Fossil footprints of different ancient humans found together for the first time

On the shores of an ancient lake more than 1.5 million years ago, two species of early humans might have set eyes on each other.

As giant storks flew overhead and ancient zebras waded through the sand, it’s impossible to know what Homo erectus and Paranthropus boisei were thinking. It might have been the first time they’d seen each other, or perhaps a day-to-day part of their lives.

Either way, this meeting was recorded in the soft sand beneath their feet. Long after both species went extinct, their preserved footprints have been rediscovered in what is now Kenya.

Professor Craig Feibel, co-author of new research describing these fossils, says they provide the first concrete evidence, aside from our own species, that two ancient humans lived in the same area.

“Their presence on the same surface, made closely together in time, places the two species at the lake margin using the same habitat,” Craig says. “It proves beyond question that two different hominins were walking on the same surface, literally within hours of each other.”

“While the idea that they lived contemporaneously may not be a surprise, this is the first time it’s been demonstrated. I think that's really huge.”

The findings of the study are published in the journal Science.

Ancient human co-existence

For the past 80 years, the rise of different dating techniques has allowed scientists to estimate the age of a variety of human fossils. They’ve revealed that many different species of humans were alive at similar times in the distant past.

However, being alive at similar times isn’t the same as living together. As Professor Fred Spoor, one of our Research Leaders in human evolution, explains, it’s hard to prove in even the most well-studied areas of the world.

“Volcanic ash in the earth layers means that sites in Kenya and Tanzania are some of the best dated sites in human evolution,” Fred explains. “The estimated dates normally have a range of around 10,000 years or so, which is very good when dating 1.5-million-year-old fossils.”

“However, it means we couldn’t be certain that different species were living at the same time, even if their bones were dated to the same period. It’s a similar length of time between a human alive today and their ancestor living before the invention of agriculture.”

The only first certain signs that ancient humans were living together came in 2010, when researchers published the complete genome of the Neanderthals. It revealed that a small percentage of our genes originally came from this species as the result of ancient interbreeding.

Another ancient human species, known as the Denisovans, is also known to have bred with both the Neanderthals and our own species.

The footprints found in Kenya’s Turkana basin, however, are the first physical evidence of co-existence of much older human species. They consist of a trackway formed by one species walking along the shore, with isolated footprints from the other species. There are also a variety of tracks from birds and hooved animals.

These impressions were buried quickly after being made, meaning they occurred within a short period of time.

“The footprints were made on a lake margin in silty sand that was probably a centimetre or two underwater,” Fred explains. “The water keeps the footprints from cracking, or being eroded, while they are covered in a new layer of sediment brought by surrounding rivers.”

“This process takes hours or days, meaning we can be sure that these different individuals would have been near each other. Even if they didn’t meet, they passed over the same ground in a short space of time.”

How are ancient human footprints identified?

Identifying which footprints belong to which species isn’t easy. Fortunately, advances in 3D imaging technology are allowing researchers to study the shape of fossil footprints in more detail than ever before, providing more clues as to who made them.

While the isolated footprints were distinctly human-like, and likely belonged to Homo erectus, the ones forming the trackway were different. The foot that made these was flatter than a modern human’s and had a big toe that was somewhat angled away from the rest of its digits.

A comparison with a sample of 340 living people, walking across a variety of different surfaces, found it would be impossible for a direct human relative to have made them. They weren’t similar to chimpanzee tracks either, meaning the answer had to be somewhere in between.

Comparisons with foot bones showed that the mysterious walker was probably Paranthropus boisei, a distinct branch on the family tree that was dedicated to eating plants. This very different diet may have allowed it to co-exist with Homo erectus.

“For both species to co-exist, they must have lived different lifestyles that weren’t in direct competition with each other,” Fred says. “One option is their diet. Paranthropus ate plants, whereas Homo erectus was an omnivore which either actively scavenged or hunted for meat.”

“This large contrast in their diet would have been an effective way of niche partitioning these species, but it’s just one possibility out of many.”

This co-existence appears to be more than a one-off, with a similar pattern of footprints found around 40 kilometres north near the village of Ileret. As these tracks were made within the same 200,000-year period as the Turkana fossils, it suggests Homo erectus and Paranthropus lived together for some time.

It’s likely that further evidence will be discovered in the coming years as researchers re-examine other sites with fossilised footprints.

“Recognising footprints is all about knowing what to look for,” Fred explains. “The teams in Kenya are very well trained to look for skulls and teeth, but spotting footprints is difficult if you’re not specifically looking for them.”

“Now it’s known these tracks are out there, I expect that there will be more discoveries in the coming years. Improvements in how we research footprints will help to improve our knowledge of our ancient relatives.”