I’m very short on time, so I will only do a basic introduction instead of a full post. In popular science documentaries or books, it is always said that all animals descend from a single common ancestor. (Typically followed by a horrible line leading straight to humans, hooray.) In other words, the animals are monophyletic, forming a taxon called the Metazoa. This is now accepted as being true.
Behind the scenes though, this matter has not always been so simple. Two theories for the hypothetical last common ancestor exist: the syncytial theory (Hadzi, 1963) in which the animals evolved from some multinucleated cilliate, and the colonial theory (Haeckel, 1868) in which the animals evolved from a colonial flagellate. However, these two theories have the assumption that the animals are polyphyletic: the problem comes from the sponges.
The sponges are now universally accepted as the basalmost animals, but for a long time – due to the above two theories – they have not been included as animals, because sponges have a whole load of unique cell types (in fact, most of the autapomorphies that allow us to characterise the sponges are these cells). So given that the sponges seemingly have different cells, they must have had a different origin from the other animals.
In the mid-1990s, this view began to be seriously challenged (e.g. Gamulin et al., 1994; Müller, 1995; Müller et al., 1994, 1995) by a shift in our thinking. Instead of looking at the cell types, we started looking to their interactions. After all, the key to animal evolution was the clumping together of cells to form a multicellular organism. Instead of looking at the identity of the cells, the more rational way is to compare how the cells in sponges and in animals interact with each other.
The framework for this rationale was provided by Willmer (1994), who examined what would have allowed the evolution of multicellularity from a unicellular ancestor. He concludes that the critical step is (obviously) the aggregation of many unicellular cells and them sticking together. In order to research the origin of such an interaction, we would thus have to look for adhesion molecules and their receptors, which all put together from the extracellular matrix (ECM). And wouldn’t you know it, not only do sponges have an ECM, their cellular adhesion mechanisms are more or less the same as in the rest of the animals. And given that the ECM is the main apomorphy for the Metazoa, it is thus inevitable to conclude that despite the sponges having unique cell types, they are indeed animals. And by virtue of their extreme simplicity and the very basic version of cell adhesion and ECM int hem, they are placed as an ancestrally divergent member of the Metazoa – but definitely as member of the Metazoa.
In summary: yes, animals are monophyletic.
Gamulin V, Rinkevich B, Schäcke H, Kruse M, Müller IM, Müller WEG. 1994. Cell adhesion receptors and nuclear receptors are highly conserved from the lowest Metazoa (marine sponges) to vertebrates. Biological Chemistry Hoppe-Seyler 375, 583–588
Hadzi J. 1963. The Evolution of the Metazoa.
Haeckel E. 1868. Natürlich Schöpfungsgeschichte.
Müller WEG. 1995. Molecular phylogeny of Metazoa (animals): monophyletic origin. Naturwissenschaften 82, 321-329.
Müller WEG, Müller IM, Gamulin V. 1994. On the monophyletic evolution of the Metazoa. Brazilian Journal of Medical and Biological Research 27, 2083-2096.
Müller WEG, Müller IM, Rinkevich B, Gamulin V. 1995. Molecular evolution: evidence for the monophyletic origin of multicellular animals. Naturwissenschaften 82, 36-38.
Willmer P. 1994. Invertebrate relationships: pattern in animal evolution.