If we look at fish, they should have all the basics for early tetrapod life on land. Their fins could have become modified like mud skippers; their eyes must have changed for the refractive index of air; and their osmoregulation would be unnecessary, apart from retaining more water. Amphibians evolved the urea excretion system that some of us have found useful ever since.
For 3 billion years, the oceans supported invertebrate life, then vertebrates, sheltering them from ultra-violet light and other hazards. With ozone layer in the atmosphere, land became more habitable and less vulnerable to the U/V. Unsuccessful colonisation must have been rife, but plants made it to land around 450 mya, making it a likely landing point for herbivores and the niches they sought.
Arthropods were the likely invaders from the ocean, perhaps 5 times, according to rumour. Once was spiders, another landing was by the insects, so what happened to the others is any invertebrate's guess. 380mya, the fish made their move. We think the coelacanth,tetrapods, is close to the first tetrapods because it has suitable limb-like fins. The genome of the zebra fish, Danio rerio, is more familiar and should show the bony fish as possible relatives.
On Ellesmere Island in Canada, a group of researchers led by Neil Shubin of Chicago found transitional fossils, including Tiktaalik. This was a mud skipper look-alike, so Shubin wrote a book called, "Your Inner Fish," relating its descendants as being fish, amphibian, reptilian avian and ourselves! The coelacanth is a slowly evolving fish, so its genome will possibly resemble some of the first fish on land, although a recent paper we reported found the lungfish closer to, for example, humans than the typical bony fish, the goldfish! From the coelocanth's terrestrial descendants, 50 genes have been left in the water. That means they were found surplus to requirements on land. In contrast, there are 44,000 elements such as genes that are conserved in tetrapods after the split from coelacanth-like fish.
10,000 genes are shared between all of human, moue, chicken and zebra fish, but the fish have more unique genes that the others don't possess. One important point is that 75% of human disease-related genes are present in this fish. What does this mean? The fish is obviously very useful for medical research and more amenable than a land animal. We all have an, "inner fish," it seems and we can be cured of genetic diseases via the zebra-fish.
The article was written by Michael Gross in the latest issue of Current Biology.