Andrzej Boczarowski
A set of ancient footprints has been found on a Greek island. They are extremely old – 5.7 million years – yet they seem to have been made by one of our hominin ancestors.
At that time, hominins are thought to have been confined to Africa. The discovery supports the controversial suggestion that they may also have been living in eastern Europe.
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The earliest stages of hominin evolution are still mysterious. Our lineage split from chimpanzees, our closest living relatives, between 7 and 13 million years ago. The oldest undoubted hominin fossils were found in east Africa and date back about 4 million years – but there are a few older, possible hominin fossils from 6 to 7 million years ago. These include Orrorin from Kenya, and Sahelanthropus from Chad – locations that are roughly 2500 kilometres apart.
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The ancient footprints discovered by Gerard Gierliński of the Polish Research Institute in Warsaw are a further 2500 kilometres away from Chad – this time to the north-east, on the tiny island of Trachilos near Crete. Gierliński teamed up with colleagues, including Per Ahlberg at Uppsala University, Sweden, to analyse the tracks.
The team found they could recognise two distinct sets of footprints, both apparently left by an animal that walked upright on two legs.
Out for a walk
The shape of the prints suggests similarities with hominin feet. Most obviously, they were clearly left by an animal that walked on the soles of its feet, as hominins do, rather than just on its toes. The prints show the track-maker had five toes, with the big toe particularly well developed – another hominin feature. And there is no evidence of claw marks, consistent with the fact that hominins have toenails rather than claws.
But surprisingly, fossil and geological evidence indicates that the footprints are 5.7 million years old. That means they predate the period during which hominins are conventionally believed to have left Africa by about 4 million years.
“They are almost without doubt actual footprints of a bipedally-walking animal,” says Robin Crompton at the University of Liverpool, UK, who was not involved in the study but who has analysed other hominin footprints.
However, exactly what sort of animal is still unclear.
Mystery feet
The oldest hominin foot fossils we have are from the 4.4-million-year-old Ardipithecus ramidus. But Ardipithecus had an opposable big toe that would have left a distinctive pattern in its footprints. The Trachilos track-maker appears to have held all five toes parallel.
“There’s something slightly funny about the big toe,” says Ahlberg. “Its position and shape are very similar to those of a modern human, but it seems to be more mobile,” a bit like the opposable thumb of a chimpanzee.
David Begun at the University of Toronto, Canada, wonders about this point too. “It’s theoretically possible that Ardipithecus could use its muscles to squeeze its big toe in line with the rest of its foot and provide a more modern-looking footprint,” he says.
An alternative is that the prints belong to an ape, unrelated to hominins, that also evolved to walk upright at least some of the time. Both Crompton and Begun point out that modern gorillas produce footprints that are far more similar to human than chimpanzee prints, so perhaps another distantly-related ape evolved similar feet 5.7 million years ago.
Apes from Europe
The announcement of the footprint discovery comes just months after Begun and his colleagues presented evidence that Graecopithecus, a poorly known extinct ape found in 7.2-million-year-old rocks in eastern Europe, may in fact have been a hominin.
The suggestion proved to be highly controversial. Some prominent researchers savaged the idea that early hominins could have lived outside Africa. “There are people who simply dismiss the idea because they think it’s just not possible,” says Begun. “But it’s widely accepted that what became the African savannah fauna – giraffes, antelopes, rhinos – those species lived in the southern Balkans and migrated from there into Africa.”
In principle, Begun says hominins might also have originated in eastern Europe and migrated south with these other species. “It doesn’t mean it’s true – but it’s certainly plausible,” he says.
But even discussing this idea in formal academic papers is difficult, he says. It took months for Begun to find a journal prepared to publish the Graecopithecus research – even though the papers were written by well-regarded researchers. Ahlberg says the paper on the Trachilos footprints proved similarly difficult to publish.
Journal reference: Proceedings of the Geologists’ Association, DOI: 10.1016/j.pgeola.2017.07.006
Read more: Key moments in human evolution happened far from our Africa home
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