Dating early animal evolution using phylogenomic data
Information about the geological timeframe during which animals radiated into their major subclades is crucial to understanding early animal ecology and evolution. Unfortunately, the pre-Cambrian fossil record is sparse and its interpretation controversial. Relaxed molecular-clock methods provide an alternative means of estimating the timing of cladogenesis deep in the metazoan tree of life. So far, thorough molecular clock studies focusing specifically on Metazoa as a whole have been based on relatively small datasets or incomplete representation of the main non-bilaterian lineages (such as sponges and ctenophores), which are fundamental for understanding early metazoan evolution. Here, we use a previously published phylogenomic dataset that includes a fair sampling of all relevant groups to estimate the timing of early animal evolution with Bayesian relaxed-clock methods. According to our results, all non-bilaterian phyla, as well as total-group Bilateria, evolved in an ancient radiation during a geologically relatively short time span, before the onset of long-term global glaciations (“Snowball Earth”; ~720–635 Ma). Importantly, this result appears robust to alterations of a number of important analytical variables, such as models of among-lineage rate variation and sets of fossil calibrations used.

We all learned in school that animals suddenly radiated in the Cambrian. Some weird animals preceded them in the Eidiacarian, the Vendobionta, but they were oddballs. The real story started with the Cambrian Explosion. Before the Cambrian, even before the Ediacaran was the Cryogenian. The “Snowball Earth” which was a wall. Before the Cryogenian was just bacteria. All of the exciting stuff happened afterwards.

This paper, building on other preceding publications, shows that the earliest animals, the earliest phyla INCLUDING BILATERIA developed before the Cryogenian. Animals appeared, and started to radiate ~800mybp, even the first phyla appeared. They were ready to take over some 200 million years early. Then at ~720mybp the world froze over and they retreated into refuges to survive through the Snowball Earth. Advancement paused. When the ice finally retreated animals came out of their refuges. They were ready to aggressively take over first the seas and then the land.