Ardipithecus ramidus a common ancestor between humans and great apes?

Seventeen years ago, in 1992, in the Middle Awash River Valley in Ethiopia, the first fossils of a new hominid species – a 4.4-million-year-old Ardipithecus ramidus – were found. Nearly two decades later, after hundreds of hours of research, and now with more than 100 specimens represented in the fossil record, the researchers studying the species claim it’s the closest yet discovered to a “common ancestor” between hominids, to which group humans belong, and the great apes, such as chimpanzees and orangutans.

This exciting claim and many other details about Ardipithecus ramidus, are revealed in a special series of 11 articles published in the October 2 2009 edition of the prestigious journal Science. The articles are written by a diverse international team.

The authors argue that Ardipithecus ramidus is older and more primitive – yet is still clearly a hominid rather than an ape – than the genus Australopithecus, which the Cradle of Humankind in South Africa is renowned for, along with sites in East Africa.“Mrs Ples”and “Little Foot” – both famous fossils from Sterkfontein Caves in the Cradle of Humankind, belong to the genus Australopithecus. They estimate that Ardipithecus ramidus lived from approximately 6-million years ago to 4-million years ago, while Australopithecus lived from about 4-million years ago to about 1-million years ago. The genus Homo lived from approximately 2.5-million years ago (thus coinciding for some time with Australopithecus), to the present, with humans, Homo sapiens, the sole surviving hominid species.

“This species [Ardipithecus ramidus], substantially more primitive than Australopithecus, resolves many uncertainties about early human evolution, including the nature of the last common ancestor that we shared with the line leading to living chimpanzees and bonobos,” says one of the Science papers, titled Ardipithecus ramidus and the Paleobiology of Early Hominids.

“Our last common ancestor was neither human nor a chimpanzee, it was something entirely different,” says a lead scientist in the project, Professor Tim White from the University of California, Berkeley, in an accompanying Science video interview titled, The Analysis of Ardipithecus ramidus—One of the Earliest Known Hominids.

A partial skeleton of Ardipithecus ramidus, nicknamed “Ardi”, provides much of the basis for these claims. Analysis of her skeleton has revealed that the 1.2m (4ft)-tall hominid, weighing about 50kg (110lb), walked upright, and possessed characteristics linked with Australopithecus, the hominid genus that followed her.

“The partial skeleton ARA-VP-6/500 is identified as female based on probability assessments of canine size (its canines are among the smallest of those of 21 available individuals). This interpretation is corroborated by its small endo- and exocranial size, as well as its superoinferiorly thin supraorbital torus,” says White in the Science paper.

Skeletons are vital to the study of palaeoanthropology as they give scientists insight into things like the stature and limb proportions that the hominid might have had. Ardi’s feet, for example, show that she had an opposable large toe, which stabilised her walking pattern. “This is the first time we’ve ever seen this in a fossil hominid,” says White.

Ardi’s hands also support the theory that she walked upright according to Science correspondent Ann Gibbons in the Ardipithecus ramidus video. “If you’re knuckle-walking, you need to have very stiff wrists and fingers that can handle all that weight,” says Gibbons, adding that Ardi’s remains show a much more flexible wrist and hands, meaning she did not use them in walking.

The discovery of Ardi also challenges the traditional viewpoint that hominids arose out of the grassy savannah. About 6000 specimens of animals, from elephants to small rodents, have been discovered at the site in Ethiopia. There is also evidence of fossil wood and seeds, millipedes, birds and many other small mammals. “All these very sensitive environmental indicators build up to a picture of a woodland habitat, very different from what it is today,” says White.

According to White, the biggest lesson that we can learn from Ardipithecus ramidus is that we can’t take what we know now to be a modern day chimp or gorilla and use it as a representative for the last common ancestor to modern humans. “What we’re seeing [in Ardi] is something we couldn’t have predicted from a modern human or chimpanzee,” he says.

To read more about the discovery, please click here.