When trying to solve a mystery, it is very important to look at the right place. Evidence can be erased, though, which may lead to misleading clues. This is true both for crime novels and evolutionary hypotheses. And in the case of the evolutionary origin of metazoa, we may well be in one of the most intriguing noir movies.
Metazoans are characterized by a great degree of cell specialization and complex tissues. Neurons, muscles, epithelia and many more are finely orchestrated by a huge array of regulatory that decide when, where, what and how much cell types will change into each other. Metazoans are related to a group of single celled or colonial protozoans called choanoflagellates; and both share high similarity with the vast group of fungi. Choanoflagellates and fungi lack the type of cell diversity that characterized metazoans, a fact that is reflected in the absence in their genomes of a good portion of the metazoan regulatory proteins. Metazoans invented themselves, truly an evolutionary miracle. But this story had some gaps, and many of these protein families were being discovered in other, much less related eukaryotes. What was going on?
Enter the scene Capsaspora ocwarzakii. This organism is a member of filasterea, a protozoan group sister to both choanoflagellates and metazoans. While it has a complex lifestyle, alternating between an aggregative colonial form, single amoeboid cells and a cysts phase. Its arrival served to solve the seemingly miraculous origin of animals. Its genome contains many of the regulatory elements that were thought to be exclusive to metazoans, proving that these mechanisms were more ancient than previously thought, and that they had an homologous function in cell differentiation prior to the development of complex multicellularity.
Electron microscopy picture of Capsaspora ocwarzakii. Arnau Sebé-Pedrós & Iñaki Ruiz-Trillo. Creative Commons 2.0 license.