Researchers date the oldest known human skull at 233,000 years

The oldest known Homo sapiens fossil is about 36,000 years older than previously thought, according to a recent study. Volcanologists matched a layer of ash above the fossil skull to an eruption of southern Ethiopia’s Shala volcano 233,000 years ago. Their findings seem to line up well with other recent research about when our species’ branch of the family tree split from that of our nearest hominin relatives, the ancestors of the now-extinct Neanderthals and Denisovans.

Geochemical fingerprints on a Pleistocene crime scene

Finding the oldest member of our species hasn’t been easy for paleoanthropologists. There's only a handful of sites in Africa where early Homo sapiens fossils—anything older than about 100,000 years—have turned up, and some of the samples have been nearly impossible to pin a precise date on. At other sites, the fossils don’t quite have all the features that distinguish our skulls from those of our now-extinct hominin cousins: things like a high, round cranium (the round part of your skull that holds your brain) and a chin.

One fossil, a skull found near the Omo River in southern Ethiopia, does have all the hallmarks of anatomically modern humans; among other traits, Omo I has a chin and a tall cranium. The skull was buried (probably not on purpose) in a layer of sediment that was later covered by ash from at least one volcanic eruption. In theory, that ash should make it easy to measure the fossil’s minimum age.

“The age of Omo 1 was very uncertain,” Cambridge University volcanologist Celine Vidal told Ars. “In the last two decades, scientists have tried to date the ash layer found above the fossil, and this triggered a lot of debate.”

The radioactive isotope Argon-40 decays into the stable isotope Argon-39 at a steady rate, so by comparing the ratios of the two isotopes, scientists can measure how old a volcanic rock is. But the ash layer at the Omo Kibish site, called the Kamaya’s Hominid Site Tuff, is too fine-grained for argon isotope dating, a problem that has helped fuel the debate about Omo I’s age.

Vidal and her colleagues measured the relative amounts of several trace elements—chemicals that make up only a tiny fraction of the material—in a thick layer of volcanic ash that sat above the Omo I fossil. The chemical makeup of the ash grains at Omo Kibish closely matched rocky debris found near the Shala volcano, the largest caldera (volcanic lake) in the region.

Unlike the fine ash at Omo Kibish, the pumice at Shala is made of chunks of rock piled up to 20 meters thick thanks to ancient pyroclastic flows, and that is very date-able. The new work shows that the Shala volcano erupted violently about 233,000 years ago, collapsing into a caldera and scattering a 2-meter-thick blanket of volcanic ash across a wide swath of eastern Africa.

When the volcanic ash fell from the sky at what’s now the Omo River Valley in southern Ethiopia, it covered ground that already held the buried remains of at least one dead human. Omo I, in other words, must have died sometime before the eruption 233,000 years ago. “It confirms that our species was present in the Ethiopian Rift before 233,000 years ago,” Vidal told Ars.

Recent genetic and archaeological findings have shown that we probably became noticeably ourselves sometime between 300,000 and 200,000 years ago, so it makes sense that the oldest fossil member of our species would come from that time frame.

The floor really used to be lava—sort of

Connecting the ash layer at Omo Kibish with the Shala caldera eruption makes the oldest known Homo sapiens skull about 36,000 years older than a previous study suggested. Based on argon isotope dates from another ash layer, that study concluded that Omo I couldn’t be more than 197,000 years old—still a venerable age, unmatched by any known Homo sapiens fossil so far.

The ash layer originally used to date Omo I didn’t lie directly beneath the skull the way the KHS Tuff lies directly above it. Instead, geologists found the layer in another part of the site, but its position relative to other sediment and rock layers made it look like it should be older than the layer that held Omo I. It’s an example of how difficult it can be to reconstruct our species’ origin story, especially when sediment layers don’t stack up as neatly in the real world as in textbook diagrams.

That's partly why Vidal and her colleagues were trying to reconstruct the volcanic history of the whole East African Rift System region, from about 300,000 to about 60,000 years ago.

Much of our evolution happened in the shadow of volcanoes. In East Africa—which includes the famous hominin fossil sites in Kenya, Tanzania, and Ethiopia—the Earth’s crust is slowly pulling itself apart. The African tectonic plate is breaking into two smaller pieces, and the rift between those chunks of continental plate grows by about six millimeters a year. The split will take another 10 million years, and it’s been going on since long before our ancestors started walking around on two legs.

That’s why fossil dating in East Africa depends so much on argon isotopes in volcanic ash layers: There are many ash layers to work with. Sometimes paleoanthropologists get very lucky; a series of hominin footprints at Laetoli records where at least two groups of Australopithecines walked through fresh, muddy volcanic ash about 3.6 million years.

Other times, volcanic ash above or below the layer of sediment that a fossil is buried in can provide useful brackets. A layer below a fossil must have been deposited first, so the fossil can’t be any older than that. And a layer above must have been deposited after, so the fossil must be at least that old.

“Hopefully, our reconstruction of the eruptive history of the Ethiopian Rift will help link all the important archaeological sites of the region together,” Vidal told Ars. “Volcanic ash layers are a remarkable tool to date ancient environments.” The team hopes that future work will provide a solid maximum age for Omo I.

Nature, 2022 DOI: 10.1038/s41586-021-04275-8  (About DOIs).