Some thoughts on why we stood up.
There are a bunch of hypotheses on why we are the only primates who are fully and exclusively bipedal. I don’t really know exactly how I feel about any of them, so I’m gonna hash out some thoughts on a few of them here. All of them have their merits, some more and some less than others.
The hypotheses I want to look at are: the savannah grass hypothesis; the retreating forest hypothesis; the heat displacement hypothesis; the tool-maker hypothesis and the climatic-shift hypothesis.
The retreating forest hypothesis is interesting because at roughly the time of our shift to obligate bipedality, climatic changes were causing increased desertification or at least expansion of grasslands along with the retreat of forests. That our ancestors (or something representative of them), by about 4.4 million years ago had anatomical features that suggested at least some form of faculative bipedalism well before the climatic instability at around 2.8ish million years ago also suggests that their ability to walk upright would have allowed them to more efficiently traverse and populate the grasslands. This can also be lumped with the savannah grass hypothesis, because that further postulates that standing up allowed us to see over the tall savannah grass. In turn, this lead to an emphasis on standing upright, yadda yadda, so on and so forth. I’ve also read/heard about another hypothesis in which we stood upright to reach higher hanging fruit. This seems pretty close to stupid to me, because if we’re trying to reach fruit–in trees–why the hell would we move away from an arboreal lifestyle/suite of adaptations in order to be less efficient at getting our food?
All well and good, but this seems way too much of a leap for me. There is more to it than that, but this seems an older version of the climatic-shift hypothesis that I’m going to get into later. Mostly, I dislike this model because hominids were the only critters to stand up on two legs to get around the savannah. Lions and hyenas do quite nicely.
It has also been found that human, obligate bipedalism better conserves energy than faculative bipedalism or obligate quadrupedalism. It would be a balance, really. Comparing modern human anatomy to that of a chimpanzee and saying “we’re more efficient bipeds” is a.) obvious and b.) almost stupid. First of all, of course we’re more efficient bipeds. We’ve been doing it for millions of years. If there is a “right” way to do it, I think we’ve figured it out. Also, we’re using half the amount of limbs. We would use more energy per limb, but we’re only using two of them. The involvement of their entire bodies couldn’t help but be less efficient overall. This also cannot take into consideration the efficiency of our ancestors, as they varied (sometimes greatly) in their anatomy, and thus we may provide a poor model.
And, like I said, it’s a trade off. Lumbering around on two legs may use fewer calories and conserve more energy, but we’re also slow as hell. Go outrun a cheetah. Who gets more calories then?
The heat displacement hypothesis doesn’t really do it for me, either. As we moved out of the aforementioned retreating forests and fanned out over the savannahs, it was hot. No tree cover. The critters that stood upright absorbed less of the punishing African sun, and those individuals–and their children as a result–would tend to overheat and tire less quickly, thus fairing better in general. Poof, here we are.
I don’t really care for this one so much, either, because it still begs the question of why the hell we stood up in the first place. I can accept that it increased the odds of survival, but to me it’s a reversal of the order of causality in that it suggests that we developed this adaptation SO THAT we could displace heat better. We evolved “for” something, rather than because of something. To me, the heat wouldn’t have made us stand up; however, once we did stand up, the heat would affect us less. Dig? This type of reversal is common, even among people who feel they understand evolution well. It’s a subtle confusion, and has been so persistent I doubt it’s going anywhere..
The tool-maker hypothesis seemed pretty compelling to me, at first. This model suggests that as we developed more skill at tool making, it began to play a larger part in our overall survival. I agree. But it also suggests that freeing our hands to manipulate tools somehow drove us to bipedalism.
Wow, so simple, so elegant.
Wait a minute, though. If we are the most efficient tool-makers around (I’m on a computer right now…so I think that’s a fair assumption), we should take a look at us. Do you typically walk around using your tools? I don’t mean a walking stick. Do you walk around whittling? Do you walk around while cooking? How about while texting? How well does that work out for you? I bet you have to look up to keep an eye on where you’re going. I usually end up stepping off to the side of the crowd so that I can avoid trampling/being trampled, or walking in front of traffic or something.
If we’re so damn adept at making and using tools, yet our walking around gets in the way, how would it have been for these ancient critters who had far less precision in both their grasps and gaits?
Homo habilis made some fun tools. I can’t picture them walking around with a tool core and hammer stone. When looking at our close cousins using tools, they’re usually not locomoting while doing so. Chimps will break a branch off and sharpen the tip, but not while brachiating. Orangutans poke sticks into the water to test for depth, but they’re hanging there while doing it.
Why, then, would an early hominid be walking around while napping stones? It seems a lot more likely that they’d sit down and use their already well-developed hands. Which makes sense, because whenever we find these manufacture sites, the discarded cores and flakes are in a pile. If tool making caused bipedalism, we would see flakes in a row, as our ancestors were walking along and napping away.
The climatic-shift hypothesis seems the most intriguing to me. The same climatic fluctuations that inspired the retreating forest bit–more fleshed out–developed this hypothesis. In this model, the changing climate preserved the suite of adaptations in our lineage, because our adaptability allowed our ancestors to be both biologically and socially dynamic.
Sediment cores have supported the existence of severe climatic instability for extended periods of time. During this time, not only did we flourish, but all the other hominids died out.
This is neat and tidy, but still falls short in one way–why the hell did we get up in the first place!? Ok, we stood up. Ok, we make lots of tools. Ok, our access to meat as a concentrated food source (and our domestication or creation of fire) placed positive selective pressures on tool-making to process meat and eventually hunt regularly; cognition for the forethought and planning involved in coordinating a hunt or following and anticipating migration routes; and once we were traveling reasonable distances in order to find a carcass to scavenge or animal to hunt…bipedalism would leave our hands free to carry it back “home.” The domestication of fire would have provided incentive to do just this, too–a fresh or aging kill would quickly attract some critters we weren’t prepared to deal with. If we could use a hand axe to dismember the carcass into manageable chunks, our freed up hands (thanks to bipedalism) to carry it back to camp, and fire to cook it, we would likely avoid losing meat to scavengers, friends and food to predation, and meat to spoilage. All of these adaptations would allow us to better navigate the climatic fluctuations of the time.
I should point out that one weakness of this is the time span that may have elapsed between shifts. If it takes 4,000 years for a serious shift in climate, where’s the selection pressure? However, I think the big pictures painted by paleoclimatic models, like the big pictures painted by radiometric dating or generalizations of species represented by fossilized individuals, the fact that any major weather fluctuation is evidenced in the geological record is likely indicative of a more frequent pattern. How many individuals of a species fossilize? How many weather events can be conclusively read from the geological and sediment record? How much more refined will techniques get in the future, either backing me up or making me laugh while reading this back to myself in 15 years?
But. We had to have had this trajectory already. Evolution doesn’t “give” us anything, for any reason. Sometimes we “get” something–sometimes we figure out how to use our physiology to better take advantage of our environment. An isolated population of, say, Ardipithecus ramidus was able to use it’s greater capacity for bipedalism for some form of crude tool use (like chimpanzee termite fishing), or carrying armfuls of picked fruit from a good site so that they could avoid competition from others. But, they could still climb the shit out of some trees. They weren’t fluid on the ground, and there were better-suited critters in the trees. But they were pretty damned good at both. It’s also worth mentioning that Sahelanthropus tchadensis, about 2 million years older than Ardipithecus ramidus, had a forward placed foramen-magnum, suggesting a more upright posturing than many other primates (though not necessarily indicative of bipedalism just yet, though I think it is).
This is completely in the realm of speculation, but it seems a tenable model to me. It synthesizes climatic and other environmental models with societal models and takes into consideration the dynamic social structures and cultural developments that make humans so unique among the animal kingdom. Furthermore–and perhaps most importantly, it provides a REASON.
A trend toward bipedalism, once establish, would have been conserved by the suite of adaptations we were already developing. The climatic shifts would give further reason to conserve that, both because they worked in allowing us to adapt, and because the other hominids roaming around at the time were unable to adapt…leaving us with less competition.
The positive feedback loop created by this hypothetical group’s ability to have a reason to be bipedal would have selected toward bipedalism and against (or at least neutrally for) arboreal traits. But, regardless of anatomical arboreal vestiges, the development of culture would have ignored them even if they may have still be effective.
All of the other models fit nicely into this one: we could cope with the high savannah grass and in doing so, displace heat; we could cope with the retreating forest, because even though we may have lived there, we weren’t doing much arboreal stuff anyway. Additionally, our hands were freed up for carrying stuff, enabling and encouraging the transportation of meat and eventually complex tools. This would also provide selective pressure on cognition as we eventually started tracking herd movements and stashing caches of tools where we could then anticipate the appearance of them. As tool industries and hunting or shelter-building techniques developed, communication systems must have developed to cope with the increased need to convey abstract ideas.
Bipedalism made us slower, but conserved more energy (and displaced more heat). The consumption of more meat would have provided us even more net calories/nutrition combined with our conserved energy from bipedalism would have allowed evolution to act on our trend towards enhanced cognition. And, as already mentioned, the crazy climate shifts would have helped hone these adaptations. Everything I toss into this mix just makes the snowball bigger; each component seemingly more inextricably linked. Any one can be considered the “cause” or catalyst. And that’s why this is so appealing to me. It just is. It’s random, but not arbitrary. This stuff didn’t HAVE to happen.
In reality, we aren’t going to definitively find out anytime soon. Paleo anything (and to a certain extent scientific anything) requires inductive reasoning–rarely do we find absolutes with something so ancient. As we find more, as we build on the previous generation of scientist’s innovations and continue on the tradition of skeptical inquiry, we can only further refine and flesh out images from the past. But, they will likely only ever be snapshots. This is what draws me to science–the search for knowledge. What draws me to paleoanthropology is the lack of, well…anything. We have a big pile of bones, and some modified rocks. Eventually we start finding hearths. More recently we’ve found better bones than we ever thought. That line from “ape” to “human” is becoming blurrier. Genetics pushed the date from our split with our common ancestor with chimps and bonobos back farther than ever expected–to roughly 6 million years. Then Michel Brunet found Toumai, Sahelanthropus tchadensis, who neatly straddles that date with his blend of hominid and ape characteristics. Geneticists have extracted DNA from seven Neanderthal specimens, and additional DNA from a finger bone–the latter of which has never been seen before. We don’t know who that DNA belonged to, but it doesn’t exist in modern humans, or in the Neanderthals.
Science continuously pushes the boundaries of both what we know and what we CAN know. No generation of scientists thought they were ignorant. One of the bigger realizations of science may be to realize the depths of our ignorance. Darwin wrote of the impoverished fossil record, and how it could prove fatal to his theory. He has only been vindicated (and not just by the fossil record–we can now watch evolution in real time through a microscope). I can only hope to add something to that legacy.