Climb On

Ancient bones tell a swinging story about our tree-climbing ancestors — study

Evolving to walk on two legs doesn't mean we stopped hanging out.

Humans may associate scaling branches with childish behavior — classic monkeying around — but in truth, climbing trees is a big part of our collective story.

Ancient bones unearthed in Africa go a long way in telling the tale.

Millions of years after hominins began walking on two legs, some early human species were still hanging out in trees, according to a new analysis of fossilized femur bones discovered in South Africa’s Sterkfontein Caves.

Scans of two leg bones, both of which seem to both be from bipedal hominins, show that one of the species actually spent much of its time climbing trees, not walking on the ground.

Evidence of early human species walking on two legs dates back 6 million years. At a glance, the bones in the new study — each about 2 million years old — seem to align with a primarily bipedal way of moving.

But using CT scans and comparing the femurs to modern human and great ape bones, researchers have peered below the surface for the first time, revealing that the bones, while similar, were used in very different ways.

The study was published Monday in the journal Proceedings of the National Academy of Sciences.

Old bones, new tales — One of the femurs came from the hominin Australopithecus africanus, the same species as Lucy. This early human had a mix of human- and ape-like traits, but it seems they were quite happy on their feet. The CT scans reveal that the way the hip bone was used to carry weight aligns with two-legged walking, based on the density of the head of the femur.

The other bone tells a different story, however. Researchers are not sure which ancient human species it belonged to, but the likeliest culprits are either an individual in the Homo genus, or the hominin Paranthropus robustus.

Whatever early human relative the bone in question belonged to, they weren't doing too much walking on the ground, it seems. The scans show a different pattern in its bone density, indicating it was more frequently flexing the hip joint, rather than remaining upright.

That regular hip flexing suggests regular tree climbing, the study authors say.

CT scans allow researchers to infer how two human-like femur bones were used by early ancestors. Leoni Georgiou

The new analysis points to the difference between physiology and behavior, and complicates the story of how our ancestors learned to walk on two legs.

Similar pressures, different results — We know that, in the past, many different hominin species lived at the same time, sometimes encountering and possibly even mating with one another.

Living in the same time and place means experiencing similar environmental pressures, feeding adaptations that evolved over time to produce us.

But the new research shows how evolutionary traits are far from clear-cut.

The two bones came from individuals that both lived in Sterkfontein Caves, which are situated within the aptly-named "Cradle of Humankind," a large area that's been the site of numerous ancient human discoveries. One of the most famous, and oldest, individuals found there is Little Foot, a nearly full skeleton from the Australopithecus genus. New research shows that Little Foot had a combination of human- and ape-like features — with a little of each just in its ear.

But though they lived in the same place, the two individuals in the new study existed hundreds of thousands of years apart. The Australopithecus africanus bone is between 2.8 and 2 million years old, while the (possibly) Paranthropus robustus femur is likely younger, about 2.2 million years old.

That timeline does not suggest a linear progression from tree-swinging to ground-walking. Rather, the unidentified hominin proves that having the ability to walk on two legs doesn't necessarily mean a species is spending most of its time on the ground.

Plus, while that mystery hominin seems to have been a habitual climber, the bone density scans suggest it did not climb with the same frequency as nonhuman apes.

The discovery challenges the “prevailing view of a single transition to bipedalism."

Even after humans began walking on two legs, they were still climbing trees regularly.

In South Africa's Sterkfontein Caves, scientists have discovered hominid and animal fossils up to four million years old.Shutterstock

“It has been challenging to resolve debates regarding the degree to which climbing remained an important behavior in our past,” study co-author Matthew Skinner, a paleoanthropologist at the University of Kent, said in a statement accompanying the research.

“Evidence has been sparse, controversial and not widely accepted, and as we have shown in this study, the external shape of bones can be misleading.”

What's next — Skinner says that further analysis of these bones could upend more common beliefs of how different parts of human bodies worked — including “hands, feet, knees, shoulders and the spine.”

The analyses could paint a brighter picture of how humans evolved, including important developments like being able to make and use tools from stone.

Abstract: Bipedalism is a defining trait of the hominin lineage, associated with a transition from a more arboreal to a more terrestrial environment. While there is debate about when modern human-like bipedalism first appeared in hominins, all known South African hominins show morphological adaptations to bipedalism, suggesting that this was their predominant mode of locomotion. Here we present evidence that hominins preserved in the Sterkfontein Caves practiced two different locomotor repertoires. The trabecular structure of a proximal femur (StW 522) attributed to Australopithecus africanus exhibits a modern human-like bipedal locomotor pattern, while that of a geologically younger specimen (StW 311) attributed to either Homo sp. or Paranthropus robustus exhibits a pattern more similar to nonhuman apes, potentially suggesting regular bouts of both climbing and terrestrial bipedalism. Our results demonstrate distinct morphological differences, linked to behavioral differences between Australopithecus and later hominins in South Africa and contribute to the increasing evidence of locomotor diversity within the hominin clade.
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