just an extra half second of glide time when you jump.
But there *are* adaptations in this class of things, where it’s great if you have full-blown X but shitty to have half-baked X. As you might imagine, they are quite rare, because as the creationists correctly observe, if half-X is maladaptive there is no path to arrive at X through gradual adaptation to an environment. And yet bipedalism is of this class. How?
Well, you wanna know what it looks like to have enough bipedal foot structure that you decide to go adventuring around in the savannah on two feet, but you haven’t got the pelvic structure to make it efficient yet? YOU CAN’T RUN. You are literally incapable of moving faster than a kind of slow awkward lope. Your back kills all the time because your bones are all pointed the wrong way and your back muscles are trying to keep you upright. Your ankle and leg bones take far more pounding than they were ever optimised before and occasionally shatter. You’re unbalanced and ungainly and frankly sort of pathetic, and at very high risk from predators (to repeat: RUN AWAY IS NOT AN AVAILABLE STRATEGY).
Why would anything go through a long gradual process of getting much shittier and then eventually getting better, since evolution can’t plan or foresee? WRONG QUESTION. Whoever told you evolution was a slow gradual constant drift was a dirty rotten liar, just like all your other teachers from when you were twelve. More commonly, evolution involves long periods of relative stability where the organism is pretty much as adapted to its niche as it’s going to get, and then something changes and there’s a very rapid response. Or it involves successful populations dispersing widely over a landscape, then becoming distinct reproducing populations which lost genetic contact with each other and diverging, and then there’s an environmental change and they reconnect and sometimes they happily interbreed and sometimes one of the divergent branches drives the others extinct and disperses itself widely and rinse and repeat.
What happened was, basically:
Hi we’re early hominins and we just love hanging around in trees and we’re proud to say we’ve been hanging around in trees now for a couple million years and we haven’t changed a bit, slightly bigger skulls aside, we’re basically just per- what the fuck? WHAT THE FUCK? WHERE DID THE TREES GO?? WHY IS IT SUDDENLY SO DRY???? oh my God I can see nothing but grass and I am having to walk around on my hind legs all the FUCKING time and FUCK FUCK FUCK THAT’S A LION FUCK PANIC RED ALERT oh okay we’re bipedal now I guess, that was quick, oh well, all fine, carry on
Somehow we survived when a change in environment pushed us into a new ecological niche. The selection pressure was strong enough to make us acquire a really quite extensive range of mods to make bipedalism work, but not strong enough to make us dead.
Of course, “strong pressure to adapt somehow” doesn’t necessarily mean “strong pressure to adapt in this specific way we know is really good”. Early hominins who lived before the forest shrinkage have been shown to have a few bipedal adaptations. We weren’t sure what the hell they were doing with them, so we looked at chimps. Turns out chimps display short-distance carrying behavior - as in, picking up an object and carrying it. They don’t carry tools and can’t move far bipedally, but what they do do is pick up a valuable resource like a choice bit of prey and haul it off with them, away from the group of moneys fighting over the rest of the prey. So before the forests collapsed, there was a mild selection pressure to be able to use only your hind legs for a short stretch so that you could carry something in your arms, and when they collapsed, individuals good at that behavior were better at surviving the savannah and evolution just slammed its foot on the gas pedal until you get obligate bipeds.
So, a species that wasn’t forced into a rapid niche change like that, wouldn’t evolve an initially-painful thing like bipedalism. What about all the other species that made the same change as the same time as us? Eh, many went extinct, that happens a lot with ecological change, but the ones who survived didn’t do bipedalism.
Points to those who said it was about evolution having different starting points to build on, y'all were correct. No matter how awesome and efficient and optimal bipedalism is, evolution only cares about whether the next tiny step in some random direction increases or decreases how many offspring are produced. Evolution “looks” for the NEAREST solution that counts as a solution, not the best solution.
For a species of monkeys that were forced to spend less time in the forest and range wider and already had some variable locomotion abilities, evolution went for bipedalism. Bipedalism may have enabled the future awesomeness of humans with its efficiency and head stability and what have you, but evolution made it happen just because it was the local maxima - its awesomeness is a lucky side effect.
But where monkeys used short bursts of bipedal movements to carry things, another species might use something more convenient for them - say, a lion might pick up and carry things in its mouth, and if there was a selection pressure to be better at carrying the lions might end up with bigger mouths, but “become bipedal” is very unlikely because half bipedal is worse than no bipedal at all.
Basically, monkeys had the preconditions for bipedalism, nothing else did. (Note that this does not make monkeys special - the ancestor of any species with an unusual adaptation, from giraffes’ long necks to penguins’ Arctic-water-proofing feathers, was a thing that had the preconditions for that adaptation when nothing else did.)
Bipedalism didn’t happen because it was awesome, it became awesome because the range of adaptations it supports turned out to be a package that turned into, well, us.
…Notice that we are not actually the only bipedal species. Notice what they mean when they say things like, “Bipedalism leads to the ability to carry things leads to tool use leads to bigger brains”. On a naive reading, it means “bipedalism is a part of the tech tree and once you’ve bought it you can get hands optimised for holding tools”, and if it says this then you are right to be confused as to why perfectly good bipedal emus do not also have spears and control of fire.
When you realise that evolutionary studies is so full of ridiculously many caveats and preconditions that lecturers just omit them and assume you know they’re there, you start interpreting what they say more like, “In a species that already dabbled in just a tiny bit of bipedalism, bipedalism was the only way to go when the niche changed, it was way better for the new niche then the old way of locomotion, and given the likely presence of some proto-tool-like behaviors like throwing rocks or poking things with sticks, it created an adaptive opportunity to better fit this particular environment by improving on the tool behaviours using the new physiological advantages.”
Also god I learned a lot in that hour. Why does time spent *not* talking to biological anthropologists have to be a thing? Talking to biological anthropologists is the BEST.
Epistemic status: my recollection of a conversation an hour ago between me and an academic in this field, any misunderstandings are because I’m an undergrad who didn’t get what he was trying to say.
