Fossil Pushes Upright Walking Back 2 Million Years, Study Says

Hillary Mayell
for National Geographic News
September 2, 2004
Computer analysis of a fossil thigh bone indicates that a chimp-size human-like creature walked on two legs as early as six million years ago. Walking on two legs, known as bipedalism, is considered by scientists to be a distinguishing characteristic in what sets humans apart from apes.

Until now, the most widely accepted date for the advent of bipedalism was about four million years ago. That's when the hominids known as Australopithecus anamensis, lived. Hominids include humans and extinct near humans.

"Dating the beginnings of bipedalism is very important in the human story because, for many experts, it would mark a clear divergence from the ancestral/ape pattern and show that the human lineage had really begun," said Chris Stringer, director of the Human Origins Program at the Natural History Museum in London.

Although this finding puts the development of bipedalism back by another two million years, it is not necessarily a surprise. The chimpanzee-human divergence has been estimated to have occurred between five and seven million years ago, based on genetic data.

"Now, for the first time, we have solid evidence dated to six million years ago of an intermediate creature between humans and the apes that demonstrated upright posture and bipedalism," said Robert Eckhardt, a developmental-genetics and evolutionary-morphology researcher at Pennsylvania State University. "And the dating of this fossil is unusually secure."

Morphology is a branch of biology that deals with the form and structure of animals and plants.

The fossil was found nearly four years ago in Kenya's Lukeino Formation. The thigh bone is about the same size as a chimpanzee's, but CT scans of the bone's interior show that its owner had adapted to walking on two legs.

"In present-day chimps and gorillas, the thicknesses in the upper and lower parts of that bone are approximately equal. In modern humans, the bone on top is thinner than on the bottom by a ratio of one to four or more," Eckhardt said.

The ratio in this fossil is one to three, suggesting a biomechanical transition to upright posture and bipedalism, the researchers say.

Eckhardt is one of a team of researchers reporting on the findings in the September 3 issue of the journal Science.

Walking on Two Legs

Scientists hoping to identify the last common ancestor shared by humans and apes think walking in an upright posture could be a key to connecting the dots of human evolution.

"Bipedalism probably does represent a fundamental first step in human evolution," Stringer said. "As Darwin recognized, walking on two legs frees up the arms and hands for tasks like carrying, tool making, and tool use. And much of what happened in human evolution later on stemmed from it."

Darwin postulated that tool use led to greater intelligence. Several theories exist to explain how and why bipedalism and upright posture emerged.

Toward the end of the Miocene epoch (23.8 to 5.3 million years ago), much of Africa's climate and landscape changed due to an extended drying period. The change in climate and landscape precipitated a change in food availability and distribution, necessitating a transition in how the animals lived.

Some scientists argue that the earliest hominids to walk with an upright posture were tree-dwelling primates forced to the ground as the African forests shrank and became more savanna-like. While many ape species probably became extinct during this period, the creatures who adapted to walking on two legs survived.

Others argue that the change in climate forced early forms of hominids to change to a diet based on insects, nuts, eggs, small reptiles, and fish found on the forest floor and at lake and stream edges, rather than foliage and hanging fruit.

The shrinking forests caused the tree-dwelling primates to leave the trees in the daytime in search of food. However, they returned to the trees at night, where they would be less exposed to predators on the ground. Walking on two legs would give them a distinct advantage in enabling them to carry the food they found back to the trees.

Owen Lovejoy, a biological anthropologist at Kent State University in Ohio, argues that bipedalism may have evolved to enhance the reproductive strategy of the species.

Since walking on two legs makes it possible to carry more food, bipedal families could practice a division of labor, according to Lovejoy's theory. One parent could go in search of food and return with a meal for the entire family. The other could remain "home," focusing more energy on child rearing.

This arrangement, Lovejoy suggests, would result in better survival rates for offspring—given that the four-legged alternative is to forage together as a family, eating as you go.

"Both males and females would benefit from intensified cooperative care of offspring under such conditions, as this would reduce the burden on females and enhance infant survival rates," Lovejoy said. "Over time, females began to choose males who regularly offered food."

CT Scans

"What's amazing is that we can use the same equipment that doctors would use in a hospital if you had a hip problem," Eckhardt said. This equipment has been used "to identify structural adaptations to new behavior patterns in an intermediate creature that no longer exists."

CT scan analysis was conducted using a new and more advanced software program. The software revealed that the density of the fossil thigh bone, (known as BAR 1002'00 Orrorin tugenensis) closely matches the structure of human femurs. External qualities of the fossil also indicate an upright posture.

However, the scans, which were taken several years ago, could be made sharper by improved techniques. So while a consensus seems to be gathering that the owner of this bone was bipedal, some questions remain.

"I'm not sure the scans, which have some problems with them, are very convincing," Lovejoy said. "I am convinced, based on morphology and anatomy, that [the owner of] Orrorin was bipedal at least frequently, if not habitually. The question is to what extent."

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