Published online by Cambridge University Press: 24 September 2020
Here, I examine the link between intelligence and life. Unlike a skeleton, which is a requisite for any large organism, intelligence is less crucial for survival. It is much more thinly spread in the animal kingdom than are skeletons, and is absent entirely from the plant kingdom. After considering how intelligence might be defined, I consider the question of where it is found in the animal tree of life. I then focus on four examples – tool use by octopuses and crows, mirror self-recognition in certain mammals, and space travel by apes (both humans and chimps). I finish by considering the link between intelligence and Darwinian fitness. Over the course of animal evolution, some groups have prospered without having brains, others have evolved small brains, and others still – notably humans – large ones. Strangest of all, perhaps, is the case of the starfish group (echinoderms), where all current species are brainless, in contrast to their ancestors, which possessed brains, albeit small ones. This combination of evolutionary trajectories in brain size shows that the link between intelligence and fitness is complex.