The cradle of our species was in Africa, but when the first representative of the anatomically modern Homo sapiens arose there is unclear. Now a re-dating of the oldest Homo sapiens fossil from East Africa provides new clues. The fossil found in the Omo Kibish Formation in Ethiopia is therefore already at least 230,000 years old – and thus around 35,000 years older than previously assumed. In addition, unlike the even older Homo sapiens fossils from Morocco, the Omo man does not display any archaic features. It could therefore have belonged to the first truly modern representatives of our species.
Like many early human species, Homo sapiens evolved in Africa—that much seems clear. However, where the first representatives of our species emerged there is largely unclear due to a lack of fossils – so far only eight sites of relics of early Homo sapiens are known, several of which are in East Africa. This region on the once fertile East African Rift Valley was therefore long regarded as the cradle of our species. Based on the age of the fossils, the origin of Homo sapiens was dated to almost 200,000 years ago. In 2017, however, scientists in Morocco discovered fragments of human skulls that were as early as 300,000 years old and showed clear characteristics of Homo sapiens, although some characteristics still seemed rather archaic. These finds, along with complementary genetic studies, suggest that our species’ roots go further back in time – and possibly not just in East Africa.
Volcanic eruptions as dating aids
To gain more clarity, Céline Vidal of the University of Cambridge and her colleagues have revisited one of the oldest and most famous human fossils from East Africa. The Omo 1 fossil is fragments of a Homo sapiens skull found in the 1960s in the Omo Kibish Formation of southwestern Ethiopia. The region used to be characterized by frequent volcanism, which is why there are several layers of volcanic deposits and ash underground. “The fossils were discovered under a thick layer of volcanic ash, but no one could accurately date it using standard radiometric methods because the ash was too fine-grained,” explains Vidal. Radiocarbon dating was also out of the question. However, from the dating of some larger chunks and similarities to volcanic ejecta, it was concluded at the time that the Omo 1 skull must be around 197,000 years old.
However, Vidal and her team re-examined the fossil’s site in more detail and took new samples of the ash above the skull’s layer of discovery. In parallel, they also sampled two volcanoes in the Ethiopian rift system, Shala and Corbetti, which are known to have erupted multiple times between 250,000 and 170,000 years ago. To date the ash from Omo Kibish, scientists compared the geochemical features of this ash sheet to those of different ages of eruption relics. “Each eruption has its own fingerprint, shaped by the development and path of the magma below the surface,” explains Vidal. By determining and comparing the chemical composition of the minerals contained in these volcanic materials, the team were able to more accurately date the age of the ash layer at Omo Kibish for the first time.
32,000 years older than thought
The analyzes showed that the ash layer geochemically corresponded best with an eruption of the Shala volcano, around 400 kilometers away. “But we didn’t know the exact age of this eruption at first,” explains Vidal. So they sent a sample of the minerals from this volcanic material to colleagues who did argon dating of the crystals. “When I got the results and realized that the Homo sapiens fossil had to be older than I had previously thought, I was thrilled!” says Vidal. The re-dating revealed that Omo 1 is not 197,000 years old as assumed, but at least 230,000 years old. This also shifts the first appearance of Homo sapiens in East Africa further into the past. In addition, “Unlike other fossils that have been assigned to early stages of Homo sapiens evolution, Omo 1 has unequivocally modern features, including a large, domed skull and a prominent chin,” explains co-author Aurélien Mounier of the Musée de l’Homme in Paris. “The new dating also makes Omo 1 the oldest clearly anatomically modern Homo sapiens in Africa.”
As the scientists explain, it cannot be ruled out that Homo sapiens developed much earlier in East Africa. “We can only date the evolution of mankind from the fossils that we have,” says Vidal. “It is therefore almost impossible to say: This is the definitive age of our species.” New discoveries and findings could change the previous timeline again at any time.
Source: Celine Vidal (University of Cambridge, UK) et al., Nature, doi: 10.1038/s41586-021-04275-8