5. The Evolution of Metazoa

5.0 Introduction

The evolutive direction taken by the metazoa was improvements of energy utilization and refined perception of the internal as well as external world by the endocrine and nervous systems. The development of a nervous system with the accumulation of a nervous material in a brain is the key to evolution. I mentioned in chapter 4 the epistemological error of gene determinism applied to complex systems. The error is the attempt to fit dynamic non-linear changes into a linear theory of the gene. A second epistemological error is the exclusion of developmental biology from evolutionary synthesis, in favor of genetic programs. Influenced by the theory of the gene, proposals were made that sponges, starfishes, and lungfishes are the forefathers of man. Based on developmental biology, the mesenteric hypothesis defended by Vandebroeck 1 suggests a phyletic line from jellyfishes to man that seems more realistic.

Besides the sponges, the most primitive metazoans are the coelenterates (corals, jellyfishes). They represent the first divergent evolving phase of metazoa that conquered the seas and dispersed into innumerous different species. From coelenterates evolved, after the stress of the Precambrian Ice-Age, various phyletic lines such as that represented by sea stars, that represented by worms and that represented by scorpions. After a brief dominance, scorpions receded in favor of fishes. The fishes represent a divergent phase of evolution, which irradiated in various phyletic lines along the chordate phylum of which one, under the stress of dry climate, succeeded in the invasion of land. A convergent phase (the amphibians) led to the appearance of the reptiles. The huge dinosaurians represent the third divergent phase. After long dominance on land, air and seas, birds (i.e. the last representatives of the Dinosaurians) and mammals replaced reptiles. Mammals and birds immediately diverged in innumerous species, occupying all available niches. Birds enjoyed a short time of dominance, followed by mammals. Among these, the monkeys and apes evolved a neural system and an anatomy that led to a convergent line, the hominids. The end product of this line is the human species, which embodies the sixth, ultimate divergent line.


1. Eléments d’anatomie comparée des chordés, edited by himself in 1954

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