Astronomer Scott Kelly spent a year in space and returned a changed man.
Kelly, who lived on the International Space Station while his identical twin Mark Kelly stayed on Earth, returned after 340 days with 7% of his genes altered, according to NASA.
The shocking transformation means Scott is no longer genetically identical to his twin.
Scott, who shared his own surprise over the latest NASA twin study results, tweeted, “What? My DNA changed by 7%! Who knew? I just learned about it in this article.”
“This could be good news! I no longer have to call @ShuttleCDRKelly my identical twin brother anymore,” he added.
Some changes — like the lengthening of Scott’s telomeres, or the endcaps of chromosomes that shorten over time — reversed once he was subject to Earth’s gravity again.
But other changes persisted after six months. Researchers found a deficient amount of tissue oxygenation in Scott’s cells, along with stress to mitochondria, which transform nutrients into energy, and increased inflammation.
Scott, who became the first American astronaut to spend a year in space, returned to Earth in March 2016.
The Twin Study, seen as the ultimate test of nurture versus nature, “may prove useful in the development of new treatments and preventive measures for related health issues on Earth,” according to NASA.