The story of the brain begins in the ancient oceans, long before the first animals appeared. The single-celled organisms that swam or crawled in them may not have had brains, but they did have sophisticated ways of sensing and responding to their environment. "These mechanisms are maintained right through to the evolution of mammals," says Seth Grant at the Wellcome Trust Sanger Institute in Cambridge, UK. "That's a very deep ancestry."
The evolution of multicellular animals depended on cells being able to sense and respond to other cells - to work together. Sponges, for example, filter food from the water they pump through the channels in their bodies. They can slowly inflate and constrict these channels to expel any sediment and prevent them clogging up. These movements are triggered when cells detect chemical messengers like glutamate or GABA, pumped out by other cells in the sponge. These chemicals play a similar role in our brains today