ABSTRACT
When thinking about the origin of animals (Metazoa), many questions arise. Most fundamentally: What were the first animal lineages to evolve? Many of the early invertebrate lineages are soft-bodied, with unreliable to non-existent fossil records. So, to help answer this question, most researchers review animals that are alive today. Formally speaking, this scientific inquiry is usually focused on finding the sister to all other Metazoa (SOM), determining which lineage branched off first from the rest of the animal evolutionary tree. This remains a field of highly active research, with diverse and contentious results being produced constantly. The data we have to answer this question using extant animals are still limited. Anatomical and genetic clues are often obscured due to the hundreds of millions of years of evolution since the first animal lineages originated. Despite great difficulty in untangling the relationships among the earliest diverging animals, the implications for animal evolution cannot be overstated. Answering the SOM question informs the evolution of fundamental features, such as body symmetry, nervous systems and muscles, and allows us to search for “natural classification” of the animal kingdom.
