Correction Appended
Sometimes the maturity of a field of science can be measured by the heft of its ambition in the face of the next daunting unknown, the mystery yet to be cracked.
Neurobiology probes the circuitry of the brain for the secrets of behaviors and thoughts that make humans human. High-energy physics seeks and may be on the verge of finding the so-called God particle, the Higgs boson thought to endow elementary particles with their mass. Cosmology is confounded by dark matter and dark energy, the pervasive but unidentified stuff that shapes the universe and accelerates its expansion.
In the study of human origins, paleoanthropology stares in frustration back to a dark age from three million to less than two million years ago. The missing mass in this case is the unfound fossils to document just when and under what circumstances our own genus Homo emerged.
The origin of Homo is one of the most intriguing and intractable mysteries in human evolution. New findings only remind scientists that answers to so many of their questions about early Homo probably lie buried in the million-year dark age.
It is known that primitive hominids — human ancestors and their close kin — walked upright across the plains of Africa at this time. They were presumably larger members of the genus Australopithecus, the best known of which was the Lucy species, Australopithecus afarensis, that had thrived up to three million years ago.
At about 2.6 million years ago, some clever hominids were knapping stone tools. Then, or some time later, scientists suspect, the first Homo appeared, but there is no confirmed evidence of this step.
Subsequent finds, from a time beginning after 1.9 million years ago, revealed an early Homo identified as Homo habilis, the “handy man,” a species with a somewhat larger brain and a more humanlike face, teeth and stature than the apelike australopithecines.
Habilis was generally accorded an important place as the first of the species, preceding the more advanced Homo erectus and, ultimately, modern humans — Homo sapiens. But certainty has been elusive. A report last month in the journal Nature renewed debate over the habilis’s place in human evolution.
William H. Kimbel, a paleoanthropologist at the Institute of Human Origins at Arizona State University, said that the million-year period “has long been the source of frustrating gaps” in the hominid fossil record. “It’s not that sites containing rocks this age are particularly rare, or that the time period in eastern Africa has not been searched by several groups,” Dr. Kimbel said. “The problem is that the fossil yield has thus far been low or poorly preserved, compared to the time periods on either side of this interval.”
A succession of recent discoveries has extended evidence of hominids reaching back from three million to beyond six million years ago, close to the estimated time of the divergence of the human and chimpanzee lineages. The hominid trail from two million years forward has been fairly well worked, by fossil hunters as well as geneticists and archaeologists tracking migrations out of Africa and across Eurasia. Researchers have determined that anatomically modern Homo sapiens emerged in Africa less than 200,000 years ago.
G. Philip Rightmire, a specialist in habilis and erectus research at Harvard, said searches into the mystery period had yielded mostly the remains of various species of Australopithecus, the genus that came to a dead end around one million years ago.
Bones were found in 2.5-million-year-old sediments associated with some of the earliest known stone tools, used to butcher animals. A coincidence, or evidence of the first toolmaking species? Hard to tell.
A skull and other fossils, uncovered by a team led by the Ethiopian anthropologist Berhane Asfaw, were named the new species Australopithecus garhi. The researchers said the specimen had the projecting apelike face, small braincase and limb bones suggesting descent from the much earlier Lucy species. But if this was a candidate ancestor of early Homo, “a lot of evolution had to take place rather quickly” to complete the transition, a scientist said at the time.
With one possible exception, no fossils that are conclusively Homo have appeared in that period, Dr. Rightmire said. “That suggests there was not much Homo around then,” he said.
Correction: September 21, 2007
A picture caption in Science Times on Tuesday with an article about the lack of a fossil record to document the emergence of the genus Homo described Homo habilis incorrectly. Habilis is the species — not the genus — thought to be first in human lineage.
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