WASHINGTON — A 2 million-year-old flat-faced skull pulled from sandstones of East Africa has shored up claims that at least three species of early humans coexisted in an ‘‘evolutionary experiment’’ that saw an explosive brain-size increase paired with radically different faces, teeth, and jaws.
While the partial skull and two newly found jawbones look very different from modern humans, they match an enigmatic, nearly complete skull found 40 years ago that paleoanthropologists have struggled to fit into the human family tree.
Together, the new finds and the puzzling skull describe a species of early humans clearly distinct from two others known from fossils from the same period, said Meave Leakey, the paleoanthropologist who led the team that discovered the fossils.
The ‘‘base of the human lineage was indeed diverse,’’ Leakey, 70, said from her home at the Turkana Basin Institute in Kenya. Her colleagues made the finds near there.
Long thought to be the seat of human origins, East Africa was once ‘‘quite a crowded place with multiple species,’’ said Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, a co-author on a report describing the finds.
Leakey and her colleagues stop short of assigning the fossils a species name. But 20 years ago, others scientists classified the 40-year-old mystery skull as Homo rudolfensis.
An associate of Leakey’s noticed a jawbone sticking out of a block of sandstone in the arid region in 2007. After hauling the block to their laboratory, the team whittled away with dental drills and revealed a face, its right cheek and upper jaw intact. The small fossil probably came from an adolescent, Leakey’s team reports in this week’s journal Nature.
Nearby, the team also found two partial jawbones that match both the new skull and the mystery skull, Spoor said. All of the fossils date between 1.78 and 1.95 million years old.
At that time, East Africa was a roiling hotbed of human evolution. Other fossils show that the long-lived species thought to be our direct ancestor, Homo erectus, thrived in the region, which was undergoing rapid changes in plant cover, rainfall, and, in all likelihood, availability of various foods.
Meanwhile, another group of early human fossils from the region has been classified as Homo habilis, which means ‘‘handy man,’’ as these creatures were thought to create primitive stone blades.
Another more primitive hominid species, called Paranthropus bosei, lived in the area.