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White pencil made out of a strand of DNA, on a blue background

Millions of amateur genealogists assembling family trees on Ancestry.com probably figure they’re just finding lost relatives and assessing their genetic proximity to Prince Harry, but in fact they have unintentionally made a significant contribution to science. An analysis of 54 million of the website’s public family trees finds that the heritability of life span, a hot research topic for decades, is considerably less than widely thought.

Scientists reported on Tuesday that genes accounted for well under 7 percent of people’s life span, versus the 20 to 30 percent of most previous estimates.

That low heritability “implies that it would be harder” to affect life span through genetic tinkering or other life extension ideas, said computational geneticist J. Graham Ruby of Calico Life Sciences, lead author of the study published in the journal Genetics. Google founded Calico in 2013 to find ways to combat aging.

Scholars who study the contributions of genes and environment to health weren’t surprised at the tiny heritability. “Welcome aboard!” said Dr. Sandro Galea, dean of the Boston University School of Public Health, who has studied the limits...