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In evaluating the brothers' data, the researchers noticed some striking differences in various biological markers, such as gut bacteria and even chromosomes.

Last March, NASA astronaut Scott Kelly returned to Earth after spending almost one year in space. Thanks to a pair of twin astronauts, NASA has been able to study the changes to the bodies of those who spend long periods of time in space to a degree that wouldn't otherwise be possible.

Spending a year in space affected former NASA astronaut Scott Kelly's body in subtle but potentially significant ways, new research suggests. The level of methylation, a process that can change the activity of a DNA segment without changing its sequence, decreased in Scott's white blood cells during flight and increased in Mark half-way through the study.

"However, upon his return to Earth, they began to shorten again", the officials added.

Another research team found an apparent decrease in bone formation during the second half of Scott's space mission, and another group identified a slight decrease in cognitive ability (thinking speed and accuracy) shortly after he touched down. The entire time, Mark stayed on Earth. The study said the hormone, linked to bone and muscle health, was, "likely impacted by heavy exercise countermeasures during flight". Scott and Mark hosted different gut bacteria, or the "bugs" that aid in digestion, throughout the year-long study.

After sequencing the twins' genomes scientists found that each twin had hundreds of unique mutations.

"RNA (transcriptome) sequencing showed more than 200,000 RNA molecules that were expressed differently between the twins", NASA officials wrote. In testing both Kellys over that period of time, NASA scientists found that Scott Kelly's level of chemical DNA modification slowed while in space.

Scientists said they are unsure if the results indicate that genes are sensitive to changing environments.

Through further research integrating these preliminary findings, in coordination with other physiological, psychological, and technological investigations, NASA and its partners will continue to ensure that astronauts undertake future space exploration missions safely, efficiently and effectively.