 Understanding details of the evolutionary history of life is not simply a stand-alone problem (as interesting as that problem would be). The history of life and the history of the planet are tied together such that knowledge in one domain aids our inferences in the other. Knowing what types of fossils are embedded in rocks together with a phylogeny can help scientists date the rocks just as dating the rocks through independent means aids in dating the fossils and thus in estimating aspects of the phylogeny (Grantham 2004). And just as the physics and chemistry of the planet have shaped living things, life has shaped the planet as well. Photosynthetic cyanobacteria were partially responsible for the great oxygenation event roughly 2.4 billion years ago, where oxygen build up in the Earth’s atmosphere fundamentally altered the chemistry and physics of the earth.