What’s the secret to living a long life? After four years of exploring just that, a team of geneticists at Stanford University have found the answer is more elusive than one might think, reported SFGate.com.

“We were looking for a really simple explanation in a single gene,” study author Stuart K. Kim, a Stanford geneticist and molecular biologist, told SFGate.com. “And we know now that it’s a lot more complicated, and it will take a lot more experiments and a lot more data from the genes of more supercentenarians to find out just what might account for their ages.”

Kim and a team of scientists at Stanford studied the genes of 17 “supercentarians,” or people who lived to the age 110 and beyond. After four years of studying an estimated 21,000 human genes, they could not find a gene or group of genes that were linked to the study participants’ long life span.

In the study, published Wednesday in the journal PLOS One, the researchers compared the genes of 17 supercententenarians and those of 4,300 “normal” subjects recorded earlier in a National Institutes of Health (NIH) study. They used a technology called whole-genome sequencing to identify genes linked with disease, immunity, and others whose functions are unclear.

None of the 17 supercentenarians in the study is alive. Their ages ranged from 100 to 116, and the study authors confirmed their life spans by referencing official documents such as birth certificates and passports, Kim told SFGate.com.

A vast majority of the supercentenarians appeared to be exempt from age-related disease at the time of their death. Only one of the subjects, who had Alzheimer’s disease, was not. One subject worked as a pediatrician until age 103 and died at age 114, while another drove a car until the age of 107.

“As would be expected,” Kim told SFGate.com, “supercentenarians have escaped many age-related diseases,” and their rates of cancer and heart disease and stroke are not significantly different from the general population.

The researchers found that the family members of the centenarians also have a longer than average life span, despite the fact that their lifestyle choices— like smoking and diet— aren’t especially different when compared with the habits of ordinary people.

“Taken together,” Kim told SFGate.com, “these findings provide ample evidence that extreme longevity has a genetic component.”

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