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Fish Evolution
Why did fish develop lungs? And legs?!

by Scott Teresi

Are you curious about so-called “missing links” in the theory of evolution? Why would a fish want to breath air? How did fish evolve legs and climb out of their prehistoric fish tanks? Let's find out!


A web site describes why lungs are better (but gills are probably cheaper if you catch them on sale at Wal-Mart): “Extracting oxygen from water is more difficult and requires a greater expenditure of energy than does extracting oxygen from air. Water is a thousand times more dense (heavier per unit volume) than air, and at 68 degrees Fahrenheit it has 50 times more viscosity (resistance to flow) than air and contains only 3% as much oxygen as an equal volume of air. Fishes, therefore, have necessarily evolved very efficient systems for extracting oxygen from water; some fishes are able to extract as much as 80% of the oxygen contained in the water passing over the gills, whereas humans can extract only about 25% of the oxygen from the air taken into the lungs.” [1]

My dad, a high school biology teacher, gave an explanation along similar lines—that gills require more energy than lungs. He said that animals with gills have more of a problem with keeping the water in their bodies balanced against the water around them. In the ocean, the difference between the salt content inside the fish and outside the fish causes osmosis; saltier water sucks up fresh water somehow. So in salt water, fish are constantly thirsty. The salt water sucks water out of their bodies (mostly through the gills which leave the fish’s body exposed). In fresh water, the opposite happens. Water enters into the fish all the time (because the inside of the fish is “saltier”), so fish are constantly going to the bathroom! (They need good kidneys.)

Anyway, the upshot of this is it takes more energy to survive in water. You’d think the opposite would be true. If you lived in water, you’d never have to look for water to drink! But it creates the equalization problem explained in the murky description above.

So if you have lungs, you can get oxygen more efficiently and have extra energy left over and you can evolve into a bigger animal, like a dolphin or a whale. Animals of that size don’t work as well with gills.


The first amphibians were fish with poorly-developed legs that crawled out of the water around 300-390 million years ago, during the Devonian era. Reasons for evolving legs (and becoming a “tetrapod”) include needing to migrate often from pools of water which often dried up, or to find more food. Being the first animals on land, they’d also be safe from predators! Unfortunately not many fossils have been found of the species which directly link fish and amphibians. [2]

Amphibians evolved from one of three orders of lobe-finned fishes: either lungfish (which have lungs and gills), coelacanths, or extinct rhipidistians. Scientists haven’t quite decided which yet. They’re all from the same subclass, Choanichthyes or Sarcopterygii [3]. A group of six coelacanths were discovered in November 2000, six feet long, a living link to our first ancestors who had designs on crawling out of the water! Previously, one or two had been seen as far back as 1938, when they were first discovered to still be living. Coelacanths never leave the water, but they do have four arm-link fins, resembling the beginnings of legs. [4]

So we have three groups of fish that could’ve turned into amphibians: coelacanths (which can’t breath air), lungfish (which don’t have the right fin structure), and rhipidistians (which most likely were the ones to lead to amphibians, according to one web site). The rhipidistians lived in shallow waters and probably had lungs, and they had similar fin structures to amphibian legs, among other similarities. [5]

Unfortunately, the only animals for which fossils are widely available after the rhipidistians are the Ichthyostega, and those already had well-developed legs and toes. One Creationist page from 1995 had this to say: “The fossil record supplies... none of the supposed links of plant to animal, fish to amphibian, amphibian to reptile, or reptile to birds and mammals.” [6] I doubt that’s very true at all, but maybe those missing links didn’t exist for very long once their more efficient descendents had evolved and took over, leaving few fossils for us to find.

On further investigation, I found a site which apparently cites a whole list of fossils filling the “missing link” between lobe-finned fish and tetrapods, most of them discovered just since 1987. Apparently some fish, such as Acanthostega, developed legs which could help propel them along the ground in shallow water but couldn’t quite support their weight if they were completely out of water. And they had seven or eight fingers! (Here’s some pictures: [7]) There’s also Panderichthys, a fish which already had very similar skull and fin bones to the earliest amphibians, indicating that they later evolved to become amphibians. It looks like an amphibian (and has nostrils and lungs), but it’s a fish and has fins instead of legs. It’s similar to both fish and amphibians, showing a path of ancestral lineage. Sounds like a winner! [8]

“Hominids” are the link between apes and humans, but not many hominid skeletons have been found, at least according to the Creationist page. I’ll have to take on that topic some other day! There’s so much out there to spend all day reading about, but I’ve gotta get off this computer.



[2] [no longer online]





[7] [no longer online]



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