Researchers believe they have solved the riddle of an enigmatic set of footprints that were left more than 3.6 million years ago in volcanic ash at Laetoli, Tanzania, and have confounded experts for decades.
The strange tracks were apparently made by a previously unknown hominin, who cross-stepped its way like a ballerina through the African savannah, concludes a study published Wednesday in Nature.
Laetoli is a prehistoric treasure trove that has provided us with seminal finds ever since it was discovered by renowned paleontologist Mary Leakey in 1976. Particularly known are the sets of cemented footprints that were left there by early humans, which are believed to be the earliest clear evidence of one of our ancestors walking upright. It is generally accepted that these tracks were left by Australopithecus afarensis, the same species of the famed Lucy, the 3.2-million-year-old skeleton discovered in Ethiopia just a few years before Laetoli was excavated.
The footprints at Laetoli have survived because they were left in wet volcanic ash, which then cemented and was quickly covered by a fresh layer of ash spewed by a nearby volcano. But not only hominins left their mark in the 3.66-million-year-old ash bed at Laetoli, and there are tracks (and fossils) left by thousands of different animals.
One of the first set of tracks discovered, labeled Laetoli A, included five prints left by an animal that had been clearly walking bipedally and had a fairly wide foot compared to humans. The other unusual feature was that the prints looked inverted, with the mark of the largest toe appearing on the outward side of the foot, rather than inward. Think about it: Normally, when you leave tracks in the sand the marks of your big toes always face your body’s midline – only if you cross your legs as you walk they will face the outside.
Bear foot or bare foot?
Leakey initially interpreted the footprints as having been made by a hominid, whose “gait was somewhat shambling, with one foot crossing in front of the other.” But later researchers dismissed this conclusion and interpreted the footprints as belonging to a young bear who had taken a few steps on its hind legs.
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In bears, the fifth toe of the foot is the largest, whereas in humans it’s the smallest. The bear interpretation neatly explained the appearance of the tracks, with the impression of the fifth toe being the largest, without having to believe that an early hominin had been doing a prehistoric version of Monty Python’s “Ministry of Silly Walks.”
Thus the Laetoli A set was attributed to a bear cub and was soon covered up by sediments and forgotten when the other, more clearly human-made tracks were discovered.
For the new study, a team of researchers re-excavated the Laetoli A set and reanalyzed the mysterious five prints by comparing them to those left by modern bears, humans and chimpanzees – as well as the other hominin tracks from the site. Given that over the last decades we have greatly improved our knowledge of how human locomotion developed, they thought these footprints deserved a second look.
Led by Ellison McNutt, a paleoanthropologist from Ohio University, the team concluded that the bear explanation didn’t hold water. First of all, bears don’t appear among the 25,000 fossils attributed to 85 mammalian species found at Laetoli. Secondly, after recording dozens of hours of footage of modern American black bear behavior, they found that this plantigrade only spends 0.09 percent of its time on two feet – and less than half of that is done in motion. Finally, there are no signs of bear claws and the impressions left by heels and toes simply don’t match with those of ursids, the study reports.
Humans and our close relatives, like chimps, have a very big first toe compared to the surrounding digits, while bears have only a slightly larger fifth toe compared to the others, McNutt explains.
“That’s why we looked at the proportions of the very large big toe impressions and by far they match those of a hominid foot,” she says.
So, apparently, track A did indeed belong to a bipedal ape and the only way to explain the unusual pattern in the footprints is to accept that this individual was cross-stepping in the way that we do when trying to keep our balance on a treacherous surface – or if we are models stomping down a catwalk.
That is further evidence that we are dealing not just with an ape, but with a hominin, McNutt notes. When bears and chimps do walk on two legs they tend to waddle around while keeping a wide stance because of the way their hips and knees are positioned. Only humans have the right anatomical features to keep a tight gait or even cross one foot over the other without falling over, she says.
Just like humans, these hominins probably didn’t walk like this all the time: perhaps they were just gingerly stepping over the unfamiliar ash deposited by the volcano, or maybe they were simply playing around.
Footprints of a ghost
The question then is whether these footprints belong to Lucy’s species, A. afarensis, as is most probably the case for the other human tracks at Laetoli. The answer is no, because the features and proportions of the various parts of the foot, as can be evinced from the prints, are just too different. For example, the Laetoli A footprints have a slightly divergent big toe: less so than what is found in chimps, but more than in modern humans or A. afarensis.
“This foot is as different from humans and the other Laetoli tracks as a chimp’s is,” McNutt tells Haaretz. “But it is as also equally different from a chimp’s.”
In other words, McNutt and colleagues believe they have stumbled upon an entire species of early humans whose skeletal remains have yet to be identified, but who lived in roughly the same time and area as Lucy’s people.
Evidence pointing at unknown hominins has been cropping up more and more in the study of human evolution in recent years. These unknown extinct relatives of humanity are often referred to as “ghost hominins” although they are usually identified not by footprints but by the traces of DNA they left in our genome, by interbreeding with our distant ancestors.
Advances in the last ten years have enabled DNA to be extracted from (some) fossils; traces of interbreeding with super-archaic ghost hominins have been detected in Neanderthals and Denisovans as well. (Long story short – all three species, Homo sapiens, Neanderthals and Denisovans bred with each other and all three also mixed with ghost hominins.)
Now the discovery of the existence of yet another unknown hominin type from the footprints adds to a growing body of evidence showing that our evolution was not like the linear progression tree we see in school textbooks, but more like a messy bowl of spaghetti.
Throughout our development, different types of hominins coexisted in the same place and time, often interbreeding with each other, and there are probably still many more hominin species that have yet to be found in the fossil record.
McNutt says her team plans to continue excavating at Laetoli in the hope of finding more tracks, or fossils remains that can be matched to the footprints.
“The magic of every time we find a new hominin, whether it’s through footprints or skeletal remains, is that it fills a gap in our story,” she says. “But it also makes the picture of human evolution more complicated.”