Physics Colloquium 7th November 2014. Delivered by Professor Steve Balbus, Savilian Professor of Astronomy, Head of Astrophysics, University of Oxford. The very similar angular sizes of the Sun and Moon subtended at the Earth is generally portrayed as coincidental. In fact, close angular size agreement is a simple mathematical consequence of even roughly

comparable lunar and solar tidal amplitudes. I will argue that the latter was a biological imperative. Comparable tidal amplitudes, sharing close but distinct frequencies, leads to beats and strongly modulated forcing. This tidal pattern must be understood in thecontext of paleogeographic reconstructions of the Late Devonian period. As seen below, two great land masses were separated by a broad western opening to the Rheic Ocean, tapering to a very narrow, shallow-sea strait. A classic WKB wave analysis suggests that the combination of this geography and modulated tidal forces would have been conducive to forming a rich inland network of shallow and transient tidal pools at an epoch when tetrapods were evolving. I will discuss the fossil evidence showing that important transitional species lived in habitats strongly influenced by intermittent tides. When the waters became anoxic, perhaps from sustained inwash of organic debris, a mass extinction ensued. The tetrapods endured, however, and we are their legacy.