Bipedal – The Savanna Theory

Bipedal skeleton
See also parts two and three.

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.

rjb

About arjaybe

Jim has fought forest fires and controlled traffic in the air and on the sea. Now he writes stories.
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11 Responses to Bipedal – The Savanna Theory

  1. mixedupmeme says:

    Just a comment on the deer standing on their hind legs. It has been so dry and I see many deer reaching really high branches to feed on the green leaves. I would not have thought they could do it so easily and with good balance too.

    A neighbor trimmed a lot of his trees recently. Many branches full of leaves. In less than a day the deer had stripped every leaf off. He just had to cut up the branches and dispose of them. It was one time we were glad for the deer feeding on them. They eat almost everything in sight. 🙁

  2. arjaybe says:

    It’s the same here. They really seem to like the ornamental cedars, which all look like mushrooms now. They like the rose blossoms in our yard, too. Unfortunately we’re getting more deer attacking people, with some serious injuries.

    rjb

  3. mixedupmeme says:

    I have seen more bucks out in the open this year. Usually they are alone. But recently seeing maybe 4-5 in a group. I try to head in the other direction.

  4. arjaybe says:

    Around here it seems to be the females attacking people, usually when they have young, and very often when there’s a dog involved. The dog will go barking at the deer, then get scared and run back to the human, followed by the deer.

    rjb

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  9. The conventional view – that human ancestors became bipedal by moving from the forests to the plains (schematically: ape=>human = forest=>plain = 4-legged=>bipedal) – is biologically & physiologically impossible, e.g. primates that move from forest to savannah become not less, but more quadrupedal (“the baboon paradox”); sweating requires salt & water (both scarce in arid grasslands); etc. An account of human evolution which mentions possible arboreal & terrestrial but not shallow-aquatic milieus is incomplete.
    Comparative, paleo-environmental & other data show:
    (1) Plio-Pleistocene australopithecines were typically wetland species (K.Reed 1997). This helps explain the remarkable combination of bipedality (e.g. for wading) & curved hand-bones (for climbing aems overhead). Human fetuses never have hand-like feet, but prenatal African apes have more humanlike feet (with longer & adducted big toes) which later become more hand-like (C.Coon 1954). This suggests Pan & Gorilla had more bipedal ancestors (e.g. for parttime wading for papyrus, frogbit, waterlilies etc.), google e.g. bonobo wading, or gorilla bai.
    (2) Our Pleistocene ancestors (archaic Homo) did not disperse intercontinentally walking or running over the open grasslands, but followed African & Eurasian coasts & rivers (coastal dispersal model, S.Munro 2010), walking & wading bipedally & parttime diving for waterside, littoral & shallow-aquatic foods (which are richest in brain-specific nutrients such as DHA, e.g. S.Cunnane 2005), even colonizing islands overseas: Flores, Crete, Cyprus etc. Homo’s diet included animal (e.g. shellfish to be opened with hard tools, waterside carcasses of herbivores & marine mammals, salmon & other fish) as well as plant foods (e.g. traces of waterlily roots in neanderthal dental calculus & of cattails on their tools).
    Homo’s brain enlargement (DHA) & parttime shallow diving (which requires breathing control) were preadaptive to human spoken language.

  10. Savannah apes became bipedal mainly to allow them to carry weapons such as clubs fashioned from tree roots – or stones to throw at animals. I don’t think bipedalism would have ever allowed them to outpace an antelope. I am inclined to think that as a small group, and there is a tribe in Sudan that still does this today, they would shoo off a lion from its fresh kill. This is done by throwing stones at the lion or charging at it with their clubs and screaming loudly. Bipedalism is also needed for carrying the kill – or parts of it – back to the community.

    Bipedalism also allows a vision above the elephant grass which can be relatively high in some areas and is also essential for wading in the East African lakes or the wet lands of the upper Nile region.

  11. arjaybe says:

    Good points, Andrew. Did you read the other posts linked in this one?

    rjb

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