  The nearly equal lunar and solar angular sizes as subtended at the Earth is
generally regarded as a coincidence. This is, however, an incidental
consequence of the tidal forces from these bodies being comparable. Comparable
magnitudes implies strong temporal modulation, as the forcing frequencies are
nearly but not precisely equal. We suggest that on the basis of paleogeographic
reconstructions, in the Devonian period, when the first tetrapods appeared on
land, a large tidal range would accompany these modulated tides. This would
have been conducive to the formation of a network of isolated tidal pools,
lending support to A.S. Romer's classic idea that the evaporation of shallow
pools was an evolutionary impetus for the development of chiridian limbs in
aquatic tetrapodomorphs. Romer saw this as the reason for the existence of
limbs, but strong selection pressure for terrestrial navigation would have been
present even if the limbs were aquatic in origin. Since even a modest
difference in the Moon's angular size relative to the Sun's would lead to a
qualitatively different tidal modulation, the fact that we live on a planet
with a Sun and Moon of close apparent size is not entirely coincidental: it may
have an anthropic basis.
