How did we first walk?
The excretory system of fish varies within the freshwater and marine groups but is not the same as land vertebrates. The urea excretion essential for life on land must have evolved before emergence from water. New modifications on the genes involved can be found in ancestral "four-legged creatures" or tetrapods. The limbs, fingers and toes and placenta were also innovations in the new vertebrates. Immune response is very different on land, according to the genes controlling it and our landlubber senses became much more chemosensitive through other gene modification. The sense of smell was obviously critical in their lives.
While scientists have struggled with the enormous size of the lungfish genome this paper on the coelacanth, Latimeria chalumnae, from the Comoros Islands, is much more comfortable in its conclusions. For 70 million years this group of fish, with two living representatives, has retained a similar chromosomal complement to humans, but with remarkable little mutation. We reported on the tulip tree, Liriodendron the other day, which has also retained its genome with little mutation. The paper does conclude however that it is the lungfish, as seen above that is the more likely ancestor! According to this concise chart, the poor old coelacanth went out on a limb and has stayed there for 70 million years.
Credit: © Nature
This chart above shows most vertebrates on earth with convenient links showing who's who in terms of your ancestors 22 vertebrates are shown starting with cartilaginous fish, and shows that the lungfish is more closely related to tetrapods than the coelacanth, and that the protein sequence of coelacanth is evolving slowly. Pink lines (tetrapods) are slightly offset from purple lines (lobe-finned fish), to indicate that these species are both tetrapods and lobe-finned fish. Thanks to the journal, "Nature."
Chris T. Amemiya and many fellow researchers contributed to the "Nature" paper, "The African coelacanth genome provides insights into tetrapod evolution." The immune system was found to be lacking the immunoglobulin-M of tetrapods, although it had the lungfish and cartilaginous fish protein, immunoglobulin-W. Perhaps the bony fish that has IgM will be a potential ancestor?
The living fossil reputation of the coelacanth is undisputed then, but it is without living descendants that are different morphologically. It is extremely helpful here because so far the lungfish genomes enormous size has prevented normal analysis. Excretion, limbs and immune responses have been investigated in order to give us an idea of how changes could take place to suit this fish to life above water . Its lungs are likely to place it on the first step to land, but the living fossil has proved much more useful in proving these two fish were close to the evolution science wants to investigate the most: vertebrates conquering the hazards of "life in the dry."
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