AbstractThe evolutionary emergence of the first animals is thought to have been intimately associated to the formation of a primitive endomesodermal gut (i.egastrulation) from ancestral multi-cellular spheres, blastulae, more than 700 million years ago. However, the biochemical cues having been at the origin of endomesoderm formation remain a mystery.Here we find that hydrodynamic mechanical strains developed by sea wavelets on pre-bilaterianNematostella vectensisand pre-metazoanChoanoeca flexarepresentatives, which common ancestor dates back to more than 700 million years ago, can trigger gastrulation in a Myo-II dependent mechanotransductive process. Gastrulation in turn induces endomesoderm first biochemical specification through the mechanical activation of the βcat pathway in pre-bilaterianNematostella vectensis, like in Drosophila and zebrafish embryos, which common ancestor dates back to 600-700 million years ago.These observations converge to animal emergence that has been mechanotransductively triggered by wavelet mechanical strains on the sea-shore in multicellular choanoflagellates through Myo-II more than 700 million years ago, a process achieved in first metazoan through mechanosensitive Y654-containing βcat evolutionary emergence found as conserved in all metazoan.One sentence summaryMarine hydrodynamic strains have activated first gastric organ formation from ancestral pre-animal cell colonies.