Why hasn鈥檛 evolution reintroduced gills in the land-based mammals that returned to the sea?
Graham Smith
Melbourne, Australia
Evolution cannot make fundamental changes in a single step, no matter how logical it may sound to a 鈥渄esigner鈥� (such as adding gills to a marine mammal). Evolution proceeds via small steps, each of which must add to the individual鈥檚 chances of survival. If lungs plus gills had a significant advantage, then it is more likely that gills would have been retained during the transition to lungs.
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The fact that gill function wasn鈥檛 maintained by evolution during the transition to air-breathing animals is probably because the efficiency of transferring oxygen from air to blood is hugely more efficient than transferring oxygen from water to blood. The higher rate of oxygen intake enables a much higher metabolism of air-breathing animals, and this extra oxygen allows for fast animals (seals), much more complex brains (dolphins) and much larger bodies (whales).
Evolution used a much simpler tactic: it came up with ways that let the animals stay underwater for longer
It is unlikely that gills could support any warm-blooded animal, because the transfer rate of oxygen is partly determined by the concentration of oxygen in the surrounding fluid, which is around 7 parts per million in water and 210,000 parts per million in air.
Herman D鈥橦ondt
Sydney, Australia
It鈥檚 all because of a simple fact about evolution: it builds on what already exists, and prefers to take the easy, low-cost way.
Swim bladders allow fish to effortlessly float at different depths. Bony fish developed their swim bladders from ancestors that used air sacs. In environments where oxygen was lacking, the fish gulped air into those sacs. Some fish like eels still have them. Sharks emerged before air sacs originated, and hence they don鈥檛 have swim bladders.
After the fish started gulping air, their oesophagus gradually widened until it became an air sac. That, in turn, was then modified to become a swim bladder. The sequence shows the gradual progression that evolution uses to build new organs. Land animals developed their lungs from the same air sacs. That was (relatively) easy: just modify the sacs so they can absorb oxygen into the blood.
However, converting lungs back into gills that can let water stream through them would be very difficult, and the intermediate steps might well lead to an evolutionary dead end. The same arguments preclude the creation of brand new gills, unless some existing organ can be converted into them. Instead, evolution used a much simpler tactic: it came up with ways that let the animals stay underwater for longer, and that let some of them even sleep there without drowning.
Note that other land animals that have returned to the water, like ichthyosaurs, did not develop gills either: they all kept their lungs.
That implies that the creation of gills, either from lungs or ex nihilo (out of nothing) is difficult indeed.
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