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Thursday, March 23, 2023
Dr. Brown: Footprints and the evolution and genetics of bipedalism
Dr. William Brown File Photo

I am fascinated by footprints left behind by our prehuman and modern ancestors. Those tracks stir the imagination in ways that bones, including the skull, do not — at least for me.

One such track was left by a woman carrying a small child on her left hip 8,000 years ago in what is now New Mexico.

Periodically, perhaps because the child was wiggling or the woman was tired, she put the child down, leaving two sets of prints, one much smaller, only to return to her prints alone when she once more picked the child up.

Their path was crossed by a mammoth who kept going and a giant sloth who paused and turned toward them before continuing. Who was she? Where was she going? And what became of them? All questions those tracks pose for us in our time.

What about the 37 or so separate pairs of footprints left on a south coast of a Spanish beach 110,000 years ago?

Most were adults leisurely walking along the beach, perhaps talking to one another, while other pairs of smaller prints suggested several children probably playing with one another, all of whom had to be Neanderthals given the age and the shape of the prints.

Or what about the tracks left in east Africa by two adults walking side by side with a child whose prints wove back and forth through their path much as human children are wont to do when they play?

The prints were probably made by Australopithecus afarensis, to whom the name Lucy was attached after the Beatles song “Lucy in the Sky With Diamonds” and dated to 3.4 million years ago. 

In the last few decades many similar tracks have been discovered, sometimes by returning to previous sites only to find more tracks in the same area or nearby areas, some uncovered by erosion or radar or satellite images.

The latter images revealed the shores of ancient lakes in northwest Saudi Arabia and led to the discovery of footprints of modern humans or perhaps Neanderthals from 100,000 years ago.

But why the fascination with ancient tracks? Because all the prints and tracks I’ve referred to were left by fluidly bipedal species.

Bipedalism among apes can be traced back at least to the common ancestor for what would much later be modern humans, chimpanzees and the latter’s very close relative, the bonobos.

It took several million years for all the closely related elements in the spine, pelvis, hips, knees, ankles and feet to evolve to the point where species became fully bipedal.

Coupled with these changes were other adaptive changes to make room in the pelvis for the birth of increasingly larger skulls (think brain here).

Human babies are born far more dependent and remain so for much longer than children of other apes, which suggested to some experts that all normal-term births in humans are developmentally premature, relative to newborn children of apes.

As Jeremy DeSilva pointed out in his 2021 book, “First Steps,” bigger brains in primates, go hand-in-hand with skeletal adaptations for both bipedalism and making room for those bigger brains in the birth canal.

Bipedalism also freed the forelimbs for other tasks such as the creation of tools and eventually go on to serve a wide variety of other functions such as playing musical instruments.

Bipedalism was accompanied with the development of increasingly large social groups and the greater challenges for navigating larger, more complex social networks.

This would have placed a premium on the evolution of social intelligence and the closely related evolution of increasingly sophisticated oral language and matching changes in the brain, such as the lateralization of speech.

The genetic basis of bipedalism was recently explored by studying legally aborted human fetuses, with the mother’s permission.

The investigators showed that the pelvis begins to take shape around four to eight weeks and involves many thousands of genes many of which are regulatory ones — that is, they dictate what changes take place in what specific regions of the forming pelvic cartilage (there is no bone that early).

By comparing the genetic control of the human pelvis with that of the mouse, they were able to show that many of the regulatory genes for the pelvis in humans evolved rapidly in the last few million years and early on were under intense selective pressure for the evolution of bipedalism.

Similar reasoning and findings would probably be true for other highly valued systems such as social intelligence, sophisticated oral speech and fine manipulatory skills with the hands.

Bipedalism was important in our evolution but as part of a much larger package of changes involving the brain and other systems and we’re just beginning to get a glimpse of how complex and integrated those changes were and are.

Nature is far more marvellous than we imagine but the gap is closing.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.  

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