Radiometric dating of volcanic ash has pushed back the age of human fossils found in Ethiopia to at least 233,000 years ago.
In a remote area in southwestern Ethiopia, the Omo River and its long-lost arms formed bare rugged cliffs and slopes, revealing a layer of ancient sediments and the remains of the first humans.
Before the Covid pandemic, Céline Vidal and her colleagues went to this site known as the Kibish Formation to work at scorching temperatures reaching up to 110 Fahrenheit to learn more about some of the oldest members of our species.
“It was an adventure,” said Vidal, a volcanologist at the University of Cambridge who has studied how ancient eruptions affected climate and civilizations.
One of the reasons Vidal and his colleagues came to the site was to learn about Omo I, one of the oldest known examples of Homo sapiens. Using geochemical clues to match the layer of volcanic ash that covered the fossil to a specific volcanic eruption, they discovered that Omo I was 36,000 years older than previously believed.
Ash from the massive eruption of the Shala volcano in the Ethiopian Rift was poured over a layer of sediment containing the fossil Omo I about 233,000 years ago, which means Omo I and his species lived here at least that long ago.
“Each explosion has a unique geochemical composition, such as a kind of fingerprint. From here, we can try to find out exactly which eruption in the Ethiopian Rift constitutes a layer of volcanic ash,” he said.
“We found a match for the ash layer that covered the fossils, so we know which eruption produced that ash and the age of that eruption.”
The findings, published in the journal Nature, suggest that Omo I’s remains should be older than the layer that later fell from the sky, but do not show the maximum possible age. If the team can similarly detect another volcanic layer from underneath the fossil, then it may be possible to determine the earliest possible date for Omo I.
Renowned paleoanthropologist Richard Leakey and his colleagues found Omo I near the southern Ethiopian town of Kibish in 1967. Initially, scientists dated the shells of freshwater mollusks found along with the skull, concluding that the remains were about 130,000 years old. They also saw very clearly from the beginning that the flat face, protruding jaw and high forehead of the skull are distinctly modern, and that this ancient person should be classified as a member of our own species.
For more than half a century, the fossil has been known as one of the oldest Homo sapiens skulls found anywhere in the world. (The partial skull and skeleton were considered the oldest from Jebel Irhoud in Morocco until the 2017 discovery of 300,000-year-old skull, jaw and tooth fragments). In 2005, a radioactive dating study significantly retracted the age of the fossil skull, dating back 195,000 years. But today’s study shows that Omo I is actually tens of thousands of years older.
The period when Homo sapiens first appeared and gradually evolved in Africa between 360,000 and 100,000 years ago was a period of severe volcanic activity. Massive explosions shook the area and accumulated thick layers of ash that would make some areas uninhabitable. Since changing environments sometimes push early humans to adopt new behaviors and tools, these explosions may have actually played a role in shaping the evolution here. Perhaps they caused older groups of people to move, meet each other, and change everything from genes to technologies before they left again.
As a more precise result, volcanic ash helped create a record of what occurred during the turbulent period.
In the Kibish Formation, researchers were stunned to find a huge layer of ash more than 1.5 meters thick, just above the sediments where Omo I and other fossils were found. About 200 miles from the nearest ancient volcano, ash was like flour, so thin that it didn’t have crystals big enough to be used for radiometric dating. Radiometric dating can provide an age estimate by measuring how much of the mineral’s radioactive potassium degrades into radioactive argona. “This material was not suitable for the techniques we normally use,” Vidal explains.
However, Vidal and his colleagues were able to determine the age of the eruption that accumulated the ash by sampling rocks closer to their volcanic sources where ash debris contained a large number of larger crystals suitable for radiometric dating.
“It’s really nice to be able to go to volcanic complexes, collect samples directly from the source, and connect them very precisely with those found chemically at the fossil site,” said Rick Potts, director of the Smithsonian’s Human Origins Program, who wasn’t involved in the study.
The history of important fossils such as Omo I is extremely important to scientists who put together the evolutionary timeline of Homo sapiens. They provide a solid framework to help track changes in evolution, such as human emergence, or behaviors such as tool technologies. They also give context to events such as dramatic climate change, which can help drive these adaptations.
“In any region, it is very useful to determine the earliest appearance of something very, very similar to the skull of Homo sapiens. And this is Omo I.”
Omo I’s fascinating skull shape suggests that 230,000 years ago, people living in East Africa evolved to look a lot like us. But that’s not the whole story. Leakey’s team found a second set of remains in the area called Omo II, which appeared to be the same age but had a rather different and more archaic appearance, sparking debate about whether it was really a Homo sapiens.
Human fossil records from about 350,000 to 160,000 years ago show a mix and match of different features, some more primitive and others more modern, at different times and places. Potts notes that this paradigm makes the remains of Omo I and Omo II particularly interesting, since such variations can be seen side by side.
“Whether it’s the same gene pool or two neighboring hominin groups, this foundation for combining archaic and modern-looking features was documented by two fossil individuals collected in Kibish by Richard Leakey in the 1960s. As with many animals, the origin of our own species was not an event, but a more gradual process that took place over time,” he said.
Smithsonian Magazine. 12 Ocak 2021.
Vidal, C.M., Lane, C.S., Asrat, A. et al. (2022). Age of the oldest known Homo sapiens from eastern Africa. Nature.
