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World's oldest human fossils identified
The oldest human fossils are 195,000 years old, about 40,000 years older than the existing oldest known remains of modern humans, according to an ANU earth scientist who has done the dating.
The fossilised skulls, recovered from sediments of the Kibish Formation, adjacent to the Omo River in southern Ethiopia — found in 1967 but not precisely dated until now — currently are the oldest known remains of modern humans.
ANU geochronologist, Professor Ian McDougall, and a team from the University of Utah and Stony Brook University in the United States published their findings in the latest edition of Nature.
“These are the oldest well-dated fossils of modern humans known from anywhere in the world. They are about 40,000 years older than fossils from Herto, another palaeoanthropological site also in Ethiopia, which was previously presumed to have been home to the earliest known human fossils,” Professor McDougall said.
“The age of 195,000 years for modern humans is very close to that estimated from genetic studies, related to the origin of Homo sapiens. The Kibish fossils also indicate that already at this time there was considerable diversity among the hominids living in Ethiopia.”
In 1967, a team of palaeontologists under Dr Richard Leakey discovered ancient human remains, which they called Omo I — a skull and part of a skeleton very similar to modern humans, and Omo II — a nearly complete, but more primitive human skull.
At the time, the sediments containing the fossils were dated at 130,000 years old, although the validity of the dating was questioned at the time — it was then thought unlikely modern humans were over 100,000-years-old.
“The fossils became important evidence for the ‘Out of Africa’ hypothesis; however, the evidence was questioned because of the morphological differences between the two skulls and doubts about the dating techniques used in 1967,” Professor McDougall said.
Beginning in 1999, the research team made a number of trips to the Kibish Formation, recovering hominid bones, remains of other animals, stone tools and information on the geology of the region.
“The team recovered bones that fit together with those found in 1967 as well as other previously unknown parts of the skeleton of these early hominids,” Professor McDougall said. “We can confirm the original reports that the Omo I and Omo II fossils are from the same geological levels and are essentially the same age.”
Professor McDougall was able to precisely measure the isotopic ages on feldspar crystals in pumice produced in explosive volcanic eruptions. The beds of volcanic material are called ‘tuff’ layers, deposited soon after the eruptions. The human fossils were found just above the first tuff layer, which Professor McDougall dated at 196,000 years old.
The research team also made surprising geological findings about the environment of the region in which these early Homo sapiens lived, which have helped to confirm the age of the Omo fossils.
For instance, the Omo River now enters Lake Turkana 100 kilometres south of the outcrop of the Kibish Formation. But the geology of the formation indicates that the layers of sediment were laid down quickly in a deltaic environment.
According to Professor McDougall, at the time the Kibish Formation was formed, the level of Lake Turkana was between 35 and 85 metres higher than now.
“These high lake levels indicate much greater precipitation and runoff from the highlands of Ethiopia — the source of the waters in the Omo River — which caused the rapid deposition of the sediments of the Kibish Formation,” he said.
The researchers also were able to correlate the deposition of members [each of which is a continuous set of sediments separated from the succeeding set by an erosional interval] of the Kibish Formation with distinctive sediments known as sapropels in the Mediterranean Sea. Sapropels are unusually dark layers of sediment, thought to be caused by a lack of oxygen.
“In many cases the low oxygen levels which caused the sapropels is thought to have been the result of major influxes of fresh water to the Mediterranean from the Nile River system during intervals of extremely high monsoonal rainfall in the Ethiopian highlands,” Professor McDougall said.
“The Omo and Nile river systems share drainage divides; thus, the high rainfall is reflected in the high levels of Lake Turkana, and in the location of the Kibish Formation. The ages of three members of the Kibish Formation closely match those of three sapropel layers in the Mediterranean, which have been dated by other means, effectively linking the systems.”
The study was funded by the National Science Foundation, the L.S.B. Leakey Foundation, the National Geographic Society and The Australian National University.
Francis H. Brown, Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah
John. G. Fleagle, Department of Anatomical Science, Stony Brook University, Stony Brook, New York
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