This is the most obvious change that needed to be made in order to move from an aquatic to a terrestrial mode of life. To obtain an adequate supply of oxygen from water organisms had special structures such as gills which had both an extensive surface and a rich blood supply. These are usually delicate structures that cannot support themselves out of water and cannot get enough oxygen because the gill filaments become stuck together (Paul, 1980). Oxygen is more abundant in air at 8.65mol m¯³ as opposed to 0.262mol m¯³ in water (Seldon, 2001), and so the real challenge of breathing in air is to evolve a structure that will not collapse under its own weight. Furthermore, this problem is made worse by water molecules being smaller than oxygen and carbon dioxide which causes membranes that are essential for gas exchange to 'leak water'. 'This means that respiratory surfaces need to be internalized and valves are required to regulate air flow - stomata in plants, spiracles in insects, etc.' (Seldon, 2001) This was relatively easy for vertebrates because several groups of early fish had already developed functional lungs, even before the first amphibians appeared. Terrestrial arthropods developed small tubules in their body called trachea, through which they could obtain oxygen. It is thought that these could be the only exclusive development of air-breathing devices for life on land (Paul, 1980).
Obtaining and Retaining Water on Land
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Marine or freshwater habitats have water all around them so they have no problem at all in obtaining it. However, on land water is only intermittently available and adaptations have evolved so that adequate supplies can be obtained. Two basic ways of solving this problem were both behavioural and actually developing special modifications to make sure water is stored or conserved during times of shortage. Vascular plants have roots which are specialized to extract water and nutrients from the soil, so are a good example of this. They also have a system of tubes within the stem that can transport water from the roots to the leaves where it is required for photosynthesis to take place. In contrast, most animals don't have specific organs which are primarily specialized to obtain water, preferring to take it in with their food or drink it directly.
Water is constantly lost from the bodies of organisms through respiration, excretion and temperature regulation. As carbon dioxide diffuses out of the respiratory system so do water molecules, and there are various mechanisms to reduce this. An example would be the two guard cells of each stoma in plants, which can open or close the pore depending on the humidity of the surrounding air in order to allow or limit the loss of water. To achieve this however, the rate of both photosynthesis and respiration has to also reduce (Paul, 1980).
Excretion is another way in which water is lost, but it is also related to another problem which is controlling blood concentration. Organisms living in the sea have the same blood concentration as the sea water which stops any unwanted effects from the process of osmosis by ensuring that there is no diffusion gradient, and therefore no energy has to be wasted maintaining blood concentration. In freshwater environments any organism that has blood more concentrated than its surroundings must use its kidneys to maintain it. Living on land requires a very efficient kidney because as little must be lost as a result of the process as possible, even if the skin is impermeable (Paul, 1980).
The risk of drying out on land is even higher for a single cell, which is exactly what is needed in the form of sperm and eggs in order to reproduce. Most marine invertebrates reproduce by just shedding many eggs and sperm in the sea and simply relying on chance in order for fertilisation to take place. The water is providing both the medium for fertilization and also the mechanism of dispersal, but because the surrounding water is absent on land fertilization is not possible this way. The only exception is wind pollination that some plants use which is very similar to the previous method. Copulation is the solution that animals evolved to use to overcome the problem of reproduction. This keeps the sex cells protected and provides the right environment for fertilization to take place, but needs special organs so that the transfer to be possible and also very highly modified behaviour. It suddenly becomes very important for recognition of the opposite sex to be possible and also to know if they are ready to mate (Paul, 1980).
Lack of Buoyancy
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With the lack of water to provide a strong upwards force of buoyancy, the structure of an organism becomes far more important because its body is virtually the only thing providing support. This problem increases exponentially as the size of the organism does, and so more and more efficient load-bearing postures must be adopted which in turn needs changes in locomotion. Early reptiles for example, were not well adapted to life on land and had splayed legs which reduced the time that they could move before having to rest their weight directly on the land. Reptiles such as dinosaurs as well as mammals and birds were much more successful because the legs were positioned underneath the body (Paul, 1980).
Forms of feeding that were appropriate in water such as ciliary filter feeding could no longer be used in the air. Some birds and bats can filter feed from aerial plankton, but this is the animal's own means of filter feeding - no exclusively filter-feeding aquatic group such as bivalves has made the transition to land (Paul, 1980). The evidence of the early terrestrial animals shows that the food chain was based on detritivory (the consumption of decomposing organic matter (Wetzel, 2001)) and also included carnivores, with little evidence for herbivores. A detritivory food chain is common in soils today (Seldon, 2001).
Temperature Control and Sense Organs
Water absorbs the sun's rays much more than air does, so even in the clearest seas they cannot penetrate more than around sixty metres. This is less of a problem for plants because they thrive on sunlight, but leaves still need to be able to combat overheating. To achieve this the chloroplasts adopt different positions in leaf cells on sunny days compared to days of shade. Animals deal with their higher sensitivity to light by developing protective pigments in their skins. There is also a much higher range of temperature fluctuation on land than water, so behavioural, structural and physiological modifications have to be made. To solve the former problem for example, burrowing underground in deserts or seeking out shade can provide cooling. The latter two are the only real options for plants, but are also utilised by animals. Features such as dorsal fins allow for temperature regulation - side on to the sun and heat would be absorbed, in contrast to end on to the sun which would radiate heat out of the body. Homiothermy (warm-bloodedness) is the most obvious change and allows the body to stay at the same temperature regardless of the outside conditions as long as they are not too extreme. To combat cold weather insulating material can be developed, while sweat glands can aid in heat loss during warm weather. Due to the difference in physical properties of air and water, light, sound and smells behave differently in both the mediums. Eyes need to be specially adapted for sight on land but still needs to be kept moist, which leads to the development of tear glands and ducts. Sound is not conducted as well through air as it is through water so complex, delicate ear structures have to be developed (Paul, 1980).
Example - Tetrapods
Support and locomotion were gained by modifying paired fins, pelvic and pectoral girdle, and the vertebral column allowed limbs to be strongly connected to a vertebral system that was very strong. Lungs were already in the osteolepiforms and were just enlarged as the gills disappeared, and the important bones of the limbs were already in the fin supports of the osteolepiforms. Water loss was prevented by development of dermal scales and he eyes were protected by lids and tear glands. The lateral line system which detects vibrations in the surrounding water was replaced by one with a small bone (the stapes) which was moved between a membrane at the back of the skull and the inner ear. Originally it was thought that tetrapods developed in freshwater conditions and then moved onto land in arid periods when there was cause to move from one body of water to another. Recently, suggestions are that they were originally marine because osteolepiforms lived mostly in marine conditions. Whichever one is correct, the move to land may have been because of the availability of food in the form of arthropods which had already made the move. (Elliot, 2000).
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It can be seen that there were many complex and difficult problems to overcome in order to make the transition from water to land.