Molecular clock
However, the development of methods of calibrating rates of change in DNA sequences has given the ability to estimate the last common ancestor of various lineages. It also allows the obtaining of molecular clock dates for lineages without a fossil record, which shows that zero-fossil phyla are also ancient. Such information comes with important caveats in relation to methodology, including the underlying assumptions of each method. "However, even the youngest dates from molecular clocks place the origins of the deepest branches in the tree of animal life—where the sponges and jellyfish branch off from the other early animals—over 600 million years ago, and so quite deep in the Precambrian." (p. 125) Gould accepted this, but noted that this does not negate the Cambrian Explosion. Molecular clocks date origins, while fossils date geographical spread and morphology. Molecular clock data cannot decide between gradual morphological change and rapid evolutionary bursts after initial species divergence. "Moreover, Gould argues that the fossil record supports the model in which the lineage splits much earlier than the distinctive morphologies evolve. For that explains why we find no Precambrian proto-arthropod fossils. In short, the 'hidden history' hypothesis remains an open option, but so does Gould's guess that the Cambrian explosion was genuinely explosive rather than an illusion generated by incomplete preservation." (pp. 125–126)