Â鶹´«Ã½

It might seem like there is no end to nature’s inventiveness but there are some features that could probably never evolve, at least on Earth

It often seems that nature invented pretty much everything that can be invented long before humans arrived on the scene – including the wheel, . There is a living in the Californian mountains that coils itself up and when threatened, for example. The goes one better and can along a flat surface for four or five revolutions to escape predators.

Nevertheless, there are structures that would clearly be useful but have never evolved. would rarely be bothered by lions, some point out. So why can evolution invent some things but not others?

This is an extremely difficult issue to tackle: how can we study something that has not happened? One way to approach it is to start with a question used by those who deny evolution and believe that many of nature’s inventions, such as the eye or the bacterial flagellum, are simply too complex to have evolved. What use is half a wing, they ask? (see Half a wing is no use)

Very useful, it turns out. The wings of insects might have evolved from that were for on the surface of water. This is an example of – structures and behaviours that evolved for one purpose but take on a wholly new one, while remaining useful at every intermediate stage.

Come in, over

Turn this argument around, however, and it suggests that some features cannot evolve because a half-way stage really would be of no use. For example, two-way radio might be useful for many different animals, for making silent alarm calls or . So why hasn’t it evolved? The recent invention of suggests it is not physically impossible.

The answer might be that half a radio really is useless. Detecting natural radio waves – from lightning, for instance – would not tell animals anything useful about their environment. That means there will be no selection for mutations that allow organisms to detect radio waves. Conversely, without any means of detecting radio waves, emitting them would serve no useful purpose. Radar might not be able to evolve for similar reasons.

The contrast with visible light could hardly be greater. It is clear that simply would be advantageous in many environments, that even a blurry picture is better than nothing at all, right up to hawk-eyed sharpness.

Seaweed skies

Emitting visible light can be helpful too, even for creatures that cannot detect it themselves. For the bioluminescent phytoplankton that , for instance, it is a way of summoning predators that eat the phytoplankton’s enemies. A similar argument applies to sound: it is not hard to see how forms of echolocation evolved independently in groups such as bats, cave swiftlets and whales.

One might also wonder why like balloons have never evolved. The idea does not seem too far-fetched at first glance: many seaweeds have floats called , filled with oxygen or carbon dioxide. Other algae can produce hydrogen. So fill a large, thin pneumatocyst with hydrogen and perhaps a seaweed could fly. Flying plants would beat water and land plants to the light, giving them a big advantage, so why aren’t our skies filled with living green balloons?

Perhaps partly because large pneumatocysts with extremely thin membranes would be far more vulnerable to predators and damage from waves, so an intermediate stage could never evolve. What’s more, algae produce hydrogen only when there’s a lack of sulphur in the water, and in any case the molecules of hydrogen gas are so tiny that they would leak out of any pneumatocyst. Half a hydrogen balloon doesn’t look very good for anything, . Even evolution has its limits.

Read all the myths in our Evolution Special