Is it possible that organisms adapted to aquatic life (such as fish) could evolve into terrestrial animals that breathe air?


If evolution is correct, a number of separate aquatic lineages could develop adaptations for terrestrial life and air breathing independently. No one set of adaptations would be absolutely required; a number of alternate forms would be adaptive. Transitional states would be possible in which some, but not all, of the adaptations required for full terrestrial life would be beneficial.


If the creationism model is correct, aquatic lineages can not evolve adaptations for terrestrial life. Transitional states are not expected, nor are multiple separate origins of comparable adaptations.


If intelligent design is correct, it is not expected that the "design" of fish would be need to be reworked for terrestrial life or air breathing. It is certainly not expected that the design of separate lineages would all need to be reworked to similar ends. If the adaptation to terrestrial life is an example of irreducible complexity, aquatic lineages should not exist which benefit from some, but not all, of the traits required for terrestrial life.


If there was only a single animal body plan which would accommodate life on land, it could be argued that variations within populations of aquatic animals might never achieve terrestrial existence without an "intelligent design". Our understanding of life's diversity has demonstrated that there is no such limit on the adaptability of aquatic organisms. A great variety of aquatic organisms have modified their originally aquatic "design" for terrestrial life. Worms first diversified in the oceans but different lineages (such as nematodes and segmented worms) have successfully invaded the land. While most snails and slugs are aquatic, some are fully terrestrial. A number of arthropod groups have adapted to terrestrial life including myriapods (centipedes and millipedes), scorpions, spiders, the ancestors of insects (such as Devonohexapodus), and many crabs. Among fish, there are a variety of lineages which can venture onto land for brief periods. These fish would undoubtedly evolve greater adaptations for terrestrial life over time were it not for the richness of modern terrestrial vertebrates that would limit their success. The rhipidistian ancestors of amphibians did not develop the only possible suite of anatomical features which would allow life on land, they simply evolved one of many possible sets of terrestrial adaptations.

Life on land requires a minimum of two modifications: the ability to support oneself on land and the ability to breathe oxygen from the air rather than water. Although terrestrial organisms can evolve respiratory, skeletal, and muscular systems which are dramatically different from their aquatic ancestors over great expanses of time, aquatic animals may require few if any modifications for their initial ventures onto land.

Many aquatic animals have adapted their respiratory systems to allow the breathing of oxygen from the air as a way of enhancing survival in purely aquatic environments. The ability of aquatic animals to breath oxygen from the air is thought to have evolved at least 67 times (Maina, 1998). A number of crustaceans can do so. The sea cucumber Holothuria tubulosa can breathe in stagnant water by rising through the surface and performing gas exchange through its cloaca. (Hoar, 1983, p. 505) There are lung-like structures in some snails. Some oligochaete and polychaete worms perform gas exchange through their anus (Maina, 1998).

A considerable number of fish can breathe oxygen from air. The majority do so as a supplement to the oxygen transported by the gills but some are actually obligate air breathers that drown in water without access to air. The chamber in which air is held for gas exchange with surrounding blood vessels can be simple but many fish have adapted it for more efficient exchange through the branching of the chamber to increase surface area and the development of a very thin respiratory membrane. Increased vascularization of caudal and pelvic fins function in respiration in the walking goby (Periopthalmus) and the South American lungfish, respectively. (Weichert, 1970, p. 244) Some fish perform gas exchange with water moved through their highly vascularized rectum. Some aquatic turtles possess 2 sacs opening into the cloaca which can function in gas exchange (Weichert, 1970, p. 244).

Some fish perform gas exchange with air through their buccopharyngeal apparatus (mouth and throat), an opercular structure, the swim bladder, the intestine, pharyngeal lungs, the skin, and the gastrointestinal tract which allow gas exchange using swallowed air. Some of these fish are obligate air breathers and may travel for short distances over land.(Maina, 1998, p. 220). An air-breathing species of the family Loricaridae is pictured in the following photos.


While the fine filaments of most fish gills collapse in air, a few fish possess modified gills which can breathe atmospheric air (Johansen, Kjell from Hoar, 1970). Some placoderms seem to have possessed lunglike structures (Perry, 2001). The air/swim bladder of bony fish functions as a respiratory organ in lungfish and in basal actinopterygians, suggesting that this was its primitive function in the ancestral bony fish. In other words, swim bladders seem to have evolved from lungs rather than lungs from swim bladders. Basal actinopterygians even possess blood vessels which correspond to the pulmonary circuit of lungfish and tetrapods, although there is no separation of this blood from that of the systemic circuit in the heart (Johansen, Kjell from Hoar, 1970).
Note that there are capillaries in the swim bladder of fish which bring the blood close enough to the epithelial lining of the bladder to perform gas exchange with the air contained within.

swim bladder

If a lung is defined as a vascularized air sac, then lungs are known in some spiders, decapods, chilopods, isopods, and snails (and in all scorpions). (Hoar, 1983, p. 508). The ancestors of bony fish evolved lungs as an outpocket of the esophagus (Romer, p. 363). The gar pike and Amia will suffocate without atmospheric air; as will sarcopterygian lungfish (Romer, p. 360, 364). The placoderm Bothriolepis possessed a pair of pharyngeal pouches which extended posteriorly and probably functioned as simple lungs. Some sharks possess a vestigial structure which may be related to this. (Weichert, 1970; Kardong).
Among vertebrates, lungs are not unique to tetrapods, they actually evolved in fish before the adaptation to life on land. Some fish alive today, appropriately called lungfish, possess true lungs, the homologs of tetrapod lungs. The Australian lungfish, the most primitive of the three lungfish, possesses a single lung while the African and South American lungfish possess paired lungs. Once African lungfish (Protopterus) reach more than 100g, more than 85% of their oxygen is obtained through gas exchange in the lung (Orgeig, 1995). While the Australian lungfish is an opportunistic air breather, African and South American lungfish are primary air breathers during several months of the year which they spend on land. (Hoar, 1970) The African lungfish secretes a mucus cocoon where it waits in the mud for the next rainy season. During this time, it depends on oxygen from the air which is breathed through its lungs. Experimentally, these lungfish have been able to survive 5 years in this state.(Webster, 1974, p. 372) The lungs of lungfish actually function better than those of many amphibians. (Weichert, 1970, p. 221).
In lungfish, the lungs are long structures which join with the esophagus (just as the swim bladder of actinopterygian fish joins with the esophagus).



Why would a fish evolve lungs? Lack of oxygen in water can cause mortality in fish and still waters can have very low levels of dissolved oxygen (especially at warmer temperatures). Any fish with lungs has an advantage in these environments since it can supplement its oxygen supply from the gills. A variety of modern fish can breathe oxygen from atmospheric air (which is why a goldfish in a fishbowl doesn't need an air pump to survive).



Locomotion on land does not absolutely require new structures other than those which are already present in aquatic organisms. Terrestrial snails, myriapods, spiders, scorpions, and crabs are not fundamentally different from their aquatic relatives. A number of different lineages of fish can move on land without any obvious structures which are lacking in their purely aquatic relatives. Stout, robust fins have evolved in a number of fish lineages ranging from placoderms (such as Bothriolepis), actinopterygians (such as mudskippers and many catfish), and extinct sarcopterygians. The limb bones which land vertebrates utilize today are simply modified versions of the bones which supported fins in purely aquatic fish. Increased mobility of these elements (such as the evolution of jointed digits in Sauripteris) could occur in fish which were adapted to aquatic life.
The first amphibians do not appear to have been intelligently designed for life on land. Their primary adaptations for terrestrial existence were the lungs and robust appendicular skeleton that had evolved first in their fish ancestors as adaptations for aquatic life. Given the variety of aquatic animals which have successfully made the transition onto land, it is evident that early amphibians simply achieved one of multiple possible sets of adaptations for terrestrial life.