Abstract
This manuscript summarizes recent hypotheses to explain the distribution of regenerative abilities among animals. The hypothesis is based on evolutionary considerations pointing out that while organ or even body regeneration is present in marine animals, limited regeneration or no regeneration is manifested in terrestrial animals. This loss derives from the terrestrial conditions for life, primarily including shortage of water, high UV and ROS exposition. These conditions are incompatible with regeneration after injury, a process that requires formation of soft embryonic-like tissues. While marine animals include variably complex larval forms and metamorphosis in their biological cycles, these characteristics were lost in terrestrial animals that evolved a direct development. Regeneration was lost during land colonization, initiated in the Paleozoic Period. Therefore, developmental gene pathways governing metamorphosis and regeneration in water could no longer be expressed on land. During the water to land transition the changing environment characteristics determined modification in the genomes of marine animals through epigenetic mechanisms that modified developmental gene pathways, including those operating for organ regeneration. Broad regenerative ability is still present in numerous fish and amphibians that live in submerged or amphibious environments and possess larval forms and metamorphosis, while amniotes, including humans rapidly form scars after organ injury.
Keywords
Animals, Human, Healing, Regeneration, Biological evolution