A new discovery in the ongoing quest to determine when our human ancestors migrated out of Africa has shifted the timeline back – once again.
Writing in Nature, a team of geologists, archaeologists, and paleoanthropologists describe a rich cache of stone tools unearthed from a steep hillside in the Loess Plateau of north-central China. The 96 shaped flakes and unmodified hammerstones were found in 17 distinct sediment layers that date from 2.12 to 1.26 million years ago (Ma), providing evidence that a species of hominin – the lineage of bipedal humans that includes our genus, Homo, and its extinct ancient relatives – had settled in the region about 400,000 years earlier than previously collected fossils and tools indicate.
“Until now, the oldest known hominin site outside Africa was in Dmanisi, Georgia. Excavations at that site uncovered spectacular finds of the roughly 1.85 million- to 1.78-million-year-old remains of multiple hominins and stone tools,” wrote John Kappelman, a biological anthropologist and geologist at the University of Texas, in an accompanying article. A variety of other sites across western Europe to eastern Asia have confirmed that diverse populations of humans were established shortly after the Dmanisi settlement.
In an email to IFLScience, Professor Kappelman explained the significance of the findings made by lead author Zhaoyu Zhu and his colleagues: “[T]here was previous evidence for early hominins outside of Africa and across Asia at less than 2 million years ago. This new work moves the date back in time but more importantly shows that the dispersal was widespread across Asia.”
Zhu’s team dug at the picturesque Shangchen site for over a decade (2004 to 2017), uncovering the wealth of stones as well as bone fragments from antelope, deer, and pig family animals – though these have not been analyzed for signs of butchering. During the careful extraction process, the research group documented where in the fossilized layers of silt and soil each item lay, then examined the magnetic polarity of the sediment minerals in each layer. Because the dates of Earth’s many magnetic field reversals during the past 5 million or so years have been determined, the age of the sediment can be estimated by comparing the preserved polarity of the charged minerals within to a reference called the geomagnetic polarity timescale (GPTS).
The GPTS dating of the plateau’s layers shows that the artifacts were discarded by human inhabitants over a 0.85-million-year period, during which time the area’s climate switched back and forth from warm and wet to cold and dry more than two dozen times. Five times more tools were found in the warm-weather soil layers than in the glacial silty layers, advancing an intuitive theory that the Loess hominin population thrived in tropical phases and faced increased hardship during ice ages.
“[I]t appears that these early populations were limited by climatic extremes, and only were able to expand north during warmer intervals,” Kappelman added. “What they were doing with the tools remains to be demonstrated, but the circumstantial evidence suggests that they might have used them to process food items.”
Given the lack of fossils, it is impossible to say what species of hominin made these tools, and given the many gaps in our understanding of the human evolutionary tree, it would be difficult to assign certainty even if Zhu’s excavation had uncovered human bones. Currently, the oldest confirmed specimen of an individual belonging to the modern human genus Homo is a 2.8 Ma jawbone found in Ethiopia, suggesting the lineage may have arisen around that time. According to Kappelman, because no fossils of earlier hominins, like Australopithecus, have been found outside of Africa, many in the field believe that some unidentified species of Homo was the first to leave. The Dmanisi skulls support this, as they have many morphological similarities to the earliest Homo species in Africa.
Until the next exciting discovery, the mysteries of our origins abound.