Deep in Ethiopia’s Afar region, researchers have uncovered new evidence of early human relatives, belonging to two distinct genera – Homo and Australopithecus – dating to between 2.5 and 2.8 million years ago in Ledi-Gararu. This could be a revolutionary find, since it could mean that different Australopithecus species may have overlapped at the same time and in the same space.
Before this discovery, scientists had not found many fossils of Australopithecus that are dated this recently, nor Homo fossils that are dated so far back. In fact, scientists posited that Australopithecus afarensis had disappeared by around 2.95 million years ago, only to be replaced later by Homo or Paranthropus.
Now we know that Homo and a distinct form of Australopithecus lived side-by-side in the same region. The paper authored by Dr. Brian Vilmoare et al suggests that at least four hominin lineages (Homo, Paranthropus, Australopithecus garhi, plus the new Australopithecus from Ledi-Geraru) coexisted between 3.0 and 2.5 million years ago.
The Australopithecus specimens cannot yet be identified to species level. But the researchers are confident that this is a different species, within the same genus. “We interpret these as related to [A. afarensis and A. garhi] as they share primitive features, such as megadont teeth. Because we don’t have a skull to give us more features, our hypothesis is that our teeth that we call Australopithecus is [a different species] because of the shape and cusp positions of the teeth (not the size).” said Dr. Kaye Reed, Professor at Arizona State University and a co-author.
“If we would find a skull of an Australopithecus species that has shared derived features with either afarensis or garhi, then we would have to look further to see if there were enough other derived features on our skull that made it a separate species.”
The Geological Backdrop: Fossil Layers with Precision Dating
The fossils came from three sedimentary layers, Gurumaha, Lee Adoyta, and Giddi Sands, in the Ledi-Geraru site, tracked with precision by dating volcanic layers. One key ash layer, the Gurumaha Tuff, is dated to 2.782 million years old. Followed by the Lee Adoyta tuffs around 2.63 million years ago, and the Giddi Sands Tuff at about 2.59 million years ago. These tightly dated horizons give a clear timeline to the fossil finds.
Lingual (left) and labial (right) views of the Ledi-Geraru LD 760-115979 canine (left) with Hadar A. afarensis specimens A.L. 763-1 (middle) and A.L. 333x-3 (right). Note that the LD 760 canine is a right canine, whereas the A. afarensis canines are from the left and are mirrored in these images. Image credit: Brian Vilmoare et al.
One tooth from the Gurumaha level (a lower premolar, LD 302-23) resembles early Homo teeth and suggests its presence there by 2.78 million years ago. From the Lee Adoyta layer comes a set of larger molars and a canine (LD 760) that share features with Australopithecus afarensis. But it has some distinct traits, so the researchers call them Australopithecus sp. indet.
Left molars from Ledi-Geraru specimen LD 760 (left) compared to A. afarensis (right). Image Credit: Brian Vilmoare et al.
In the Giddi Sands layer, a molar shape matches that of early Homo. All this confirms that these ancestral species shared the same landscape, in overlapping ranges.
The discovery fills many gaps in our knowledge
These findings reshape how we see human evolution. There wasn’t a straight line from one species to another.
Instead, multiple lineages coexisted and adapted to drier, more open environments, well before Paranthropus entered the scene. “It adds to the body of information that we just didn’t have one hominin species at a time that evolved into the next and the next”, Dr. Reed adds. “I hate those graphics that go from a chimp-like creature to modern humans—it is not how evolution worked. One of the species we have likely didn’t make it—especially since there are no more Australopithecus species that we know of after 2 million years ago in Africa.”
The time interval between about three and two million years ago is a critical period in human evolution. This is when the genera Homo and Paranthropus first appear in the fossil record and a possible ancestor of these genera, Australopithecus afarensis, disappears.
Unfortunately, there aren’t many fossiliferous exposures in eastern Africa that would allow us to test theories as to why. These new fossils are critical in filling this gap, making this a very exciting discovery indeed.
The next step is analyse the teeth, and dig more
The researchers will now examine the enamel on the teeth to look for Carbon, Oxygen and Nitrogen isotopes. These isotopes form in teeth as they are developing, and can give us insight into what the owners of the teeth ate.
Dr. Reed explains, “Carbon would tell us whether it ate foods from trees, bushes, and shrubs, or something derived from grass. Oxygen would tell us if either was an obligate drinker or got water from plants (although primates are all pretty much obligate water consumers). Nitrogen will tell us if one of them ate meat.”
Their diets could give us clues about their relationships with each other. “If these are different, then it is likely they were not in competition. If they are the same, they may have fought over resources, and one population could have driven the other population away.”
In addition to the isotope analysis of the enamel, the scientists hope to conduct more excavations in the field, and conduct landscape archaeology to further understand the behavior of probably one or both of the species. They also hope to look for more fossils of both species that are not teeth.