Walks of life
Walking is a complex, evolutionary experiment that began in prehistoric times. Fossils overcome what we think of evolution of human bipedalism showing that for most of human evolution multiple species with different ways of walking upright coexisted.
Long before our ancestors evolved large brains and language, they started doing something no mammal had done before: walking on two legs. Moving on two legs rather than four set the stage for subsequent evolutionary changes.
The March of Progress
Human evolution is usually represented as a procession of ancestors starting with a chimplike creature ambling on all fours that gives way to a series of ever more erect forebears, culminating in a fully upright Homo sapiens striding on two legs. This image, called the March of Progress, was first popularized in the 1960s and its variants are decorating books, T-shirts, bumper stickers and coffee mugs. But paleoanthropological discoveries made over the past two decades are forcing scientists to redraw this traditional, linear imagery. We now know that various hominin species living in different environments throughout Africa, sometimes contemporaneously, evolved different ways to walk on two legs.
Mysterious footprints
Fossil footprints from Laetoli, Tanzania, show that two different hominin species walked bipedally in this area 3.66 million years ago. Paleontologists Kay Behrensmeyer and Andrew Hill, who were visiting archaeologist Mary Leakey’s fossil site of Laetoli in Tanzania, on July 24, 1976, spotted fossilized elephant footprints and raindrop impressions hardened in an exposed layer of volcanic ash that fell 3.66 million years ago.
For the next few weeks Leakey and her team explored an area they called Site A, brushing sediment to reveal thousands of footprints, mostly made by ancient animals. Hoping to find hominins in the mix, Leakey told the group to be on the lookout for bipedal footprints. That September Peter Jones and Philip Leakey discovered five consecutive footprints made by something traveling on two legs. The footprints were strangely shaped and cross-stepped, moving the left foot over the right like a model on a runway. The Site A bipedal trackway was a mystery.
Two years later two other members of Leakey’s team, Paul Abell and Ndibo Mbuika, discovered another bipedal trackway two kilometers west of Site A at a location dubbed Site G. Two or three, perhaps even four, individuals had walked through the muddy ash, leaving 69 humanlike footprints. Most scholars agree these tracks were made by Australopithecus afarensis—Lucy’s species—fossils of which have been found at Laetoli. These tracks were decidedly different from the ones at Site A. If a hominin made the tracks at Site G, then what kind of creature made the bipedal trackway at Site A?
In the mid-1980s University of Chicago anthropologist Russ Tuttle tried to solve this mystery. After comparing the shape of the Site A footprints with those made by humans, chimpanzees and bears, Tuttle concluded that the prints were either made by a second species of hominin or by a bipedally walking bear. Perhaps because a linear view of the evolution of human bipedalism was the dominant paradigm, other researchers embraced the bear hypothesis. As a result, whereas the Site G hominin footprints were exhaustively studied and became world-famous, the footprints at Site A fell into obscurity. Three decades passed before anyone focused on them again.
In 2017 paleoanthropologist Jeremy DeSilva and then graduate student Ellison McNutt, teamed up with local black bear expert Ben Kilham to collect footprints from cubs whose feet were similar in size to the tracks at Laetoli Site A and discovered that their footprints were no match for Site A. Bears’ heel impressions are narrow, and their steps are widely spaced because their hip and knee anatomy causes them to wobble back and forth when walking bipedally. They started to have doubts about the bear hypothesis.
Different Hominins tracks
More than 40 years have passed since the discovery of the Site A trackway. In that time, seasonal rains have slowly washed sediment from the barren hills at Laetoli, exposing tens of thousands of fossils. Charles Musiba of the University of Colorado Denver, Terry Harrison of New York University and Denise Su of Arizona State University have recovered many of these fossils. An extinct bear called Agriotherium did roam Africa during the Pliocene, but not one of the animal fossils they have recovered at Laetoli is from a bear. In 2019 Charles Musiba of the University of Colorado Denver and paleoanthropologist Jeremy DeSilva traveled to Laetoli and used detailed drawings to identify the precise location where the mysterious bipedal footprints should be. Then they began to dig. After several days Tanzanian team member Kallisti Fabian had found the footprints covered and preserved with a layer of sediment. They cleaned the prints, revealing never before seen details of the toe impressions, which they captured with high-resolution, 3-D laser scans. The heel impressions of the Site A footprints are large, and the big toe is the dominant digit, as it is in humans and our ape cousins. This was no bear. A hominin made these tracks.
Family tree shows time spans associated with our evolutionary ancestors and highlights foot structures of various hominins. Maybe the footprints at Sites A and G were showing normal variation within a single species of hominin. If so, the small size of the Site A footprints might indicate they were made by a child of Lucy’s species.
They found that the Site A footprints had a shape that was as different from the Site G and S prints as a chimpanzee’s footprints are from yours and mine. The Site A footprints were short and wide, the big toe stuck out to the side a bit, and there was some evidence the walkers had a more flexible middle portion of the foot.
In their paper describing these findings, they claimed that not only were the Site A footprints from a hominin, but they also were evidence of a second species at Laetoli. This is the best evidence we have that different Pliocene hominin species not only were contemporaries but shared the same landscape.
Evolutionary experimentation
The rediscovery of the Laetoli Site A footprints and the conclusion that they were made by a second species are the latest additions to a growing body of evidence that the evolution of upright walking was a lot less linear, more complex and more interesting than we once thought. Many of these new discoveries sample a pivotal period in human evolution, between five million and three million years ago, when our ancestors were becoming committed upright walkers.
As bipedalism evolved in our earliest ancestors, there was a burst of evolutionary experimentation that resulted in different hominins having different foot forms. The pattern of locomotor diversity is not limited to these early chapters of human evolution. The pattern of diverse walking styles persisted even after Australopithecus and Paranthropus went extinct. As recently as 60,000 years ago, by which point H. sapiens was well established, the small human species Homo floresiensis, roamed its island home of Flores in Indonesia on relatively giant, flat feet and short legs with small joints.
It's possible that bipedalism evolved multiple times at the base of the hominin family tree, perhaps for different reasons, in different hominins living in slightly different environments throughout Africa. The diversity of foot forms found in Pliocene fossil sites across the continent supports such a scenario.
Once our ancestors got moving on two legs, they kept on walking, and that journey has continued right up to today. In a lifetime, the average person will take about 150 million steps—enough to circle Earth three times. We stroll, stride, plod, traipse, amble, saunter, shuffle, tiptoe, lumber, tromp, lope, strut and swagger. After walking all over someone, we might be asked to walk a mile in their shoes. Heroes walk on water, and geniuses are walking encyclopedias. But rarely do we humans think about walking. It has become, you might say, pedestrian. The fossils, however, reveal something else entirely. Walking is anything but ordinary. Instead it is a complex, convoluted evolutionary experiment that began with humble apes taking their first steps in Miocene forests and eventually set hominins on a path around the world.
Abridged and simplified from:
DeSilva, Jeremy. "Fossils upend conventional wisdom about evolution of human bipedalism." Scientific American, 1 Nov. 2022, scientificamerican.com/article/fossils-upend-conventional-wisdom-about-evolution-of-human-bipedalism/
Learn more:
Watch Professor Jeremy DeSilva talking about his research on the origin of upright walking
