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Understanding the chrononology of early migrations to Asia by hominids (beginning with Homo erectus) as well as their subsequent activities has continuously been hampered by lack of suitable dating methods. Such early archaeological materials are far too old for radiocarbon dating (limited to organics up to about 60,000 years old) and require geological dating methods. Much of the Asian region, however, particularly at cave sites, lacks the volcanic deposits usable (as in East Africa) for potassium/argon or argon/argon dating, which can reach back to date strata formed millions of years (myr) before present.
Every so often, however, researchers can find a way out of such predicaments by using other techniques. Simple flaked stone tools, discovered over 20 years ago in the ancient lake sediments of Xiaochangliang in Chinas Nihewan Basin (fig.1), were recently redated by paleomagnetism to 1.36 myr by geologist Rixiang X. Zhu of the Chinese Academy of Science in Beijing. Although not the oldest clues of hominids in Asia (Java in Indonesia and Dmanisi, Georgia date to 1.7- 1.9 myr; AR 1,1; 2,4), they add to the very few early and securely dated materials on the continent and they also contribute a new piece to the Asian puzzle - evidence for higher latitude living.
The method of high resolution, paleomagnetic dating used by Zhu and his team to determine the tools age relies on known shifts in the Earths magnetic field, which get recorded in sediments that contain iron particles. By analyzing the direction in which iron grains are aligned in a specific layer of sediment, one can know where magnetic north was at the time the material was deposited. Geologists have recorded such ancient changes in the magnetic field in places where they can also directly radiometrically date the sediments - such as at East African sites of Olduvai Gorge, Tanzania and Lake Turkana, Kenya - to produce the known dates of such global events (fig.2). This allows researchers to tie archaeological remains to a specific time period even in places where direct dating is not possible.
[Fig.1: China's Nihewan Basin (after Zhu et al. 2001).]
Zhus team scaled the Nihewan Basins steep slopes, taking sediment samples at 25-35 centimeter intervals in a verticle line, intersecting the horizontal layers of sediment which lie above, in, and below the one in which the stone tools were found. After analyzing the position of the grains in this series of samples, the whole Nihewan (Chinese) sequence was compared to and matched up with the known magnetic shifts in a Plio-Pleistocene section from East Africa. As it turned out, the Nihewan artifact level lay in a reversed-polarity layer (deposited at a time when magnetic north had shifted to the South Pole). It fell in between a normal-polarity period (where magnetic north was actually near the North Pole) dated radiometrically at Olduvai Gorge to 1.77 to 1.95 million years ago, and one dated there to one million years ago (fig.2). From the position of the Nihewan artifact layer within this band of reversed-polarity sediment, using estimated sedimentation rates, Zhu and his colleagues, including geophysicist Ken Hoffman of California Polytechnic State University, conclude that the tools should date from at least 1.36 myr.
Project colleague and anthropologist Richard Potts (Smithsonian Institution) surmises that the climate may have been milder than the harsh temperatures known today in the region, but nevertheless, migrating here would have been a challenge. The hominids who made the tools, presumably Homo erectus, had to traverse the Tibetan Plateau and navigate the Himalayas to reach this site which lies between the North China Plain and the Inner Mongolian Plateau at 40 degrees north latitude, 150 kilometers west of Beijing. Furthermore, the formidable Qinling (Kunlun) Mountains prevented southern migrations from this area.
[Fig.2: East African stratigraphies in Olduvai, East Turkana, and the Omo Delta, with paleomagnetic phases]
This research facilitates new perspectives about the early human migrations to Asia. It provides the oldest unambiguous evidence for hominids in East Asia at 40 degrees north latitude, suggesting that Homo erectus was able to adapt to a broad range of habitats. It also clearly shows, according to archaeologist Kathy Schick (Indiana University) among others, that early on in the Homo lineage, our ancestors had the ability to spread from Africa into other continents over vast and perhaps arduous terrain with relatively simple tools.
[Bower, B. Science News 160, 29 Sept. 2001; Gibbons, A. Science 293, 28 Sept. 2001; Zhu, R.X., Hoffman, K., Potts, R., Deng, C., Pan, Y., Guo, B., Shi, C., Guo, Z., Yuan, B., Hou, Y., Huang, W. Nature 413, 27 Sept. 2001]
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