for National Geographic News
When it's cold outside, modern humans don a sweater to ward off the chill. But how and when early humans began to develop an ability to cope with different climates has been a great puzzle in the study of human evolution.The answer is important because it suggests when early humans were able to migrate out of tropical Africa and settle all corners of the globe.
Now, researchers have determined that stone tools found in a region of northern China are 1.36 million years old, which provides direct evidence of the earliest human occupation of eastern Asia as far as 40 degrees north.
The stone tools were found in China's Nihewan Basin. During the period when they were used, 1.36 million years ago, much of the area was covered by a large lake that was ringed with forests of birch and elm trees. Mammals such as hippopotamuses, hyenas, rhinoceroses, and horses roamed the area.
While the climate was probably humid and warm most of the time, the area is thought to have experienced bouts of cold and dry weather. To settle in the region, early humans would have had to adapt to this climate fluctuation.
The stone tools are an indication of that early ability to thrive in a variable climate. They show that "early humans could live in a wide range of climate conditions," said one of the researchers, Rixiang Zhu of the Institute of Geology and Physics at the Chinese Academy of Science in Beijing. He and his collaborators published a report on their findings in the September 27 issue of Nature.
The stone tools from the Nihewan Basin were found more than 20 years ago, but until now their age was unknown. Anthropologists routinely determine the age of materials through a process known as isotopic dating, a technique based on knowledge about the rate of decay for certain radioactive elements.
Dating the stone tools from the Nihewan Basin was a challenge because they lacked suitable material for isotopic analysis. Zhu and his colleagues overcame the hurdle by correlating the magnetic polarity of the sediments in which the tools were found with a known timeline of when Earth's magnetic field shifts its polarity, or attraction toward a specific direction.
"We know that Earth's magnetic field flips polarity from time to time, and for the last several polarity reversals, the ages are rather precisely known," said Kenneth Hoffman, a paleomagnetist at California Polytechnic State University in San Luis Obispo and a co-author of the paper in Nature. "Sediments record the magnetic polarity of the field more or less as they are deposited," he explained.
The stone tools were found in a section of sediment that correlates with a known era of reverse polaritywhen the needle in a compass would have pointed south instead of norththat lasted from 1.77 million years ago to 1.07 million years ago.
Working under the assumption that the sediment was deposited at a constant rate, the researchers calculated that soils in which the stone tools were found were deposited 1.36 million years ago. Consequently, the stone tools must be 1.36 million years old.
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