What is it, precisely, about our anatomy that sets our species apart in the animal kingdom? Opposable thumbs? Big brains, which enable tool use, self-awareness and complex language structures? Or is it our ability to walk upright?
Of course, the answer is ‘all of the above.’ However, in his new book First Steps, paleoanthropologist Jeremy DeSilva argues that bipedalism —the ability to walk on two legs—set off a series of changes that affected our anatomy and thus our evolutionary journey.
Bipedalism was around far before Homo Sapiens arrived on the scene 300,000 years ago. In the opening chapters, DeSilva references ancient crocodile relatives that lumbered about on two legs 106 million years ago. In fact, most early dinosaurs were bipedal, until traits like heavy armor and large stomachs necessitated a four-legged posture.
Walking upright has never exclusively been restricted to humans. Go far enough back into the evolutionary timeline, and you will see that bipedalism predates the evolution of hominins.
Photo credit is courtesy of sciencenews.org
DeSilva points to Sahelanthropus, one of the oldest known species in the human family tree, dating back six to seven million years. In examining the crania of fossils discovered in 2000, scientists found the foramen magnum (the large opening where the spinal cord exits out of the cranium from the brain) located in a forward position relative to other primates. That forward position is a tell-tale sign that the head of Sahelanthropus was held on an upright, standing-on-two-legs, body.
And then there’s Danuvius guggenmosi. A study of a 11.6 million year old fossil reveals anatomy that is both human- and ape-like. D. guggenmosi was the size of a baboon, with flexible arms like a bonobo, and strong hands that they used to swing from branch to branch. Its lower limbs, however, featured extended hips and knees, with knees and ankles that could bear weight—all of which suggest that D. guggenmosi was capable of “both chimp-like suspension and unassisted bipedalism.”
These discoveries have huge implications for our understanding of human evolution. The common assumption is that humans evolved to walk upright only after the evolutionary split between humans and chimpanzees, six million years ago. D. guggenmosi upends that assumption, instead suggesting that the great apes must have evolved from a creature with bipedal capabilities.
“Given what we know about the relationships between humans and the African great apes, then gorillas and chimpanzees would have had to have independently evolved knuckle-walking. That would have happened twice,” DeSilva says. “That is unsettling. It's disruptive to what we once thought.”
The findings serve to remind us that when it comes to evolution, nothing is set in stone. DeSilva point out that there are huge gaps in the evolutionary timeline, fossils yet to be discovered. Rather than thinking of evolution as a linear progress to ever-better traits, we can think of it as a messy mosaic whose branches sometimes lead elsewhere, and sometimes hit a dead end. Bipedalism, as a means of human locomotion, was just one of many evolutionary possibilities.
Order First Steps from your local bookstore to learn more about how bipedalism affected our anatomy. And if you want to delve even further, there is a free online EdX course, from Dartmouth University, entitled Bipedalism: The Science of Upright Walking.