The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

Gastrulation is something that all animals do early in their life. Typically, before gastrulation, an animal embryo consists of a hollow ball of cells, the blastula, whose wall is one cell thick. During gastrulation the ball indents to form a cup with two layers. The opening of the cup closes in to form a small hole called the blastopore. Almost all animal embryos go through this stage, which presumably means it is a very ancient feature indeed. You might expect that so fundamental an opening would become one of the two deep holes in the body, and you’d be right. But now comes the big divide in the animal kingdom, between the Deuterostomia (every pilgrim who arrived before Rendezvous 26, including us) and the Protostomia (the huge throng who are now joining at Rendezvous 26). In deuterostome embryology, the eventual fate of the blastopore is to become the anus (or at least the anus develops close to the blastopore). The mouth appears later as a separate perforation at the other end of the gut. The protostomes do it differently: in some, the blastopore becomes the mouth, and the anus appears later; in others, the blastopore is a slit that subsequently zippers up in the middle, with the mouth at one end and the anus at the other. Protostome means ‘mouth first’. Deuterostome means ‘mouth second’.