‘Rosetta Stone’ Fossil Shows Australopithecines Still Had Apelike Shoulders

The evidence is clear that this ancient human ancestor could and did walk 3.5 million years ago, but its shoulder and arm structures were still suited to the arboreal lifestyle

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A reconstruction of Lucy.
A reconstruction of Australopithecus: Lucy.Credit: ©Neanderthal Museum

In a cave in South Africa, an almost complete skeleton of an australopithecine was found in a process that would take years, starting in 1994. Hominin fossils are rare enough; complete ones are even rarer, and extracting them from the surrounding rock is extremely painstaking and time-consuming. First, the archaeologists found its foot, based on which it was determined that “Little Foot,” as it was dubbed, could walk upright.

Most of the rest of its skeleton was found in time. Now, a high-tech analysis of its upper body has determined that whether or not it strolled around on its feet, Little Foot’s shoulder structure was apelike. It still had the ability to swing though the treetops.

The report by Kristian Carlson and colleagues, from the University of Wisconsin, the University of Liverpool and the University of the Witwatersrand in South Africa, was published this week in the Journal of Human Evolution. 

We cannot say for sure that australopithecines were directly ancestral to us, let alone which species of Australopithecus is in our distant line, if any. But, they’re definitely in the Homo family tree, midway between the chimp and us – and now we know they were morphologically sort of half-way as well.

The Homo line apparently split from chimps about 7 million years ago. The australopithecine found in the Sterkfontein Cave lived about 3.7 million years ago, the researchers say, and was apparently an aged female. She was about four feet tall, which is bigger than earlier australopithecines that have been found.

The scan of her pectoral girdle, which includes collarbones, shoulder blades and joints, was done at the Keck School of Medicine of USC. The bottom line is that while her feet and legs indicate the ability to walk, her pectoral girdle is more similar to apes, mainly the gorilla, than to ours. This reflects adaptations to arboreal behaviors, especially those with the hand positioned above the head, rather than the human-like manipulatory capabilities that would evolve later, the team explains.

Her shoulder structure supported arms suited for hanging from branches, swinging from them, or shimmying up and down trees. Later hominins and we folk have a pectoral girdle more suited to just dangling by our sides, or throwing rocks. The fossil provides the best evidence yet of how human ancestors used their arms halfway down our evolutionary path, the researchers say.

“Little Foot is the Rosetta stone for early human ancestors,” stated Prof. Kristian Carlson, lead author of the study and associate professor of clinical integrative anatomical sciences at the Keck School of Medicine. “When we compare the shoulder assembly with living humans and apes, it shows that Little Foot’s shoulder was probably a good model of the shoulder of the common ancestor of humans and other African apes like chimpanzees and gorillas.”

Sequence of images showing Little Foot's shoulder reconstruction.Credit: Kristian Carlson

The upright australopithecine 

Swinging from the trees is fun, but swing a baby by its arms too much and you’ll dislocate its shoulder. Our evolutionary course placed our feet firmly on the ground, but we didn’t start out that way. In fact, Purgatorius, the most distant ancestor of primates, was a sort of arboreal rat that coexisted with the dinosaurs. The earliest origins of the proper simian line are lost in time, tens of millions of years ago; they may have arisen in what is today Eurasia and spread to Africa, and much later to South America. However, the one thing we can say is about them is that they lived in the trees. And Little Foot, her remarkably complete skeleton indicates, could still do that, too, as well as walk upright.

To be clear, australopithecines weren’t one “thing” – there were several types, whether defined as different species, subspecies or just different types. Collectively the australopithecines spanned more than three million years: fossils of them date from over four million years to 1 million years ago, found in Chad, in eastern Africa and in South Africa.

Little Foot is therefore one of the earlier ones and was still in the trees, it seems, though she could walk. This is not a surprise, more a confirmation: “Lucy,” a different australopithecine from the Afar region in Ethiopia, who lived 3.2 million years ago, is also believed to have been both able to walk upright and have arboreal capabilities.

On the other hand, the discovery of a foot of a toddling australopithecine – the same type as Lucy – about 2 to 3 years old who lived 3.3 million years ago had chimp-like characteristics, separate research reported. That team posited that Lucy and her kind lived mostly on the ground but the kiddies stayed in the trees for protection. The postulation is that as the individual australopithecine grew up, its feet would lose the chimp-like structures and gain more human-like shape. The toddler, dubbed Selam, was also found practically whole.

Not everybody agrees that Lucy and Selam were the same species; many an argument is over what species this or that skull or bone belonged to. Since there is such a wealth of data in Little Foot, she is being studied “piecemeal,” with different groups studying different parts – like in the fable of the blind men and the elephant, “each examining one part in coordination with others to explain the whole of something that’s not fully understood,” the researchers write. The USC-led research team chose to study Little Foot’s shoulder assemblies because hers are the oldest and most intact example of this anatomy ever found.

It bears adding that the oldest stone tools, which were very crude, are about 3.4 million years old, but the point is, that’s about the same time frame as Lucy (and Selam); Little Foot lived hundreds of thousands of years earlier. As the team says, in our evolutionary course, our shoulders changed from swinging from trees to being able to use our arms for functions other than supporting our weight as we swung, such as butchering animals. Large animal bones with cut marks from those oldest stone tools, which were found in Ethiopia.

So, australopithecines were evidently hunting, making tools and butchering. Yet Little Foot’s collarbone has a distinctive S-shaped curve commonly found in apes but not in us. Australopithecines were truly sort of half-way, it seems: arboreal in capability, but clearly also got about on foot. Multiple lines of evidence include a 27-meter-long trail of footprints preserved in volcanic ash in Laetoli, Tanzania from 3.6 million years ago (shortly after Little Foot but before Lucy and Selam). Why do we think the footprints were those of an australopithecine? They might have been made by some other bipedal hominin, but the only one we know of from that time for sure is australopithecine.

Little Foot was discovered in the 1990s in a cave in South Africa and is the most intact ancient skeleton of any human ancestor.Credit: Paul John Myburgh

Also, adult australopithecine feet, the few we have of them, are more like us bipedal humans than our cousins the chimps. Lucy’s skeleton wasn’t complete, but a fair amount was found showing she had a massive heel, hominin expert Yoel Rak has explained to Haaretz; knuckle-walking great apes don’t have heels like that because their feet don’t strike the ground like ours do. Yes, Selam the baby had articulated feet like a chimp, but either he was something else altogether or he would have grown out of it. It is possible that australopithecines were born in a primitive state and descended from the canopy during maturation. The bottom line, says Carlson, is that the apelike shoulder persisted longer than had been thought down the Homo line.

“We see incontrovertible evidence in Little Foot that the arm of our ancestors at 3.67 million years ago was still being used to bear substantial weight during arboreal movements in trees for climbing or hanging beneath branches,” he said. “In fact, based on comparisons with living humans and apes, we propose that the shoulder morphology and function of Little Foot is a good model for that of the common ancestor of humans and chimpanzees 7 million to 8 million years ago.”

Finally, we note that in contrast to the actors playing Tarzan, these archaic humans were small. Little Foot towered all of four feet above the ground, while Lucy was about three-and-a-half feet tall. Not that much weight to hang from a tree – even if sometimes, like us today, they could have a bad fall. There’s even a theory that Lucy died falling off a tree. Anyway, if we ran into one, we probably wouldn’t have recognized the family connection. Apelike shoulders, tiny stature – and it turned out in 2019 that one of the oldest australopithecines, anamensis, living 3.8 million years ago had a ridged skull. We don’t.