"How multicellular animals (metazoans) emerged from their single-celled ancestor remains a long-standing evolutionary question. Recent genome data has shown that the unicellular ancestor of metazoans already had a complex gene repertoire for genes involved in cell adhesion, cell signaling and transcriptional regulation, including integrins, cadherins, T-box genes, and protein tyrosine kinases. Thus, besides a few metazoan-specific genes, gene co-option and, probably, an increase in gene regulation played important roles into the origin of Metazoa. However, the lack of genetic tools among metazoan’s closest relatives has so far precluded further investigations at the molecular level. Our recent establishment, for the first time, of transgenesis methodologies in two close unicellular relatives of metazoans (both the ichthyosporean Creolimax fragrantissima and the filasterean Capsaspora owczarzaki), allow us to approach these questions in ways that were not previously possible. Thus, we aim to push forward these two model systems and infer, by cell biology and functional genomics, the ancestral function of those genes key to multicellularity in order to understand how they were co-opted for new multicellular functions. In addition, we will analyze the regulation of the different cell stages and the colony formation (syncitial and aggregative multicellularity) in these two organism by functional genomics and identify when and how the metazoan histone code (an important regulatory layer of gene expression) evolved by analyzing the histone code in these taxa. Finally, to understand the ecology, distribution and adaptation of these unicellular taxa we will obtain the complete genome sequence of uncultured lineages by using single-cell genomics. This research will not only markedly improve our understanding of a major biological question (the origin of metazoan multicellularity) but will also generate new data relevant to a broad range of researchers."