Why are there eggs?

A description and update of the "egg-as-novelty" hypothesis is presented. It is proposed that the major animal phylum-characteristic suites of morphological motifs first emerged more than a half-billion years ago in multicellular aggregates and clusters that did not exhibit an egg-soma divergence. These pre-metazoan bodies were organized by "dynamical patterning modules" (DPMs), physical processes and effects mobilized on the new multicellular scale by ancient conserved genes that came to mediate cell-cell interactions in these clusters. "Proto-eggs" were enlarged cells that through cleavage, or physical confinement by a secreted matrix, served to enforce genomic and genetic homogeneity in the cell clusters arising from them. Enlargement of the founder cell was the occasion for spontaneous intra-egg spatiotemporal organization based on single-cell physiological functions - calcium transients and oscillations, cytoplasmic flows - operating on the larger scale. Ooplasmic segregation by egg-patterning processes, while therefore not due to adaptive responses to external challenges, served as evolutionarily fertile "pre-adaptations" by making the implementation of the later-acting (at the multicellular "morphogenetic stage" of embryogenesis) DPMs more reliable, robust, and defining of sub-phylum morphotypes. This perspective is seen to account for a number of otherwise difficult to understand features of the evolution of development, such as the rapid diversification of biological forms with a conserved genetic toolkit at the dawn of animal evolution, the capability of even obligatory sexual reproducers to propagate vegetatively, and the "embryonic hourglass" of comparative developmental biology.