4.4 Million-Year-Old Skeleton Can Answer How Humans Evolved To Walk Upright

Based on their anatomies, humans are considered to have a close evolutionary relationship with chimps and gorillas since long. There are, however, some unanswered questions in this regard, like how did early humans get to move on two limbs from four. A new research sheds some lights on these unsolved mysteries.

Published in the current issue of Science Advances, the new study has been conducted by a Texas A&M University professor Thomas Cody Prang and his team. The scientists examined the skeletal remains of Ardipithecus ramidus ("Ardi"), which were found in Ethiopia and date back to 4.4 million years.

One of the Ardi's hands was exceptionally well-preserved over the millions of years. The researchers compared the shape of this hand to hundreds of other specimens of recent humans, apes and monkeys. The scientists looked at bones in museum collections around the world to make the comparisons, as mentioned in a university report.

The idea was to find the locomotor behavior used by the earliest hominins (fossil human relatives). From the results, the team of scientists generates clues about how early humans began to walk upright and make movements that all humans perform today.

4.4 million years old skeletal remains of Ardi (Image: Wikimedia Commons)

"Bone shape reflects adaptation to particular habits or lifestyles¡ªfor example the movement of primates¡ªand by drawing connections between bone shape and behavior among living forms, we can make inferences about the behavior of extinct species, such as Ardi, that we can't directly observe,¡± Prang said.

"Additionally, we found evidence for a big evolutionary 'jump' between the kind of hand represented by Ardi and all later hominin hands, including that of Lucy's species (a famous 3.2 million-year-old well-preserved skeleton found in the same area in the 1970s).¡±

Prang explains that this 'evolutionary jump' took place at a critical time when hominins are ¡°evolving adaptations to a more human-like form of upright walking.¡± The earliest evidence are observed at the time too, like hominin stone-tool manufacture and their use found through marks on animal fossils.

The findings also shed light on the kind of ancestor from which humans and chimpanzees evolved. "Our study supports a classic idea first proposed by Charles Darwin in 1871, when he had no fossils or understanding of genetics, that the use of the hands and upper limbs for manipulation appeared in early human relatives in connection with upright walking," he said.

Being an ancient species, the skeletal remains of Ardi might be retained and in line with those of the last common ancestor of humans and chimpanzees. If true, researchers could also find clues to the origin of the human lineage in addition to the reasons for their upright walking.

"It potentially brings us one step closer to an explanation for how and why humans evolved our form of upright walking," Prang said.