Probable appearance of Ardipithecus ramidus, or Ardi. Illustration credit: J H Matternes/ Science/ AAAS

New Delhi, Oct. 1: Scientists today announced the discovery of the remains of a 4.4 million year old creature, the earliest known member of the human family tree that challenges some key ideas about the evolution of humans and apes.

An international research team digging in Ethiopia since 1992 has described a female skeleton and other fossil fragments of a species that has pushed science closer than ever to the elusive last common ancestor of humans and chimpanzees.

The scientists say the creature which stood 4 feet tall and weighed 50kg was a mosaic species — neither human-like, nor chimpanzee-like, yet with a few anatomical traits of both. Their findings will appear in the US journal Science tomorrow.

The species named Ardipithecus ramidus, or Ardi, is more than a million years older than Australopithecus afarensis, or Lucy , the earliest known bipedal ancestor whose remains were also discovered in Ethiopia 35 years ago. Researchers say the new fossils may throw fresh light on walking behaviour, environment, diet, and even social behaviour of the earliest known member of the human family line.

“It’s not a chimpanzee, and it’s not a human either. It’s a small head atop this body, but the body is erect, and it (walked) on two feet,” said Tim White, a paleoanthropologist at University of California, Berkeley who led the 47-member team.

The team from Ethiopia, Europe, North America and Japan says the creature had capacity for terrestrial bipedality and climbing. It had feet that could grasp, long arms, and long fingers that allowed it to comfortably negotiate an arboreal habitat.

The discovery of Ardipithecus began in 1992 when White and his colleagues found a single molar tooth embedded in ancient sediments in Afar Triangle, a desert region of Ethiopia abundant in fossils. Over the next two years, they found 110 more fragments, including a partial skeleton with skull, hands, feet, limbs and pelvis, which they have analysed over the past 15 years.

Scientists believe humans and chimpanzees shared their last common ancestor more than six million years ago. “What we have found is the closest we’ve ever come to that ancestor along our own line,” White said.

“This is a spectacular, unprecedented find — so many bones and so many individuals,” said Carol Ward, professor of anatomical sciences at University of Missouri in the US, who was not associated with the discovery but has been interpreting the earliest fossils of bipedal creatures for many years. “Ardi is a convincing relative of humans,” Ward told The Telegraph.

The finding challenges a long-held assumption that bipedality evolved in a savannah terrain with few trees. “The new skeleton and other evidence from the area shows this was [about 4.4 million years ago] a woodland area with lots of trees,” said Andrew Hill, professor of anthropology at Yale University who was also not a team member but has been studying human evolution for three decades.

Its skeletal features also question another belief — that chimpanzees, gorillas, and orangutans have retained most traits of the last ancestor they shared with humans, and this ancestor would look somewhat similar to modern-day chimpanzees.

But Ardipithecus wasn't a knuckle-walker like chimpanzees or gorillas, nor did it swing and hang from tree branches. “The skeleton suggests that chimpanzees and gorillas too have been evolving independently since the last ancestor,” Hill told The Telegraph.

An analysis of its teeth and fossil and pollen remains from the area suggest that Ardipithecus was an omnivore that would gather small animals in the woodland or climb trees for fruits, the researchers have said.

US team member Owen Lovejoy from the Kent State University has attempted to link anatomy with social behaviour, suggesting that parenting may have emerged in the human family during the reign of Ardipithecus.

Lovejoy has interpreted that the absence of large projecting canines in males and relative similarities in male and female body sizes could mean that males no longer fought for females, but were part of the parenting process.

But Ward cautions that this interpretation would need more testing.

“In general, differences in body size are associated with males fighting, competing for females, but we have examples of a modern-day spider monkey where we see no size differences, but both males and females are promiscuous,” she said.