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The evolution of the development

Biological journal of the Linnean Society 1990 v.39 no.2 pp. 109-124
Animalia, Protozoa, animals, asexual reproduction, cell cycle, embryonic structures, energy, eukaryotic cells, evolution, gastrulation, stem cells
The evolution of development required few new features not already present in the eukaryotic cell, as exemplified by the cell cycle. Moreover, the protozoa possess many features of spatial organization and regulation present in metazoan embryos. The earliest multicellular organism could have been reproduced by a stem cell mechanism or by fission, the latter requiring cell-to-cell interactions that may have favoured cell-interactions and regulation. Regeneration can be considered as a meta-phenomenon related to asexual reproduction and retention of embryonic characters. The origin of embryonic structures like the gastrula may be accounted for in terms of Haeckel's ‘Gastrea’ theory. Mechanisms based on selection at the level of cell lineage are rejected. It is not clear what selective forces act on development itself, as distinct from the requirement for reliably producing a functional orgainsm. There is, for example, a major problem why gastrulation should be so variable in related animals. Selection for rate of development in relation to energy utilization may play a role. If many variants are neutral this may facilitate the evolution of novelty. In general terms there is a requirement for a continuity principle for the evolution of each form in development. Most groups pass through a phylotypic stage with considerable diversity before and after.