1050 words

I wrote about bipedalism last week, however, in a conversation with a commented on PumpkinPerson’s blog, I came to a slightly different conclusion than I did in my previous article on the matter.

I quoted Daniel Liberman, author of the book The Story of the Human Body: Evolution, Health, and Disease:

As one might expect, other selective pressures are hypothesized to have favored bipedalism in the first hominins. Additional suggested advantages of being upright include improved abilities to make and use tools, to see over tall grass, to wade across streams, and even to swim. None of these hypotheses bear up under scrutiny. The oldest stone tools appear millions of years after bipedalism evolved.(Lieberman, 2013: 43)

However, after being linked to this journal article Shallow-water habitats as sources of fallback foods for hominins, I began to rethink my position on this matter.

Now, the climate change when humans and chimps diverged is still the primary cause of bipedalism, but after reading this abstract, I came to think that the climate change (getting warmer) rose the sea levels which then drove Man to walk on two legs, gather food, and, eventually, wade in water to find more food which led to some selection for bipedalism in our ancestors. Humans needed to become bipedal to find more food as the climate change made their primary food more scarce. This then drove early Man into shallow waters to look for food.

As the climate was getting warmer, the same foodstuffs we ate were not as readily available. So what drove us to be bipedal was 1) the need for immediate food, i.e., looking for food on the forest floors and 2) when adequate food could not be found on the floors of the forests, Man then had to go into the water. This had multiple advantages. One could escape from predators, find more food with adequate nutrients which, in turn, had as evolve bigger brains, and the most important aspect, it’s much easier to be bipedal in water as it’s easier to stand in water.

This paper, The evolution of the upright posture and gait—a review and a new synthesis, concludes:

Wading was an appropriate trigger not only to stand up but also forced the primate to walk on. It seems likely that habitual bipedalism began not long after the separation from the gorilla and chimpanzee clade(s). From that time onwards, throwing could be evolved with free upper extremities much more successfully than before. Selective factors related to the reduction of incoming solar radiation became effective. Endurance running and adaptations to carry tools (like weapons) started their evolutionary improvements. If these processes took about 4 Ma, the wading hypothesis is consistent with a rather perfect bipedal anatomy as shown, e.g., in Homo ergaster (WT 15000), about 1.6 Ma ago. In this way, many of the hypotheses competing in the past may be harmonised, as some of them have yielded important contributions to the understanding of the evolution of the human habitual upright gait.

The first sentence corroborates what Lieberman says in The Story of the Human Body. The final sentence brings together all of the theories that drove bipedalism in humans into a ‘new synthesis’.

Now, climate change (the earth getting warmer) is still the ultimate cause, but a proximate cause of bipedalism is wading in the water to 1) find more food and 2) escape predators.

This ‘new synthesis’ of how Man became bipedal is a great way to unify a lot of theories of bipedalism that gave us great understanding of human evolution in regards to bipedalism. In that vain, it’s like E.O. Wilson’s Sociobiology: A New Synthesis in which he sought to unify the evolutionary mechanics of altruism, aggression, and nurturance– our main social behaviors. This ‘new synthesis’ in the study of how we became bipedal unifies competing theories into a more understandable theory.

Moreover, bipedalism made it easier to consume more kcal which led to bigger brains. To quote Suzana Herculano-Houzel from her book The Human Advantage: A New Understanding of How Our Brain Became Remarkable:

“The remaining way to work around an energetic constraint to the number of neurons in the brain involves dietary changes that would allow for more calories to be obtained in the same amount of time, or even less. Some first changes in that direction probably took place 4 million years ago when our australopithecine ancestors stood upright and became habitual bipeds. As Daniel Lieberman explores in detail in The Story of the Human Body, bipedality potentially increases the amount of calories that can be amassed in a day by extending the range of food picking, for it is much easier and costs four times fewer kilocalories to walk on two feet, as humans do, than on all fours, as modern great apes do and the ancestor from which australopithecines originated must have done. Roaming away from home to find food, is the definition of a food gatherer, as opposed to a food picker, which is what great apes remain to this day. Bipedality made food gatherers of our ancestors.” (Herculano-Houzel, 2016: 189)

This graph from her book shows that bipedality preceded cooking which increased our brain size (I will write on that soon).

More neurons in the cerebral cortex is the cause for our amazing brains. But we are NOT unique!! This kcal increase led to more neurons in our cerebral cortex which then allowed for reasoning, finding patterns, developing technology and passing it on through culture. Cooking is why we are so ‘unique’ in comparison to other animals. As shown in the graph above, the increase in brain size happened around the time of H. Erectus. They show smaller teeth at that period, which shows that the selection was already occurring. The smaller teeth to break down food more to extract nutrients from the food they gathered shows that bipedalism evolved alongside the evolution of smaller teeth.

In sum, the ultimate cause of bipedality is still climate change, but the proximate cause, in part, was wading in the water which led to our ancestors to find more food. And, over time, we were selected for bipedalism as I wrote in my previous article on the matter. We can see that bipedalism slightly predated cooking, which the ultimate cause of which was to find more food. This is seen in the records we have. I will write more on this in the future as I read into this more.