Early hominins ventured out into the world beyond Africa even earlier than we've given them credit for, according to a new stone-tool find on the southern edge of China's Loess Plateau.

Hominins—the lineage of apes that eventually came to include humans—began making recognizable stone tools about 3 million years ago. Before that date, we know that our early relatives inhabited a place only if we find their bones or, in rarer cases, their footprints. But stone tools offer a more durable, more abundant calling card. Pick up a stone flake or scraper—or a core of flint or chert with obvious scars from flintknapping—and you know that someone made this object. Someone was here.

And that's exactly what archaeologists led by Zhaoyu Zhu of the Chinese Academy of Sciences found in a 2.1 million-year-old layer of ancient wind-blown sediment in China's southern Loess Plateau: a collection of stone cores, flakes, scrapers, borers, and points, as well as a couple of damaged hammerstones. The tools' style strongly resembles stone tools found at sites of about the same age in Africa, made by early human relatives like Homo erectus.

The find pushes back the earliest evidence for hominins outside Africa, which had been a 1.85 to 1.77 million-year-old group of Homo erectus bones and stone tools at a site in Dmanisi, Georgia, not far from the Armenian border. The location in China means that hominins may have ventured beyond the warm tropics of Africa into the less-certain environments of Eurasia a few hundred thousand years earlier than archaeologists previously thought.

The layers of sediment that make up the Loess Plateau (located west of Beijing and south of Mongolia) don't lend themselves to radiometric dating, but they're perfect for paleomagnetic dating. Earth's magnetic field reverses its poles at random intervals, and geologists have a good record of when those reversals have happened in the past. Magnetic minerals in sediment align with Earth's poles, and when those sediments harden into rock layers, the alignment of those magnetic minerals gets locked into place, creating a fossil record of Earth's past magnetism. Archaeologists can compare the sequence of those alignments in a series of rock or sediment layers to a record of Earth's polarity reversals. They can then come up with a reliable date for each layer.

Off-and-on occupation

Stone tools also turned up in several layers of sediment above the oldest one, spanning a range of time from 2.1 to 1.26 million years ago. That means ancient hominins used this landscape, though not necessarily continuously, for about 850,000 years. In fact, their presence seems to have varied with the climate.

Across the 168,000 square miles of the Loess Plateau, layers of wind-blown silt called loess alternate with heavier soils, now compressed into rock, called paleosols. The paleosols, according to ancient-climate reconstructions, got deposited during warmer, wetter periods. During these times, the Loess Plateau would have been a temperate grassland crossed by streams and dotted with small lakes, offering rich grazing for horses, rhinos, deer, elephants, and ancient relatives of cattle. Those big grazers would have supported wolves, hyenas, and, apparently, early hominins.

Most of the stone tools at the site—80 artifacts out of 96—appeared in 11 of those paleosol layers. Just 16 stone tools turned up, spread across six layers of loess, deposited in colder, drier, windier times when the plateau would have been a steppe grassland with less ample grazing and much colder winters. It seems that fewer hominins lived on the Loess Plateau during colder, drier periods, although archaeologists don't have enough information to tell if they moved elsewhere and then returned in warmer times or if they simply died out to be replaced by another wave when the local climate improved.

"We evolved in the Tropics, and these hominins did not have fire or sewn clothing," University of Exeter archaeologist Robin Dennell, a co-author on the paper, told Ars Technica. "Some argue that they simply moved to warmer places (refugia) until conditions became more favorable; others argue that there was a lot of local extinctions, and then new groups moved in. I think it was probably a mixture of both: some waxing and waning, some new pulses of migration."

Snapshots of the past

It's hard to say exactly what our early relatives were doing here 2.1 million years ago.

"We don't have a site in the sense of a living floor—what we have is a very long sequence of hominins in a landscape," Dennell told Ars. "Imagine that you take one photograph once a year, at a predetermined time, of a country road. Most of the time, it wouldn't show anything, but just occasionally, you might see a figure. This type of fieldwork is a bit like that on a much longer timescale and over a much larger area. You just get the occasional glimpse that a hominin was there." It's hard to tell whether a stone tool is a one-off or part of a larger site with a detailed story to tell.

Even a one-off is proof that, 2.1 million years ago, someone was here. But it's not enough to reveal what they did or how they lived. There's no flaking debris at the site to suggest that the hominins who made these stone tools did so at Shangchen. And the nearest source of stone would have been on the slopes of the Qinling Mountains, between 3 and 9 miles to the south, which is about the same distance early tool-making hominins in eastern Africa traveled to acquire their own raw stone.

"I think they are carrying artifacts and a few hammerstones that they have made elsewhere, and then discarding them when they don't [need] them," Dennell told Ars. "The main artifact type, in a very general sense, are scrapers; those with sharp edges could be used for slicing or cutting."

None of the animal bones found nearby bears cut marks or cracks from hammerstones to suggest that people were butchering any of those potentially tasty grazing animals at Shangchen. Finding that kind of evidence, according to Dennell, would require a much larger excavation.

Cliffhanger archaeology

A larger excavation is likely to be a challenging prospect at Shangchen. For one thing, much of the surrounding land is being farmed. And despite its name, the Loess Plateau isn't flat terrain. Archaeologists have to contend with steep, soft-sided, slippery slopes.

"Working on those slopes is OK if you are careful. The sediments are soft, so it is not like climbing rock, unfortunately (I used to do a lot when much younger). We sometimes cut food holds," Dennell told Ars. "The slopes can be very slippery after rain, but we never had any accidents. There were times when I was working in Iran and Pakistan in the 1970s and 1980s when I used rope and a climbing harness to inspect sections, but we didn't need [them] at Shangchen."

Because of that difficult terrain, archaeologists can't be sure whether they've gotten all the way to the bottom of hominin presence on the Loess Plateau. They can't rule out the tempting possibility that even older stone tools may lie waiting in deeper layers. "We don't know if there are even older stone artifacts, but it is certainly worth looking for them! The hard part will be in finding sections that expose layers between 2.0 and 2.5 million years old. This is a decision for my Chinese colleagues to make," Dennell told Ars.

It's not likely that archaeologists will find any tools older than about 2.8 million years at Shangchen or any other side outside eastern Africa. The earliest evidence we have of a member of the genus Homo is a 2.8 million-year-old jawbone from a site in Ethiopia, and according to University of Texas archaeologist John Kappelman, all the evidence we have indicates that the first hominins to leave Africa were probably from the genus Homo, not hominin species that evolved earlier like Australopithecus or Paranthropus (although these were still around in Africa when the first members of Homo started chipping flint into handy stone tools).

And once those first hominins ventured out of Africa, their expansion would have been relatively slow.

"The dispersal of hominins was probably facilitated by population increases as they moved into new territories and filled empty niches," wrote Kappelman in a paper commenting on Zhu's study. Modern hunter-gatherer communities have a daily foraging range of between 3 and 9 miles, and if early hominins expanded into new territory at that same pace, it would have taken between 1,000 and 3,000 years to cover the 8,700 miles between eastern Africa and eastern Asia.

Eventually, of course, the descendants of those first hominin explorers would encounter long-lost relatives: the first anatomically modern humans, who ventured out of Africa in another wave sometime around 175,000 years ago.

Nature, 2018. DOI: 10.1038/s41586-018-0299-4 and 10.1038/d41586-018-05293-9 (About DOIs).