The human placenta may be a biological curiosity, but it has a bit of history behind it - evolutionary history, that is. A study conducted by Dr. Julie Baker and Kirstin Knox suggests that the placenta may have evolved from much simpler tissue found in birds and reptiles.

Baker and Knox set out to test study the history of the organ by attempting to determine which genes are active in placenta cells of pregnant mice. Their research revealed that in two distinct steps.

In the first stage, the placental cells primarily activated genes that mammals have in common with birds and reptiles. This suggests that the placenta initially evolved through repurposing genes that early mammals inherited from their immediate ancestors more than 120 million years ago.

In the second stage, cells of the mice placenta switched to a new wave of species - specific genes. Different organisms tend to produce different genes according to their needs. For instance, mice activate newly evolved mouse genes and humans activate human genes.

Given that cloned mice are at high risk of dying soon after the placenta’s genetic transition takes place, these can lead to some particularly interesting findings. For one thing, the tissue doesn't change in appearance despite the dramatic shift taking place in the placenta.

Baker and Knox intend to pursue another study as a follow-up to this one. They are particularly interested in learning how the placenta protects the brain of the unborn baby.