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Such a long span of cryptic evolution strains credulity. The g

'phylogenetic fuse' might be a reasonable explanation for 5 to ee

10 million years of hidden evolution, but it is unlikely that a group g could sustain itself for huge spans of time and not either diversify s or go extinct.

The theoretical debate about the 'phylogenetic fuse' was brought to a rapid end by more recent studies, by Kevin Peterson from Dartmouth University and others. These used new molecular evidence to show that the estimated date of metazoan origins was really 650-600 million years ago, older than the oldest fossils, but not much older than the enigmatic Ediacaran faunas for example. The first analyses suffered from a variety of problems with the genes and the calculation methods. The key problem had been, however, that all the dates were extrapolated from known fossil dates for splits among early fishes and other vertebrates. And, unknown to the earlier analysts, the vertebrate molecular clock ticks rather more slowly than that for other metazoan phyla. Hence, extrapolating with a slow clock, but assuming a fast rate, extends the estimate far too deep; in fact this accounts for the virtual doubling of the estimate from 650-600 to 1,200 million years.

So, the Cambrian Explosion is back on track, more or less! There is still debate about whether, as seems likely, all the metazoan groups had appeared as naked forms well down in the Neoproterozoic, at the time of the Ediacaran faunas or even a little earlier. So, 50-100 million years of the early history of these groups is missing. And the problem of whether the Cambrian Explosion really represents the rapid acquisition of skeletons by all and sundry, or is in some way a preservational artefact, is still open.

The Cambrian Explosion is still wonderful and mysterious in equal measure. The new life that became established in the oceans - all the trilobites, brachiopods, echinoderms, chordates, 5 molluscs, and others - continued to evolve and develop ever-more o t complex ecosystems through the Cambrian and into the | subsequent Ordovician and Silurian periods. But something else 1= was happening at this time - some life forms were already exploring the margins of the oceans and making the challenging leap onto land.

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