Sunday, March 28, 2010

From Water to Land

On page 35 Coyne begins to describe the transition of organisms from water to land (more specifically from fish to amphibians). What adaptations, through natural selection, occurred to the organisms to better suit survival on land? What were the selective advantages to living on land instead of in the water? Neil Shubin, and his team, unveiled the discovery of the Tiktaalik roseae in 2006 in the Canadian Arctic ( How does this discovery tie into the transition from fish to amphibians?


  1. Two major adaptations that allowed for vertebrates to begin the transition to land include sturdy ribs and fewer, sturdier limb bones (instead of the many, tiny bones usually found in the fins of fish). Sturdy ribs allowed helped these transitional animals to pump air into their lungs, and to move oxygen from their gills, thus allowing oxygen exchange both below and above water. Sturdier limbs meant better support/balance on land.
    Selective advantages to living on land meant escape from predators and competitors found in the water, and access to the many incests found only on land, as a new source of food.
    The Tiktaalik serves as a transition from fish to amphibians because of it's physical features and the placement of its fossils. Tiktaalik were flat-headed predatory fish that lived in the shallow waters of steams. They had both lungs and gills, and they had sturdy ribs that allowed them to utilize these organs and breathe both above and under the water. It's limbs are part-fin, part-leg. So it had fish features, (gills and fin-ish limbs, along with scales) and amphibian features (lungs and leg-ish limbs, along with a neck). However, it couldn't be as successful of a fish in deeper waters, because it's flathead put it's eyes closer to the top of it's head; therefore it wouldn't of been able to spot predators that lurked below it, which happens a lot in deep as opposed to shallow waters. Also, it couldn't of been successful if it spent large amounts of time out of the water and on land - it's limbs didn't allow for walking. So Tiktaalik ties the transition from fish to amphibian by having both fish and amphibian features, yet at the same time not being able to survive in environments where most fish or most amphibians can.
    The Tiktaalik fossil record also ties it into the transition from fish to amphibians. It's fossils were found in strata around 375 million years old. As there were lobe-finned fish but no terrestrial vertebrates 390 million years ago, and clearly terrestrial vertebrates 360 million years ago, the transitional form between fish and terrestrial vertebrates would be found in rock around 375 million years old. Also, it's fossils were found in freshwater sediments. Since both late lobe-finned fish and early amphibians lived in freshwater, the transitional form between the two would be found in freshwater sediment. As Tiktaalik fossils were found in 375 million year old freshwater sediment, it clearly has ties to the transitional form from fish to amphibians, and thus itself ties into the transition from fish to amphibians.

  2. One of the adaptations that required water living species to evolve to land living creatures is, as Polly said, an organ that required oxygen to be delivered throughout the somatic cells for cellular respiration. In water, the gills are used to diffuse oxygen into the body. By moving water across the gill surface and having a large surface area, species with gills can process a gaseous diffusion of countercurrent exchange. This is when a process in which substance or heat is exchanged between two fluids moving in opposite direction. In order for an animal to survive on land, this must occur. In modern animals, the gas exchange mainly occurs in the lung of the tracheal system. Few amphibians can travel both on land and water due to different gas exchange. Instead of lungs, their gas exchanges are mainly in the skin. This skin diffusion allow the gaseous oxygen and Carbon dioxide to be diffused in and out of the body system. This fits one of the biological theme of structure and function. The structure of the gills allow its own function to gas exchange in the water. However, gills cannot perform gas exchange on land, which is why fishes cannot walk on land. Another adaptation that a specie must have is the type of excretory system. Aquatic animals excrete their metabolic waste as ammonia. However, ammonia requires much fluid in order to excrete. Thus, aquatic animals can excrete their waste as ammonia. However, on land, one of the special challenges is lack of water. Since land animals do not have abundance of water, they must convert their nitrogenous waste into urea or uric acid. These require less amount of water in order to excrete. Therefore, a land animal requires a process in which they convert ammonia into other nitrogenous waste material. One of the striking fact that scientists discovered from Tiktaalik roseae is that Tiktaalik roseae has both fish-like and tetrapod like characteristics. Fish-like characteristics are anatomical structures such as scales, fins and gills. Tetrapod-like characteristics include neck, ribs, flathead, fin skeleton and ear notches. Containing two different characteristic shows that the Tiktaalik roseae is a evolutionary discovery that contained two of the different species. It served as a evidence that land creatures were evolved from the fishes millions of years ago.

    Jerry Coyne: Why Evolution is True

    Niel Campbell and Jane Reece: Biology: AP Edition 8e

  3. Nutrition has traditionally been one of the driving factors in determining why animals move the ways they do. There are many well documented examples today of populations of large predators migrating to follow smaller animals they prey on because they need an energy source. One of the primary reasons that fish may have started to move onto the land is that they needed a more abundant food source. With the seas becoming crowded with more predators and more species competing for food, it could be expected that some individuals would look to other sources for food: particularly the land. The land offered plants and insects that lived close to the shore that the fish could feed on. Because fish did not have a developed method for gas exchange out of the water, early movements to the shore would be quick, with the individual poking its head out of the water to grab a bite. The Tiktaalik roseae shows many of the adaptations that early fish moving to the shore would need to have to be successful at getting food. Its more muscular, stronger front fins would allow it to prop itself up on the shore to extend its head out of the water. Its neck gave his head a greater range of movement, allowing it to track insects flying around it before taking a bite. It also had a longer mouth that could have allowed it to reach for plants growing close to the shore. Natural selection would then take its course, developing these traits to be stronger and more prominent, allowing later species which evolved from the Tiktaalik roseae to be more efficient at obtaining food from the land, eventually leading to full migration onto the land


    Why Evolution is True - Jerry Coyne

    Seasonal migration of predators and prey--A study of pythons and rats in tropical Australia - Thomas Madsen and Richard Shine (

  4. On page 36, Jerry Coyne describes certain traits of the amphibious transitory organisms: "flat heads and bodies, a distinct neck, and well-developed legs and limb girdles...[and also have] large bony fins that enabled them to prop themselves up on the bottom of shallow lakes or streams" (Coyne). Early tetrapods also exhibited fishlike structures ie. scales, limb bones, and head bones. The flat head of these amphibious creatures allows for the eyes and nose to be on top. This generally means that the organisms can peer and breathe above the surface. The robust fins would allow the organism to flex up and survey its surroundings. The neck is completely different as fish don't have necks. The sturdy ribs helped the animal pump air into its lungs. The sturdy fin limbs allowed the animals to heave themselves onto the land (most likely done by animals wanting to travel to a nearby stream with more food, escape a predator, or feed on insects on land).
    Some selective advatages to life on land may include a more plentiful food supply (more nutritious and abundant) or protection from predators. It would be advantageous to be able to live on a separate surface offering those things (foraging opportunities and protection).
    The discovery of Tiktaalik rosea was a large milestone in the sense that it made a "direct link between the earlier lobe-finned fish and the later amphibians." This fish had several traits that resembled the transitional forms previously mentioned. It provides the necessary outline for further transitory organisms but shows that there was a clear transition between amphibious and land animals.

    Why Evolution is True