A quarter of a million years ago, the East African Rift zone was a fertile causeway, where ample rainfall collected in basins among the hills, and fertile soils were laced with ash from a series of active volcanoes. This region is also thought to be the birthplace of our species, Homo sapiens, based on a rich nexus of archaeological sites.
Now, scientists have dated a famous set of remains from the rift zone in Ethiopia, known as Omo I, firmly identifying them as the oldest modern human remains in eastern Africa, and the oldest unquestioned evidence of H. sapiens in the world.
“It’s probably no coincidence that our earliest ancestors lived in such a geologically active rift valley – it collected rainfall in lakes, providing fresh water and attracting animals, and served as a natural migration corridor stretching thousands of kilometres,” says Clive Oppenheimer, a volcanologist at Cambridge University, UK, who is attempting to date volcanic eruptions from the region at the time of our species’ origins.
“The volcanoes provided fantastic materials to make stone tools and from time to time we had to develop our cognitive skills when large eruptions transformed the landscape.”
The Omo I remains were uncovered in the late 1960s, and have been the subject of intense debate; scientists have attempted to date them precisely using the layers of volcanic ash found above and below the sediments in which they lay.
Earlier attempts to date the fossils placed them at less than 200,000 years old, but a new study in Nature from researchers at Cambridge has pushed back their origin story.
“The generally accepted age of the Omo fossils is under 200,000 years, but there’s been a lot of uncertainty around this date,” says lead author Céline Vidal, from Cambridge’s Department of Geography. “The fossils were found in a sequence, below a thick layer of volcanic ash that nobody had managed to date with radiometric techniques because the ash is too fine-grained.”
Vidal’s new work on the Omo remains is part of an epic four-year effort among her colleagues and led by Oppenheimer to date all the major volcanic eruptions in the Ethiopian Rift around the time H. sapiens emerged, in a period known as the late Middle Pleistocene.
Geological sleuthing to date modern human origins
To date each eruption, the researchers collected rock samples from the deposits and ground them down to sub-millimetre size. Once ground down, the rocks from each eruption carry unique chemical fingerprints that identify their point of origin.
“Each eruption has its own fingerprint – its own evolutionary story below the surface, which is determined by the pathway the magma followed,” says Vidal. “Once you’ve crushed the rock, you free the minerals within, and then you can date them, and identify the chemical signature of the volcanic glass that holds the minerals together.”
The Omo fossils are found within a layer of ash known as the Kamoya Hominin Site (KHS) ash. By undertaking a new geochemical analysis, researchers linked this layer to the eruption of the Shala volcano, some 400 kilometres away. Pumice samples from that volcano were then dated to 230,000 years ago, meaning the human remains enclosed beneath the ash layer must be older.
“First I found there was a geochemical match, but we didn’t have the age of the Shala eruption,” says Vidal. “I immediately sent the samples of Shala volcano to our colleagues in Glasgow so they could measure the age of the rocks. When I received the results and found out that the oldest Homo sapiens from the region was older than previously assumed, I was really excited.”
The contested nature of human origins
The origin of our species is a hotly debated topic in science. While the researchers say their hominin is the oldest uncontested evidence of H. sapiens, there are older fossils that are thought to belong to our species, though these are often subject to intense debate.
These include the remarkable discovery of what looks like a modern human from a 300,000-year-old site in Morocco.
“Unlike other Middle Pleistocene fossils, which are thought to belong to the early stages of the Homo sapiens lineage, Omo I possesses unequivocal modern human characteristics, such as a tall and globular cranial vault and a chin,” says co-author Aurélien Mounier, from the Musée de l’Homme in Paris. “The new date estimate, de facto, makes itthe oldest unchallenged Homo sapiens in Africa.”
And while this study sets a new minimum age for H. sapiens in Africa, the authors say new discoveries may yet push our origin story further back in time.
“Our forensic approach provides a new minimum age for Homo sapiens in eastern Africa, but the challenge still remains to provide a cap, a maximum age, for their emergence, which is widely believed to have taken place in this region,” says co-author Professor Christine Lane, head of the Cambridge Tephra Laboratory where much of the work was carried out. “It’s possible that new finds and new studies may extend the age of our species even further back in time.”
“We can only date humanity based on the fossils that we have, so it’s impossible to say that this is the definitive age of our species,” agrees Vidal. “The study of human evolution is always in motion: boundaries and timelines change as our understanding improves. But these fossils show just how resilient humans are: that we survived, thrived and migrated in an area that was so prone to natural disasters.”
Amalyah Hart has a BA (Hons) in Archaeology and Anthropology from the University of Oxford and an MA in Journalism from the University of Melbourne.
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