The First Animals
AT THE TIME unicellular phototrophic algae started to assemble into the first primitive seaweeds, heterotrophic protists were also led by the vagaries of mutation to experience the virtues and drawbacks of multicellular association, leaving it to natural selection to pronounce the final verdict. Because of the overwhelming need for food that dominates heterotrophic life, the selective advantages that drove animal evolution were different from those that propelled the evolution of plants, depending mainly on improved feeding and reproduction through cooperative association among cells.
The outcome is an amazing diversity of life forms, which the combined efforts of taxonomists, comparative anatomists and physiologists, paleontologists, and, more recently, biochemists and molecular biologists have ordered into a majestic genealogical tree depicting the evolutionary history of extant and extinct animals.1
The first elaborate animal tree was drawn by the nineteenth-century German naturalist and philosopher Ernst Haeckel, an early and enthusiastic disciple of Darwin, as well as an imaginative master in the art of weaving sparse facts into daring, persuasive generalizations. The most famous of these is summed up by the aphorism "ontogeny recapitulates phylogeny," by which is meant that animals, in the course of their embryological development (ontogeny), go through successive stages that recall the stages of their evolutionary history (phylogeny). Known as the recapitulation law,2 this statement, though not to be taken literally, expresses a profound truth. Recent acquisitions of molecular biology have shown that development is the main key to animal evolution, which has proceeded largely by way of genetic changes affecting body plans.
A possible misapprehension must be corrected. Many of us, when looking at a