This book deals with a classic problem in macroevolution: how do we reconcile the Darwinian ‘tree of life’, which implies that every single branch or living species is historically and adaptively unique, with apparent differences in levels of complexity between organisms such as bacteria and humans? The first author to deal with the issue was Darwin himself, at the same time a believer in the power of natural selection to create biological diversity and in evolutionary ‘progress’, or the emergence of complex organisms from humbler beginnings. Besides Darwin, I was also inspired by Erwin Schrödinger and his insightful definition of life; by J. B. S. Haldane, Julian Huxley, John Maynard Smith and Eörs Szathmáry and their aggregation model; by Marcello Barbieri, Eva Jablonka and Marion Lamb and the study of biological codes of inheritance. This book is an attempt at melding together those ideas so as to present one possible answer to the problem of evolutionary complexity.

My intention was to offer at the same time an update on classic themes (including the evolution of sex and the role of genes in adaptive evolution) and an introduction to new debates (in particular the controversies surrounding the comparisons between social behaviour in humans and other species). Many important topics could not be addressed: plants are rarely mentioned, as emphasis was given to animals, behaviour and communication; Price's equation and Kimura's neutral theory were not discussed; and the chapter on evolution and development is necessarily tentative due to the constant transformation of the field.