Some 3.6 million years ago, in Laetoli, Northern Tanzania, a volcano erupted, spewing ash and soot that came to settle in the plains below and, for a period of perhaps a few days, thickened into a layer of mud. This pyroclastic goo would prove a boon for archaeologists millions of years later, thanks to the thousands of animal tracks found pressed into it, each of which offers a tantalising snapshot of a day lived in the deep past.
Most famously, the Laetoli trackways would produce the earliest evidence of bipedalism in hominins, thanks to a set of footprints at Laetoli site G, excavated in the late 1970s. These prints belonged to an individual Australopithecus afarensis, the hominin ancestor most famously known from the fossilised specimen called Lucy.
Now, a re-analysis of footprints at Laetoli site A, long thought to have been laid down by an upright bear, has revealed they almost definitely belonged to another bipedal hominin ancestor – albeit one with a strange and shuffling gait. The new study, out today in the journal Nature, cements the reality that Africa 3.5 million years ago was populated by a far more diverse range of human ancestors than once thought.
“It’s kind of odd,” says Ellison McNutt, corresponding author of the study and a PhD researcher in bipedal evolution. “It didn’t quite match up with the fossils, which were admittedly few at the time; it’s an unusual set of prints that had some interesting affinities with bears; and it came to ‘well, maybe it’s a bear?’ And then it got ignored for years because there were other prints.”
McNutt says since the original discovery of the prints in the 1970s, conventional wisdom now accepts that multiple hominins were moving across the landscape in the early days of our evolution. That’s what compelled her and the research team to reinvestigate these strange, long-forgotten prints.
Fortunately for McNutt, she was living at the time just down the road from the Kilham Bear Centre in New Hampshire, US, which rehabilitates black bear cubs whose mothers have been killed. “We had this opportunity where there were young juvenile bears about the same size as the footprints, so we could actually test some of these hypotheses.”
What they learned about the mechanics of bear movement convinced them that the prints could not have ursid origins, so they went back to the source, to re-excavate at Laetoli.
“It turns out that the original excavation didn’t completely reveal these prints, so our return allowed us to finish and get a really good sense of the anatomy,” she says. Crucially, they noted key features of the creature’s anatomy, like the impression of the second toe, which disproved the bear theory once and for all.
But the prints were still highly unusual, unlike most hominin tracks ever found.
“These footprints display an example of what’s called cross-stepping – it’s sort of a catwalk where one foot goes all the way in front of the other,” says McNutt.
“And that’s a really interesting thing, because the ability to do that and remain standing requires some adaptations to your hip and your knee that humans have, but something like a bear or a chimpanzee can’t do that.”
But why would a creature develop what might seem a deeply impractical gait? McNutt says there’s a few potential explanations, one being that this was a one-off movement to compensate for uneven ground.
“Other than a catwalk, we humans don’t usually do cross-stepping much in our day-to-day life,” she says. “But the moment it does happen is if we’re walking on an uneven surface, you can use it as a sort of compensatory mechanism to keep yourself from falling over.”
“Another potential explanation is that maybe the individual who made these prints is walking with an odd gait, or, and this is maybe a little more far-fetched until we have more data, maybe it’s something that the species did.”
But McNutt says that won’t become clear unless researchers can find a fossil that matches this unusual print, or another footprint with the same loping gait.
What is clear from the prints at Laetoli is that ancient hominins were diverse, and must have not only coexisted but interacted with one another.
“The nature of the fossil record is that the dating of anything has an age range, and so even when ranges overlap, we don’t actually know for sure that these two species existed in that place at the same exact time,” she says.
“What’s cool about Laetoli is that footprint sites like this are made up of really short timescales. So, the volcano exploded, laid down a layer of ash which turned to mud, and all these animals walked across it. We’re talking timescales more like hours to days.
“It’s not unreasonable at all to imagine that the maker of the Laetoli A prints could have looked across the landscape and seen an afarensis walking by. So, this becomes one of our oldest unequivocal areas that has evidence of two hominin species actually coexisting in the same place at exactly the same time.”