Early Hominins Were Fully Erect 3.6 Million Years Ago, Footprints Found in Tanzania Show

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A favorite insult to this day is knuckle-dragger – implying primitive, and probably stupid to boot. The fact is that walking on two legs evolved well before our ability to philosophize about it.

“Lucy,” our postulated Australopithecine ancestor who lived nearly 3.2 million years ago, was bipedal with arboreal skills, say experts. In fact, the hominin line leading to human beings may have begun walking on two legs as much as 7 million years ago. The question is when they stopped hunching over and augmenting their bipedalism with knuckle-walking, and became fully upright.

Reconstruction of Lucy: A reconstruction on exhibit in the National Museum of Natural History, Washington, D.C.Credit: Mpinedag

Now, Prof. David Raichlen of the University of Arizona has reported on footprints found in Laetoli, Tanzania, attesting to full-blown bipedalism 3.6 million years ago.

For one thing, the footprints simply look like those of people walking upright – and the team showed so empirically. Raichlen and the team compared the depth and shape of the Laetoli footprints to those left by eight people living today, some walking properly and some walking with bent knees and angled hips. The team measured the impression of the feet, from toe to heel – which is a function of how the center of pressure moves along the foot while taking a step.

The theory that the early walkers retained a bent-legged walking posture for some time is based on the way other primates evolved. But by the time the Laetoli footprints were left, they were evidently walking upright.

“By 3.6 million years ago, our data suggest that if you can account for differences in size, hominins were walking in a way that is very similar to living humans,” said Raichlen, who presented the study at the 2018 Experimental Biology conference in San Diego this week.

“While there may have been some nuanced differences, in general, these hominins probably looked like us when they walked,” he said.

A reconstruction of the bipedal Australopithecus 'Lucy': 40% of her skeleton has been foundCredit: 120

Who the bipedalist was remains to be argued about. Human, it obviously wasn’t: The earliest Homo sapiens remains, recently found in Morocco and still controversial in identification, date to almost 300,000 years. However, Homo genus – the family of apes that led to humans – is generally believed to have emerged about 2 million to 2.5 million years ago in Africa. (“Hominins” includes the broader set of pre-human ancestors.) There is no reason to assume this was some sort of “early human.”

Why would hominins have gotten to their feet in the first place? Because strolling along, swinging one’s legs, expends less precious energy than walking crouched over or on feet and knuckles. The advantages of that are myriad.

Raichlen for one suggested that the development of bipedalism could have extended the foraging range. “This work suggests that, by 3.6 million years ago, climate and habitat changes likely led to the need for ancestral hominins to walk longer distances during their daily foraging bouts,” he said.

Selection may have acted at this time to improve energy economy during locomotion, generating the human-like mechanics we employ today,” he added.

Footprints: Modern man, stooped modern man and Laetoli footprintsCredit: David Raichlen / University of A