Human behavior is very different and unique
We’re used to partial evidence, dead ends and red herrings. It’s especially hard to reconstruct ancient behaviors, something we are particularly interested in. We must rely on either skeletal remains or the physical things left behind by ancient people to deduce anything about their lives, be it what they ate, how they moved or the origins of complex behaviors like creating tools or communicating with language.
Prehistoric footprints are a remarkable and precious source of evidence for the behavior and biology of ancient organisms, capturing a snapshot of their lives in deep time. In a new paper in Nature Scientific Reports, our research team documented and interpreted an extraordinary site in northern Tanzania called Engare Sero, where hundreds of human footprints were preserved in volcanic ash many thousands of years ago. It’s the most abundant assemblage of ancient human footprints currently known from Africa and suggests this ancient community had a division of labor between the adult females and males. Footprints are unique in that they are a preserved moment in time when an animal moved across a landscape and left traces of its movements imprinted in the ground. While they cannot tell you too much about how an animal looked, they can be surprisingly useful for reconstructing many other aspects of their biology. Footprints can tell you how fast an animal was running, where it was going and sometimes even if the animal was solitary or moved in herds. For the human lineage, footprint sites have been especially important in furthering scientists’ understanding of our own evolutionary journey. The iconic 3.66-million-year-old paleontological site of Laetoli in Tanzania, for instance, provided some of the earliest definitive evidence of upright walking in our ancient ancestors.
While Engare Sero is much younger than Laetoli, it provides a fascinating snapshot of a time period when our own species, Homo sapiens, was on the rise. When you walk along wet sand on a beach, your footprints might last for a few minutes, or a few hours, before being washed away by the ocean. But if you were to walk in wet volcanic ash and leave your footprints behind, and the ash then dried nearly as hard as concrete, your footprints could last thousands – or even millions of years. That’s exactly what happened thousands of years ago when a group of at least 20 prehistoric modern humans walked through a volcanic mudflow produced by the Oldoinyo L’engai volcano, still active today in what’s now Tanzania. The local Tanzanian Masai have known about this set of ancient human footprints at Engare Sero for a while. When our research team leader, geologist Cynthia Liutkus-Pierce, first visited the site in 2009, only 56 footprints had been exposed by natural surface erosion – but she was still floored by what she saw. Our research group including archaeologists, anthropologists, biologists, geologists, statisticians and other scientists went on to uncover, document and analyze an astonishing 408 total human footprints. Using two different dating techniques that measure radioactive decay in the material that preserved the footprints, we dated the footprints to between 6,000 and 19,000 years ago. In this time period, modern humans were likely the only hominin species left in Africa, and had already spread to many other parts of the globe.
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