A team of anthropologists that studied chimpanzees trained to use treadmills has gathered new evidence suggesting that our earliest apelike ancestors started walking on two legs because it required less energy than getting around on all fours.
"When our earliest ancestors started walking on two legs, they took the first steps toward becoming human," said lead researcher Michael Sockol of UC Davis. "Our findings help answer why."
"This is the first time anyone has succeeded in studying energetics and biomechanics in adult chimps," said Sockol, who worked for two years to find an animal trainer willing to coax adult chimps to walk on two legs and to "knucklewalk" on all fours on the sort of treadmill found in most gyms. The five chimps also wore face masks used to help the researchers measure oxygen consumption.
While the chimps worked out, the scientists collected metabolic, kinematic and kinetic data that allowed them to calculate which method of locomotion used less energy and why. The team gathered the same information for four adult humans walking on a treadmill.
The researchers found that human walking used about 75 percent less energy and burned 75 percent fewer calories than quadrupedal and bipedal walking in chimpanzees. They also found that for some but not all of the chimps, walking on two legs was no more costly than knucklewalking.
"We were prepared to find that all of the chimps used more energy walking on two legs -- but that finding wouldn't have been as interesting," Sockol said. "What we found was much more telling. For three chimps, bipedalism was more expensive, but for the other two chimps, this wasn't the case. One expended about the same energy walking on two legs as on four. The other used less energy walking upright."
These two chimps had different gaits and anatomy than their knucklewalking peers. And when the researchers examined the early hominid fossil record, they found evidence of these traits – skeletal characteristics of the hip and hind limb that allow for greater extension of the hind limb -- in some early bipeds.
Taken together, the findings provide support for the hypothesis that anatomical differences affecting gait existed among our earliest apelike ancestors, and that these differences provided the genetic variation natural selection could act on when changes in the environment gave bipeds an advantage over quadrupeds.
Fossil and molecular evidence suggests the earliest ancestors of the human family lived in forested areas in equatorial Africa in the late Miocene era some 8 to 10 million years ago, when changes in climate may have increased the distance between food patches. That would have forced early hominids to travel longer distances on the ground and favored those who could cover more ground using less energy.
"This isn't the complete answer," Sockol said. "But it's a good piece of a puzzle humans have always wondered about: How and why did we become human? And why do we alone walk on two legs?"
Sockol, a doctoral candidate in anthropology, has been pursuing his research for four years as part of his dissertation. He conducted the research at UC Davis and at a private animal refuge and training facility in Northern California with colleagues Herman Pontzer of Washington University in St. Louis and David Raichlen of the University of Arizona.
His work was featured in the July 2006 issue of National Geographic. A video of the chimpanzees walking and knucklewalking on the treadmill can be seen at http://www7.nationalgeographic.com/ngm/0607/feature5/multimedia.html
The research appears in the online early edition of the Proceedings of the National Academy of Sciences and in the July 24 print edition.
FAQs
But almost all the theories suggest that bipedalism is an adaptation to getting around on land. It's clear that early bipeds evolved when savanna grasslands became increasingly common as forests retreated 4-8 million years ago. Walking on two legs made it easier to forage and travel on the ground.
Why don't humans walk on four legs? ›
Anthropologists and evolutionary biologists are now agreed that upright posture and two-legged walking - bipedality -- was the crucial and probably first major adaptation associated with the divergence of the human lineage from a common ancestor with the African apes.
Why do we walk on two legs and we dont fall? ›
Our ancestors went through many body modifications to shift from four legs to two: The pelvis shortened, the thighs became longer, the angle of the thigh bone changed to point inwards allowing the knees to come together under our center of gravity!
Why is walking on 2 legs better than 4? ›
Walking on two limbs was also more energy efficient than walking on four – giving early hominids more energy to reproduce and therefore more chance of producing offspring bearing this unique trait. But even with these advantages, these transitional hominids probably spent time in the trees as well.
Why are humans the only mammals that walk on two legs? ›
Savannah-based theory. According to the Savanna-based theory, hominines came down from the tree's branches and adapted to life on the savanna by walking erect on two feet. The theory suggests that early hominids were forced to adapt to bipedal locomotion on the open savanna after they left the trees.
Do humans naturally walk on two legs? ›
Early humans evolved skeletons that supported their bodies in an upright position. Modern humans have bodies adapted for walking and running long distances on two legs.
When did humans start to walk on two legs? ›
Ardipithecus ramidus lived about 4.4 million years ago in Africa. It is the first hominin we are sure walked on two legs. Ardipithecus was what we call a facultative biped, which means it was capable of walking on two legs but only did it sometimes. We know this from Ardipithecus' hip bones.
How did humans begin to stand erect? ›
Our prehuman ancestors likely started and adapted to bipedal locomotion while living in trees; this process is referred to as the generalized arboreal activity model. When humans began terrestrial locomotion, they likely performed proficient bipedalism from the first step.
Why did humans evolve to stand up? ›
He argues that hominids became upright and then were able to expand their territory into the grasslands, where our bipedalism helped us thrive by protecting us from overheating, allowing us to collect and carry food, look over tall grass, and more. Us humans have a unique anatomy that sets us apart from other apes.
Can a person walk with one leg? ›
Generally speaking, the median distance that someone with a lower limb amputation could walk was about 67 meters which equates to about 219 feet. Some patients were only able to walk 22 meters (72 feet) while others could walk as far as 93 meters (305 feet).
Five of the 19 children in the Ulas family have walked on all fours since infancy. But the affected siblings have intellectual disabilities and imbalance issues, suggesting their way of moving was more of an opportunity for them to more easily navigate their world.
Did humans originally walk on all fours? ›
We say that humans 'evolved' from 4 legs to 2 legs because the available evidence suggests our genetic ancestors walked on four legs, and at some point developed bipedalism, which was selected for among our ancestors until it became our default method of locomotion (at least as adults).
Are humans the only animals that walk upright? ›
Humans, birds and (occasionally) apes walk bipedally. Humans, birds, many lizards and (at their highest speeds) co*ckroaches run bipedally. Kangaroos, some rodents and many birds hop bipedally, and jerboas and crows use a skipping gait.
Why don't people walk on all fours? ›
The answer, according to a new study published today in Proceedings of the National Academy of Sciences USA: to save a few calories.
Can humans walk on four legs? ›
Five of the 19 children in the Ulas family have walked on all fours since infancy. But the affected siblings have intellectual disabilities and imbalance issues, suggesting their way of moving was more of an opportunity for them to more easily navigate their world.
Did humans ever walk on four legs? ›
We say that humans 'evolved' from 4 legs to 2 legs because the available evidence suggests our genetic ancestors walked on four legs, and at some point developed bipedalism, which was selected for among our ancestors until it became our default method of locomotion (at least as adults).
