STANFORD, Calif., April 16 (UPI) -- U.S. medical scientists say they've determined the placenta of humans and other mammals evolved from simple tissue that attached to the inside of eggshells.
Researchers at the Stanford University School of Medicine said the evidence suggests the placenta enabled the embryos of our distant ancestors, birds and reptiles to receive oxygen.
"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 Assistant Professor Julie Baker, senior author of the study.
The placenta grows inside the mother's uterus and serves as a way of exchanging gas and nutrients between mother and fetus. It is expelled from the mother's body after the birth of a baby. The placenta is the only organ to develop in adulthood and is the only one with a defined end date, Baker said, making the placenta of interest to people curious about how tissues and organs develop.
The research conducted by Baker and graduate student Kirstin Knox, the study's first author, is to appear in the May issue of the journal Genome Research.