Timings of major glaciations, inherent selection schemes and the origins of cold hardy animal phyla.

BACKGROUND: Knowing the origin dates of specific phyla may help in understanding the intricate relationships between environments and their biota. Past extreme environments would have challenged biota and led to the evolution of phyla that were and possibly still are able to withstand extreme conditions. OBJECTIVE: We test the hypothesis that major glaciation events imposed strong selection schemes, ultimately leading to the origins of cold hardy phyla. METHODS: We identified dates of past major glaciation events, cold hardy phyla and their origin dates and synthesized these data in a phylogenetic context. RESULTS: Origin dates of cold hardy phyla do not correspond with major glaciation events, falsifying the hypothesis. CONCLUSION: An alternative hypothesis is proposed in which varying degrees of cold hardiness evolved at other taxonomic levels within these phyla.