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

by Richard Dawkins

Plot the logarithm of brain mass against the logarithm of body mass for lots of species of different sizes. The points will probably fall around a straight line, as indeed they do in the graph here. If the slope of the line is 1/1 (brain size exactly proportional to body size) it will suggest that each brain cell is capable of servicing some fixed number of body cells. A slope of ⅔ would suggest that brains are like bones and muscles: a given volume of body (or number of body cells) demands a certain surface area of brain. Some other slope would need yet a different interpretation. So, what is the actual slope of the line? It is neither 1/1 nor ⅔ but something in between. To be exact, it is a remarkably good fit to ¾. Why ¾? Well, that is a tale in itself, which will be told, as you will no doubt have guessed, by the cauliflower (well, a brain does look a bit like a cauliflower). Without pre-empting the Cauliflower’s Tale, I will just say that the ¾ slope is not special to brains, but crops up all over the place in all sorts of living creatures, including plants like cauliflowers. Applied to brain size, and with the intuitive rationale that must wait for the Cauliflower’s Tale, this observed line, with its ¾ slope, is the meaning we are going to attach to the word ‘expect’ as it was used in the opening paragraphs of this tale.