The sex of baby mice—and quite likely baby humans—is determined by a virus that inserted itself into the mammalian genome 1.5 million years ago, Live Science reports.

Yale researchers published their surprising findings on March 30 in Nature. According to a press release, more than 40 percent of the human genome is comprised of remnants from viruses that invaded it millions of years ago.

Researchers found some of that viral material on mice X chromosomes—the chromosomes that determine the sex of embryos. As long as that viral remnant is active, males and females are born at equal rates.

But if the viral remnant is turned off, which UPI reports happens in some embryos, males are born at twice the rate of females. While it's unclear if the same process holds for humans, the human X chromosome contains the same viral material as its mouse counterpart.

What's more, researchers believe the method by which mice embryos turn off the viral remnant could be used to keep cancer from spreading. That's because the same viral material is also found in tumors and could be helping them grow.

But really they're still trying to figure this whole thing out. "Why mammalian sex ratios are determined by a remnant of ancient virus is a fascinating question," says esearcher Andrew Xiao.

(Thanks to genetics, girls in this town become boys at puberty.)

This article originally appeared on Newser: Ancient Virus Could Determine the Sex of Your Baby

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