In the 13th century, the Dutch poet Jacob van Maerlant envisioned a fish that had sprouted arms to lift itself to shore, all set for life on land. Recently genetic studies made this fantasy look surprisingly prescient.
Combined, a trio of studies published in Cell suggest that in terms of genes, the aquatic ancestors of four limbed land animals, or tetrapods, were pre-equipped with genes that could be turned to making limbs, efficient air breathing lungs, and a nervous system tuned to the challenges of life on land. In other words, the origin of tetrapods was something waiting to happen. Everything needed was already there before vertebrates first came ashore.
Fossils reveal that lobe finned fish moved into shallow water about 375 million years ago. The recent studies instead analysed the genes of living fish to gain insight into their genetic ancestors. One set of clues came from a mutagenised zebrafish. Specific mutations led to the development of two bones resembling the forelimb bones of land mammals in their front fins. These bones were complete with muscles, joints, and blood vessels. The genes responsible activate the Hox11 pathway which is usually suppressed in fish.
Other genetic clues are derived from the living relatives of ancient fish: lungfish and coelacanths. Geneticists at the University of Copenhagen sequenced the genomes of African lungfish. They discovered that the groundwork for terrestrial traits, like limbs and lungs, was laid deep in the fish family tree. All of the fish sequenced had the regulatory element which helps to form synovial joints. These makes limbs flexible and are essential for terrestrial locomotion.
The findings show that genetic material believed to only exist in land animals is also found in these fish. The lungfish genome also contained DNA for specifying five toes, connecting nerves to limb muscles and for sensitising the brain to react quickly. All those genes were previously believed to be unique to tetrapods.
The researchers believe the transition to lands occurred by using existing genes to adapt to terrestrial living, rather than building new structures. These studies demonstrate the extent to which the fish-tetrapod transition was achieved by modifying existing systems, rather than creating new ones.
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