Butz contacted scientists at a laboratory in Miami that specializes in radiocarbon dating who suggested he first conduct a simple test to determine if the soot stains occurred from more recent fires.

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"Radiocarbon testing in this type of situation provided a very strong test of growth rates and ages, " says Dr Ebert, from Oregon State University.

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My quest took me to three continents, from the US, to Italy, to Jerusalem and the radiocarbon dating laboratory in Oxford, which was part of the original test 20 years ago.

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Those elevated levels of radiocarbon were tracked carefully, and they began to drop off quickly after a test ban treaty was signed in 1963.

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