Analysis of available data on the lunar torque and the mechanism and rate of tidal dissipation and studies of growth lines on Paleozoic corals imply that the Moon became an Earth satellite between 0.5 and 2.0 billion years ago. Origin of the Earth-Moon system by fission or close-capture should have left obvious indications in the geologic record. The absence of such indications is compatible with an origin by capture-at-a-distance or the aggregation of several smaller moons. Evidence of a noncatastrophic origin may nevertheless be present in the stratigraphic record as a consequence of greatly increased tidal ranges and tidal currents and the introduction of a lunar light cycle. Shallow-marine invertebrates, which are highly sensitive to changes in environment, would have been affected by the origin of the Earth-Moon system, and a causal relation with the appearance of hard-shelled marine organisms in Lower Cambrian strata is conceivable. Protective exoskeletons would be an adaptive characteristic for shallow-water bottom-dwellers suddenly confronted with powerful tides. If the origin of the Earth-Moon system provided the impetus for the evolution of hard-shelled organisms, the event occurred in late Precambrian time.

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