Here's the next question in the evolution debate: We know roughly how the sequence of life ran forward in time. What about running it backward? How would you feel about rewinding human evolution to a species that's almost like us, but not quite?

Last week in Nature, scientists reported major progress in sequencing the genome of woolly mammoths. They reconstructed it from two fossilized hair samples. One was 20,000 years old; the other was 65,000 years old. Now, according to Nicholas Wade of the New York Times, biologists are discussing "how to modify the DNA in an elephant's egg so that after each round of changes it would progressively resemble the DNA in a mammoth egg. The final-stage egg could then be brought to term in an elephant mother."

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