Easter and Passover are closely linked festivals , and this year they overlap, with Passover beginning on Good Friday, April 14. Both are rooted in ancient traditions of blood sacrifice, which in turn has its evolutionary roots in the behaviour of predators; when they have killed one prey animal and have enough to eat, the others are safe for a while. The death of one protects the others. These archetypes run very deep in many religions and cultures. They are still with us today in secular societies, on an unprecedented scale: in the US alone, about 25 million vertebrate animals are sacrificed every year on the altar of science in biomedical research. This is a good time to reflect on how much we owe to those who have died for our sakes, and the Easter archetype of death and rebirth.