At Walvis Bay in Namibia, a scouring wind pushes sand dunes across an ancient mud flat. Sometimes when the dunes shift, the tracks of long-gone people and animals are exposed — much to the delight of Matthew Bennett, an ichnologist at Bournemouth University in England.
Ichnologists study tracks and traces and other signs of living creatures, including the footprints left by our human and pre-human ancestors. As rare and prized as hominin bones in the fossil record, footprints evoke a different response.
“A track is extremely emotive,” says Bennett. “If you see an ancient footprint, you are automatically drawn to it.” Indeed, at Walvis Bay the tracks emerge from the landscape as if they were just created. “One of the sites has [the tracks of] very small children in it,” he adds, “and there’s little doubt that they’re playing.”
The Namibian tracks are only about 1,500 years old. Africa has footprint sites that are far older, the oldest and most famous being Laetoli in northern Tanzania. Some 3.6 million years ago, three or possibly four individuals, probably Australopithecus afarensis, a predecessor of our Homo line, walked across a rain-spattered plain of volcanic ash that hardened soon afterward. More ash fell, and over eons it was compressed into a soft rock known as tuff. Erosion eventually exposed part of the roughly 80-foot-long trackway, and, beginning in 1978, archaeologists working with paleoanthropologist Mary Leakey excavated the rest.
The prints are “fossilized locomotion,” says Bennett. “They capture a lot more than is possible by simply looking at a collection of foot bones. The foot is a wonderful machine made possible by the soft tissue that wraps those bones. Footprints help you understand the whole item, not just the skeletal parts.”
After studying casts of the Laetoli prints for decades, scientists decided that A. afarensis, though a primitive hominin, walked with a surprisingly modern gait that was not like an ape’s. Bipedalism, in other words, evolved earlier than believed.
That theory is now being re-examined, however, by new analyses of the footprints using photogrammetry, which renders three-dimensional digital images. The photographer takes a series of pictures of the original footprints from different vantage points. The camera captures the depth, length and width of the impressions from all angles. A computer then integrates the images. Unlike the wet blotches you make stepping out of the shower, three-dimensional footprints show where force is being applied as the foot carries through the step.
Kevin Hatala of the Max Planck Institute of Evolutionary Anthropology, and a postdoctoral researcher at the Center for the Advanced Study of Human Paleobiology at George Washington University, recently analyzed one set of prints at Laetoli using photogrammetry. He and his colleagues at the American Museum of Natural History and Stony Brook University compared the track with footprints made by the area’s present-day residents, who don’t wear shoes. Their findings challenge the consensus that the Laetoli gait is fully humanlike.
“We wouldn’t necessarily say that [the tracks] looked apelike,” he says. “If you saw the maker of the Laetoli prints walking alongside a modern human, you might not notice any dramatic differences. But with careful observation, you could probably pick up distinctions in the ways they moved.”
Photogrammetry not only has provided new data to interpret and debate, but also a means of site preservation. Bennett and other ichnologists have called for the “digital rescue” of ancient track sites. The footprints face destructive forces from the moment they’re uncovered, such as weathering, erosion and disruption by humans or animals. Working with the host countries, scientists have tried to protect sites by covering them after study. Laetoli, for instance, has been exposed to view only twice since the prints were excavated.