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PRECAMBRIAN PERIOD

600-555 million years ago

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Ancestral coelomates possessed homologs of several of the hormones that are important in higher vertebrates such as oxytocin, GHRH, and insulin. Coelomates also share nuclear receptors, a gene family which includes receptors for estrogen and testosterone.


A number of peptides similar to oxytocin/vasotocin are known in invertebrates such as Arg-conopressin-S in mollusks, Lys-conopressin-G in mollusks, cephalotocin in mollusks, annetocin in annelids, Lom-DH in arthropods, and Stp-OLP in tunicates. Mollusk conopressin genes are homologous to those of vasoticn/oxytocin in vertebrtates (Hoyle, 1998; Youson, 1999; Kesteren, 1992). In worms, annetocin functions in egg-laying and the contraction of neprhidia (which propel both wastes and gametes) (Ivell, 1999).


GHRH is produced by the hypothalamus to affect the secretion of growth hormone from the pituitary. Homologs of gene family members are known in flies. It is also a paracrine/autocrine hormone produced by the lung and gastrointestinal tract. Beta cells and the insulin they secrete are known in basal deuterostomes and some protostomes (Hoar). Mollusks have an insulin-like protein and the insect hormone bombyxin is also similar to insulin. (Chan, 1990).

Ftz-F1, a protein which activates the ftz homeodomain gene in Drosophila which is involved in the segmentation of the embryo is a member of the nuclear receptor superfamily. Some members of the family only contain the DNA-binding region of the mammalian nuclear hormone receptors (such Knirps and Embryonic gonald) while Ftz-F1 possesses both the DNA-binding and ligand binding domains found in the mammalian hormone receptors (Lavorgna, 1991)