Which animal groups are involved in the earliest split in the animal tree?

Three major hypotheses have been proposed in recent decades about the earliest split in the animal tree. Given the structural simplicity of sponges and their similarity to some animal outgroups (especially choanoflagellates), for many years biologists considered sponges to form the sister group of all other animals. When placozoans were first described, their structural simplicity also led other biologists to suspect that they, rather than sponges, were the sister group of other animals. In contrast, the ctenophores share some superficial similarities with cnidarians and were traditionally considered to be more closely related to them, rather than splitting earlier at the base of the animal tree. However, as biologists collected complete genomes from all these lineages, it became clearer that the simplicity of sponges and placozoans derives from a mix of ancestral retained features and secondarily derived losses of complexity. For example, although placozoans, with only four cell types and no true organs, are structurally less complex than ctenophores, this structural simplicity is now thought to represent an evolutionary reversal in the placozoan lineage. In contrast, some of the “advanced” features of ctenophores (such as their nervous systems) were gained through independent gene duplications and specializations compared with the nervous systems of cnidarians and bilaterians. As we explored in Investigating Life: Reconstructing Animal Phylogeny from Protein-Coding Genes, phylogenetic analyses of many genes now suggest that ctenophores are the sister group of all other animals.

Future directions

As biologists collect more genomes from new species of animals, it is becoming possible to reconstruct the various gene duplications and changes that have led to the major structural and functional differences among animal groups. Having several independent origins of nervous systems is a boon for understanding how nervous systems arise and evolve. The understanding and study of these genomes is just beginning, but already complete genomes are shedding considerable light on the origins of animal complexity.