The pelagic lifestyle is characteristic of plankton, a community of organisms that share the common condition of staying suspended in the water without the possibility of actively abandoning one water mass for another. The plankton components are divided according to size classes, ecological roles, and/or life cycles. An intriguing matter regarding plankton has affected scientists for a long time: why are there so many species in such a relatively simple and homogeneous habitat as the water mass? (Hutchinson 1961). From this point of view, plankton suggest that the solution to such a question is not only in the field of ecology and that the history of habitats and communities can play a role in justifying the presence of many species. The number of species living in each site is probably dependent on the age of the site, not only for plankton but also in general for the whole fauna, and a new interpretation of current scientific rules (such as the theory of island biogeography) is proposed. In the sea, many organisms spend a part of their life cycles in plankton before settling in the benthos and reproducing as sessile species. The larval stages that stay in the plankton (the so-called meroplankton) are the mirror of the simple and probably archaic conditions of living organisms. Pelagic life cycles, plankton larvae, and evolution of plankton lifestyles are presented and discussed on the basis of recent theories on the development of multicellularity on planet Earth.