MINNEAPOLIS—Newly discovered fossil footprints at a site in northern Tanzania on the shore of Lake Natron capture a moment in time around 120,000 years ago when a band of 18 humans—early members of our own species, Homo sapiens—traipsed across wet volcanic ash to an unknown destination. Brian Richmond of George Washington University unveiled the stunning find here on April 13 at the annual meeting of the Paleoanthropology Society.

Footprints are extremely rare in the human fossil record, and highly prized for the unique information they can reveal about the anatomy and behavior of our ancient relatives. Richmond and his colleagues found the new prints—more than 350 in all—in 2010 at the site, called Engare Sero. No animal prints were among them.

Richmond reported that one group of prints head east and, judging from the stride lengths evident in the trails, appear to have been left by individuals who were variously walking, running, or moving at an intermediate pace.* The different speeds indicate that these people were not traveling together. The other group of prints, however, were made by 18 individuals walking together to the west.

To learn more about the these early travelers, Richmond and his colleagues compared the fossil footprints to a set of prints obtained experimentally from modern-day men and women from Ileret, Kenya, moving at a variety of speeds. Based on these measurements, the team concluded that the ancient human group was composed of men, women and children, with more women than men.

"This is evidence of what an early modern human social group traveling together looked like," Richmond said. "It offers a glimpse of actual behavior in the fossil record."

*An earlier version of this story stated incorrectly that the prints heading east were made by 14 people.