Written in stone
Lawrence Edwards, the U's 'cave man,' discovered a way to read 500,000 years of history in the rock record
In 2007 a jawbone and two molars with some a modern human features turned up in a cave about 100 miles from the Gulf of Tonkin, in southeastern China.
Modern humans, it was thought, had moved out of Africa and spread into eastern Asia by 40,000 years BCE.
It could have happened tens of thousands of years earlier than that date; the fossil record was too sparse to tell. But when a team led by University of Minnesota researcher Lawrence Edwards, using methods he devised, dated the 2007 remains, they turned out to be an astounding 100,000 years old.
In that one fell swoop, Edwards and his colleagues raised the question of whether modern humans could have made it across the vast expanse of Asia far earlier than suspected. Because the remains also bore characteristics of more primitive humans who were already in the area, it is possible that the two groups could have coexisted for some time the way moderns and Neanderthals did in Europe.
But the discovery, published in the Proceedings of the National Academy of Sciences, is only the tip of the iceberg. Edwards has used his dating methods to uncover a whole library of information about the last 500,000 years, a period in which the dearth of data has been especially frustrating.
"A series of ice ages, the most recent stages of evolution, and human history all happened during this time, as well as many episodes of abrupt climate change," says Edwards, a professor of geology and geophysics. "When applied to certain types of rocks, [our methods] are better than the best carbon-14 dating, which is limited to dating materials from the last 50,000 years."
No more stony silence
In dating the human remains, Edwards and his team examined not the remains themselves but the rock laid down on top of them as minerals grew from flowing water in the cave. Information stored in such rock, called flowstone, and in stalagmites—both born of minerals precipitating out of water—makes caves one of Edwards's favorite sources of material.
To measure the ages of rock, he uses the "uranium-thorium" (also called the "thorium-230") dating method, in which he must detect incredibly small amounts of the elements uranium and thorium (see sidebar). And by measuring the proportions of different forms of oxygen, he can tell how much rain fell at the time the rock was deposited.
In previous work, Edwards and a colleague used those techniques to trace variations in the strength of monsoon rainfall in China and linked weak monsoons to the fall of several historical dynasties. Since then they have tracked the monsoons with great accuracy back 400,000 years, when Homo erectus, not Homo sapiens, inhabited the region.
"The monsoon history is an exciting piece of work coming out of Minnesota," says Edwards. "It's the best-dated climate record covering this time period. We pieced it together from many stalagmites of varying ages."
Strong monsoons correlate well with climatic warming in Europe, and weak ones with cooling, such as the low temperatures caused by "calving" of glaciers that raised the amount of sea ice in the northeastern Atlantic Ocean. Such events can be dated with greater accuracy by determining the strength of monsoons in the general time frame when the events occurred.
In another example, about 1,500 years ago an abrupt, worldwide climate change sent the average temperature in Greenland soaring by a whopping 16 degrees C in the space of a decade. Edwards and his colleagues detected the "signature" of this warming in Chinese cave stalagmites, where it appeared as a strengthening of the monsoons during that period. Labs around the world have adopted Edwards's work with cave rocks to study climate and to plot the course of climate change in time.
"It's amazing to me what you can find in the natural world if you know how to look," he says. "For instance, … looking at stalagmites from a cave and learning about climate change."