The oldest most complete skull of a human ancestor ever discovered was found by chance by a local herder tending to his flock of goats in Ethiopia.
The rare fossil is all that remains of a hominin with a brain the size of chimpanzee’s, that roamed shrublands surrounding a lake 3.8 million years ago.
“This specimen is the most complete cranium ever found from sediments older than 3 million years,” said Ethiopian palaeoanthropologist Yohannes Haile-Selassie, who co-led the scientific team.
The discovery of the near complete skull puts a face on a critical gap in human evolution.
With its wide cheekbones, long protruding jaw and large canine tooth, the fossil dubbed MRD, is the first to reveal the face of Australopithecus anamensis — the oldest-known species definitively part of the human evolutionary tree.
The skull, detailed in the first of two papers in Nature, is set to rewrite our understanding of where A. anamensis fits between primitive hominins that lived more than 4 million years ago, and Australopithecus afarensis, the species made famous by the Lucy skeleton.
“It fills a gap in the fossil record from 3.6 to 3.9 million years and highlights some of the changes that took place from one species to another,” Dr Haile-Selassie said.
A very lucky discovery
The fossilised skull is likely to become a celebrated icon of human evolution, but its discovery came about by sheer good fortune.
Dr Haile-Selassie, a world authority on ancient human evolution at the Cleveland Museum of Natural History, was on a dig at Woranso-Mille in the Afar region of Ethiopia in 2016 when he was approached by the local herdsman.
The man had walked 3 kilometres from where he was camped with his family to show Dr Haile-Selassie a jawbone he’d picked up from the ground while tending to his goats on the harsh, rocky plateau.
Dr Haile-Selassie returned to the site with the man, who pointed out where the fossil came from.
“About 3 metres away from where the upper jaw was found the whole cranium was sitting there,” he recalled.
“That’s when I realised it was something really, really significant.”
Ethiopia’s Afar region is known as the cradle of human evolution with a variety of fossils spanning a period of about 7 million years.
While there is debate about whether all these species are part of the human family tree, traits such as walking on two legs are clearly seen in the fossils of Australopithecine species.
Until now evidence of A. anamensis was limited to fragments of jawbones, teeth, skull and limb bones dated between 4.2 million and 3.9 million years old.
A. anamensis is thought to have given rise to A. afarensis, which arrived on the scene around 3.9 million years ago.
But without a face or cranium the relationship between the two species had been unclear.
“If we didn’t find this specimen we wouldn’t have any idea whether this species’ face looked like A. afarensis or something like its predecessors,” Dr Haile-Selassie said.
Putting a face to Lucy’s ancestor
The team used micro-CT scans and 3D reconstructions of the skull to replace the missing eye socket and cheekbone to pin down the fossil’s lineage.
They found it had a curious mix of primitive and more advanced features.
“The protruding mid face and the lower face … is a very old adaptation in these early human ancestors going back all the way to 6 million years ago,” Dr Haile-Selassie said.
But it also had the same eye sockets as Lucy’s species, and wide, prominent cheekbones that resembled an even younger species that lived about 2 million years ago.
Previously it had been thought that A.anamensis had given way to Lucy’s species, but dating of the volcanic sediment surrounding the fossil, showed the two species overlapped by 100,000 years.
“This mostly happens when a small population of the parent species separates from the main population and undergoes some change to adapt to the environment it’s living in”, Dr Haile-Selassie said.
It is “extremely rare” to find an intact fossil that old, said palaeontologist Justin Adams of Monash University, who has worked on other early hominin discoveries in South Africa.
“If they had even gone out maybe this year or a couple of years from now, erosion could have completely obliterated that skull,” he said.
What does this tell us about human evolution?
“We have this concept that a species persists for a period of time and then another species arises from it and it moves in a linear trajectory,” he said.
“Rather A. anamensis was kind of unique, and not all parts of that species necessarily led directly into subsequent species on the hominid lineage.”
He said one of the defining features of the skull that put it in the Australopithecine camp was the shape of its canine tooth.
“It’s something that very clearly links it with that species to the exclusion of say earlier or later hominins,” Dr Adams said.
Renaud Joannes-Boyau, a geochronologist at Southern Cross University, who has studied the teeth of other species of Australopithecines agreed.
“The teeth give no doubt that this belongs to the early type of A. anamensis,” Dr Joannes-Boyau said.
“This is a massive contribution to understanding early, early human evolution. And especially the emergence of this genus Australopithecus.”
What remained to be seen was whether A. anamensis and Lucy’s species were directly related or they travelled different paths via a common ancestor, he said.
“It’s possible that A. anamensis is not directly related to us, but [Australopithecines] are really on the path to Homo sapiens.”