One of the nice things about having a reliable knowledge of the relationships between groups of organisms is that it lets us figure out when different traits have evolved. This can be of great practical importance – for instance, if we see that a particular mutation has evolved independently in several species that are all parasites, then it suggests that the mutation might be important for parasitism. It also lets us make more general statements about the history of life on earth – for example, we can say with great certainty that complex eyes have evolved independently multiple times.
Another trait that has evolved multiple times is powered flight. We know that there are four groups of organisms that can fly – birds, bats, insects, and (extinct) pterosaurs. And we know that each of these groups evolved flight independently because, in each case, the most closely-related group cannot fly. For example, bats are most closely related to carnivores like dogs and cats than they are to any other flying animal. (The situation with birds is slightly more complex because their closest relatives are all extinct, but the principal is the same). We can be pretty confident that powered flight evolved independently four times.
This seems on the low side; after all, many traits that sound just as complicated have evolved much more often than that – sophisticated eyes, complex social living arrangements (as seen in bees) and multicellularity have all been estimated to have arisen independently at least a dozen times. You might expect that something like flying, which offers such obvious benefits – escaping from predators; access to food; being able to migrate long distances – would have evolved many times as well. So why is it so uncommon? Perhaps it requires a degree of specialization that is rarely justified by the benefits it confers. Perhaps it requires a very unusual set of anatomical adaptations to happen simultaneously – this idea is given credence by the fact that gliding, as opposed to powered flight, has evolved independently many times. Maybe gliding offers enough of the benefits of flying, without the need for radical specialization, that it is more adaptive for the majority of animals.
Of course, perhaps it only seems rare to us because of our land-bound nature – flying seems like a big deal because we can’t do it. Let’s forget flying for a moment; how many times has walking evolved? I’m going to rule out animals like tardigrades, that live in tiny films of water – they don’t count. Neither do annelid worms; some of them have leg-like protuberances, but they still move by wiggling. By my count, walking has evolved independently just three times in the history of life – in arthropods (like insects and crustaceans), tetrapods (like humans and crocodiles) and velvet-worms (rather than try to describe them, here’s a wikipedia link. In fact, it seems likely that velvet-worms and arthropods form a closely-related group, which means that walking has evolved independently just twice. So as per the title of this post walking is, from an evolutionary point of view, twice as difficult as flying!
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