3. The animal LCA likely possessed many different cell types

A major feature of animals is the spatial distribution of labour between coexisting cells within a single multicellular entity. The building blocks of multicellular bodies are different kinds of cell types, each specialized in different roles within the whole organism. Cell types have their own sets of expressed genes used in different processes (e.g., contraction, secretion, signaling and reception), that are normally regulated by well-defined genetic programs (a set of TFs and other specific regulatory mechanisms). This implies that some genes are expressed by certain cell types but not others – in other words, each cell type expresses a limited number of genes encoded in the genome. On the other hand, the totality of genes in the genome of a unicellular organism may be used by a single cell throughout its life. The genome partitioning into functional modules accessed by different cell types reflects an increase in regulatory mechanisms to determine diverse cell fate genetic programs (Arendt et al., 2019). Understanding the origin of animals, therefore, requires asking questions about the evolutionary origins of cell types, and their mechanisms of differentiation.

In the previous entry we learned a bit about the “historical” context of the origin of animals, both in the evolutionary and the chronological meaning of the word. We saw how the fossil record was the first resource explored to unraveling the origin of animals, and how it helped to pinpoint the geological period when this lineage emerged. We also learned that comparing the genomes of different animal species has become a new paradigm of research, as it helps to clarify the phylogenetic relationships of animals, as well as it allows to infer the what genetic information needed to build animal bodies was present in the ancestor.

In this entry, we will explore what was the gene content like in the animal LCA in two ways: firstly, by broadly looking at the changes occurred in the genomes of animals during their evolutionary history; secondly, by looking at the genes found scattered across the early branching animals, which together are like pieces of the puzzle that is the animal LCA.