Life's Vital Link: The astonishing role of the placenta

The development of the placenta was a pivotal event in evolution. Without it, we would still be laying eggs instead of giving birth to live offspring. It represents the critical link between the foetus and the mother, but its character is extraordinary -- it is, in effect, a foreign tissue that invades the mother's body. Compared to many other animals, the human placenta represents a particularly aggressive body. But how is it managed and controlled? How did such an organ evolve in the firs...