It’s definitely one of those Eureka moments . Fossil hunters have uncovered a remarkably complete hominin skull nearly 3.8 million years old. This unexpected specimen sheds some light on the very little known early members of the human evolutionary family.
The East African skull, which appeared at the Woranso-Mille site in Ethiopia, has been classified as Australopithecus anamensis . It is the oldest known species in a hominid genus that includes Australopithecus afarensis , best known for Lucy’s 3.2 million-year-old partial skeleton.
The research team, led by paleoanthropologist Yohannes Haile-Selassie of the Cleveland Museum of Natural History (USA), describes their analysis of the skull in two papers published in the journal Nature .
“This specimen provides the first glimpse of the face of Australopithecus anamensis , “ said Haile-Selassie. The skull, which is slightly larger than the fist of a modern adult human, also includes the earliest example of a brain cavity from A. anamensis.
The expected find
Until now, A. anamensis fossils consisted only of partial upper and lower jaws, isolated teeth, and some bones from the lower body. Those specimens, previously unearthed in Kenya and Ethiopia, date to between 4.2 million and 3.9 million years ago.
Later, on February 10, 2016, a member of the Woranso-Mille team discovered that the underside of a hominid skull protruded from the eroded sediment. That same day, Haile-Selassie found the skull lying on the ground about ten feet from the initial find. Sifting the soil produced additional skull fragments.
Geoscientist Beverly Saylor of Case Western Reserve University in Cleveland led the team tasked with dating the fossil by estimating the ages of nearby layers of volcanic rock. Known reversals of the Earth’s magnetic field in the Woranso-Mille sediment also aided the dating.
A habitat surrounded by active volcanoes
Geological evidence indicated that the individual A. anamensis fossil had been covered in sandy deposits in a region crossed by several rivers. The surrounding area was largely dry, but included some wooded areas. Volcanic eruptions occasionally covered the lake and its surroundings.
A digital reconstruction of Woranso-Mille’s skull helped establish its species. The skull has particular features, such as a long, narrow shape and a chimpanzee-sized brain, similar to those of even older proposed hominins such as Sahelanthropus tchadensis and Ardipithecus ramidus . In contrast, the forward-projecting cheekbones are reminiscent of those of later hominins, such as the 2.5-million-year-old Paranthropus aethiopicus. That species belonged to an African line of large-jawed, small-brained creatures that died about 1 million years ago. It is difficult to know if these shared traits evolved independently, or if the traits signal an evolutionary relationship.
Coexistence for about 100,000 years
The Woranso-Mille skull differs enough from a hominid forehead bone about 3.9 million years old discovered in East Africa in 1981 to assign that older find, known as the Belohdelie frontal, toA. afarensis,Haile-Selassie holds up. If this is so, the speciesA. anamensis,now located between 4.2 million and 3.8 years ago, and the species of Lucy, which dates from between 3.9 million and 3 million years ago,overlapped for at least 100,000 years.That scenario contradicts an earlier hypothesis thatA. anamensisit evolved directly into the Lucy type, with the previous species disappearing as it transformed into its descendant species.
A large group of individuals fromA. anamensiscould have isolated itself from its fellow species and then evolved to an early version of A. afarensis, Haile-Selassie speculates.In that case, other groups ofA. anamensisthey would have coexisted for a time with Lucy’s species.
While the newly discovered skull“fills a critical gap in Australopithecus evolution”,the evolutionary state of the frontal of Belohdelie remains unknown, says paleoanthropologist William Kimbel of the Institute of Human Origins at Arizona State University in Tempe. More skulls are needed fromA. anamensisto assess whether the frontal of Belohdelie shows features more typical of that species or of Lucy, says Kimbel.
Researchers have been working at this site for 15 years, and they are eager to see what else this discovery may reveal. “It’s still covered with a hard matrix,” Haile-Selassie said over the base of the skull. “This region of the skull in early hominins is extremely important, but little known, and I would like to see how the condition of the apes has changed.”
Referencia: Yohannes Haile-Selassie et al. “A 3.8-million-year-old hominincranium from Woranso-Mille, Ethiopia” Nature, 28 de agosto Doi 10.1038/s41586-019-1513-8
Beverly Z. Saylor et al. “Age and context of mid-Pliocene hominin cranium from Woranso-Mille, Ethiopia”. Nature, 28 de agosto. Doi 10.1038/s41586-019-1514-7
