In a discovery that several colleagues describe as "spectacular" and "extraordinary," an international team of researchers has uncovered fossils in Ethiopia that fill a crucial gap in the record of human evolution.
Judged by their physical characteristics, the 160,000-year-old-fossils - nearly complete skulls of two adults and a child found near the village of Herto - teeter on the razor-thin edge of change between anatomically early and modern humans. The team also found skull pieces and teeth from seven other individuals.
The discoveries dovetail with an expanding body of genetic evidence indicating that modern humans first evolved in Africa about 150,000 years ago.
Over the years, paleoanthropologists have gathered fossils that open windows on key periods in the history of human evolution in Africa dating back millions of years, notes Tim White, co-leader of the team that made the discovery. But "the record has been mute" on what was happening 150,000 years ago, he says.
"We're opening the first window on the continent in this time period," he adds. And the view from the window is "very consistent with the predictions made by genetic evidence" that modern humans originated in Africa.
Combined with existing fossil and genetic evidence, researchers say, the new find seriously undermines hypotheses that modern humans evolved roughly simultaneously in different regions of the world and that in Europe, Neanderthals gave rise to anatomically modern Europeans.
In one sense, the finds at Herto come as little surprise to some paleoanthropologists who work in Africa.
Given the pattern of development in fossils that date to either side of the Herto group, these are the kind of hominids one could expect from that time and that region of Africa, according to Curtis Marean, professor of anthropology at Arizona State University's Institute for Human Origins.
What makes them special, he says, is their "outstanding state of preservation" and their tightly defined dates. The technique that Dr. White's team used allowed it to date the fossils between 160,000 and 154,000 years ago. The age of other potential near-modern human fossils, such as those found at Jebel Irhoud in Morocco, is more uncertain.
The White team, which details its results in Thursday's issue of the journal Nature, first came across the site where the fossils were found on Nov. 16, 1997. It's a landscape that today looks as much like a Martian plain as anything on Earth.
Pebble-strewn, virtually treeless, scorchingly hot, the terrain along the rift valley containing the site stretches in subtle undulations toward distant mountains. But 160,000 years ago, when much of the Northern Hemisphere sagged under a sheath of ice two miles thick, this patch of Africa was the site of a shallow freshwater lake teeming with catfish, crocs, and hippos, researchers say.
According to White, he noticed a butchered hippo skull and related artifacts in an eroding slope near Herto. Eleven days later, he and colleagues returned to survey the site.
As White worked on a shaded enclosure for a lunch break, he says, "I sent two people off to begin the survey near where we had found the hippo." When they came back about a half hour later, both had found hominids, he says.
And the painstakingly slow process of collection and excavation began.
Of the specimens included in Thursday's report, the child's cranium presented perhaps the biggest reconstruction challenge. More than 200 pieces were scattered over some 4,300 square feet of the valley floor, presenting Ethiopian scientist and team member Berhane Asfaw with a 3-D jigsaw puzzle to assemble.
In addition to the hominids, stone tools also emerged from the team's excavations that signal a shift in toolmaking technology to the flake-based features of the Middle Stone Age. Moreover, one of the adult skull fragments shows evidence of parallel surface cuts that could indicate some form of primitive burial rites, the team reports.