There are many hypotheses about the fact that humans walk on two legs. We aren’t the only ones to do so. Birds are bipedal, that is they have only two feet. Many dinosaurs were bipedal, for instance the Tyrannosaurus Rex and the velociraptors. Fossils of the earliest dinosaurs are from small predators with two legs. Birds are believed to have descended from at least one line of bipedal dinosaurs. But amongst mammals humans are with only a few who regularly get around on two feet.
Other mammals include the giant pangolin, an anteater. When it’s down on all fours it carries most of its weight on its hind legs, and it often walks upright, using its tail for balance. Also in this group are the macropods, meaning animals with big feet. These are the kangaroos, wallabies and other hoppers. Other mammals use limited forms of bipedalism while using their front paws, for example rats and beavers. Some do it to better view their surroundings, like ground squirrels and meerkats. Some antelopes and deer stand on their hind legs to feed from trees.
Although we have a lot of company, no other mammal has specialized its bipedal locomotion to the degree of our upright stride. Our feet can’t grasp things like those of our primate cousins. Our legs are so long and our arms so short that we can’t walk on all fours with any efficiency. Our spines, pelvises, knees and ankles are best adapted for an upright striding gait.
So, how did we get this way? Any explanation for how we diverged from our primate cousins has to account for a lot of obvious differences between us and them. Our closest relatives among Earth’s other animals are the primates – that’s all the monkeys, lemurs, gibbons, apes and so on. Among the primates, the other apes are most like us – orangutans, gorillas, gibbons and chimpanzees. Our closest relatives of all are the other great apes – orangs, gorillas and chimps – none of which is primarily bipedal. The gibbons are classified as lesser apes. Although they are more similar to us than, say, a trout, there are obvious differences between us and our ape relatives. While we move on the ground on two long, straight legs, the others show more quadrupedal locomotion, having longer arms and shorter legs. We have longer, stronger thumbs and shorter, straighter fingers. We have larger brains relative to our body mass. We have much less hair than the rest of the great apes.
There have been many suggestions for our uniqueness, with wildly varying levels of plausibility. It’s likely that no single explanation can account for it on its own. The most widely held consensus is the Savanna Theory, which holds that we developed bipedal locomotion to deal with the grassland, or Savanna, that was spreading about that time. Before about seven or eight million years ago all of the great apes lived in a great forest that covered much of the equatorial and sub-equatorial Earth. Then there was a change in the climate. It got drier and the forest began to recede, with patches of grassland growing between the trees. While the other great apes continued with their traditional lifestyle as forest dwellers, the Savanna Theory holds that our ancestors began to exploit both the old habitat in the trees and the new opportunities in the grass. That’s when our forebears and those of the chimpanzees split from a common ancestor and evolved in different directions. As our ancestors had to move farther between clumps of trees as their forest was shrinking, an upright posture would have helped in many ways. It would improve their view in tall grass. It would reduce the amount of their skin exposed to the sun. It would get their heads above the hot boundary layer near the ground, helping them stay cooler. Also for cooling out there under the blazing sun, they lost their body hair. In addition, they might have used their hind limbs for standing and walking more as they used their hands for carrying things across open ground.
Some other advantages came from this adaptation. It provides a better posture for feeding from trees while standing on the ground. Being upright makes wading in water easier and safer. Our long, strong thumbs and short, straight fingers evolved as we swung in the trees less. Other great apes have curved fingers which facilitate hanging from branches. The unique anatomy of our hands makes them stronger and more versatile in a wider range of activities. Our larger brains are partly the result of a positive feedback loop between our nimble hands and a growing intelligence. As our greater dexterity improved our diet, our better brains improved our abilities.
The question of human bipedalism holds an unspoken assumption that we evolved the trait while the rest of the apes didn’t. We may have to examine that assumption. To this day large primates like orangutans walk upright on their tree branches. Their knee joints are so similar to ours that we’d have to say that they evolved for an upright stance. It’s an adaptation that shows up in the fossil record twenty-one million years ago, long before our ancestors were walking on the ground. There’s a growing suspicion that other primates like gorillas and chimpanzees evolved their quadrupedal locomotion on the ground while our forebears stuck with the bipedalism learned in the trees. Maybe we should be asking why they did that instead.
That’s a brief look at some of the features of the Savanna Theory. Next time we’ll look at another explanation, the Aquatic Ape Theory.