The first apes that walked upright may have evolved in Europe

Apes may have walked upright in what is now Bulgaria 7.2 million years ago. That’s according to 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 developed in Europe, rather than Africa.

“The oldest indications for bipedality are found in Europe,” says Madelaine Böhme of 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 meters thick.

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

“It’s a very, very beautiful discovery,” says Clément Zanoli of the University of Bordeaux in France, who was not involved in the study. “This femur is pretty well preserved, and it’s 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, all our closest ape relatives live in Africa. Similarly, all the earliest ancient humans, or hominins, are from Africa.

The only monkey identified from Azmaka is Graecopithecus freybergia poorly understood species known only from a damaged jaw bone from Greece, a tooth from North Macedonia and from Azmaka, a single tooth. Böhme’s team therefore suggests that the femur probably is Graecopithecus.

“It’s the most parsimonious choice we can make for now,” says Zanoli, but he emphasizes that more fossils are needed before we can be sure.

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

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

For example, at the top of the leg, a short neck extends 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 at the bottom of the neck than at the top, which also helps it bear weight.

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

Other scientists are intrigued but not convinced. “This femur shows a number of features,” says Zanoli. “Some are bipedal-like, but others are quadrupedal-like. So it’s quite difficult to know exactly what the locomotor behavior was.”

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 that researchers must find out which features are truly diagnostic, and identify more from each species. “We’re raising the bar on what’s required,” she says, and for now, the femur isn’t enough to convince her.

Much of this re-evaluation has come about because of an ongoing dispute over Sahelanthropus tchadensis. Known from a single site in Chad, this species is the earliest generally agreed to be a hominin rather than an ape. It lived 7 million years ago, slightly more recent than the Azmaka specimen. One Sahelanthropus femur has been discovered and paleoanthropologists have argued for years about 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 was disputed because the bone is so damaged. They have also described another European monkey, Danuvius guggenmosi from 11.6 million years ago, such as being able to stand upright and walk along tree branches.

Then these early hominins could have migrated into Africa—perhaps driven by climatic changes 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 Zanoli. “If the fauna can do it, why not hominins?”

However, Pugh says we need to strengthen the evidence for bipedalism in Europe and find more specimens Graecopithecusso we can figure out how it is related to other apes and hominins. Without that, it is too early to create detailed scenarios, she says.

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