Human ancestors scrambled to their feet, a new explanation for bipedalism asserts

WHOOSH. Scrambler Man ascends steep cliffs in a single burst, leaving his barbaric enemies in the dust. Then, while chasing angry beasts uphill and down, Scrambler Man is transformed into — POOF — The Biped, who can outwalk, outlast and outsmart them all.

Scrambler Man and The Biped could be a weird comic book superhero tale, but it’s really a different kind of story about how humans came to be: A group of researchers proposes that maneuvering in rough, rugged terrain drove the evolution of upright walking.

The question of bipedalism’s roots is one of the most fundamental — and contentious — in human evolution. It’s worth fighting over because a two-legged gait is the primordial feature separating humans from other apes. Charles Darwin recognized its importance, reasoning that when our ancestors started relying on tools and weapons to hunt, a form of walking that freed the hands would have been beneficial. One problem with Darwin’s theory: Upright walking emerged 6 million to 7 million years ago, but stone tools don’t show up in the archaeological record for millions of years after that.

Since Darwin, scientists have conjured a lengthy list of explanations for how and why upright walking arose in hominids. The savanna theory was popular: As Africa dried out millions of years ago and grasslands replaced forests, two-legged walking may have offered a more efficient way for an ape to travel across an open landscape. That idea hit a snag when researchers started digging up early, upright hominids that lived in woodlands. Proponents then tried to salvage the claim by proposing that upright walking was the most efficient way to travel between clumps of shrinking forests as the landscape changed.

Bipedalism has also been tied to the beginnings of monogamy (male providers needed free hands to bring home the Pliocene bacon), thermoregulation (an upright posture helped dissipate heat in sweltering environments) and warning displays (standing tall scared off predators and competitors). People have even claimed that a bipedal stance allowed our ancestors to wade in deep water during a watery phase of human evolution. This “aquatic ape” theory has been thoroughly rejected by anthropologists, if not by Animal Planet.

Just when you thought scientists couldn’t possibly come up with yet another idea, along come Isabelle Winder of England’s York University and her colleagues. They argue in the June Antiquity that previous hypotheses fall short because they ignore an important factor: topography.

Past bipedalism research considered the benefits of upright walking only on flat ground. But early hominid fossils are found in places where frequent tectonic activity created rocky, jagged environments. Scrambling up steep hills and uneven terrain might have given early hominids evolutionary advantages, such as finding new food supplies and escaping predators that couldn’t climb.

Scrambling would have favored an upright posture, shorter arms relative to an ape’s and a more rigid foot that could act as a lever while scaling rough surfaces, Winder and her colleagues say. These features are also beneficial for bipedalism. As savannas took over, hominids might have spent more time there, perhaps hunting prey. Chasing quick animals would have favored long-distance walking and running, and the features related to scrambling would have been further modified for habitual bipedalism. So, the savanna theory is once again resurrected, albeit in a new form.

It’s not as if scientists have completely ignored the importance of topography in human evolution. In 2011, Ryan Higgins and Christopher Ruff, both of the Johns Hopkins School of Medicine, suggested that living in the mountains explains Neandertals’ short legs.

Compared with Homo sapiens, Neandertals were small statured with relatively short shins. Many anthropologists attribute the size difference to the colder environments that Neandertals lived in. Compact bodies conserve heat better. Higgins and Ruff thought that living in Eurasia’s mountains also shaped Neandertals. On flat land, individuals with long legs walk more efficiently than those with shorter legs because they don’t have to take as many strides to cover the same distance. That advantage disappears in sloped terrain. People with shorter lower legs don’t have to swing their legs as high with each step uphill as lankier people do, Higgins and Ruff reported in the American Journal of Physical Anthropology.

The pair tested the idea by simulating the efficiency of walking in different environments. They also found corroborating evidence in mountain-dwelling gazelles, which tend to have shorter lower legs than their counterparts living on smooth terrain. The relationship holds up for gazelles living in both cold and warm environments.

The Scrambler Man scenario is an elaborate hypothesis that’s difficult to test. One problem is that tectonic activity, volcanoes and other geological forces continue to mold Africa’s landscape, so it’s hard to reconstruct exactly what the topography was like millions of years ago. But throwing out new ideas — even outlandish ones — is the only way to advance a discipline like human evolution, where controlled laboratory experiments are impossible. 

Erin Wayman is the managing editor for print and longform content at Science News. She has a master’s degree in biological anthropology from the University of California, Davis and a master’s degree in science writing from Johns Hopkins University.

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