Resolving the tree of life is a consequential goal of evolutionary biology and over the last several years there has been much research devoted to determining the relationships among the earliest branching animal lineages. As a scientist who studies sponges and cnidarians (taxa near the base of the animal tree), I have much enjoyed seeing new results come in and watching the debate evolve.
One hypothesis is that sponges (the Porifera) are our most distant animal relatives. This view has been supported by Phillippe et al. (2009), Pick et al. (2010), Nosenko et al. (2013), and Dorhmann & Wörheide (2013). The alternative hypothesis that ctenophores, the comb jellies (phylum Ctenophora), are sister to all other animals is supported by work from Dunn et al. (2008), Hejnol et al. (2009), Ryan et al. (2013), Moroz et al. (2014), Whelan et al. (2015), and Borowiec et al. (2015).
These studies included different ingroup and outgroup taxa, different genetic markers, and have used different phylogenetic methods, often yielding conflicting results. For example, Nosenko et al. (2013) found the position of the ctenophores varied between two multi-gene partitions (see figure below).
The latest paper in the mix by Pisani et al. (2015) reinstates sponges (for the moment, at least) as the sister group of all other extant animals. Recently, Casey Dunn, one of the major players in the effort to resolve the animal phylogeny, wrote a great perspective on the Pisani et al. paper and the quest to find the sister group to all other animals. Check out his post here: Who is our most distant animal relative?