Jun. 2nd, 2017

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so you see, humans evolved to be bipedal on account of how our ancestors transitioned from the forest environment to the savannah environment, and in the savannah environment bipedalism was more adaptive because it provides better thermoregulation and allows you to carry things, but most of all because bipedal locomotion is highly energy efficient and energy efficient locomotion would have been very strongly selected for on account of how time budgets are a limiting factor on home range which is a limiting factor on diet quality and breadth which is really quite important

my lecturers have been very clear and very insistent that bipedalism evolved first and then allowed tool use, tool use did not spur a transition to bipedalism, the fossil record is Clear On This Point

and what I do not understand is: if bipedalism is so completely wonderfully energy-efficient and optimal, why are there so few bipedal things? How come lions and gazelles and giraffes and buffalo aren’t bipedal? Why aren’t other savannah species selected for energy-efficient locomotion too?

I am sure there is a good explanation for this but my lecturers have still not provided it and I must know please god just somebody explain this to me or I will die of curiosity

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.


(Why do I not live on a university campus D:)

SO YES and also, I’m going to pull out my Vaclav Smil* for a second here.

Human locomotion is not particularly energy efficient! It takes us more energy to walk or run than it does for most mammalian quadrupeds, but our energy use curves look pretty different from theirs. 

If a horse goes for a trot, its trot (like all its gaits) has a U-shaped energy curve. It costs more to trot at slower speeds, goes down to a most-efficient pace, and then comes back up. At a certain point, it crosses over the energy curve for the horse’s next gait, and the horse will (left to its own devices) start to canter or gallop.

Human WALKING has a U-shaped curve like that, but human RUNNING does not, and that is damned strange for a mammal. Our friend Smil says: “the energetic cost of human running is relatively high, but humans are unique in virtually uncoupling this cost from speed”. That particular aspect of things is a direct side-effect of bipedalism: we can vary our breathing in ways that quadrupedal animals (who have supporting legs all attached to their breathing apparatus) cannot. Basically, we are the evolutionary equivalent of cartoon characters who can spin their legs really fast. So we aren’t as efficient at running as a horse who is going at its optimum pace, but we can speed up and slow down and it won’t cost us much, which is not true of the horse.

Not incidentally, this is why many humans practiced (or still practice) persistence hunting. If you are less efficient than that delicious antelope, but you can make it run at its least-efficient panic speed while you trundle along at a nice constant rate, you can exhaust it.  

* Smil, Vaclav (2007-12-21). Energy in Nature and Society: General Energetics of Complex Systems (MIT Press). The MIT Press. Kindle Edition. 

I love that my tumblr dash has both romantic smut and bipedalism theories in the same evening.

And yes, I am now thinking about hominids in flower crowns.
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“Wake up Steve!! Tony is gonna swim away!!”

Credit: @烛原


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