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Ever since the discovery the couple have devoted themselves to chopping away at that stubborn little word if.In the face of the entrenched skepticism of their colleagues, it is an uphill task. In those same four years since the first harpoon was found at Katanda, a breakthrough has revived the question of modern human origins.The fossilized remains of extinct animals found near the object also provide a biostratigraphic record that can offer clues to a new find’s relative age.(If a stone tool is found alongside an extinct species of horse, then it’s a fair bet the tool was made while that kind of horse was still running around.) Sometimes the tools themselves can be used as a guide, if they match up in character and style with tools from other, better-known sites.But some is made up of the unstable, radioactive form carbon 14.When an organism dies, it contains about the same ratio of carbon 12 to carbon 14 that exists in the atmosphere.After death the radioactive carbon 14 atoms begin to decay, changing into stable atoms of nitrogen. Scientists can look at the amount of carbon 12 and--based on the ratio-- deduce how much carbon 14 was originally present.Since the decay rate of carbon 14 is constant and steady (half of it disappears every 5,730 years), the difference between the amount of carbon 14 originally in a charred bit of wood or bone and the amount present now can be used as a clock to determine the age of the object.
In design and workmanship the harpoons were not unlike those at the very end of the Upper Paleolithic, some 14,000 years ago. Brooks and Yellen believe the deposits John was standing in were at least five times that old.
The breakthrough is not some new skeleton pulled out of the ground.
Nor is it the highly publicized Eve hypothesis, put forth by geneticists, suggesting that all humans on Earth today share a common female ancestor who lived in Africa 200,000 years ago.
In France alone there must be three hundred well-excavated sites dating from the period we call the Middle Paleolithic, Brooks says.
In Africa there are barely two dozen on the whole continent. On an afternoon in 1988 John Yellen--archeology program director at the National Science Foundation and Brooks’s husband--was digging in a densely packed litter of giant catfish bones, river stones, and Middle Paleolithic stone tools.
Plants take in carbon from the atmosphere to build tissues, and other organisms take in plants, so carbon ends up in everything from wood to woodchucks.