The Protistan Cellular and Genomic Roots of Animal Multicellularity

Abstract

Metazoans evolved from a single-celled eukaryotic ancestor more than 650 Ma ago. While the investigations on the origin of animal multicellularity remained purely theoretical for over a century, the recent discovery of the closest extant unicellular relatives of metazoans opened the way to the comparative study of this major evolutionary transition. In this chapter, we will describe how the sequencing of the genomes of multiple protists phylogenetically related to animals and the study of the biology of these species are illuminating the emergence of metazoans. We will show that recent data support a scenario with a complex unicellular ancestor of all animals. From comparisons of animal and protist genomes, we can infer that this unicellular ancestor had many of the genes involved in animal multicellularity (adhesion, signalling, transcriptional regulation). Moreover, the regulatory biology and diversity of cell behaviours in extant unicellular relatives suggest that the unicellular ancestor of animals had multiple temporally separated cell types. These cell types might have become spatially integrated in the first multicellular animals through the advent of genomic regulatory novelties.