The migration of vertebrates from water to land was one of the key events in the history of the biosphere, but it also had negative consequences. In a new study, scientists found that the loss of the ability to regenerate in reptiles, birds and mammals was a kind of price for adapting to terrestrial life.

Photo: flickr.com by Brian Gratwicke, CC BY 2.0

For a long time, all life on Earth was limited to the aquatic environment. Only in the Paleozoic did the first plants, fungi, and invertebrates begin to appear and colonize land. Chordates, the predecessors of modern vertebrates, came to land later, when ecosystems were already partially formed. One of these pioneers was Tiktaalik, an ancient creature that was on the border between fish and amphibians.

Later, vertebrates became the dominant inhabitants of land. From the first amphibians came reptiles, then mammals and birds. However, this evolutionary success was accompanied by the loss of the ability to restore lost body parts, which is characteristic of most land vertebrates.

While fish and amphibians display amazing regenerative abilities, such as regenerating limbs, hearts, and even brains, reptiles, birds, and mammals have very limited regenerative abilities. The exceptions are a few species, such as lizards, which can regenerate their tails. But in general, terrestrial amniotes (reptiles, birds, and mammals) have lost the ability to fully regenerate.

Researchers from the Institute of Bioorganic Chemistry of the Russian Academy of Sciences and the Russian National Research Medical University have found that the loss of regenerative abilities was caused by the complication of physiological processes, such as increased immunity, keratinization of the skin, transition to warm-bloodedness, and an increase in body size. These changes led to the loss of many genes and regulatory DNA elements that played a key role in regeneration.

Interestingly, vertebrates lost their ability to repair themselves long before their genomes lost the genes needed to do so. About 150 such genes in fish and amphibians are active in the initial stages of regeneration, immediately after injury. Their loss in higher vertebrates, including humans, did not have a fatal impact on survival because they do not play a key role in embryonic development.

The article also highlights that restoring a lost limb in humans may be possible through tissue engineering. This would require first creating an embryonic limb rudiment that could later grow into a fully functional organ.

Clarifications

Regeneration (Latin regeneratio – restoration) — the ability of living organisms to restore damaged tissues over time, and sometimes entire lost organs. Regeneration is also called the restoration of an entire organism from its artificially separated fragment (for example, the restoration of a hydra from a small fragment of the body or dissociated cells).

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