Researchers at the Stanford University School of Medicine have uncovered the first clues about the ancient origins of a mother's intricate lifeline to her unborn baby, the placenta, which delivers oxygen and nutrients critical to the baby's health.

The evidence suggests the placenta of humans and other mammals evolved from the much simpler tissue that attached to the inside of eggshells and enabled the embryos of our distant ancestors, the birds and reptiles, to get 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,"