The site in northern Tanzania is the largest assemblage of ancient human footprints in Africa and one of the biggest on the planet, Liutkus-Pierce and her colleagues reported in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. The 400-odd footprints, which cover an area the size of a tennis court, were imprinted in deposits from an ancient flood, dried, then covered up with a second layer of mud and preserved for as many as 19,000 years.

Now, excavated and exposed to the sun, they offer an unprecedented window into an ancient world. Anthropologists at the site plan to use the footprints to understand social dynamics at the end of the Pleistocene era, a time when the climate was changing and Homo sapiens was on the brink of settling down and learning to farm.

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"It'll give us a sense of the group size and structure of these ancient hunter-gatherers," said Briana Pobiner, a paleoanthropologist at the National Museum of Natural History and a member of Liutkus-Pierce's team. "What's the composition of this group? How many males, how many females and kids, and how many directions are they going? Are they running? Are they walking? Are they walking side by side?"

"For people who work in prehistory, it's incredibly rare to get that kind of snapshot in time," she continued. Most knowledge about ancient communities is reconstructed from exhumed skeletons, scattered tools, animal bones dug up from bygone garbage pits. But the Engare Sero prints have the potential to tell us exactly who lived in this spot, how they related to one another and where they may have been headed.

The site was discovered by a resident of the nearby village of Engare Sero about a decade ago, but the scientific community didn't learn of the prints until 2008. Liutkus-Pierce was sitting in her office at Appalachian State University in North Carolina when she got a call from a colleague Jim Brett, who was working in East Africa.

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"I think I've come across some amazing hominid footprints," he said.

Liutkus-Pierce glanced at her calendar: It was April 1. Half worried she was being fooled, she told him, "Jim, I'm going to need you to call me back and say this again tomorrow."

But Brett was adamant — this was no joke.

A year later, Liutkus-Pierce and a small crew of colleagues were standing at the edge of the footprint site. The shallow, saline waters of Lake Natron shimmered behind them and the mountain loomed up ahead. And there they were: Some 50 footprints pressed into a dried-up riverbed, clearly defined and unmistakably human. She teared up looking at it.

There were also tire tracks zig-zagging across the site — a reminder of how vulnerable this spot really was. The scientists needed to get to work.

With a grant from the National Geographic Society, Liutkus-Pierce assembled a team of geologists, archaeologists and anthropologists to excavate the site. (Their discovery was first reported by National Geographic this week.) They worked to expose the rest of the prints, elucidate their origins, document their context and interpret their significance. Each print was photographed and 3-D scanned. Soil samples were sent off to labs for radiometric analysis.

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When researchers first arrived at Engare Sero, they thought that the site might be hundreds of thousands of years old. The Laetoli footprint trails discovered by paleontologist Mary Leakey and Paul Abell are just a few dozen miles away; those impressions were made by two Australopithecines walking through volcanic ash 3.6 million years ago.

But attempts to date the rock layer turned up confusing results. The volcanic ash around the footprints looked to be 120,000 years old, yet other dated materials yielded ages that were much younger. Efforts to date plant material embedded alongside the prints proved futile.

Eventually, Liutkus-Pierce and her team concluded that the mud must have been deposited during a flooding event, rather than a volcanic eruption. But since floods jumble materials of different origins and ages together, that meant the scientists had to date dozens of different minerals. The youngest crystal in the footprint layer would represent the oldest possible age for the prints; the oldest crystal in the layer above it would represent the youngest they could be.

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Using the argon-argon dating technique, by which scientists measure the decay of an isotope called Argon-40 into Argon-39 in order to find the age of crystals, they came up with a rough approximation of the footprints' age: 19,000 years at the oldest, 10,000 or 12,000 years at the youngest.

Hatala, a paleoanthropologist at Chatham University, is part of a subset of the Engare Sero team aiming to tease out the anthropological questions that the prints raise. He and his colleagues are working on another study, which they hope to publish in the next few months, that uses the footprints to examine the size and composition of the group that made them. Already, the researchers have distinguished at least 24 distinct trackways (series of steps that can be attributed to a single person) going in two directions. They've established the age and genders of some of the footprint-makers, and sorted out who was walking and who was running.

"Knowing that somebody was walking through this exact spot, at this moment in time, thousands of years ago," said Kevin Hatala, another co-author on the study, "it does provoke lots of questions about what were these people doing there, who were they with?"

And how did they survive? The environment at Engare Sero is hot, dry and austere. The nearby lake is fed by hot springs that make its water undrinkable. The volcano, Ol Doinyo Lengai, is active and still spews strange, silvery lava into the valley below. This is probably the same landscape that the footprint-makers would have encountered as they walked through the mud.

For part of their research, the team invited volunteers to match their own feet to the prints and attempt to follow the tracks, including residents of the nearby village.

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"Everybody was trying to put their foot in the footprint, seeing, 'is this person the same size as me?' You try to walk out the steps," he said. "It definitely is this uniquely interactive form of data."

Liutkus-Pierce recalled helping one elderly woman fit her foot into one of the impressions. Eyes on the ground, the woman followed a trackway across the site, matching each of her steps to one taken 19,000 years ago, quite literally following in the footsteps of her ancestors.