There has been debate for a number of years about whether sponges (the simplest, non-motile, filter-feeding multicellular organisms) or the comb jellies (multicellular, motile organisms with a rudimentory nervous system) represent the 'sister' clade that gave rise (from flagella-bearing protozoans) to all other types of complex animal (https://www.theguardian.com/science/2017/nov/30/evolution-row-ends-as-scientists-declare-sponges-to-be-sister-of-all-animals). Scientists have now declared that the totalled evidence supports the 'lowly' sponge rather than the more 'flash' comb jellies as the first stage of multicellular animal development. One telling piece of support is that, otherwise, the sponges would have had to have lost some of the more 'advanced' attributes seen in comb jellies (this seems unlikely but not impossible- see the parasitic flatworms).
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