In two papers published in the journal Science, an international team of anthropologists reported the discovery of a partial hominin jaw with teeth from the Ledi-Geraru area, the Afar region of Ethiopia, that establishes the presence of our human genus at 2.8 – 2.75 million years ago.
LD 350-1, a partial mandible of early Homo. Image credit: Arizona State University.
Labeled LD 350-1, the fossil – the left side of a lower jaw with five teeth – predates the previously known fossils of the Homo lineage by 400,000 years.
For years, anthropologists have been searching for fossils documenting the earliest members of the genus Homo.
They have found fossils that are 3 million years old and older. The most famous example of those human ancestors is the skeleton of Lucy, found in northeastern Africa in 1974.
Lucy and her relatives, known as Australopithecus afarensis, were smaller-brained and more apelike than later members of the human family tree.
Scientists have also found fossils that are 2.3 million years old and younger. These ancestors are in the genus Homo and are closer to modern day humans.
But very little had been found in between – that 700,000-year gap had turned up few fossils with which to determine the evolution from Lucy to the genus Homo.
Because of that gap, there has been little agreement on the time of origin of the Homo lineage.
With the 2.8-million-year-old LD 350-1, that mysterious time period has gotten a little clearer.
“The record of hominin evolution between 3 and 2.5 million years ago is poorly documented in surface outcrops, particularly in Afar, Ethiopia,” said team member Dr Erin DiMaggio of Pennsylvania State University, the lead author of one of the two papers.
The analysis of LD 350-1 revealed advanced features such as slim molars, symmetrical premolars and an evenly proportioned jaw, which distinguish early species on the Homo lineage, such as Homo habilis at 2 million years ago, from the more apelike Lucy (Australopithecus afarensis). But the primitive, sloping chin links the jaw to a Lucy-like ancestor.
“It’s an excellent case of a transitional fossil in a critical time period in human evolution,” said team member Dr William Kimbel of Arizona State University.
The scientists determined the age of LD 350-1 by dating various layers of volcanic ash or tuff using Ar-40/Ar39 dating.
“We are confident in the age of LD 350-1. We used multiple dating methods including radiometric analysis of volcanic ash layers, and all show that the hominin fossil is 2.8 to 2.75 million years old,” Dr DiMaggio said.
“The area of Ethiopia where LD 350-1 was found is part of the East African Rift System, an area that undergoes tectonic extension, which enabled the 2.8 million-year-old rocks to be deposited and then exposed through erosion.”
In most areas in Afar, Ethiopia, rocks dating to 3 to 2.5 million years ago are incomplete or have eroded away, so dating those layers and the fossils they held is impossible. In the Ledi-Geraru area, these layers of rocks are exposed because the area is broken by faults that occurred after the sedimentary rocks were deposited.
By dating volcanic ash layers below the fossils and then above the fossils, geologists can determine the youngest and oldest dates when the animal that became the fossil could have lived.
Other fossils found in this area are – those of prehistoric antelope, water dependent grazers, prehistoric elephants, a type of hippopotamus and crocodiles and fish – fall within the 2.84 to 2.54 million years ago time range.
The scientists analyzed these fossils to learn about the ecological community in which the LD 350-1 early Homo lived.
The results suggest that the area was a more open habitat of mixed grasslands and shrub lands with a gallery forest – trees lining rivers or wetlands. The landscape was probably similar to African locations like the Serengeti Plains or the Kalahari.
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