The Footprints in the Ash: Laetoli's 3.7-Million-Year Walk

In 1976, on a stretch of dusty ground in the Enduleni ward of Ngorongoro District in Tanzania's Arusha Region, paleoanthropologist Mary Leakey and her team noticed something extraordinary embedded in ancient volcanic ash: a trail of footprints, unmistakably hominin, stretching 75 feet (24 meters) across the ground at a location that came to be called Site G. By 1978, the site had been fully excavated, revealing one of the most significant discoveries in the study of human origins.

The footprints were dated to roughly 3.7 million years ago, placing them within the Upper Laetolil Beds, layers of volcanic tuff that geologists identified using potassium-argon dating and stratigraphy; that upper unit of the Laetolil Beds has itself been dated to a broader range of 3.6 to 3.8 million years ago. At the time of discovery, the prints represented the oldest known evidence of bipedalism—walking upright on two legs—in hominins. Though older Ardipithecus ramidus fossils were later found suggesting bipedal traits, the Laetoli prints remained a landmark because they showed, in astonishingly direct physical form, exactly how an ancient relative of humans moved across the earth.

The story of Laetoli's discovery stretches back further than 1976, though. In 1935, a local man named Sanimu persuaded archaeologist Louis Leakey to investigate the region, leading to the collection of mammalian fossils, including a tooth later identified in 1979 by P. Andrews and T. White as the site's first hominin fossil. German archaeologist Ludwig Kohl-Larsen carried out extensive study of the site in 1938 and 1939, uncovering hominin premolars, molars, and incisors. Interest faded until 1974, when George Dove's discovery of a hominin premolar drew Mary Leakey back to the site—setting the stage for the footprint find two years later.

Geologically, the footprints were preserved through a remarkable sequence of events: soft rain fell on a fresh layer of volcanic ash roughly 15 centimeters thick, cementing it just enough to hold impressions before it hardened into rock. Researchers originally suspected the nearby Sadiman volcano, some 20 kilometers away, as the source of the ash, though a 2011 study by Zaitsev and colleagues cast doubt on that connection, showing Sadiman was not the source of the Laetoli Footprints Tuff.

Determining exactly which species left the tracks has proven difficult, but researchers have tentatively settled on Australopithecus afarensis. That conclusion rests on detailed anatomical work: Tim D. White and Gen Suwa of the University of California reconstructed the foot skeleton of a female A. afarensis, while Russell Tuttle of the University of Chicago conducted comparative footprint analysis, examining step length, stride length, stride width, and foot angle against those of humans and other bipedal creatures such as bears and primates. Tuttle's conclusion was striking: the gait was far more human-like than ape-like.