Human mothers may have a longer and much more noticeable pregnancy than many mammals due to a virus that infected our ancestor’s DNA millions of years ago and has been passed down through the ages, according to a new study published in the journal Nature Genetics.
To determine why some mammals give birth after only a few weeks, while others carry their infants for several months, researchers compared the DNA from the opossum (which gives birth at two weeks), the armadillo (which gives birth between 60 to 120 days), and humans. Looking at the genes that were activated in each species during pregnancy, they found new stretches of DNA – called transposons – were present in the armadillo and human but not in the opossum.
The researchers hypothesize that mammals like the armadillo and humans, who carry fetuses longer and use a placenta to nourish them, may have picked up this different DNA from an infection while we were evolving into different species.
The theory doesn’t entirely address why we came to develop a placenta or how the immune system knows not to reject the developing fetus, which is half foreign DNA. It may, however, at least be an important piece in the evolutionary puzzle.
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