Modern humans' distant relatives left Africa earlier than previously thought—rewriting a key chapter in humankind's epic prequel, according to a discovery unveiled on Wednesday in Nature.
Nearly a hundred stone tools found at the Shangchen site in central China may push back the spread of our ancient cousins—hominins—out of Africa by more than a quarter million years.
The toolmakers lived at Shangchen on and off for 800,000 years between 2.1 and 1.3 million years ago, leaving behind tools that are unprecedented outside of Africa. The site's oldest tools are roughly 300,000 years older than Dmanisi, a 1.8-million-year-old site in the Republic of Georgia with the oldest known fossils of our extinct cousin Homo erectus.
Some of the stone artifacts from Shangchen's oldest sediment layers. (Photograph by Zhaoyu Zhu)
"Finding artifacts that you knew were around two million years old—and therefore the oldest outside Africa—was for me, as a palaeoanthropologist, really exciting," says study coauthor Robin Dennell, a professor at the University of Exeter.
"More people have climbed Everest than found stone tools that old."
"I’ve always said that once the Chinese researchers start looking for evidence on a similar scale as all the money spent in Africa, things will turn up!" exclaims Gerrit van den Bergh, a paleoanthropologist at the University of Wollongong who wasn't involved with the study.
"It again shows how little we actually know."
Today's modern humans, Homo sapiens, trace back to a migratory pulse that left Africa some 60,000 years ago. But that migration was hardly the first to leave the continent—nor were modern humans the only hominins to make the trip. Remains of Homo erectus have been found from Georgia to Java. Neanderthals' ancestors trekked to Europe roughly half a million years ago. At least 700,000 years ago, early hominins somehow swept through the South Pacific, giving rise to the “hobbit” Homo floresiensis and other island toolmakers.
Some sites have hinted at an even older hominin presence in Asia. In the 1980s, researchers suggested that stone tools in Pakistan could be as old as two million years old. In 2004, a Chinese team found 1.66-million-year-old stone tools in north China's Nihewan basin. And in 2015, researchers made the case that a Homo erectus skull found less than three miles from Shangchen was more than 1.6 million years old.
Convinced that China had even older sites, Zhaoyu Zhu, the new study's lead author and a geologist at the Chinese Academy of Sciences, started digging at Shangchen in 2004.
In July 2007, one of Zhu's colleagues at the site noticed a stone in a steep outcrop—which turned out to be a hominin-modified tool. By 2017, Zhu's team had uncovered a 240-foot-thick sequence of soil at Shangchen, with 17 layers bearing stone tools.
Nestled on a hillside in China's Shaanxi Province, this dig site exposed Shangchen's oldest stone tools. Chinese Academy of Sciences geologist Zhaoyu Zhu and his colleagues excavated the area for 13 years. (Photograph by Zhaoyu Zhu)
"My colleagues and I were all very excited,” says Zhu. “This sequence is too massive and spectacular."
But precisely when were the tools made? To find out, Zhu's team measured the varying magnetic fields in the tool-bearing soil layers.
As each layer formed, minerals within the soil preserved the orientation of Earth's magnetic field at the time. Because Earth's magnetic field sometimes reverses polarity, some of the layers' fields flipped in kind—as did the magnetic fields of similarly aged sediments all over Earth.
By comparing Shangchen's sediments to well-dated African soils that preserved the same magnetic reversals, Zhu could accurately assign ages to each Shangchen layer. Six of the 96 tools discussed in the study were found in a layer dating back 2.12 million years.
Who Made the Tools?
Since there aren't hominin fossils alongside Shangchen's tools, nobody knows for sure who made them.
Homo erectus, the toolmakers at Dmanisi, may have been responsible. The hominin species made stone tools, and it had the sort of build and walking gait needed to cross continents. But the species's oldest known fossils are about 1.8 million years old—much younger than Shangchen's oldest tools.
"It is entirely possible that Homo erectus occupied China at this time, but given the age of the site, and the possibility that artifacts may be found at even earlier ages, another member of the genus Homo may be occupying Asia, such as a Homo habilis-like ancestor," says Michael Petraglia, a Max Planck Institute paleoanthropologist who studies ancient tools from Asia.
María Martinón-Torres, the director of Spain's National Center of the Study of Human Evolution (CENIEH) and a world authority on Asian hominin fossils, says that even some Chinese hominin fossils once labeled as H. erectus are worth a second look.
"It is time to accept that not all hominins found in Asia fit in the Asian H. erectus taxon, a species that has been largely employed as a blanket term," she says. "I think that the question about the first Asian hominin identity is not closed yet."
Regardless, Shangchen's toolmakers would have had brains about a third the size of our own. While brain size isn't everything, experts say that it's astounding that such early, small-brained humans made it from Africa to China some two million years ago.
Future work will shed light on who these mysterious hominins were. Dennell says that he'd love to look at sediments older than 2.1 million years, which they couldn't readily do at Shangchen because farms now cover them. Elsewhere in Asia, more stunning finds are all but guaranteed.
"For a long time [the] scientific community has given a secondary role to Asia versus Africa in explaining relevant episodes of our evolution,” says Martinón-Torres. “With more fieldwork at Asia I am sure that more surprises are to come." (National Geographic)
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