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PRECAMBRIAN PERIOD

600-555 million years ago

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Coelomate hearts are homologous given the shared gene expression patterns which initiate their development.

How many times did animals evolve hearts? Are the hearts of invertebrate protostomes and deuterostomes homolous? At first, it might appear that they are not given the difference in their location in the body (a dorsal position in protostomes as opposed to a ventral position in higher deuterostomes) and the difference in the direction of blood flow through the dorsal and ventral vessels. These differences in the cardiovascular system, and a number of other differences in the body organization between protostomes and deuterostomes could be reconciled if a rotation of the body axis occurred in ancestral deuterostomes. As a result of this rotation from the ancestral condition, deuterostomes would possess a ventral heart and the anterior-flowing blood travels through ventral blood vessels while protostomes possess a dorsal heart and the anterior-flowing blood travels through dorsal vessels (Gerhart, 2000).
Genetic evidence supports that protostomes and deuterostomes utilize modified versions of the same ancestral body plan given the considerable number of developmental genes are shared. The last common ancestor of protostomes and deuterostomes probably possessed some type of primitive heart since homologous homeodomain genes are involved in the earliest formation of the heart (Msh-2 in Drosophila and csx in mammals) (Komuro, 1993). tinman/csk is expressed in heart progenitor cells in protostomes and deuterostomes but on opposite sides of the body (Gerhart, 2000). Several vertebrate proteins related to tinman are also involved in the development of the embryonic vertebrate heart. These vertebrate proteins can actually be substituted for tinman in Drosophila to promote the differentiation of the visceral mesoderm but they do not replace its function in promoting the development of the heart (Park, 1998). In flies, Dpp functions with wingless in the formation of the heart tube. Their vertebrate counterparts (bone morphogenetic proteins and Wnt/Wg respectively), function in the formation of vertebrate embryonic hearts (Nakamura, 2003).