Regeneration and regengrow in multicellular animals derive from the presence of processes of organ metamorphosis and continuous growth in their life cycles

The present review formulates an evolutionary hypothesis on the distribution of regeneration in invertebrates and vertebrates. Regeneration is a basal ancestral property of animals living in aqueous environment where life was generated. The specific life cycles that evolved in each phylum indicate that only adult aquatic animals with asexual reproduction, larval stages and metamorphosis, possess broad regenerative abilities, protostomes or deuterostomes. Regeneration derives from the re-utilization in different forms of numerous developmental gene pathways active during development and the transitional phases of larval metamorphosis. An injured adult animal, composed of differentiated tissues, cannot repeat the same sequence of gene activation of embryogenesis, resulting in a variable regeneration (most aquatic invertebrates and anamiotes). In contrast, species with a genome that is not programmed for producing larvae and intense metamorphosis, mainly terrestrial (numerous nematodes, arthropods and amniotes), cannot regenerate their organs after injury. It is hypothesized that during the evolution of terrestrial animals, they lost genes involved in regeneration so that they repair by wound healing associated with grow (regengrow) or by scarring. Future molecular knowledge on developmental pathways that evolved in regenerating competent animals will tell us whether or not organ regeneration in regenerative incompetent animals will be feasible.