A new find has shocked scientists who didn’t imagine the earliest critter could be so complex. “This was a complete shocker,” said study team member Casey Dunn of Brown University in Rhode Island. “So shocking that we initially thought something had gone very wrong.”
“Our data reinforce several previously identified clades that split deeply in the animal tree, unambiguously resolving multiple long-standing issues. We find strong support for the placement of ctenophores (comb jellies) as the earliest diverging extant multicellular animals. A single origin of spiral cleavage (with subsequent losses) is inferred from well-supported nodes. A diminishing number of lineages remain recalcitrant to placement on the tree.
The spiral cleavage programme, a complex and highly stereotyped mode of early embryonic development, is present in at least Annelida, Entoprocta, Mollusca, Nemertea and Platyhelminthes. If corroborated by further analyses this would have major implications for early animal evolution, indicating either that sponges have been greatly simplified, or that the complex morphology of ctenophores has arisen independently from that of other metazoans.”
Casey W. Dunn et al Nature Vol 452 10 April 2008 p745 (thanks bFast)