‘Lucy’ Walked Upright but Baby Australopithecines Lurked in Trees

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Another reconstruction of Lucy the Australophitecus, Natural History Museum, Washington DC, USA
Another reconstruction of Lucy the Australophitecus, Natural History Museum, Washington DCCredit: Mpinedag
Ruth Schuster
Ruth Schuster
Ruth Schuster
Ruth Schuster

A rare fossil foot of a toddler Australopithecus indicates that while the adults walked upright on two legs and apparently lived on the ground, their kids may have clung to the trees for protection, says a paper published Wednesday in Science Advances, by Jeremy DeSilva of Dartmouth College and colleagues.

This was deduced from primitive, chimp-like characteristics of the baby’s foot, which would have been lost by adulthood if the child had survived. Which it didn’t.

Reconstruction of Lucy, the Australopithecus afarensis, in Museon Den HaagCredit: Ellywa

Fossils of hominins are extraordinarily rare. Only 10 fossils of australopithecine afarensis have been discovered so far, none complete, says Prof. Yoel Rak of Tel Aviv University, an expert on the subject. The first was “Lucy,” the partial skeleton of a female found in Ethiopia, which was dated to 3.2 million years ago.

The remains are of a toddler around 2 to 3 years old, still with a full mouth of milk teeth. Found in Dikika, Ethiopia in 2002, the skeleton of young Selam – as the juvenile has been dubbed – has been dated to 3.3 million years.

Selam has been categorically identified as belonging to the same species as Lucy, despite being around 100,000 years older.

Fossil of Selam's foot - a toddler Australopithecus afarensis from 3.3 million years ago - superimposed on the footprint of a modern human toddlerCredit: Prof. Jeremy DaSilva

Sure, the australopithecines could have developed a lot during that 100,000 year difference, but there is no question that Selam is the same species because the archaeologists found critical mass of his remains, explains Rak, who has studied Lucy extensively. Extraordinarily, they didn’t find isolated bones scattered about; they found almost the whole skeleton in a single body of rock – skull, vertebrae, feet and all.

Lucy’s feet were not found, Rak clarifies. But the feet of other adult australopithecines were discovered, enabling the comparison with the juvenile.

Adult Australopithecus afarensis feet were very human-like. The baby’s feet were a little more ape-like. It could grasp branches with its feet.

Baby had chimp-ish feet

Archaeologists today concur that Lucy and her brethren were bipedal like us, a conclusion based on multiple avenues of evidence.

Skeleton of "Lucy" the Australopithecus afarensisCredit: 120

One is fossil footprints preserved in volcanic ash found in Laetoli, Tanzania, and dated to about 3.6 million years ago.

The 27-meter (88-foot) trail of prints is believed to have been made by early Australopithecus afarensis on the grounds that its remains were found in the same sediment layer, and no other hominins are known to have existed at that time. These proto-people were definitely walking upright.

3.32 million-year-old skeleton of an Australopithecus afarensis child, complete with one foot - note the heavy heel. Credit: Zeray Alemseged

A second avenue of evidence is the fossil feet of adult australopithecines. The adults’ feet were like ours and unlike those of our cousin the chimpanzee in two chief ways, explains Rak (who was not involved in this research).

One very human characteristic that Lucy shared was a massive heel. Knuckle-walking apes don’t have that.

Humans need robust heels because our step begins with our heel striking the ground, Rak explains: that load needs to be dissipated. Apes have relatively gracile heel bones.

Having struck the ground with our heel, we then propel ourselves forward using our big toe, which is in line with the sole and also grouped with our other toes.

“Our big toe does not deviate medially from the sole,” Rak says. “This is crucial because that’s what enables us to propel the body and progress forward with each step.”

This is the second very human characteristic that Lucy shares. That first toe of chimps and other apes does diverge medially. Their toe is sort of like our thumb. That gives chimp feet more manipulative ability than our feet have. It means chimps can grasp branches with their feet.

In other words, archaeologists are quite sure that Lucy and the gang had modern feet and were chiefly ground-dwelling, bipedal walkers. The thing is that baby Selam had articulated feet.

Born in a primitive state

We don’t actually have Selam's big toe. Such are the vagaries of fossilization. But we do have his medial cuneiform bone on which his big toe sat, and it is slightly more round at its front than that of his parent.

To people who understand foot bones, the rounded medial cuneiform indicates that his (or her) big toe had manipulative ability. More than we have and more than his mother had.

In other words, the baby Australopithecus was born in a primitive state and as he would have progressed to maturity, his feet would have lost that primitive state and become more human-like.

Rak suggests that isn't surprising. It's a rule in biology, he points out: the baby has a more primitive form than the adult. "The younger our human babies are, the harder it is to differentiate us from chimps, anatomically speaking,” he says. “It is very hard to distinguish between the skull of a very small child and the skull of a very small baby chimp. It becomes easier as they grow up.”

Mother Lucy

Here’s another fact: There is a huge argument over whether or not Lucy and the adult Australopithecines spent all their time in the ground or at least some of it in trees. For what it's worth, this was a wee species: Lucy stood at all of about 3 feet six inches. They wouldn't have been much of a burden on branches.

“Regarding Lucy and the adults, they felt comfortable climbing. She may have a modern foot, but her leg and arm proportions – and the length of her fingers – indicate that she would have been comfortable in the trees,” says Rak.

Archaeologists in one school of thought think Lucy’s primitive proportions indicate she did climb trees: there's even a theory that she died after falling from a tree. Archaeologists in the other camp think it means she retained some primitive features.

Rak thinks the australopithecines were very efficient bipeds, and they may have availed themselves of trees too, whether to escape predators or to nest at night. “But they would walk upright – this is clear. They were not bent over or intermediate in any way,” he sums up.

So, was Lucy our mother? “This is debatable, but she’s a very close candidate to our ancestor,” says Rak. “We are not aware of other hominin species around Lucy’s time.” Shortly after her, there was quite the radiation of hominins living in roughly the same time in the same places.

And why would Lucy’s baby lurk in the trees while she sat on the ground? Because he could. It’s fun. And there are more predators on the ground.