Multicellularity - a major recurrent evolutionary change
Most species in the extant world are multicellular organisms (plants and animals) living on continents. About a billion years ago, all organisms were unicellular and lived in oceans. The progressive colonisation of terrestrial biota is quite well documented thanks to fossils, comparative anatomy and phylogenetic reconstruction. This was accompanied by spectacular new phenomena such as increase in body size, diversification of life histories and ecological niches, and more complex interactions between species.
Multicellularity is a recurrent phenomenon which occurred independently several times (Baldauf et al. 2000). Multicellular animals (i.e. Metazoa) appear as a monophyletic clade. Terrestrial plants are another one, and also Fungi. Multicellularity is also observed in algae, among Protista (e.g. slime molds) or among Bacteria (stromatolithes).
Multicellularity raises a major challenge for evolutionary theory. In terms of Darwinian fitness, multicellularity implies, in a primitive stage, that some cells will not contribute to the next generation: they remain somatic for the benefit of their sisters. In other words, they become altruistic and the evolution of altruism, as observed in animal societies, was already a challenge for Darwin. Another problem is that multicellularity is accompanied by a delay in reproduction, and a decrease in the number of offspring, both resulting in an apparent decrease in the fitness of the population. In the next section, I shall try to explain how modern theory solves these apparent evolutionary paradoxes.