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Why anthropologists were so excited to discover a set of 3.66-million-year-old footprints

The scientists hypothesize one pair of prints belonged to a polygamist.

Raffaello Pellizzon

Around 3.6 million years ago, a pair of Australopithecus afarensis — a species of ancient ancestor made famous by the fossil Lucy — were walking through wet mud in Laetoli, Tanzania. This was likely just an ordinary day, but then something extraordinary happened: A nearby volcano erupted, covering their freshly made footprints in ash, preserving their shape, and locking this moment in time.

We know very little about the Australopithecus, a long-extinct hominid species that may be our ancestral link to the apes. Basic facts, like whether they could climb trees, are subjects of scientific debate. The footprints, recently discovered by archaeologists, not only give us a glimpse into how these creatures walked but also, amazingly, give us some clues about how they behaved.

“When we reached the footprint layer and started to clean it with a soft brush and saw the footprints for the first time, it was really one of the most exciting times of my life,” Marco Cherin, a paleontologist at the University of Perugia in Italy, recently told the Guardian.

Yes, footprints are really that exciting. Here’s why.

Publishing in the journal e-life, Cherin and his colleagues in Italy say the footprints at the Laetoli site represent a significant breakthrough for a few reasons.

One is that the Laetoli site is already home to footprints of three Australopithecus individuals that were discovered in 1976, which are the oldest-known footprints of an upright-walking human ancestor. These two new sets of footprints are thought to have been formed at the same time. And therefore, it’s possible these two new individuals were walking with the other three, separated by about 150 meters.

While skeletal remains give anthropologists clues to an ancestor’s biology, they hold few clues to that ancestor’s actions. Skeletons, Cherin tells me in an email, “often give only indications on the skeletal anatomy.”

Footprints, however simple they may appear, provide rich data. They “give us information not only on the ‘shape’ of their makers, but also on their behavior,” Cherin says. More so than a skeleton, footprints represent a snapshot in time. “They are a real ‘photograph’ on a precise moment of the deep past,” he says.

“Like a spotlight on a prehistoric scene, fossil tracks provide data about the locomotion, biomechanics, and body size of the extinct creatures and reveal the diversity among individuals, explaining even their reproductive strategies,” Giorgio Manzi, another author on the study, tells me in an email.

And already this snapshot is firing up the imaginations of researchers, who have put forth a flurry of new hypotheses about the Australopithecus. Such as:

  • One of the new sets of footprints appears to be made by an individual 5-foot-5 in height, the tallest Australopithecus on record (scientists can measure stride length and foot size to make a guess). This individual — the researchers dubbed it “Chewie,” after the Wookiee — would have towered over Lucy, who was about 3-foot-6. Chewie was also several inches taller than the other individuals at the site. This finding is making scientists wonder if the tall individual was male and if the shorter individuals at the site were female or juveniles.
  • If this is the case, it’s possible that the one male was mating with many females — which would mean the Australopithecus mating and social behavior are more like those of gorillas than humans. Gorillas tend to be led by one large dominant male surrounded by several smaller females. But as study co-author Jacopo Moggi-Cecchi said on a call with reporters, “we don’t know exactly” the sex of each individual. But “you find this body size difference in species like gorilla.” And you don’t see this pattern in modern humans.
  • In any case, the two newly discovered tracks appear to be from individuals who were walking together, which suggests some kind of social bond between them. If indeed all five individuals were walking together, that would imply the Australopithecus was a fairly social species.

As a story in National Geographic makes clear: All of these hypotheses are in dispute. It’s really difficult to say if the footprints belong to females or juveniles (we don’t have many examples of Australopithecus feet to know for sure how their size correspond to height). From there, it may be a stretch to assume that if a male was with more than two females, they were all his mates. And finally, there isn’t conclusive evidence that these individuals were, indeed, all with one another.

But this is what the footprints do: activate imagination. Anthropologists are in the business of finding out and writing the long-forgotten history of life on Earth. Just a footprint alone is enough to tantalize them.

“Footprints in general take on an even greater value in the case of hominins [human ancestors],” Cherin says. They “are creatures of which we know very little, even though they are so closely related to us.”

Watch: How scientists discovered a new human ancestor

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