Apes may have been walking upright in what is now Bulgaria 7.2 million years ago. So say researchers who have found a leg bone that shows signs of bipedal walking.

The leg bone is older than any known hominin fossils, including all those from Africa. It suggests that bipedality – a crucial step in human evolution – may have evolved in Europe, rather than in Africa.

“The oldest indications for bipedality are found in Europe,” says Madelaine Böhme at the University of Tübingen in Germany.

Böhme and her colleagues have been excavating at Azmaka, near Chirpan in southern Bulgaria, since 2008. The site has a layer of river-deposited sediments that is about 20 metres thick.

In 2016, the team found a single right thighbone or femur, buried in sands known to be 7.2 million years old. The femur is 21.5 centimetres long and is almost complete, with only part of the lower end missing. The team has nicknamed the individual “Diva”.

“It’s a very, very beautiful discovery,” says Clément Zanolli at the University of Bordeaux in France, who wasn’t involved in the study. “This femur is quite well preserved, and that is really, really rare in the Miocene fossil record for hominids.”

In earlier periods, Europe was home to many different apes, but by 7.2 million years ago, many of them had disappeared. Today, our closest ape relatives all live in Africa. Likewise, all the earliest ancient humans, or hominins, are from Africa.

The only ape identified from Azmaka is Graecopithecus freybergi, a little-understood species known only from a damaged jawbone from Greece, a tooth from North Macedonia and, from Azmaka, a single tooth. Böhme’s team therefore suggests that the femur is probably Graecopithecus.

“It’s the most parsimonious choice we can do for now,” says Zanolli, but he emphasises that more fossils are needed before we can be confident.

“The association between these fossils is loose,” says Kelsey Pugh at OCAD University in Toronto, Canada.

Böhme’s team measured the femur in detail and performed a CT scan. The researchers found several features that, they argue, are evidence that it belonged to a bipedal animal.

For instance, at the top of the bone, a short neck extends out to the side and then expands into a rounded bulb that would have slotted into the pelvis. The neck has a fairly long, straight section – seen in bipedal hominins, but not in knuckle-walking apes – that can support a vertical load. Likewise, the outer layer of bone is thicker on the bottom of the neck than on the top, which also helps it withstand weight.

In addition, there is a ridge on the back of the bone, where the gluteal muscles would have attached. “They are important because they hold the back upright,” says Böhme.

Other researchers are intrigued but unconvinced. “This femur shows a number of features,” says Zanolli. “Some are biped-like, but others are quadruped-like. So, it’s quite difficult to know exactly what was the locomotor behaviour.”

The more we study extinct apes from millions of years ago, the more we discover that bipedality is difficult to diagnose from isolated bones, says Pugh. Many features that were thought to be unique to bipedal hominins have been found in quadrupedal apes. This means researchers have to figure out which features are truly diagnostic, and identify several from each species. “We’re raising the standard of what is required,” she says, and, so far, there isn’t enough from the femur to convince her.

Much of this re-evaluation has come about because of an ongoing row about Sahelanthropus tchadensis. Known from a single location in Chad, this species is the earliest one generally agreed to be a hominin rather than an ape. It lived 7 million years ago, slightly more recently than the Azmaka specimen. One Sahelanthropus femur has been discovered and palaeoanthropologists have been arguing for years over whether it shows evidence of bipedality.

Böhme and her colleagues have spent years trying to establish that key steps in the early evolution of hominins took place in Europe. They have reported evidence of hominin-like features in the Graecopithecus jawbone, which were disputed because the bone is so damaged. They have also described another European ape, Danuvius guggenmosi from 11.6 million years ago, as being able to stand upright and walk along tree branches.

Subsequently, these early hominins could have migrated into Africa – perhaps driven by climatic shifts between 8.75 and 6.25 million years ago, the team has suggested – where they gave rise to all later hominins, including us.

Many other animals moved between Africa and Eurasia, says Zanolli. “If the fauna can do it, why not hominins?”

However, Pugh says we need to firm up the evidence of bipedality in Europe and find more specimens of Graecopithecus, so we can figure out how it is related to other apes and hominins. Without that, it is premature to draw up detailed scenarios, she says.

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