Multiple sets of footprints in an ancient basin in Kenya reveal that human evolution had more meandering forks in the road than highways.
An international team of researchers has uncovered dozens of 1.5 million year old footprints at an archaeological site called Koobi Fora.
The footprints are thought to belong to two species of extinct hominins that lived at the same time, the team report today in the journal Science.
Analysis indicated one set of footprints belonged to Homo erectus, an ancestor to modern humans, while the other were made by Paranthropus boisei, a great ape that is considered a side branch to human evolution.
The footprints reveal both species walked upright but had different gaits and stances.
Andy Herries, an archaeologist who was not involved with the research, said although both species were known to have lived around the same time, the paper provided new evidence they may have lived alongside each other.
"It adds another critical piece of the puzzle in understanding how different species may have interacted on the same landscape, two species in this case which had very different diets and ways of living," Professor Herries from La Trobe University and the University of Johannesburg said.
How humans learnt to walk
When researchers first discovered evidence of ancient species of humans and their relatives, it was thought that humans evolved in a linear, gradual way.
In the 1950s, renowned evolutionary biologist Ernst Mayr even argued that no two species of ancient humans overlapped.
But in the 70 years since, there has been plenty of evidence refuting that idea, with human evolution unfolding more like a sprawling family tree with side branches and dead ends littered throughout millions of years of history.
The Koobi Fora, where the new footprints were found, is part of the Turkana Basin in northern Kenya.
It's a rich archaeological site, replete with evidence of ancient human occupation like stone tools, hominin bones and animal remains that show signs of butchering.
But understanding how and when ancient humans learned to walk upright, known as bipedalism, has been harder to investigate.
"We know that lots of different hominin species lived in the same regions through time," Professor Herries said.
"The issue is that because of the dating resolution of these fossils, it's often impossible to say [if] they were there at exactly the same time."
That's because although we have skeletons of ancient hominins that show features of upright walking, confirming when different species walked, and whether they overlapped, requires footprints.
"Finding prints from two different species is not unexpected, although it is great to find," Professor Herries said.
"Footprints that form at close to the same time and are rapidly covered over do tell us [the species overlapped]."
A stroll through the mud
The team uncovered a large collection of footprints at the site including hominins, birds and ruminant mammals.
While most of the prints were dotted throughout the patch, multiple striding steps from one ancient hominin were found right through the middle, as well as three individual footprints from three other different, ancient hominin feet.
By reviewing fossil evidence, other similar trackways, and the species that were already known in the area, the team concluded the individual walking through the middle was likely P. boisei, and the three single footprints were from three different H. erectus individuals.
It would make sense for these two species to coexist alongside each other, the researchers said.
According to Professor Herris, P. boisei was "an ultra-dietary specialist that ate tough foods", so the diets of the two species wouldn't overlap enough for there to be significant competition.
But this discovery wasn't the end of the story, he said.
Footprints are hard to definitively match to a species of ancient hominins, particularly if there are only a few examples available.
"We can't definitively say … that the footprints relate to these two species," Professor Herris said.
He said more research was needed to confirm exactly what human relative made their mark.
If they were, however, confirmed to belong to P. boisei and H. erectus, they could provide an interesting look at how different species of ancient hominins interacted long before our species Homo sapiens, according to William Harcourt-Smith, an anthropologist at City University of New York.
"The study … offers a tantalising glimpse into the behavioural ecology of contemporary hominin species," he wrote in an accompanying Perspectives piece.
"It hints at aspects of palaeobiology that are difficult to reconstruct, but crucial to improve our understanding."
