Slab of sandy rock containing the ancient footprints, next to the ocean, with a mountain in the background

Tracing human origins by foot

A new study has pushed back the estimated age of 50 footprints preserved in rocks on the Mediterranean island of Crete – footprints that were already thought to be among the oldest pre-human prints in the world, and have now been dated to around 6.05 million years ago.

The international team of researchers, whose study was published this week in Scientific Reports, used refined dating techniques to more accurately place the precious imprints in history, but the claims are deeply controversial, challenging prevailing wisdom about human evolution.

One of over 50 footprints of predecessors of early humans identified in 2017 near Trachilos, Crete. Dating techniques have now shown them to be more than six million years old. Credit: University of Tübingen

Back in 2017, when these footprints were first dated to the Miocene, some 5.7 million years ago, the authors of that paper in Proceedings of the Geologists’ Association claimed they placed some of the earliest human wanderings in Eurasia as well as Africa, and challenged the hegemony of the Out of Africa theory. The authors noted what they saw as key similarities between the ancient Cretan footprints and those of humans.

The findings were controversial and rebuttals were swift, with other researchers likening the footprints to those of gorillas rather than something resembling human.

But the Cretan camp has redoubled its efforts with a new study by a mixed team of researchers – some of whom worked on the original 2017 paper – pushing the dating of the footprints back a further 300,000 years, and insisting on their morphological human likeness.

So why is the debate so controversial, and why does it matter anyway?

Ancient footprints in the sand

The footprints were discovered in 2002 near the small Mediterranean town of Trachilos by a researcher called Gerard Gierliński from the Polish Geological Institute, Warsaw. Unaccompanied by evidence of forelimbs pressed into the earth, Gierliński reckoned the prints belonged to a bipedal ape, and he returned more than a decade later to date them.

Dating the underlying sedimentary rock bed, as well as the remains of foraminifera – a type of algae that lives on the sea floor – the study found that the rocks would have been created around 5.6 million years ago, and estimated the tracks were laid down around 100,000 years before.

That made them some 2.5 million years older than the tracks of Australopithecus afarensis, otherwise known as Lucy, one of humanity’s most famous ancestors.

Crucially, by 3D printing and laser-scanning the prints, Gierliński and his research team identified key features that appeared to liken them to ours, including a ball region, a pulling-up motion of lifting the foot, and evidence of a hallux or big toe.

Whoever these bipedal ancients were, Gierliński and team reasoned, they could have been the ancestors of modern hominins, representing a step in the chain to the modern bipedal ape.

“The oldest human foot used for upright walking had a ball, with a strong parallel big toe, and successively shorter side toes,” says Per Ahlberg, professor at Uppsala University and co-author of both the 2017 and 2021 studies. “The foot had a shorter sole than Australopithecus. An arch was not yet pronounced, and the heel was narrower.” But, according to the authors, they are tantalisingly hominin-like.

In the palaeoanthropological community, this kind of statement is a massive departure from conventional understanding, which places human origins firmly in Africa. The other oldest known bipedal ape was Orrorin tugenensis in Kenya, who lived around the same time as the creature from Crete.

Madelaine Böhme, of the University of Tübingen in Germany and co-author of the new study, says “we cannot rule out” the possibility that the tracks could be linked to Graecopithecus freybergi, a species of previously unknown pre-human that the team from Tübingen uncovered in Athens, and which they say lived in Europe 7.2 million years ago, just 250 kilometres from the Cretan prints.

The researchers believe that evidence for the short-term expansion of the Sahara Desert some 6 million years ago could explain the separate but concurrent evolution of bipedalism in O. tugenensis and the mysterious Cretan hominin.

Böhme calls this theory ‘Desert Swing’, hypothesising that about 6.25 million years ago, aridification in Mesopotamia would have pushed many mammals, including primates, out of Eurasia and into Africa, while an expansion of the Sahara 6 million years ago would have sealed off the two continents, separating our ancient hominins and sending them on separate – but potentially similar – evolutionary paths.

Out of Africa: challenging the narrative

Africa is known as the cradle of humanity, because the earliest known hominin fossils – as well as some of the oldest-known stone tools – are almost invariably found there. These include the 7-million-year-old Sahelanthropus tchadensis, the 6-million-year-old, upright walking O. tugenensis, and Ardipithecus kadabba, which lived in Ethiopia some 5.8 to 5.2 million years ago.

But the oldest known pre-human footprints, excluding the contentious Cretan prints, were laid down by A. afarensis in Tanzania, and are dated to the Pliocene, around 3.6 million years ago. The authors of the 2017 study of the Trachilos footprints acknowledged that the Tanzanian prints were more human-like than their European set, but they saw this as evidence that our earliest ancestors ranged far and wide, across the land bridge to Europe and back at various times in the past 7 million years.

Accepting a pre-human footprint in Europe would thus throw the prevailing wisdom about hominin evolution out the window. It’s possible, however, that the Trachilos footprints represent an example of convergent evolution – the independent evolution of the same mechanical feature in separate species. The authors of the initial paper on the Trachilos prints disputed this, writing in a 2017 piece in The Conversation that convergence “rarely produces perfect duplicates”.

The authors, aware that their research was controversial, reported massive difficulties getting it published, with one online commentator referring to the controversy as evidence of “cancel culture in science”.

Footprints leave a fickle trace

According to Julien Louys, a palaeontologist and palaeo-ecologist at Griffith University, whose work has focused on hominin evolution over the last 2 million years, footprints are notoriously difficult to draw inferences from without accompanying bodily material such as fossils and teeth.

“The difficulty with working with footprints is it’s very difficult to tie them to a particular species,” says Louys. “With a lot of footprints, there’s a huge amount of variability that’s introduced by the substrate that the animals are walking in, then there’s all sorts of post-depositional defamation that can occur as well.

“And so, with a lot of footprints, it’s much more difficult to tell what species they belong to – to the point where footprint researchers have their own taxonomic system in place to deal with the naming and the classification of footprints.”

Having looked at the images in the publication, Louys says it’s hard to pin down exactly what made them.

“Some of the footprints look like a bipedal animal, but a lot of the other footprints are very ambiguous and variable in size. Some of them don’t look like footprints at all. So, the issue here is making a very large claim on the basis of information that’s quite open to interpretation.”

Louys believes the footprints could belong to a bipedal hominin, but they could also belong to a hominid, another apelike creature that’s not related to humans.

Despite this, Louys says it’s not necessarily controversial to suggest that migration has been a two-way street, with movements pulsing outwards and inwards over time.

“The interesting thing claimed in the new paper is that it demonstrates migration from Europe to Africa of these bipedal hominins,” he says. “All our studies of intercontinental migrations indicate that it’s not just a one-way street.

“So even if we take at face value that these are hominin footprints, there’s no indication that they have to have originated in Europe and then moved to Africa, there’s equal possibility that they could have originated in Africa and moved to Europe.”

Louys says these kinds of ranging and returning migrations are evidenced across the mammalian world.

“A lot of large-bodied mammals are making these movements,” he says. “On a geological timescale, there’s this constant interchange between Africa, Europe and Asia, so I don’t think we can assume that primates were any different from those broader movements of larger mammals.”

Why do we need to know where we come from?

We may never know what – or who – laid those prints down in soil on their journey across Crete more than 6 million years ago. But these imprints – a rare snapshot of a life that occurred so deep in the past we can’t possibly imagine it – will be causing a scientific ruckus for some time. Louys, for his part, wonders why the question of where exactly the first human walked upright is quite so important.

“Playing devil’s advocate, does it really matter where these creatures came from?” he asks.

“These ideas that we’ve found the earliest such and such in this place, I don’t think it’s scientifically interesting as much as the actual things themselves: their relationships, how they changed over time, what were the environmental factors that produced them, the abiotic factors, all those sorts of things.

“I think that those are much more interesting questions than just a simple case of, ‘the earliest human was from this place’. Those sorts of statements don’t have a huge amount of scientific interest, at least not to me.”