Study suggests human placenta has reptilian ancestry

BEIJING, April 25 (Xinhuanet) -- New research reveals the placenta, which supplies the fetus with oxygen, nutrients and ensures the fetus is not rejected by the mother's body, that spills from a woman's body after childbirth is rather reptilian in its ancestry.

"The placenta is this amazing, complex structure and it's unique to mammals, but we've had no idea what its evolutionary origins are," said researcher Julie Baker, a molecular biologist at Stanford University in California.

Now Baker and her colleague have discovered the inner lining of eggs laid by the distant ancestors of all mammals could be the origin of the placenta, and the whole setup evolved as mammals employed leftover reptilian-like genes. A better understanding of all this could shed light on pregnancy and disorders linked to it, the researchers say.

To investigate how the placenta evolved, Baker and Kirstin Knox analyzed which genes are active in the cells of the placenta throughout pregnancy.

"I was pregnant with my first daughter, and I was really interested in learning more about it," Baker said.

The researchers found the placenta develops in two distinct stages. In the earlier stage, which runs from the beginning of pregnancy through halfway through, the cells in the placenta primarily activate genes that mammals have in common with birds and reptiles.

These findings suggest the placenta initially evolved when early mammals found new uses for genes they inherited from their reptile-like ancestors. For instance, there is a simple tissue attached to the inside of eggshells that currently allows unborn reptiles and birds to absorb oxygen from the air. The placenta might have evolved from similar tissues, although the placenta is far more complex.

In the later stage the researchers found, cells of the placenta undergo a major change genetically that is specific to each species, switching on thousands of genes that were not on earlier. For instance, rodents trigger genes special to rodents and apes activate ape genes.

(Agencies)