The plant plan: multicellular life in the other Kingdom

One must conclude, then, that plants and animals may have evolved in quite different fashions. There is no doubt that they have independently evolved development, and this is demonstrated by the nonhomology of genes serving identical developmental functions in prepattern formation. There is nonetheless also no doubt that plants and animals have evolved from a common eukaryotic ancestor, as indicated by the clear homology of the genes that control the chromatin level of gene regulation. There is also little doubt that some developmentally important genes of plants, such as the ethylene and red light receptors, have derived from an event of horizontal evolutionary transfer specific to plants. And it is at least possible to think that the variation on which Darwinian evolution acts in plants results in part from phenomena that are not seen in animals, namely, the controlled appearance and Mendelian inheritance of epigenetically silenced genes. Genomic and genetic analyses of plants thus reveal a type of organism with familiar features, but profound differences from the more-studied animals. Only by further study of plants and of animals can we fully understand the differences between plants and animals, and consequently distinguish between those features of developmental pattern formation and cellular signaling that are necessary aspects of complex organisms, and those that are accidents of evolutionary history.