Before our ancestors evolved large brains and language, they started walking on two legs, which was a first for mammals. The fossils of the very oldest hominins are between seven and five million years old. The stage was set for evolution when we moved on two legs. The way we give birth and parent our children has changed because of it. Humans are unique due to their peculiar mode of locomotion.
A procession of ancestors starting with a chimplike creature ambling on all fours leads to a series of erect forebears, culminated in a fully upright Homo sapiens striding triumphantly on two legs. The March of Progress, as this image and its variant are known, has adorned countless books, T-shirts, bumper Stickers and coffee mugs.
Over the past two decades, paleoanthropological discoveries have forced scientists to rethink this traditional imagery. Various hominin species living in different environments throughout Africa have different ways of walking. Bipedalism was the start of a long phase of evolution. Our modern stride was not set in stone, with each descendant marching closer to their end goal. It is one of the forms of upright walking that early hominins tried out.
They didn't want to be hit by elephant poop. Who could do that? When the game of elephant dung dodgeball broke out, paleontologists Kay Behrensmeyer and Andrew Hill, who were visiting archaeologist Mary Leakey's fossil site of Laetoli in Tanzania, jumped into a gullies to get more fodder for the game. One of the most important discoveries in the history of paleoanthropology was made on July 24, 1976
The elephant footprints and raindrop impressions were found in an exposed layer of volcanic ash that was 3.66 million years old. The people came to see what had been found after a truce was called. Fossil footprints capture snapshots of moments in time for long- extinct animals.
Thousands of footprints, mostly made by small antelopes and hares, but also from ancient elephants, rhinoceroses, giraffes, large cats, birds and even a beetle, were discovered by Leakey and her team over the course of several weeks. The group was told to be on the lookout for bipedal footprints. Maybe they would be able to. They did in September. The footprints were discovered by Peter Jones and Philip Leakey. Ahominin? Maybe, but the footprints were oddly shaped and moved the left foot over the right like a model on a runway. There was a mystery about the site.
Two other members of Leakey's team discovered a second bipedal trackway two kilometers west of Site A at a location called Site G. Fossils ofLucy's species have been found at Laetoli. The tracks at Site A were very similar to those at Site G. What kind of creature made the bipedal trackway at site A?
Russ Tuttle, an anthropologist at the University of Chicago, tried to solve the mystery in the 1980's. After comparing the footprints made by humans, Chimpanzees, and circus bears with those made by a second species of hominin that lived in Laetoli during the Pliocene, it was concluded that the prints were made by a second species. The bear hypothesis was embraced by other researchers because of a linear view of the evolution of bipedalism. The footprints at Site A fell into obscurity because they were not studied as much as the ones at Site G. It took three decades for anyone to focus on them again.
There is a small liberal arts school in New Hampshire that is nestled between the White Mountains and the Green Mountains of Vermont. The motto of the school is vox clamantis in deserto, which means "a voice crying out in the wilderness". There is a lot of bears in the woods surrounding the campus and large swaths of sugar maples.
The footprints of black bears at Laetoli Site A were collected by Ellison McNutt and Ben Kilham, both professors at Ohio University. We were surprised that their footprints and mechanics were not up to par. The bear hypothesis made us have doubts.
The discovery of the Site A trackway was made more than four decades ago. The barren hills at Laetoli have been exposed to tens of thousands of fossils. Many of the fossils have been recovered by teams led by Charles Musiba of the University of Colorado Denver. One of the animal fossils recovered at Laetoli is not from a bear as we know from other sites. The bipedal tracks at Site A need to be looked at again by someone. The site A bipedal footprints were long gone. We were incorrect.
We traveled to Laetoli and used Mary Leakey's drawings to find the location of the bipedal footprints. We started digging. After a few days, a member of the Tanzanian team called to us, saying, "Mtu", which means "human." He was able to find the footprints. All five of them were covered and preserved by the rains. We used tongue depressors and thick-bristled brushes to clean 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 big toe is the dominant digit in humans and our ape cousins, and the heel impressions of the site are large. The bear was not this one. These tracks were created by a hominin. Which one is it?
There are a lot of footprints on the beach, and you can see them if you walk on the sand. Modern humans are all different in size and shape. It was almost certain that the same thing happened for A. afarensis. The footprints at Sites A and G could be from a single species ofhominin. The small size of the footprints indicates they were made by a child. I originally thought that was what it was.
Kevin Hatala, who helped to discover and analyze 1.55-million-year-old Homo erectus footprints at Ileret, joined our team and we compared the shape of the footprints with the best-preserved ones. The differences we observed were not in line with the variation in footprints from people of all ages.
The shape of the footprints on the site A was different from the ones on the other two sites. The Site A footprints were different in shape from those of Lucy's species and not just like a chimp. The Site A footprints were short and wide, the big toe stuck out to the side, and the walkers had a more flexible middle portion of the foot.
In a paper published last December in the journal Nature, we claimed that there was more than one species at Laetoli. Some of our colleagues did not fully embrace our interpretation. Some people think we've found another A. afarensis trail. Our field was convinced for decades that the Site A footprints were made by a bear, because they were so different from the ones at the other site.
After ash fell from the sky 3.66 million years ago, two types of hominins, walking on slightly different feet in slightly different ways, moved north toward the Olduvai Basin. It is thought that the footprint layer at Laetoli captures most of the activity, so it's the best evidence that different Pliocene hominin species shared the same environment. At this point, nobody knows how they interacted.
The rediscovery of the Laetoli Site A footprints and our 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 more complex and interesting than we thought. Fossils of the hominins themselves provide the other evidence. In the human fossil record, foot bones are rare. In the past two decades, the number of fossils from the only part of a biped's body has increased. Our ancestors were becoming upright walkers between five million and three million years ago. McNutt and I collaborated with a paleoanthropologist at the University of the Witwatersrand to understand the finds.
We wanted to evaluate the received wisdom about the evolution of bipedalism. The traditional view says that hominins started out with a chimplike foot. The fossil of Ardi, a member of Ardipithecus ramidus, shows a foot that was capable of grasping and walking. Lucy, who lived in Hadar, Ethiopia, some 3 million years ago, had a big foot and a stiff midfoot that were better adapted to living on the ground. The foot evolved shorter toes and a high arch as a result of the emergence of Homo.
We noticed a different pattern emerging from our data after studying all the foot fossils carefully. Different hominins had different foot forms as bipedalism evolved. In the two million year interval we studied, we identified five different foot morphs. There are three unique feet between the chronological bookends. The first belongs to an Ardi-type creature, about the same age as that fossil, from Gona, Ethiopia, the second is a 3.67-million-year-old hominin from South Africa, and the third is a strikingly similar creature. Although all five of the hominin feet have apelike and humanlike features, they do not follow the pattern of becoming less apelike and more humanlike over time.
Like an ancient version of the story of Cinderella, one of the recently discovered feet could reveal the identity of the track maker. As we explore these early stages of our evolution, we will see.
The pattern of locomotor diversity is not limited to the beginning of human evolution. Take the example of Australopithecis sediba. Matthew Berger was nine years old when he discovered this nearly two-million-year-old hominin. While surveying for fossils at the site of Malapa Cave in South Africa, he stumbled over a rock with a hominin clavicle and lower jaw. Berger and his team discovered two partial skeletons of a new species in the months that followed. Berger invited me to study the fossils after I finished my PhD.
I wasn't sure what I saw. The bones were not right. The midfoot, ankle, knee, hip and lower back were skeletons that were too ape like for a hominin. These bones were strange. They told the story of a hominin with a peculiar way of walking that was similar to the way humans walk today. The bones of A. sediba were thought to be the solution to the problems modern humans face when they walk this way. This species was able to walk in this way. What's the reason? The shoulders and arms of A. sediba show that it climbed trees, and its teeth show that it ate in trees. The compromise for a hominin well adapted for life in two worlds was to walk between the trees and the ground.
Two million years ago, many hominin walked around southern Africa. A team of researchers led by Andy Herries of La Trobe University in Australia discovered fossils from the Drimolen Cave system in 2020. The Paranthropus robustus and H. erectus are two other hominin species. Three different types of hominins from three different genera were interacting.
In the 1980's, a partial skeleton was found along the western side of Lake Turkana, and it is believed that it is that of H. erectus, a human. There are footprints on the east side of the lake. H. erectus would have looked across its territory to see two other bipeds from two different genera. I think the hominins had different styles of walking due to their different foot and leg bones.
Even after the extinction of Paranthropus, the pattern of walking styles remained the same. As recently as 60,000 years ago, the small human species Homo floresiensis was well established, as it was found on the island of Flores in Indonesia. I don't know if the short steps and high knee drive of a person in snowshoes would be included.
hominins may have been able to determine if a group of people in the distance belonged to their own species. If the foragers were from the same species, could the watchers tell if they were friends or not? If you knew the answer, you could have avoided conflict and invited it. It turns out that gat is more than just a way to get from point A to point B.
There are many questions about bipedalism. We don't know why upright walking was good for our ancestors. There are many hypotheses. Jean-Baptiste Lamarck was the first to theorize that humans evolved to walk upright. Charles Darwin thought that walking on two legs freed the hands. According to other scholars, it allowed our ancestors to gather and carry food. Others argue that it was a more efficient way to travel between resources. Efforts to identify the reason bipedalism evolved aren't a good idea. Bipedalism may have evolved multiple times at the base of the hominin family tree, possibly for different reasons, in different hominins living in slightly different environments. The diversity of foot forms found in Pliocene fossil sites support such a scenario.
Other unknowns are highlighted in the fossil record of apes from the early 20th century. The time period when hominins differed from other apes has been difficult to find fossils from. The bones from apes that used to live in Spain, France, Germany, Greece, Italy, Hungary and Turkey have been found in southern Europe. The European apes didn't walk like a chimp. Some of them may have been able to move on two legs more often. It is possible that the ape from which the ancestors of humans, Chimpanzees andGorillas split could be found in Germany. Bipedal walking on the ground is what the hominin adaptation would be like. If more fossils continue to support this hypothesis, rudimentary bipedalism might turn out not to be a new form of locomotion, but an old one co-opted for a new environment as our ancestors shifted from an arboreal to a terrestrial existence.
The idea is controversial and needs further testing. Between 12 million and seven million years ago, when the ancestors of today's humans, Chimpanzees, and gorillas were beginning to diverge from one another, paleoanthropologists have yet to find foot or leg bones from Africa. The apes from southern Europe are used to fill that gap. It's like trying to figure out what your great-grandmother looked like by looking at photos of your 19th century cousins. Some clues will be provided but not the whole picture. As more fossils are recovered from sites around the Mediterranean and in Africa, we'll see how this hypothesis holds up. The beginning of upright walking remains shrouded in mystery.
Our ancestors kept walking even after they got moving on two legs. The average person will take about 150 million steps over the course of their life. We could be asked to walk a mile in someone's shoes. Geniuses are walking encyclopedias. Most of the time, we don't think about walking. You could say that it has become a pedestrian. Something else is revealed by the fossils. There is nothing ordinary about walking. It is a complex and convoluted evolutionary experiment that began with humble apes taking their first steps in forests and eventually led to hominins on a path around the world.
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