These Fossilized Leg Bones Forced Experts To Rethink Our Ancestors’ Relationship With The Apes

At first glance, experts from England’s University of Kent think they have a pair of ancient leg bones that are very much human – belonging to an ancestor that walked on two legs. But a closer inspection uncovers the truth; one small feature of this species’ skeleton reveals to them that they were very different from today’s people. And it shows they were a bit more like primates than previously thought in at least one way.

For years, scientists have worked to figure out why humans evolved from primates while so many species of ape have maintained their original form. This is because many of the latter species remain as they were about a million years ago due to the fact that they had no need to evolve. They thrive in their environments and as such don’t need any adjustments to do so.

But modern humans – who share a common ancestor with primates – spent millions of years evolving into what they are today. Each adjustment has made them even fitter for life in their environment. And experts continue to chronicle these minor evolutionary tweaks to map our progress from what we were to who we are now.

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These changes can be extremely small, but each of them is important in understanding how humans have come to be. Clarifying this timeline is what pushed scientists to re-examine a pair of leg bones found more than 60 years prior to their research. These skeletal remains were thought to be anywhere from one million to three million years old.

And, as it turned out, the bones had some new information to divulge. These beings did, indeed, belong to mankind’s relatives, and they confirmed that this early human had walked upright. But another small detail showed they moved in more ways than one – thus painting a new picture of the path from primate to modern-day Homo sapien. And, crucially, this redefined the relationship between these two species.

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Nowadays, it’s easy to see the differences between humans and primates, even if we have evolved from one common ancestor. Before the lines between the species became clear, though, there was always one trait to differentiate the apes from the hominids. This was the fact that the latter walked on two legs and stood upright.

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Not too long ago – at the start of the 20th century, to be exact – experts believed that the human brain was what made them unique from the primates. It made sense at that time, as they only had the remains of Neanderthals and Homo erectus to work with. Just like us, both of these ancestors of modern-day humankind had large brains.

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But the roaring ‘20s brought a big change after the anatomist Raymond Dart made an important discovery in the realm of human evolution. He uncovered what’s now known as the Taung Child – a skull that Dart had located in South Africa. And researchers estimated the remains to be approximately three million years old.

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At first, experts thought that Dart had simply discovered the skull of a three-million-year-old ape, as its brain was much smaller than those of modern-day humans. Upon closer inspection, though, they realized that the Taung Child had a distinctly human feature. This was its foramen magnum, the opening through which the spinal cord stretches from the head downward.

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Specifically, the Taung Child’s foramen magnum had a more forward position than an ape’s. Experts could, therefore, deduce that the creature could hold its head upright, thus showing that it probably walked erect and on two feet. Furthermore, similar fossils found in the 1930s and ‘40s revealed even more apes that traveled this way.

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These latter fossils were much older than known human ancestors including the Homo erectus and Neanderthals. As such, experts realized that humans evolved into upright, two-foot walkers before their brains grew. And the discovery of a near-perfect australopithecine skeleton – who scientists named Lucy – solidified this fact.

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Researchers dated the sediment surrounding Lucy’s remains to determine that she is more than three million years old. And, like the Taung Child and other bones found before her, Lucy could walk on two feet. Because experts found her near-complete skeleton, they could see that her pelvis, thighs and knees all aligned so that she could stand up straight, as well.

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Although experts now believe that early humans walked before they developed brighter minds, what remains to be explained without dispute is why they walked on two feet. In 1871’s The Descent of Man: Hominids, evolutionary expert Charles Darwin hazarded a guess of his own. He theorized that early humans needed to walk upright in order to free-up their hands to hunt.

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However, since Darwin came up with his theory regarding why early humans didn’t walk on all fours, more fossil evidence of our early ancestors has emerged. Now, experts know that bipedal hominids – those who walked on two feet – appeared approximately seven million years ago. But evidence shows that bipedal hominids only started wielding tools about 2.5 million years ago, which means they spent 4.5 million years upright and not using them.

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It could be, however, that bipedalism had ties with humankind’s tendency to mate monogamously. Kent State University anthropologist C. Owen Lovejoy verbalized his theory after the 2009 discovery of Ardi – a 4.4-million-year-old near-complete skeleton from the species Ardipithecus ramidus. Those who found Ardi said that he had once dwelled in African forests, which helped Lovejoy to understand why he had needed to walk on two feet.

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During Ardi’s time on earth, the forests of Africa had experienced climatic shifts that created more distinctive shifts between seasons. This would have made it harder for him and his fellow Ardipithecus ramidus people to find food. So, they may have developed a gender-based system for gathering their sustenance, Lovejoy theorized.

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Lovejoy thought that the Ardipithecus ramidus might have agreed that men were in charge of getting food for the females, who raised their offspring. The women were, in turn, expected to mate exclusively with their providers. And in order to serve successfully in the role, males like Ardi needed their arms and hands to collect food, so they couldn’t move on all fours anymore.

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Of course, Lovejoy’s theory would be hard to test now that humans have evolved so far beyond Ardi’s species. But his hypothesis gained traction after researchers noted similar behavior in chimpanzees. The primates tend to hop onto two legs when they have a particularly rare or valuable type of food in their hands.

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Before Lovejoy’s assertion, Peter Rodman and Henry McHenry of the University of California, Davis, had come up with their own explanation in the 1980s. The pair thought that humans’ evolution onto two feet happened because their environment began to change and they needed to move quickly in order to survive it.

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Specifically, early hominids had to move between forests as the tree-covered areas shrank in size. To make it through the grasslands as fast as possible, they started to move on two feet. And research has backed up this theory by showing that walking upright is more efficient than traveling on all fours.

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Meanwhile, others came up with now-rejected theories for why humans began to walk on two feet. One purported that the species stood up so they could see over tall savannah grasses. This one got debunked when experts realized that ancient humans lived in forests, where they wouldn’t have to contend with extra-tall reeds blocking their view.

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The forest habitat also helped to disprove another theory, which was that humans stood upright to protect themselves from sun exposure. Indeed, less of their skin would be exposed to the sun if they were upright as opposed to walking horizontally on all fours. But ancient humans didn’t live on the savannah, where the sun would have damaged their skin.

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Human ancestors’ ability to walk on two feet clearly got plenty of attention from scientists. But the earliest species of hominids had more than one method for getting around. Their skeletons have certain features which suggest that they moved differently than we do today, as their environments forced them to adapt.

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Although scientists know that the evolution of bipedalism happened before brain-related changes, these ancient species didn’t simply decide to stand up and walk one day. They slowly evolved – and their remains prove precisely how it happened.

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In March 2020 the results of a study painted a clearer picture of how ancient humans moved and revealed how much they once resembled their primate predecessors. Experts honed in on a pair of leg bones that had been discovered more than 60 years prior to the start of their research.

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Researchers analyzed the two leg bones and determined that they were between one and three million years old. They may have been early Homo sapiens, or they could have come from a species called Paranthropus robustus, which once roamed across Ethiopia, Tanzania and other parts of East Africa. The two leg bones in question were found in South Africa, though, and they may have belong to the Paranthropus robustus species.

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Paranthropus robustus aren’t part of the Homo sapiens’ immediate family. As explained by the Australian Museum’s website, “They are our distant ‘cousins’ rather than our direct relatives.” Still, the South African leg bones would help explain how we evolved – whether they belonged to direct or distant ancestors of modern humans.

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Professor Tracy Kivell, Dr. Matthew Skinner and Dr. Leoni Georgiou – all of the University of Kent’s School of Anthropology – worked on the study into the bones. Their team included paleontologists and biomechanical engineers from around the world. And with fresh eyes on the remains, they found something striking.

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At first, the outside of both fossils looked very much like human leg bones – right down to their hip joints. The structure suggested that the Paranthropus robustus or the Homo sapien to whom the bones belonged had walked upright on two legs, which made them more like humans than primates.

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However, the research team didn’t just look at the bones from the outside. They also analyzed the legs’ internal structure in order to further clarify how these human ancestors moved. That’s because these interior systems shift during a person’s life depending on how they actually use their limbs. And it was here that the experts found something that wasn’t human-like at all.

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Whether it was a Paranthropus robustus or an early Homo sapien, the being had “highly flexed hip joints,” according to Olivia Miller from the University of Kent’s News Centre. Ape bones often have this feature built into their internal structures for one reason – the pliability helps them to climb trees. As such, the ancient human’s leg bones revealed that they, too, had spent a considerable amount of time doing just that.

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If the thigh bones belonged to a Paranthropus robustus, then further fossil evidence seems to reiterate that these human ancestors dwelled comfortably in trees and on land. Specifically, the discovery of more complete remains has revealed that the species had arms fit for climbing trees – the perfect accessory to flexed hip joints.

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For the team conducting the research, their findings did more than link ancient humans more closely to their primate relatives. They also proved that there’s always something new to learn when it comes to human evolution. Indeed, as we mentioned earlier, they had made their shocking findings by re-evaluating bones unearthed more than 60 years prior to their study.

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As Olivia Miller wrote for University of Kent, “These results demonstrate that novel information about human evolution can be hidden within fossil bones that can alter our understanding of when, where and how we became the humans we are today.” Meanwhile, project leader Dr. Leoni Georgiou credited the team’s findings to modern technology, which helped them see inside of the ancient leg bones.

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Georgiou said, “It is very exciting to be able to reconstruct the actual behavior of these individuals who lived millions of years ago and every time we CT scan a new fossil it is a chance to learn something new about our evolutionary history.” In this case, her colleague Dr. Matthew Skinner pointed out, research had helped to answer a long-standing debate about how much – or little – ancient humans climbed.

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Skinner explained, “It has been challenging to resolve debates regarding the degree to which climbing remained an important behavior in our past. 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.” But a look inside of the bones changed everything.

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Skinner then pointed out how re-examining existing fossils could change our perception of humankind’s ancestors and the ways the latter groups survived. He said, “Further analysis of the internal structure of other bones of the skeleton may reveal exciting findings about the evolution of other key human behaviors: such as stone tool making and tool use.”

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Skinner explained in the University of Kent article that he and the rest of the team would expand their analysis to include other areas of the body. He said, “Our research team is now expanding our work to look at hands, feet, knees, shoulders and the spine.” Together, these bones and joints show exactly what ancient humans were capable of.

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The experts’ future research will require them to study the spongy part of the bones they have gathered. These softer spots may show signs of how human ancestors used their bodies. For example, along with tool-making and use, they may have adapted to run on two feet. And all of this taken together will help the team map evolution in movement over the millions of years that led to modern humans.

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The team at the University of Kent will clarify the movements of both the oldest and more modern hominins. They may even be able to pinpoint the moment at which our ancestors descended from the trees and stopped climbing altogether. It was our ability to walk on two legs that separated us from our primate relatives, after all.

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