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Looking at the swimming birds building nests just across my garden, I suddenly wondered how evolution came to swimming birds and whether flying birds started swimming or whether swimming bird like animals started flying.

What came first and when and why?

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Wouldn't we have had running birds first? –  Brightblades May 7 '12 at 11:29
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2 Answers 2

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Flying came first, as far as we know. The earliest known bird (currently), Archaeopteryx lithographica, already had aerodynamic feathers (Feduccia and Tordoff, 1979). The Solnhofen Limestone, where it was discovered, is ~145 million years old, so we can place the "when" flight evolved to greater than or equal to that time.

Gansus yumenensis is regarded as the earliest aquatic bird and is dated from ~120 million years ago. The Hesperornithiformes are the sister taxon of Gansus (less derived) and also aquatic. However, they date from ~85 million years ago. It is not clear whether the ancestors of Hesperornithiformes were also aquatic. Because they branched off the main line of birds before Gansus, these ancestors would likely predate Gansus, but those relatives have not yet been discovered.

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Thanks for the extensive answer, so there's a gap of at least ~25 million years, considering the current state of knowledge. –  Abel May 7 '12 at 11:53
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Another way to look at this is: How do you evolve flight when you're aquatic? There are three proposed models (or even five) for the evolution of flight:

  • wings helped predators to better jump at prey from high positions
  • wings helped a jumping animal to make longer jumps
  • wings helped a tree-living animal to get around

All of these models assume that you have a high or solid surface to start flying from. Water, in contrast, is a difficult underground to start flying from (being flat and soft).

In contrast, to evolve swimming for "terrestrial" bird seems much more straightforward.

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Ok, seems a fair line of thought, but if this were true, why do we have flying fish (ok, not really flying, but still, they could have evolved from there)? –  Abel May 7 '12 at 11:55
    
Fish have been around for 500 million years, and no true flying fish has evolved during that time. However, in tetrapods (which evolved from fish), flight has evolved in bats, birds, and pterosaurs. In contrast, "flying fish" are actually gliding fish that accelerate in the water (and not in the air, as birds do). –  Michael Kuhn May 7 '12 at 12:12
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