Ancient Footprints Tell a Human Story

BY AYLIN WOODWARD

When Kevin Hatala asks his students on the first day of class, “What are the characteristics that make us human?” the first response they tend to give is opposable thumbs.

“But if you look at our hand anatomy compared with other apes, our hands still do many of the same things their hands are doing,” said Hatala, a paleoanthropologist at Chatham University in Pennsylvania.

Our feet, however, are a different story.

The modern human foot is one of the most unique aspects of our species, he said. What began as a tree branchgrasping organ in a long-dead common ancestor with great apes evolved into something that can carry us upright for many miles at a time. Our cushioned heels strike the ground, and an arched sole transfers weight toward the front of our feet, propelling us forward. Our robust big toes push off into the next step, the final note in an efficient stride for long-distance locomotion.

One of the most significant fossilized footprint discoveries in the past several years— multiple trackways uncovered in eastern Africa and studied by Hatala’s group—helps paint a picture of this thing that makes us special.

Differences in the tracks show, for the first time, that two humanlike species coexisted some 1.5 million years ago in what is now Kenya.

One group was our ancestor, their footprints revealed. The other wasn’t.

One set of prints closely resembled those you would spot on a beach today. The other, at first glance, did too. But a closer examination revealed these tracks had some key differences.

A dozen or so footprints headed east at the site, known as Koobi Fora, had a big toe that curved away from the other toes—similar to the big toe a chimpanzee uses for grasping. The prints were also flatter, demonstrating a different pattern of weight transfer.

About 3 feet away other prints headed north. “That cluster looked like modern humans could have left them,” Hatala said. “It shows a foot moving in a humanlike way.”

His group realized that not one but two humanlike species crossed paths within days, if not hours, of each other on this ancient lakeshore.

Footprints in lakeshore mud are preserved when they are rapidly covered by other sediments, like ash, perhaps during a flood. Scientists date the age of a footprint relative to the ash layers above or below it, a process known as stratigraphy.

A layer of volcanic ash helped researchers date the footprints at Koobi Fora to 1.52 million years ago, said Craig Feibel, a professor of geology at Rutgers University and one of Hatala’s co-authors on a recent study published in Science describing the footprints.

Using 3-D computerized models of the tracks, the researchers compared them with those made by hundreds of modern people and other ancient relatives and human ancestors.

The comparisons revealed that some of the individuals who left tracks at Koobi Fora were the long-legged Homo erectus, considered the first of our lineage to walk upright as we do. The other prints were left by human relatives known as Paranthropus boisei.

Limited competition between them for resources suggested they coexisted peacefully, Feibel said. Yet within a few hundred thousand years of their encounter, Paranthropus boisei had all but disappeared from the fossil record, while our ancestors had spread out of Africa and across Europe to the Iberian Peninsula and as far east as modern-day China.

Homo erectus, bigger in limbs and brain size than other ancient relatives in our lineage, became one of the longest-lived early human species, persisting from roughly two million years ago until 110,000 years ago.

Its modern gait and longer strides may not have been the secret to Homo erectus’s longevity as a species, according to Jeremy DeSilva, a Dartmouth College paleoanthropologist not involved in the study—but it would have helped the species expand its range significantly and increase the diversity of its diet, ultimately leading it to become cosmopolitan.

“The more territory you have, the more stuff you can munch on, and the more stuff you can munch on, the greater input of energy into the system, which can fuel a growing brain,” he added.

Studying such visually stark clues as footprints is key to understanding the diverse forms of our ancestors and relatives.

“Footprints have the power to help us time travel and tell individual stories about the past,” according to Matthew Bennett, an environmental scientist and geographer at England’s Bournemouth University who wasn’t involved in the recent work.

“They’re incredibly emotive, and people connect to them, because who hasn’t stood on the beach and watched a wave swallow their steps in the sand.”