Box 1. Box 1 How to make an animal — indirectly
From the following article:
Palaeoclimate: Loophole for snowball Earth
Bruce Runnegar
Nature 405, 403-404(25 May 2000)
doi:10.1038/35013168
In a provocative series of articles, summarized in ref. 9, Peterson, Cameron and Davidson have proposed a new model for the origins of the disparate body plans of the various bilaterian metazoan phyla — primitively mobile animals that have an anterior–posterior axis, a central nervous system, and tissues and organs derived from a middle body layer. Molecular phylogenies segregate all bilaterian phyla into three large groups: deuterostomes, ecdysozoans and lophotrochozoans, shown in the figure here with some distinctive component phyla. The last two are sister groups and they jointly share the last common ancestor of all living bilaterians with the deuterostomes. That animal, argue Peterson et al., had a free-living larval stage that bore little or no resemblance to the adult. The adult body was built from undifferentiated 'set-aside' cells that were free of growth constraints imposed on all other parts of the embryo. These cells could thus participate in a more sophisticated developmental process, involving Hox gene expression patterns, that is characteristic of all bilaterian phyla.
The key point here is that similar bilaterian larvae are thought to have given rise to very different adult body plans. A classic case is the dissimilarity of adult molluscs and sipunculan worms and the great similarity of their larvae. But here lies the difficulty. Molluscs and sipunculans are lophotrochozoans, so their origins lie well above the common bilaterian ancestor in the metazoan tree. Somehow, deployment and patterning of pre-existing set-aside cells had to be delayed until the various lineages leading to the bilaterian phyla had been separated for long periods of time. For example, distantly related members of the three bilaterian groups appear abruptly in the fossil record at the beginning of the Cambrian around 545 million years ago (arrows). However, molecular clocks10 suggest that the common ancestor of all three groups lived before the Cryogenian glaciations. Early-diverging bilaterians may therefore have been kept in 'larval mode' after the invention of set-aside cells and Hox cluster patterning by being forced to survive one or more snowball Earth glaciations in open-ocean environments.
BRUCE RUNNEGAR