Original text: Nature Science Update at http://www.nature.com/nsu/010712/010712-14.html

New early human fossils

Fossils fuel controversy about humanity's earliest days
12 July 2001

HENRY GEE

New-found fossils have fueled controversy surrounding the earliest days of our hominid ancestors, and those of chimpanzees, our closest living relatives in the animal kingdom.

Chimpanzees' and humans' most recent common ancestor is thought to have lived in Africa around 6 million years ago. This estimate is based on differences between the DNA and other biomolecules of modern apes and humans. Fossil evidence is scant, and controversial.

Until February of this year, the earliest known hominid was Ardipithecus ramidus, which lived in Ethiopia around 4.4 million years ago¹, some time after the divergence.

Then researchers working in Kenya claimed to have found a 6-million-year-old hominid; they called it Orrorin tugenensis²,³. Provocatively, they suggested that Ardipithecus ramidus was not a member of the human lineage at all, but was more closely related to chimpanzees.

The new finds, announced today by Yohannes Haile-Selassie of the University of California, Berkeley⁴, support the hominid status of A. ramidus and cast doubt on that of Orrorin. The fossils, from the Middle Awash area of what is now Ethiopia, represent an early form of A. ramidus, the team believes.

The resolution of this debate, if any comes, may have surprising consequences. At least one of these very ancient creatures could, in fact, be more closely related to chimps than humans. This would be an exciting result: fossils of hominids are notoriously rare, but fossils of chimpanzees are non-existent.

Such a fossil could even represent a creature from a time before the human and chimp lineages split - and so could be equally related to both.

Fragmentary findings

Understanding the precise anatomical and evolutionary relationships at the crucial interval during which the chimp and hominid lineages diverged could tell us much about how these creatures lived.

Did they walk upright, for example? Or did they walk on their knuckles, like chimps do? The recent suggestion that humans had a knuckle-walking ancestor⁵ has been particularly controversial.

Unfortunately, all of the fossils are extremely fragmentary - a few teeth and scraps of bone. What's more, telling the difference between an ancestral chimp and an ancestral hominid immediately after the split will be extremely hard, as they will inevitably look very similar. So it will remain difficult to establish beyond dispute that a hominid-like creature from this period is more closely related to chimps or humans.

Savannah settlers or fans of the forest?

Fortunately, researchers can say more about - and agree more upon - the environments in which these creatures lived. Fossil humans have traditionally been regarded as residents of open country, such as the modern African savannah. But many suspected that the primary heritage of humanity was the kind of forest environment in which chimps live today.

When Ardipithecus fossils were first discovered in the early 1990s, they were associated with fossils indicative of a woodland environment, such as colobus monkeys, forest-dwelling antelopes, and even discarded nut shells⁶.

The same seems to be true for the older Ardipithecus announced today. Everything points to a woodland habitat for hominids living 5-6 million years ago, say Giday WoldeGabriel of the Los Alamos National Laboratory, New Mexico, and colleagues⁷. Orrorin seems to have shared this taste³.