LONDON Aug. 29 (Xinhua) -- An international research team has reported the first direct measurement of the general rate of genetic mutation at individual DNA letters in humans.

    The team, including 16 Chinese and British scientists, published its findings in the latest edition of the journal Current Biology.

    "If we say the mutation drives human evolution as an ongoing train, now we finally get its speed measured," said Dr. Xue Yali, a Chinese scientist working in the British Wellcome Trust Sanger Institute and the first author of the team's research paper.

    The team sequenced the same piece of DNA -- 10,000,000 or so letters or 'nucleotides' from the Y chromosome -- from two men separated by 13 generations, and counted the number of differences. Among all the nucleotides, they found only four mutations.

    The research shows that humans all carry around 200 new mutations in their DNA, which is equivalent to one mutation in around 30 million nucleotides.

    Fortunately, most of those mutations are harmless and have no apparent effect on health or appearance, Xue said.

    "The accumulated mutation in history has made us evolve from ape to human, but finding this tiny number of mutations in individuals was more difficult than finding an ant's egg in the emperor's rice store," Xue said.

    In a rural village in China's Jiangxi Province, the research team found a big family that had lived there for centuries. The team studied two distant male-line relatives - separated by thirteen generations - whose common ancestor lived 200 years ago.

    To establish the rate of mutation, the team examined an area of the Y chromosome. The Y chromosome is unique in that, apart from rare mutations, it is passed unchanged from father to son, so mutations accumulate slowly over the generations.

    Researchers compared the two Y chromosomes, actually 10,149,085DNA letters, and found just 12 differences, of which eight had arisen in the cell lines used for the work. Therefore, only four were true mutations that had occurred naturally through the generations.

    By knowing the number of mutations, the length of the area that they had searched and the number of generations separating the individuals, the team was able to calculate the rate of mutation.