Fossils Upend Conventional Wisdom about Evolution of Human Bipedalism

Fossils Upend Conventional Wisdom about Evolution of Human Bipedalism

Long before our ancestors evolved large brains and language, even before they tamed fire or made stone tools, they started doing something no mammal had done before: walking on two legs. Skeletal adaptations for traveling upright are evident in fossils of the very oldest hominins–members of the human family–which date to between seven million and five million years ago. The foundation for evolutionary changes in our lineage was set by the ability to move on two legs instead of four. It allowed our predecessors to expand their home ranges ,, and it transformed the way we give birth to our children. This unique mode of locomotion was fundamental to almost all other characteristics that make us unique.

In the iconic representation of human evolution, a procession of ancestors starting with a chimplike creature ambling on all fours gives way to a series of ever more erect forebears, culminating in a fully upright Homo sapiens striding triumphantly on two legs. First popularized in the 1960s, the March of Progress, as this image and its variants are known, has decorated countless books, T-shirts, bumper stickers and coffee mugs.

But paleoanthropological discoveries over the past 20 years are forcing scientists to redraw traditional linear imagery. We now know that different hominin species have evolved different ways of walking on two legs in different environments across Africa. Bipedalism was the beginning of a long period of evolutionary riffing about this form of locomotion. Our modern stride did not evolve from a predetermined pattern. Each successive ancestor marched closer to a specific goal (evolution does not have plans). It’s actually one of many types of upright walking that early hominins tried, and the one that eventually prevailed.

Mysterious Footprints

They didn’t want to be hit by a lump of elephant poop. Who would? So paleontologists Kay Behrensmeyer and Andrew Hill, who were visiting archaeologist Mary Leakey‘s fossil site of Laetoli in Tanzania, hopped into a gully to take cover and gather more ammunition for the game of elephant dung dodgeball that had spontaneously broken out. It was July 24, 1976, the day of one of the most serendipitous discoveries in the history of paleoanthropology.

Hill & Behrensmeyer searched the ground for dung, but instead found fossilized elephant footprints and raindrop impressions. These impressions were hardened in a layer of volcanic ash which fell 3. 66 million years ago. The dung fight was ended with a truce, and everyone else marveled at the discoveries. Fossils can be used to describe an organism. For long-extinct animals, fossil footprints provide valuable snapshots of time and moments.

Leakey and her colleagues explored Site A over the next few weeks, removing sediment to reveal thousands upon thousands of footprints. These were mostly made by small antelopes or hares, but also from large cats, birds, and ancient elephants. Leakey advised the group to look out for bipedal footprints, in hopes of finding hominins. They might be lucky. They did that September. Peter Jones and Philip Leakey found five consecutive footprints that were made by something walking on two legs, rather than four. Is this a hominin? Perhaps, but the footprints were oddly shaped and the person who made them had crossed-stepped, moving the left leg over the right, much like a model walking on a runway. It was a mystery what the Site A bipedal trackway was.

Fossil footprints from Laetoli, Tanzania, show that two different hominin species walked bipedally in this area 3.66 million years ago.
Fossil footprints from Laetoli, Tanzania, show that two different hominin species walked bipedally in this area 3.66 million years ago.
Fossil footprints from Laetoli, Tanzania, show that two different hominin species walked bipedally in this area 3. 66 million years ago. The Site G trackway (bottom) is thought to have been made by Australopithecus afarensis. The Site A trackway (top) was made by a different, as yet unidentified hominin. Credit: Jeremy DeSilva (top); John Reader/Science Source (bottom)

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 stride for stride through the muddy ash, leaving 69 stunningly humanlike footprints. Most scholars agree these tracks were made by Australopithecus afarensis–Lucy’s species–fossils of which have been found at Laetoli. Site G tracks were markedly different to those at Site A. What kind of creature would have made the Site G tracks?

In the mid-1980s University of Chicago anthropologist Russ Tuttle took a crack at solving this mystery. Tuttle compared the Site A footprints to those made by unshod humans and chimpanzees. Tuttle also concluded that the prints were either made during the Pliocene epoch by another species of hominin or by a bipedally-walking bear. Other researchers accepted the bear hypothesis, perhaps because it was more consistent with the linear view of human bipedalism’s evolution. The Site G hominin footprints became world-famous and were extensively studied. However, the footprints from Site A went unnoticed. They were forgotten for three decades.

Dartmouth College is a small liberal arts college in New Hampshire, nestled in a valley that runs between the White Mountains and Green Mountains of Vermont. This is where I teach anthropology. Although the school is only two hours by car from metro Boston, its motto is vox clamantis in deserto, which translates to “a voice crying out in the wilderness.” Large swaths of sugar maples provide an ample supply of syrup, the famous Appalachian Trail abuts the campus, and bears–a lot of bears–live in the surrounding woods.

In 2017 my then graduate student Ellison McNutt, who is now a professor of anatomy at Ohio University, and I 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. To tempt them, we used maple syrup and applesauce to get the young bears to stand up on their hind legs to walk through a trackway made of mud. We were surprised to find that their footprints and gait mechanics did not match Site A. Bears’ heels impressions are narrow and their steps are wide spaced because of their hip and knee anatomy. We began to doubt the bear hypothesis.

More than 40 years have passed since the discovery of the Site A trackway. Since then, the sediment has been slowly washed away from the barren hills of Laetoli by seasonal rains, exposing thousands of fossils. Many of these fossils were recovered by Terry Harrison of New York University, Charles Musiba of University of Colorado Denver and Denise Su of Arizona State University. We know from other sites that an extinct bear called Agriotherium did roam Africa during the Pliocene, but not one of the animal fossils these teams have recovered at Laetoli is from a bear. It was necessary to look at Site A’s bipedal tracks again. However, the same seasonal rains that give us fossil bones and footprints also have a destructive power to remove them. We assumed that the Site A bipedal footprints had long since disappeared. We were wrong, thankfully.

In 2019 Musiba and I traveled to Laetoli and used Mary Leakey’s detailed drawings like a treasure map to identify the precise location where the mysterious bipedal footprints should be. Then we started digging. After several days Tanzanian team member Kallisti Fabian called to us, “Mtu“–the Swahili word for “human.” He had found the footprints. They had been protected by a layer of fine sediment, but the rains hadn’t destroyed them. Using tongue depressors and thick-bristled brushes, we fully cleaned the prints, revealing never before seen details of the toe impressions, which we captured with high-resolution, 3-D laser scans unavailable to our colleagues working in the 1970s. 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 not a bear. These tracks were made by a hominin. But which hominin made these tracks?

Family tree shows time spans associated with our evolutionary ancestors and highlights foot structures of various hominins.
Credit: Dino Pulera (foot illustrations) and Jen Christiansen

Walk on a sandy beach, and you are sure to see a variety of H. sapiens footprints–small, flat prints made by a toddler next to the long, arched prints of her mother, for instance. Modern humans come in many shapes and sizes. It is almost certain that the same holds true for A. afarensis. It is possible that the footprints at Sites G and A showed normal variation within one species of hominin. If this is the case, the Site A footprints may have been made by a child from Lucy’s species. This is what I initially thought.

Chatham University’s

Footprint expert Kevin Hatala helped to find and analyze 1. 55-million-year-old Homo erectus footprints at Ileret, Kenya, joined our team, and together we compared the shape of the Site A footprints with the best-preserved footprints from Site G and another trackway discovered in 2015 at Site S, along with hundreds of footprints made by humans and chimpanzees. We did not find any differences that were within the range of variation found in footprints made by people of all ages.

We discovered that Site A footprints were as different from the Site G, and S footprints as chimpanzees’ footprints are from mine and yours. This is not to say that the Site A footprints looked exactly like Lucy’s. Compared with those presumed A. afarensis footprints at Sites G and S, 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 our paper describing these findings, published last December in the journal Nature, we claimed that not only were the Site A footprints from a hominin, but they also were evidence of a second species at Laetoli. Our interpretation was not accepted by all of our colleagues, as is to be expected in science. Some think we just found another A. afarensis footprint trail. But it is worth repeating that the Site A footprints were so different from the Site G Australopithecus prints that our field was convinced for decades that they were made by a bear.

It seems that shortly after the ash fell from heaven, 66 million years ago, two kinds of hominins, walking on slightly different feet in slightly different ways, moved north toward the Olduvai Basin in Tanzania, perhaps in search of water. This is the best evidence that we have that different Pliocene hominins were not only contemporaneous but also shared the same landscape. It is not clear how they interacted, if at all.

Laetoli Site G footprint presumably made by A. afarensis.
Site A print is short and wide; the big toe sticks out to the side.
Compared with the Laetoli Site G footprint (top), presumably made by A. afarensis, the Site A print (bottom) is short and wide; the big toe sticks out to the side. Credit: John Reader/Science Source (top); Jeremy DeSilva (bottom)

Fossil Feet

The rediscovery and conclusion that the Laetoli Site A footprints were made by a different species is the latest evidence to show that upright walking evolved in a more complex, linear and interesting way than we thought. Other evidence comes from fossils of hominins, not footprints. Isolated foot bones are rare in the human fossil record, and foot skeletons are even more elusive. It is therefore exciting to note that paleoanthropologists have quadrupled the number fossils found in the foot skeletons of bipeds, which are the only part of the body that is in direct contact with ground in Africa’s Great Rift Valley over the past 20 years. These new discoveries represent a pivotal time in human evolution. They are between five and three million years old, when our ancestors became upright walkers. In 2017 McNutt and I teamed up with Bernhard Zipfel, a former podiatrist-turned paleoanthropologist at the University of the Witwatersrand in South Africa, to make sense of these finds.

We sought to assess the accepted wisdom regarding the evolution of bipedalism, in light of new fossil evidence. According to the traditional view, hominins began with a chimp-like foot that was used for grasping branches. This foot evolved into a transitional foot capable of both grasping and walking, as seen in the fossil known as Ardi, a member of Ardipithecus ramidus that lived in Aramis, Ethiopia, 4.4 million years ago. Fast forward to Lucy, the A. afarensis individual who lived in Hadar, Ethiopia, some 3.2 million years ago, whose foot has a big heel and a stiff midfoot that were better adapted to life on the ground. With the emergence of our own genus, Homo, roughly a million years later, the foot became even better suited to terrestrial locomotion, evolving shorter toes and a high arch.

Researchers studying Site A tracks at Laetoli,
Decades after the discovery of the Site A tracks at Laetoli, researchers returned to study the tracks again. Although the footprints are easily eroded by seasonal rains, they were protected with sediment in this case. Credit: Shirley Rubin

After examining all the foot fossils in museums across Africa, we discovered a completely different pattern. Bipedalism developed in our earliest ancestors. There was an evolutionary explosion that led to different foot forms in different hominins. In the two-million year period we studied, five foot morphs were identified, possibly indicating five distinct ways to walk upright. Three other feet are unique to the chronological bookends of Lucy and Ardi. The first is an Ardi-type creature of the same age as the fossil from Gona, Ethiopia. The second is a 3. 67-million-year-old hominin from Sterkfontein, South Africa, dubbed “Little Foot”; and the third is a strikingly primitive foot from a site called Burtele in Woranso-Mille, Ethiopia, that dates to 3.4 million years ago. Although all five hominin feet have both apelike-humanlike traits, they are not the same. They differ in each foot and don’t follow the expected pattern of becoming less apelike or more humanlike with time.

Like an ancient version of Cinderella’s story, one of these recently discovered feet may fit the mysterious hominin footprints found at Laetoli Site A. It could reveal the identity and origin of the track maker. As we continue to study these early stages of evolutionary history, we’ll be able to see.

Australopithecus sediba.
Homo floresiensis.
Multiple styles of upright walking continued to evolve even after the emergence of species with our modern human gait. Australopithecus sediba (top) had adaptations to both terrestrial and arboreal locomotion; tiny Homo floresiensis (bottom) had large, flat feet that might have required taking small, high steps. Credit: S. Entressangle and Elisabeth Daynes/Science Source (top); Sebastien Plailly and Elisabeth Daynes/Science Source (bottom)

Sustained Diversity

It is interesting to note that the pattern of locomotor diversification is not restricted to the early stages of human evolution. Take, for instance, Australopithecus sediba. Rivaling the elephant dung fight in the lore of fortuitous paleoanthropological discoveries, this nearly two-million-year-old hominin was discovered in 2008 by then nine-year-old Matthew Berger. While searching for fossils at Malapa Cave in South Africa’s Cradle of Humankind, Berger literally stumbled upon a rock that contained a hominin lower jaw and clavicle. Berger and his team excavated fossil-bearing cave walls for months and found two partial skeletons from a new species. sediba. Berger invited to study the foot- and leg fossils shortly following my Ph.D .

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I was stunned by what I saw. The bones had strange shapes. The heel bone was too apelike for a hominin from this time period. Both the skeletons also showed unusual traits in the lower back, midfoot, ankle and knee. These bones are bizarre when taken in isolation. They told the story of a hominin who had a strange way of walking. It was similar to today’s hyperpronating, which is when they transfer excessive weight to their feet. This gait can lead to joint pathologies in modern people, but Berger and I and our colleagues interpreted the peculiarly shaped bones of A. sediba as anatomical solutions to the problems modern humans face when they walk in this manner. This means that we believe this species was adapted for this type of walking. Why? The shoulders and arms of A. sediba indicate that it climbed trees, and its teeth preserve microscopic traces of plant cells derived from leaves, fruit and bark–evidence that this species frequently fed in trees. This walking style was the compromise for a hominin that was well adapted to life in two worlds. It could navigate between trees and ground long after other hominin species had fully committed themselves to terrestrial life.

A. sediba was not the only hominin walking around southern Africa two million years ago. In 2020 a team of researchers led by Andy Herries of La Trobe University in Australia reported newly discovered fossils from the Drimolen Cave system, also in the Cradle of Humankind area. These fossils came from two other hominin species: the large-toothed Paranthropus robustus and the much more humanlike H. erectus. In other words, three different kinds of hominins from three different genera–Homo, Paranthropus and Australopithecus–were coexisting.

We know from a partial skeleton discovered in the 1980s along the western side of Lake Turkana in Kenya that H. erectus had a body form nearly identical to that of humans living today. Footprints found on the eastern side confirm that these hominins walked in the same way as us. H. erectus–the likely ancestor to the lineage that led to our own species, H. sapiens–would have peered across its territory and seen two other bipeds from two different genera, Australopithecus and Paranthropus. I believe these hominins had different styles of walking due to the differences in their foot and leg bones.

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, nicknamed the Hobbit, roamed its island home of Flores in Indonesia on relatively giant, flat feet and short legs with small joints. I wonder if this resulted in the person walking in snowshoes taking shorter steps and a higher knee drive.

Perhaps hominins used gait differences to determine whether a group of foragers in the distance belonged their own species or another. If gait revealed distant foragers to belong to the same species, how could observers tell if the other individuals were family members or strangers? Knowing the answer could have made the difference between inviting conflict or avoiding it. Gait is more than just a way to get from A to B.

Open Questions

Many questions remain about the evolution and development of bipedalism. We don’t know why upright walking was so advantageous for our earliest ancestors or extinct relatives. There are many hypotheses. In 1809 French naturalist Jean-Baptiste Lamarck speculated that humans evolved upright walking to see over tall grass. Charles Darwin, six decades later, posited that walking on two feet allowed the hands to use tools. Others have suggested that it allowed our ancestors the ability to gather food and to navigate through shallow water. Still others argue that it offered a more energetically efficient means of traveling between scattered resources. It seems to me, though, that efforts to identify the reason bipedalism evolved are a fool’s errand. Instead, I believe it’s possible, maybe even probable, that bipedalism evolved multiple time at the base the hominin tree, possibly for different reasons in different hominins in slightly different places throughout Africa. This scenario is supported by the diversity of foot forms found at Pliocene fossil locations across Africa.

The fossil record of apes from the Miocene epoch (23 million to 5.3 million years ago) highlights other unknowns. Paleoanthropologists in Africa have had difficulty finding fossils of apes from this crucial time when hominins diverged. Their counterparts in Europe have uncovered a remarkable collection of bones from apes who lived in Spain, France and Germany. Judging from their hands, arms, backs, hips and legs, these European apes didn’t knuckle-walk like a chimpanzee. Some of them may have been able move on two legs more often than modern African apes. Depending on where these ancient apes–such as the 11.6-million-year-old Danuvius guggenmosi from Germany, first announced in 2019–fit into the family tree, it is even possible that the ape from which the ancestors of humans, chimpanzees and gorillas split was not a knuckle-walker at all but more upright, using hand-assisted bipedalism to “walk” through the trees. In that case, the unique hominin adaptation would be not bipedal walking per se but rather bipedal walking on the ground. If fossils continue to support this hypothesis then rudimentary pedalism may not be a new form or locomotion. It could be an old one that was adapted for a new environment, as our ancestors moved from an arboreal existence to a terrestrial one.

This idea is controversial and needs further testing. The challenge is that paleoanthropologists have yet to unearth fossil foot or leg bones from Africa during the key time period when the lineages that would eventually lead to humans, chimpanzees and gorillas were beginning to diverge, between 12 million and seven million years ago. We rely on the anatomy and physiology of ancient apes from south Europe to fill in this gap. In a way, it is like trying to figure out what your great-grandmother looked like by studying tattered black-and-white photographs of your 19th-century cousins three times removed. They will give some clues, but not the whole picture. As more fossils from Africa and the Mediterranean are discovered, we’ll see if this hypothesis holds up over the next decades. The beginnings of upright walking are still a mystery.

Once ancestors started walking on two legs, they continued to walk and this journey continues today. In a lifetime, the average person will take about 150 million steps–enough to circle Earth three times. We walk, stride, tromp, plod and traipse. We might be asked to walk a mile with someone after we have walked over them. Heroes walk on water, geniuses walk encyclopedias. But rarely do we humans think about walking. It is now pedestrian. However, fossils reveal something entirely different. Walking is not a common activity. Instead it is a complex, convoluted evolutionary experiment that began with humbl

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