How Evolution Explains Everything About Life

For those who have never had the opportunity to learn much about biology or science in general, claims about evolutionary theory made by those who believe in supernatural alternatives can appear convincing. Even among those who do accept the reality of evolution, misconceptions still abound. Most of us are happy to admit that we do not understand, say, quantum mechanics in physics, yet we would baulk at saying the same about evolution. In fact, as biologists are discovering, evolution can be stranger than their predecessors ever imagined.

Why do so many of us spend our evenings in front of the TV with a microwave meal? Could it be that television is the modern equivalent of a Neolithic fire, making TV dinners ‘the natural consequence of hundreds of thousands of years of human evolution’, as a researcher once concluded?

Stop laughing. It is very tempting to invent evolutionary ‘just so’ stories to explain almost any aspect of our body or behaviour. We all tend to assume that everything has a purpose – but we are often wrong.

Take male nipples. Male mammals clearly do not need them. They have them because females do: it doesn’t cost much to grow a nipple, so there has been no pressure for the sexes to evolve separate developmental pathways, to switch off nipple growth in males. Some researchers claim the female orgasm exists for the same reason, though this is far more controversial.

Or consider your sense of smell. Do you find the scent of roses overwhelming or struggle to smell anything at all? Can you detect the distinctive odour that most people’s urine acquires after eating asparagus? People vary greatly when it comes to smell, and this is probably less to do with natural selection than with chance mutations in the genes coding for the smell receptors.

Then there are features which do result from selection, but for another trait entirely. For instance, the short stature of pygmies might have no survival advantage in itself, but instead be a side effect of selection for early childbearing in populations where mortality is high. Similarly, since the same gene often has different roles at different times of development or in different parts of the body, selection for a variant that is beneficial in one way can have other, seemingly unrelated effects. Male homosexuality might be a side effect of genetic variants that boost female fertility. What’s more, a mediocre or even poor gene variant can spread rapidly through a population if it happens to be located near a highly beneficial gene.

Other features of plants and animals, such as the wings of ostriches, are adaptations no longer needed for their original purpose. These vestigial traits can persist because they make no difference to an individual’s chances of survival, or they have taken on another function, or because even though they have become disadvantageous, they occur in a population that is too small or has undergone too few generations for evolution to eliminate them.

A prime example in humans is the appendix. While claims abound that it has this or that function, the evidence is clear: you are more likely to survive without an appendix than with one. Another example is wisdom teeth. Having a smaller, weaker jaw allowed our ancestors to grow larger brains, but left less room for molars. Yet many of us still grow teeth for which there is no room, and the consequences can be fatal.

Evolutionary psychology in particular is notorious for attempting to explain every aspect of human behaviour, from gardening to rape, as an adaptation that arose when our ancestors lived on the African savannah. Some behaviours may indeed be past adaptations, but in the absence of any proof, claims about TV dinners should be taken with a large pinch of salt.

There are all sorts of findings and experiments that could have falsifed evolution, but in the century and a half since Darwin published his theory, not a single one has done so.

When asked what would disprove evolution, the biologist J.B.S. Haldane famously growled: ‘Fossil rabbits in the Precambrian.’ What he meant was that evolution predicts a progressive change over time in the millions of fossils unearthed around the world: multicellular organisms should come after unicellular ones; jawed fish should come after jawless ones, and so on. All it would take is one or two exceptions to challenge the theory. If the first fossil amphibians were older than the first fossil fish, for example, it would show that amphibians could not have evolved from fish. No such exceptions have ever been found anywhere.

The discovery of a mammal–bird hybrid, such as a feathered rabbit, could also disprove evolution. There are animals with a mixture of mammalian and reptilian features – such as the spiny anteater – and there are fossils with a mixture of bird and reptilian features, such as the toothy archaeopteryx. But no animals have a mixture of mammalian and bird features. This is exactly what you would expect if birds and mammals evolved from separate groups of reptiles, whereas there is no reason why a ‘designer’ would not have mixed up these features, creating mammals with feathers and bird-like lungs, or furry, breast-feeding ostriches.

A young Earth would also be a problem for evolution, since evolution by natural selection requires vast stretches of time – ‘deep time’ – as Darwin realized. Some thought evolution had been falsified in the nineteenth century when physicist William Thomson calculated that the Earth was just 30 million years old. In fact, several lines of evidence, such as lead isotopes, show the Earth is far older than even Darwin imagined – about 4.5 billion years old.

Suppose for a moment that life was designed rather than having evolved. In that case, organisms that appear similar might have very different internal workings, just as an LCD screen has a quite different mechanism to a plasma screen. The explosion of genomic research, however, has revealed that all living creatures work in essentially the same way: they store and translate information using the same genetic code, with only a few minor variations in the most primitive organisms. Huge chunks of this information are identical or differ only slightly even between species that appear very different.

What’s more, the genomes of complex creatures reveal a lack of any intelligence or foresight. Your DNA consists largely of millions of defunct copies of parasitic DNA. The inescapable conclusion is that if life was designed, the designer was lazy, stupid and cruel.

Not only that, if organisms had been designed for particular roles, they might be unable to adapt to changing conditions. Instead, countless experiments, both planned and unplanned, show that organisms of all kinds evolve when their environment is altered, provided the changes are not too abrupt. In the laboratory, tweaking organisms’ environments has enabled researchers to produce bacteria, plants and animals with all kinds of novel characteristics – even entirely new species. In the wild, human activity is reshaping many species: urban birds are diverging from their country cousins, some fish are getting smaller because fishermen keep only big fish, and trophy hunting is turning bighorn sheep into smallhorns, for instance.

Until recently, such examples were considered the exception, but it seems we may have seriously underestimated the extent to which evolution likes to simplify matters. There are entire groups of apparently primitive creatures that are turning out to be the descendants of more complex organisms. For instance, the ancestor of brainless starfish and sea urchins had a brain; why their descendants dispensed with a brain is still unclear.

Despite this, there is no doubt that evolution has produced ever more complex life forms over the past 4 billion years. This is usually assumed to be the result of natural selection, but recently some biologists studying our bizarre and bloated genomes have turned this idea on its head. They propose that, initially at least, complexity arises when selection pressure is weak or absent. How could this be?

Suppose an animal has a gene with two different functions. As a result of mutation some offspring may get two copies of this gene. In a large population where competition is fierce and selection pressure strong, such mutations are likely to be eliminated because they do not increase an individual’s fitness and are probably slightly disadvantageous.

In smaller populations where selection pressure is weak, however, these mutations have a small chance of surviving and spreading as a result of random genetic drift. If this happens, the duplicate genes will start to acquire mutations of their own. A mutation in one copy might destroy its ability to carry out the first of the original gene’s two functions, while the other copy might lose the ability to perform the second function. Again these changes don’t confer any advantage – such animals would still look and behave exactly the same – but these mutations might also spread by genetic drift. So the population would have gone from having one gene with two functions to two genes with one function each.

This increase in genomic complexity would have occurred not because of selection pressure but despite it. Yet it can be the foundation of greater physical or behavioural complexity because each gene can now evolve independently. For example, either can be switched on or off at different times or in different tissues. And as soon as any beneficial mutations arise, natural selection will kick in.

It seems there are opposing pressures at the heart of evolution: while complex structures and behaviours, such as eyes and language, are undoubtedly the product of natural selection, strong selection – as in large populations – blocks the random genomic changes that can throw up greater complexity in the first place.

It’s a theme endlessly repeated in wildlife documentaries. Again and again we are told how perfectly animals are adapted to their environment. It is, however, seldom true.

Take the red squirrel, which appeared to be perfectly adapted to its environment until the grey squirrel turned up in the UK and demonstrated that it is in fact rather better adapted to broadleaf forests.

There are many reasons why evolution does not produce perfect ‘designs’. Natural selection only requires something to work, not to work as well as it could. Botched jobs are common. The classic example is the panda’s ‘thumb’, a modified wrist bone that the animal uses like an opposable thumb to grasp bamboo. It’s far from the ideal tool for the job, but since the panda’s true thumb is fused into its paw, the panda had to settle for a clumsier alternative.

Evolution is far more likely to reshape existing structures than throw up novel ones. The lobed fins of early fish have turned into structures as diverse as wings, hoofs and hands. What this means is that we have five fingers because amphibians had five digits, not because five fingers is necessarily the optimal number for the human hand.

Many groups haven’t evolved features that would make them better adapted. Sharks lack the gas bladder that allows bony fish to precisely control their buoyancy, and instead have to rely on swimming, buoyant fatty livers and, occasionally, gulping air. Mammals’ two-way lungs are far less efficient than those of birds, in which the air flows in one direction.

Continual mutation also means that potentially useful features can get lost. Many primates cannot make vitamin C, an ability that wasn’t missed in animals that get lots of vitamin C in their diet. However, such losses can be limiting if the environment changes, as one primate discovered on long sea voyages.

Evolution’s lack of foresight also leads to inherently flawed designs. The vertebrate eye, with its blind spot where the wiring goes through the retina, is one example. Once natural selection fixes upon a bad – but workable – design, a species’ descendants are usually stuck with it.

Environments also change. In the arms race between predator and prey, parasite and host, species have to keep evolving just to maintain their current level of fitness, let alone get even fitter. As the Red Queen says in Through the Looking Glass: ‘It takes all the running you can do, to keep in the same place.’

Humans aren’t running fast enough. Evolving and adapting is a numbers game: the larger a population and the more generations there are, the more mutations will appear and the more chances there will be for natural selection to favour the beneficial and eliminate the harmful. Around 10 billion new viral particles can be produced every day in the body of a person infected with HIV; the total human population on Earth was no more than a few million until fairly recently.

A bacterium can produce 100,000 generations in a decade, but there have probably been fewer than 25,000 generations since the human lineage split from that of chimpanzees. So it’s hardly surprising that in less than a human lifespan, we’ve seen the evolution of new viruses, such as HIV.

Our evolution has accelerated in the last 10,000 years, but we are changing our environment even faster, leading to problems ranging from obesity and allergies to addictions and short-sightedness. Viruses and bacteria might approach perfection: we humans are at best a very rough first draft.

Cosmologists make precise predictions about what will happen to the universe in 20 billion years’ time. Biologists struggle to predict how a few bacteria in a dish might evolve over 20 hours. Some claim that this lack of precise predictive power means evolution is not scientific.

However, what matters in science is not how much you can predict on the basis of a theory or how precise those predictions are, but whether you can make predictions that turn out to be right. Meteorologists don’t reject chaos theory because it tells them it is impossible to predict the weather 100 per cent accurately – on the contrary, they accept it because weather follows the broad patterns predicted by chaos theory.

The difficulty in predicting the path of evolution partly springs from organisms’ freedom to evolve in quite different directions. If we could wind the clock back 4 billion years and let life evolve all over again, its course might well be different. Life on this planet has also been shaped by chance events. If an asteroid had not wiped out the dinosaurs, intelligent life might have been very different, if it evolved at all.

Nevertheless, although evolution’s predictive power might appear limited, the theory can and is used to make all sorts of predictions. For a start, Darwin predicted that transitional fossils would be discovered, and millions – trillions if you count microfossils – have been uncovered. What’s more, researchers have predicted in which kinds of rocks and from what eras certain transitional fossils should turn up in, then gone out and found them, as with the half-fish, half-amphibian Tiktaalik.

Or take the famous peppered moth, which evolved black colouring to adapt to pollution-stained trees when industrialization took place. Remove the pollution and, evolutionary theory predicts, the light strain should once again predominate – which is just what is happening.

This predictive power can also be put to much more practical use. For instance, evolutionary theory predicts that if you genetically engineer crops to produce a pesticide, this will lead to the evolution of insect strains which resist that pesticide, but it also predicts that you can slow the spread of resistance genes by growing regular plants alongside the GM ones. That has proved to be the case. Many researchers developing treatments for infectious diseases try to predict how resistance might evolve and to find ways to prevent this from happening, such as prescribing certain drugs in combination. This slows the evolution of resistance because pathogens have to acquire several different mutations to survive the treatment.

Take a look in the mirror. The face you see is rather different from that of a Neanderthal. Why? The answer could be genetic drift. With features such as the shape of your skull, which can vary in form with little change in function, chance might play a bigger role in evolution than natural selection.

DNA is under constant attack from chemicals and radiation, and errors are made when it is copied. As a result, each human embryo contains 100 or more new mutations. Natural selection will eliminate the most harmful – those that kill the embryo, for instance. Most mutations make no difference because they occur in junk DNA, which makes up the vast majority of our genome. A few cause minor changes that are neither particularly harmful nor beneficial.

While most new neutral mutations die out, a few spread through later generations purely by chance. The odds of this happening are tiny, but the sheer number of mutations that arise make genetic drift a significant force. The smaller a population, the more powerful it is (see Figure 6.2).

Population bottlenecks have the same effect. Imagine an island where most mice are plain but a few have stripes. If a volcanic eruption wipes out all the plain mice, striped mice will repopulate the island. It’s survival of the luckiest, not the fittest.

These processes have almost certainly played a big role in human evolution. Human populations were tiny until around 10,000 years ago, and genetic evidence suggests that we went through a major bottleneck around 2 million years ago.

Most of the genetic differences between humans and other apes – and between different human populations – are due to genetic drift rather than selection, but as most of these mutations are in the nine-tenths of our genome that is junk, they do not make any difference. Of those that do affect our bodies or behaviour, it is likely that at least a few have spread because of drift rather than selection.

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.

FIGURE 6.2 The power of genetic drift: natural selection is not the only force in evolution. Mutations that have little or no effect on fitness can spread throughout a population or die out due to chance alone. Each graph shows ten simulation runs from the same starting point.

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 specialized, 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 remaining useful at every stage.

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.

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 (see Chapter 7).

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.

Sometimes just one aspect of a feature can be co-opted for another use. The first hard mineralized 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.

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 does not prove it did not evolve.

But isn’t evolution just a theory?

This is a common question posed by creationists.

And yes, evolution is a theory, just like Einstein’s theory of special relativity. By theory, scientists mean an explanation backed by evidence. What creationists mean is that evolution is just a hypothesis, unsupported by evidence – which it is not. Sure, there are plenty of details to fill in. But would you jump off a skyscraper on the basis that the clash between general relativity and quantum theory means there are serious problems with our theory of gravity? It makes no more sense to question the reality of evolution because scientists are still debating about some of its finer aspects than it does to question the existence of gravity for the same reason. As surely as dropped objects fall, life has evolved and continues to do so.