4 min readHere’s what you’ll learn when you read this story:
- A fossilized femur attributed to the hominin species Graecopithecus freybergi is offering fresh clues about how human ancestors began their transition to bipedalism.
- The bone’s morphology suggests that the shift from walking on four legs to two began as early as 7.2 million years ago.
- Hominins may have begun to walk on two feet when resources in trees became scarce, and they needed to either hunt or avoid being hunted.
Humans only became human when Homo sapiens evolved—but when did our ancestors begin to show a vague semblance of what we would become?
Evolutionary advances aren’t always obvious, but subtle signs of one major leap emerged recently from the back rooms of the National Museum of Natural History of the Bulgarian Academy of Sciences in Sofia, Bulgaria. There, a fossilized femur was identified as belonging to a 7.2-million-year-old hominin, originally unearthed in 2016 at the archaeological site of Azmaka in southern Bulgaria. The bone was recently given a careful analysis that revealed it most likely belonged to the species Graecopithecus freybergi, which would make it even older than iconic Australopithecus Lucy.
Only two other fossils belonging to Graecopithecus have been found—a mandible discovered in 1944 near Athens, Greece, and a tooth unearthed in Bulgaria in 2012.Those fossils provided enough evidence to conclude that Graecopithecus was a hominin, but not necessarily a pre-human one. Unfortunately for paleontologist Nikolai Spassov and his research team, the paucity of earlier fossils meant it wasn’t possible to analyze the femur from Azmaka by fitting it into a larger skeleton. Crucially, having a complete (or nearly complete) skeleton would have made it possible to see whether the arms were needed for walking, or if the individua would have been able to rely solely on its legs. Spassov, a curator of fossil and recent mammals at the museum, needed alternative ways of determining whether Graecopithecus was capable of walking upright.
“The Azmaka hominin represents a candidate for the ancestral form of positional behavior from which bipedalism documented in later hominins evolved,” he said in a study recently published in the journal Paleobiology and Paleoenvironment. “The Azmaka femur lacks many of the specialized attributes of arboreal quadrupeds. At the same time, the combination of locomotor features of this femur indicates a complex locomotor repertoire.”
Hominins did not just climb down from trees one day and become bipedal. The process of relying less evenly on all four limbs, and more on the lower two, evolved over millions of years. Slightly younger specimens of the hominin genera Sahelanthropus and Orrorin had previously raised suspicions that the transition to bipedalism began between six and seven million years ago, but the more ancient Graecopithecus might push this timeline back even further.
The fossil evidence of Graecopithecus also suggests that early hominins may have developed bipedalism after migrating away from the African savanna, and that similar hominin fossils found in Africa might represent later lineages that were already walking on two feet before they returned. The researchers hypothesize that this return to Africa might have been caused by cooling Pliocene temperatures that drove early bipedal hominins south.
Prehistoric Bulgaria was also a savanna where trees were sparse, implying that Graecopithecus would not have most of its time in trees. That doesn’t necessarily mean these hominins were unable to climb, and there’s not enough evidence to determine how much time they spent on the ground. But it seems clear that many morphological features of the Azmaka femur—thought to have come from a female who weighed about fifty pounds—were associated with walking. These include the thickness of the bone cortex (outer layer) and an elongated neck positioned upward between the femur shaft and head. The length of the femur neck is associated with both hip joint mobility and the process of gluteal muscles moving away from the body, as they do in bipedal walking.
Another distinguishing feature is the collection of bony projections that would have been attached to the gluteal muscles. These have been found to project less dramatically in hominids than in orangutans or monkeys that mostly climb trees, and the Azmaka femur is consistent with this pattern. Its straight shaft also differs from those of monkeys and great apes.
What’s most important, though, is that the femur does not have specialized climbing adaptations, despite having some features that show echoes of the quadrupedalism out of which hominins would eventualy evolve. Spassov was particularly surprised by how much of the bone’s morphology was associated with traits indicative of bipedalism. He thinks hominins may have gravitated towards bipedalism to hunt or escape being hunted themselves.
“The morphology of the Azmaka femur is associated with a unique combination of locomotor capabilities that includes aspects of terrestrial quadrupedalism and bipedalism, but without special adaptations for suspensory arboreal locomotion,” he said. “The Azmaka femur does not show the features of habitual and obligate bipedalism of later hominins, but this is not surprising given its geological age.”
