Ancient bones discovered in a cave in Casablanca, Morocco, could fill in some of the blanks about human evolution.
The cave, known as Grotte à Hominidés, contains assemblages of jawbones, teeth, and vertebrae dating back to 773,000 years ago – a period close to when the modern human lineage began to diverge from the ancestors we share with Neanderthals and Denisovans.
Detailed analyses suggest the remains belonged to an early African hominin population living near this evolutionary crossroads, showing a mix of features later seen in modern humans and Neanderthals, alongside more archaic traits inherited from earlier members of the genus Homo.
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It's a finding that helps anchor humanity's origins firmly in Africa, away from the confusion introduced by Homo antecessor hominin fossils from Europe dating to a similar time period.
"The fossils from the Grotte à Hominidés may be the best candidates we currently have for African populations lying near the root of this shared ancestry, thus reinforcing the view of a deep African origin for our species," says anthropologist Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Germany, who led the research.
Humanity's evolutionary history is messy, complicated, and mostly lost to the ravages of time and decay. What evidence we do have is often fragmentary and difficult to interpret – a task made even trickier by the problem of accurate dating. Many hominin fossils have a broad date range for this reason.
The Grotte à Hominidés fossils are different, which makes them exciting.
Every so often, Earth's magnetic poles flip. These events are recorded geologically, as ferromagnetic materials in rock realign. The most recent flip was the Matuyama-Brunhes reversal, which took place around 773,000 years ago and may have lasted at least a few thousand years.
It's recorded very, very clearly in the sediment in Grotte à Hominidés – and the fossilized bones were found in the same layer as the signature of magnetic reversal. This dates them very cleanly and precisely to 773,000 years ago – right within the timeframe most anthropologists think the process of human divergence was underway.
So that's part of the picture. Based on the sediments in which they were found, we know these bones belonged to a population that was living at a critical moment in human history.
The next step for the researchers was to take a close look at the bones themselves and see what they reveal about what that population was like.
The hominin bones in the cave included two adult jawbones and one belonging to a very young child. There was also a small number of teeth and vertebrae, as well as part of a femur.
Although these remains are scant, they can tell us a lot. The jawbones, for instance, are long, low, and narrow, with a receding joint – traits distinct from those of modern humans and Neanderthals, and more similar to older members of the genus Homo, such as H. erectus.
The teeth themselves, on the other hand, were on the smaller side, more similar to those of modern humans.
CT scanning allowed the researchers to study a structure inside the teeth known as the enamel-dentine junction. The shape of this structure showed some similarities to both H. erectus and H. antecessor, but was distinctly different from both.
"In their shapes and non-metric traits, the teeth from Grotte à Hominidés retain many primitive features and lack the traits that are characteristic of Neanderthals," says anthropologist Shara Bailey of New York University.
"In this sense, they differ from Homo antecessor, which – in some features – are beginning to resemble Neanderthals. The dental morphological analyses indicate that regional differences in human populations may have been already present by the end of the Early Pleistocene."
Given the geographical separation of H. antecessor and the new Moroccan fossils, the researchers suspect the two groups were likely distinct from one another, with the Grotte à Hominidés population belonging to an African stem lineage that would later give rise to modern humans, and H. antecessor representing a related sister population on the Eurasian side of the early human lineage split.
Meanwhile, the mosaic mixture of traits in Grotte à Hominidés suggests a transitional period. Altogether, the mix of traits suggests these fossils should be placed on the African stem of the modern human evolutionary lineage; it's unlikely the population represents the last common ancestor between modern humans and their relatives, but it's close enough that it could provide new insights into how that divergence unfolded.
"The origin of H. sapiens, and the precise timing of the divergence of its ancestral populations from the Neanderthal-Denisovan clade, remain subjects of debate," the researchers write in their paper.
"Our findings not only align with the phylogenetic structure inferred from palaeogenetic data but also highlight the Maghreb as a pivotal region for understanding the emergence of our species, reinforcing the case for an African rather than a Eurasian ancestry of H. sapiens."
The research has been published in Nature.