Oldest Known Fossil of a Modern Human May Be Even More Ancient Than We Thought

Oldest Known Fossil of a Modern Human May Be Even More Ancient Than We Thought

Scientists say a Homo sapiens fossil found in Ethiopia in the 1960s is at least 233,000 years old, which would make it 36,000 years older than the previous estimate.

Fossils of the earliest members of our species, Homo sapiens, are exceptionally rare. To date, only eight sites in Africa have produced fossils of early anatomically modern humans, the oldest of which, known as the Kibish Omo I fossil, has now been re-dated by team of experts led by Clive Oppenheimer from the Department of Geography at the University of Cambridge.

That’s right — geography, and not archaeology or anthropology. Sure, members of those disciplines were involved in the study, published today in Nature, but the effort to re-date this fossil required scientists of the Earth, specifically experts in volcanology. For you see, the bones of Omo I were found directly beneath a layer of volcanic ash.

Previous efforts to date this ash and to distinguish it from other layers of ash have led to uncertainties about the fossil’s age, but the scientists behind the new study think they’ve finally nailed it. The new minimum age of Omo I is 233,000 years old, as opposed to the previous estimate of 197,000 years old.

“This is the best estimate we have at the moment, and it is congruent with the most recent models of human evolution, which place the emergence of our species — Homo sapiens — between 350,000 and 200,000 years ago,” Céline Vidal, a volcanologist at the University of Cambridge and the first author of the new study, explained in an email. To which she added: “The Omo I fossil is the oldest Homo sapiens we know of so far,” and until now it was “thought to be younger than 200,000 years old, but there was a lot of uncertainty on this age.”

The team sampling ash deposits at the Omo Kibish formation in Ethiopia.  (Photo: Alan Deino)
The team sampling ash deposits at the Omo Kibish formation in Ethiopia. (Photo: Alan Deino)

The Omo I skeleton was found in 1967 at the Omo Kibish Formation of southwestern Ethiopia. The site is positioned within the East African Rift valley, an area prone to volcanic activity. Vidal is part of a research project seeking to understand the timing and history of very large eruptions in the Ethiopian Rift between 300,000 and 60,000 years ago, which she described as “poorly studied.” This data, it is hoped, will help scientists to better understand possible links with human evolution.

“We have been analysing volcanic rocks from these eruptions to find out when they occurred and identify their chemical fingerprint,” Vidal wrote. “Each eruption has a unique chemical signature. Using these fingerprints, we could track the far-travelled fine ash hundreds of miles away from the volcanoes. In Ethiopia, these ash layers can be found at archaeological sites where tools and fossils of our ancestors were identified.”

Traditional radiometric, or istopic, techniques are limited when it comes to the dating of ash, but by studying the chemical composition of pumice rock samples from volcanic deposits, the team was able to date all major volcanic eruptions that happened in Africa during the Late Middle Pleistocene — the time when our species first appeared.

Omo I was found in sediments beneath a 1.83 m layer of volcanic ash that can’t be directly dated, as the ash is too fine-grained. Positioned directly above the Omo I fossil, this ash layer “is the key to constraining the minimum age of the fossil,” said Vidal. She and her colleagues analysed this ash and compared its chemical fingerprint to other eruptions in the Ethiopian rift. This allowed them to connect the volcanic deposits to a major eruption of the Shala volcano located 400 kilometres away.

“We identified the source of the ash to be a colossal eruption of Shala volcano, which occurred about 233,000 years ago,” Vidal said. “This means that Omo I is older than 230,000 years.”

The margin of error on this new estimate is significant at a plus-minus of 22,000 years, a degree of uncertainty the team hopes to refine in future work. Also, the team wasn’t able to determine the maximum age of the fossil. To that end, Vidal and her colleagues are currently trying to correlate an ash layer located beneath the fossil to other ash deposits.

The new study is interesting, but it’s far from paradigm-shattering. That the oldest known fossil of an anatomically modern human is older than 233,000 years is hardly surprising, given that genetic evidence points to an even earlier emergence of our species, perhaps as far back as 600,000 years ago. For example, certain fossils from the Jebel Irhoud site in Morocco date back some 300,000 years, but they represent archaic, not modern, Homo sapiens.

The Jebel Irhoud fossils “present some characteristics of Homo sapiens but are not considered as complete Homo sapiens,” said Vidal. Omo I, on the other hand, possesses features consistent with anatomically modern humans, such as a tall and round skullcap and a chin on the mandible.

This aside, the new work “is important in demonstrating that the Omo I skeleton is pretty certainly more than 230,000 years old, thus even older than previously suggested,” Chris Stringer, an anthropologist at the Natural History Museum of London who wasn’t involved in the the new study, wrote in an email. “And on the parts preserved, this is an anatomically modern human and thus the oldest known so far. However, it’s likely from genetic data that the Homo sapiens lineage stretches back even further, for at least another 300,000 years, although the earlier members of the lineage would not have shown all the ‘modern’ traits that evolved subsequently.”

Stringer was disappointed to see that a paper he co-wrote in 2012, which directly dated Omo I, wasn’t cited in the new study. That paper pointed to an age older than 195,000 years, but Stringer’s team could only estimate a minimum age of 155,000 years old.

When asked what excited her most about her team’s new paper, Vidal said it’s that discoveries never seem to end.

“Science is always in motion, boundaries and timelines change as our understanding improves. There is still so much to uncover — we only see what is at the surface,” Vidal said. “It is also fascinating to think that before smartphones and vaccines, we adapted, moved, and survived catastrophic eruptions and climate change. There is a lot to learn from our natural innate resilience as a species.”

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