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鈥淔rom so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.鈥� That was how Charles Darwin described the incredible diversity of life forms generated by evolution. But he never addressed the big question: if evolution is infinitely innovative, why hasn鈥檛 it produced animals with wheels? Are there limits to evolution鈥檚 creativity that mean some things can never evolve? And if not, why haven鈥檛 things like flying plants arisen or anything resembling a semi-automatic rifle?

Evolution exists because living things vary: each member of a species is subtly different to every other one. Those that are better suited to their environments are more likely to breed and pass on their genes, and so their favourable traits become more common in the population. Given enough time, this slow process can create wonders like gigantic blue whales, cooperative honeybees and towering sequoias. Evolution has produced animals that live more than 10 kilometres under the sea and bacteria that can handle doses of radiation that would kill a person many times over.

Yet some things just don鈥檛 seem to turn up. Wheeled animals are a classic example 鈥� the evolutionary biologist Stephen Jay Gould discussed them in his 1983 book Hen鈥檚 Teeth and Horse鈥檚 Toes 鈥� but there are many others. In a 2015 paper, at the University of California, Davis, identified there are no rigid structures like coral reefs in fresh water, no plant-eating snakes and no flying plants.

It is tempting to invent reasons why these can鈥檛 exist, but Vermeij is reluctant to place such limits on evolution. 鈥淚 think very few of these are what I would characterise as鈥�forbidden phenotypes,鈥� he says 鈥� a phenotype being the observable physical properties of a given organism. Given more time, evolution may well produce them.

Vermeij says we should imagine ourselves as evolutionary biologists living in the Ordovician Period, some 450 million years ago. 鈥淲e couldn鈥檛 have conceived of some of the things that have actually evolved [since],鈥� he says. 鈥淭hings like flowers and flying insects. That demonstrates to me that evolution is very much a cumulative process, and what is now possible would not have been at that time.鈥�

at the University of Essex in the UK agrees. 鈥淏iological evolution is the most unlimited process in the known universe,鈥� she says. 鈥淚ts main striking feature is its ability to produce, as far as we鈥檝e seen, infinite diversity. We haven鈥檛 run out of new animals. We have no idea where the edges of this space might be.鈥�

Hoyal Cuthill says that many of the seemingly forbidden animals are examples of a kind of selective reasoning. Take the lack of plant-eating snakes. Many groups of animals have switched back and forth between eating meat and eating plants. Giant pandas are one such example: although they depend on bamboo, their ancestors were carnivores and their guts haven鈥檛 fully adapted to eating plants yet. No known snake has done this so far, which might imply that there is something about having this sort of body that is particularly suited to meat eating 鈥� or it might just be a coincidence. Hoyal Cuthill points out that snakes鈥� closest living relatives are lizards, many of which eat plants.

Vegetarian spiders

There may also be limits to our knowledge. For decades, it was thought that all spiders were primarily carnivorous. Then, in 2009, we discovered that a species called Bagheera kiplingi mostly eats plants.

Still, it is possible to go further and imagine much wilder scenarios. A 2007 paper by Timothy Shanahan at Loyola Marymount University in Los Angeles memorably asked: It argued that zebras would benefit from evolving machine guns to repel attacks by lions, but that they can鈥檛 because evolution has to proceed by intermediate steps, all of which must be advantageous 鈥� or, at least, not harmful. A machine gun is only useful when it is complete. As a functionless organ, it would just be hogging vital nutrients.

But there may not be as strong a limit here as it first appears, says Hoyal Cuthill. 鈥淧rojectile weapons have frequently evolved. Archerfish use water as a projectile weapon. Antlions use sand as a projectile weapon,鈥� she says. These animals are all ambush predators, so it may simply be that evolution doesn鈥檛 favour projectile weapons in prey, because fleeing a predator is normally a better strategy than fighting it. The truth may not be that evolution can鈥檛 fashion a rapid-fire projectile weapon for zebras, but rather that it doesn鈥檛 do so because the existing solution 鈥� running away 鈥� works well enough.

鈥淧rojectile weapons have frequently evolved, albeit only in ambush predators鈥�

This suggests that evolution hasn鈥檛 even remotely reached its limits, and that over the next few hundred million years 鈥� and assuming humans don鈥檛 cause a global extinction event 鈥� it might lead to animals and plants that are inconceivable to us today.

Vermeij argues that evolution is getting better at producing new things. It is known, for instance, that , meaning modern flowering plants can extract more energy from sunlight than their ancestors could 100 million years ago. Similarly, 鈥渢he average metabolic rates of things like snails and clams and many other organisms have increased over time鈥�, says Vermeij.

Energetic expansion

The reason for that is competition, he says. 鈥淎s organisms become more energetic, they also have more choice, and they have a greater opportunity to modify their environments.鈥�

Vermeij says one knock-on effect may be the evolution of general intelligence in multiple animal groups: not just humans and other primates, but cetaceans like dolphins, cephalopods such as octopuses and certain birds including crows and jays. 鈥淢ore brainpower and greater intelligence are themselves products of greater energetic investment, and are again favoured under most circumstances because they give the bearer greater choice,鈥� he says. The smartest organisms on Earth may be getting smarter over the ages.

That leads us back to the original question. If evolution is so precocious, why hasn鈥檛 it created a wheeled animal yet?

Hoyal Cuthill offers a blunt answer. 鈥淲heels are terrible,鈥� she says. 鈥淭he reason so much effort went into the development of the tank is that wheels are atrocious.鈥� Sure, they鈥檙e fine on smooth roads but not so much on badly maintained ones. And in nature, smooth, flat surfaces are rare, so there is no reason for evolution to favour wheels.

The other issue is whether creating a wheel that freely rotates about an axle is beyond evolution, says Vermeij. It is hard to imagine how the wheel might receive nutrients from the rest of the body, as any blood vessels would be torn away. 鈥淭he wheel might be one of the very few things that may just not be accessible to living things, other than by making a machine as we have done,鈥� says Vermeij.

Yet he points out that even if wheels and axles aren鈥檛 possible, organisms have still found ways to travel using rotational motion. 鈥淭here are spiders that manage to actively roll down a hill by using their limbs to propel themselves as they form a ball,鈥� he says.

Hoyal Cuthill has a final suggestion. 鈥淎 ball can move rotationally with more degrees of freedom, in more directions, than a wheel on an axle,鈥� she says. 鈥淲heels are prone to getting stuck or falling over,鈥� she says; balls don鈥檛 have that problem. So the reason evolution hasn鈥檛 produced wheeled creatures might simply be that it has other solutions. That鈥檚 just how natural selection rolls.