Evolution myths: Half a wing is no use
18:00 16 April 2008 by Michael Le Page
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Just as objects designed for one purpose can be used for another, so genes, structures and behaviours that evolve for one purpose become adapted to do another
Ever used a newspaper to light fires or mop up spills? Stood on a chair to reach something? Or swatted flies with a rolled-up copy of New Scientist? Just as objects designed for a specific purpose can be co-opted for something quite different, so features that evolved to do one task can be used for another - and often are.
But what use is half a wing? It's a question that those who doubt evolution first asked more than a century ago. When it comes to insects, rowing and skimming could be the answer. Stonefly nymphs have flapping gills for extracting oxygen from water. When standing on the water's surface, early insects could have used these gills for getting oxygen and propulsion rowing simultaneously. Some stoneflies still stand on the surface and "row" across water using their wings.
Over time, flapping could have replaced rowing as the main means of propulsion, allowing insects to skim across the water's surface: low levels of friction on this scale mean proto-wings would not have had to generate much air flow to be useful for skimming.
As these proto-wings became more efficient and specialised, early insects may have taken further steps towards flying. While some skimming insects keep all six legs on the water's surface, faster skimmers keep just four legs or two legs on the water. This surface-skimming hypothesis concerning the evolution of insect flight shows how flapping gills could gradually have turned into wings while remained useful at every stage.
From T-rex to sparrow
What about the wings of birds? In some dinosaurs, the scales covering their bodies evolved into hair-like feathers, most likely to insulate warm-blooded bodies or help keep eggs warm.
Those dinosaurs with feathers on their limbs might then have started to exploit the aerodynamic properties offered by feathers, perhaps gliding between trees or running faster along the ground. Fossils show a gradual transition from downy, hair-like feathers into the rigid flight feathers that form the key part of birds' wings.
Another idea that is gaining favour is that flapping forelimbs helped the ancestors of birds to run up steep slopes or climb trees - a technique many birds still employ today.
Without a time machine it is difficult to prove exactly what early birds or insects used "half a wing" for. But it is now clear that half a wing can have all sorts of uses. Indeed, there are numerous examples of physical structures and behaviours that evolved for one purpose acquiring another one, a process called exaptation.
Reuse recycle
Evo-devo - evolutionary developmental biology - is even starting to identify the precise mutations that underlie such changes. For instance, the forelimbs of the ancestor of bats turned into wings partly thanks to a change in a gene called BMP2 that made its "fingers" far longer than normal.
The webbing between the extra long digits that makes up the bat wing is a reappearance of a long-lost feature: as embryos, all tetrapods initially develop webbed digits, a hangover from our fish ancestors. Normally, this webbing kills itself off at an early stage, but in bats this cell suicide is blocked.
Repurposing a structure does not have to involve the loss of the original structure. Reptilian jaw bones turned into mammalian ear bones, without the loss of the jaw. The neural circuitry that allows us to make fine limb movements may have been adapted to produce speech as well.
In fact, almost every feature of complex organisms can be seen as a variation on a theme. Switching off one gene in fruit flies, for instance, can turn their antennae into legs.
On the shoulders of fish
Sometimes just one aspect of a feature can be co-opted for another use. The first hard mineralised structures to evolve in our ancestors were the teeth of early fishes known as conodonts. Once the ability to form hard hydroxyapatite had evolved, it could be exploited elsewhere in the body and may have been the basis of the bony skeletons of all vertebrates.
As these examples show, there are all kinds of routes by which structures and behaviours that evolved for one purpose can contribute to new structures and abilities. Just because it is not immediately obvious how something as complex as a bacterial flagellum evolved (see The bacterial flagellum is irreducibly complex) does not prove it did not evolve.
An even more interesting question than what half a wing is good for is whether some features cannot evolve because half of them really would be useless. Such thought experiments might not prove anything but they can be fun (see Evolution is limitlessly creative).
