Between 2015 and 2016, NASA astronaut Scott Kelly spent 340 consecutive days in space while his identical twin brother Mark, a retired NASA astronaut, remained firmly planted on Earth. The pair were participating in a groundbreaking study that allowed scientists to study the impact of space travel on the body—critical knowledge for sending astronauts on increasingly long missions.

Researchers are finally reporting the initial results of the aptly-named Twins Study, which suggest that extended time in orbit can result in substantial changes to the human body—right down to the molecular level.

As Alexandra Witze reports for Nature, NASA took a range of biological samples from the Kelly brothers before, during and after the space mission, testing everything from the microbiomes in their guts to the length of their DNA. Because the Kelly brothers have “almost identical genomes and similar life experiences,” Witze writes, researchers were seeking out differences between space-bound Scott’s biological samples and those of ground-based Mark.

The study did, in fact, reveal interesting changes. While Scott was in space, his telomeres—the caps on the ends of chromosomes—grew longer than Mark’s. The telomeres returned to their pre-flight length shortly after Scott returned to Earth, but the findings were nevertheless surprising.

Telomeres shrink as a person ages, explains Rae Paoletta for Gizmodo, and scientists had previously believed that the stresses of spaceflight caused them to shorten at a faster speed. Such eroding telomeres can lead to health defects, making it crucial to understand the effects of space on telomeres.

Telomere size wasn’t the only difference the researchers identified. Scott’s DNA methylation, a chemical mechanism that controls gene expression, decreased in activity while he was in space. Methylation activity is impacted by things like diet and sleep patterns, and it shifts all the time in people on the ground—Mark’s, in fact, increased during the period of study. But the change in Scott’s DNA methylation seemed atypically large, perhaps because he was subjected to the atypically harsh conditions of space travel (freeze dried food, changing sleep patterns, exposure to microgravity, and so on).

The full results of NASA’s twin study won’t be released for another year, and scientists don’t yet know why the changes in Scott’s biology occurred. As NASA scientist John Charles put it during a news briefing, the “data analysis is only now beginning in earnest."