Evolution is a tinkerer.
FRANÇOIS JACOB, ‘Evolution and Tinkering’
Pigs look us straight in the eye and see an equal.
WINSTON CHURCHILL
Question: What do aeroplanes and television sets and lamp posts have in common with frogs and whales and people? Answer: All are highly improbable configurations of matter and all do what they do extremely well. The technological things in the first group were designed by human beings. An obvious conclusion to draw from the similarity between the two groups would therefore be that the living things in the second group were also designed. The obvious conclusion, however, is wrong.
The illusion of design in nature is so strong that it was not recognised as an illusion until the nineteenth century. In Europe at the time, there was a pretty much universal belief that living things had been created and put on the Earth in their present forms by a Supreme Being. The scientists of the day were mostly religious and the very last thing they wanted to do was question such an idea and bring down on themselves the wrath of the Church. However, scientists have no choice but to go with the evidence. And the evidence was overwhelming that the bewildering diversity of life on Earth – everything from bacteria to blue whales, fungi to flying foxes, gorillas to giant sequoias – is the consequence of a purely natural mechanism.
An important clue came from fossils. These appeared to be the relics of ancient creatures, buried by sediments settling to the bottom of lakes and seas, and somehow – nobody knew exactly – turned to stone. Fossils reveal that the creatures that inhabit the Earth today are not the same as the ones that once inhabited the planet. Some ancient creatures such as the dinosaurs have dis appeared entirely whereas other vanished creatures appear related to creatures today. The simplest, most primitive creatures appear fossilised in the oldest sediments. As the rock layers became progressively younger, the fossils became ever more complex and sophisticated.
The idea dawned on scientists that the fossil record was a time sequence of life on Earth. It was telling us that, over vast tracts of time, species of creatures gradually change their appearance, morphing from one into another and eventually becoming the species we see around us today. Life was not created on Day One by a Creator, remaining frozen and static forever after. Instead, it has evolved, gradually, from simpler ancestral forms.
Such evolution explains the striking similarities between creatures living today such as humans and chimpanzees. If all life on Earth descended from a common ancestor in the distant past, it is obvious that all creatures today are related. But what drives evolution? What causes species to change over the generations? And how have all creatures ended up doing what they do so incredibly well that they give every appearance of being designed? The man who found the answer was Charles Darwin.
Darwin embarked on HMS Beagle in 1831. During his five years as the ship’s naturalist, he made some tantalising observations of the biological world. On the Galápagos archipelago, 1,000 kilometres off the west coast of South America, the finches on different islands had different-shaped beaks. In all cases, the beaks were perfectly shaped for exploiting the nuts available locally: short, stubby beaks for cracking open big nuts, slender beaks for less formidable seeds.
An explanation began to form in Darwin’s mind when he also noticed that the birds and animals on the Galápagos were but slight variants of those common on the mainland of South America. The Galápagos, it seemed, had been colonised by creatures from the nearby continent. Some birds and animals from South America that could easily have made a living on the Galápagos were conspicuous by their absence. Only a small subset had made it across the ocean barrier on winds or mats of floating vegetation. It had been these hardy creatures that had radiated to fill all the empty niches – a single type of finch spreading to all islands and evolving beaks best suited to exploit the seeds found locally.
Darwin was now in possession of new and important clues about evolution. But he did not know what was driving the changes in species – what was pushing each to an apparent perfect fit with its environment. Back in England in 1836, and still only twenty-seven, he sat down at his desk, laid out the facts he had collected before him, and began to think.
Darwin was aware of one common way that creatures change their forms over the generations: by deliberate breeding. Plants and domestic animals inherit physical traits from their parents, and these can be enhanced. To create a flock of sheep with the thickest-possible woolly coats, for instance, breeders select sheep with the thick coats, mate them together, and repeat the process, generation after generation.
But, whereas humans select for traits they desire in an animal or plant, nature appears to select for traits that maximise an organism’s chance of survival in its environment. Such natural selection might not be as fast as the artificial selection of human breeders but it is just as effective.
After Darwin had spent eighteen months of intense concentration on the problem, a light went on in his mind. He suddenly saw the elusive mechanism of natural selection. And it was breathtakingly simple.
One of the striking things about the natural world is how profligate organisms are. Invariably, animals give birth to large litters of young. Plants produce vast quantities of seeds. But there is simply not enough food in the world to support so many young. Inevitably, therefore, most creatures starve to death. Crucially, Darwin realised, the only ones who survive to reproduce are those with traits that best enable them to make a living in their environment.1 And these traits are inherited by the next generation. So, as time goes by, the prevalence of beneficial traits in a population increases at the expense of traits that do not confer survivability.
This was it: the missing piece of the jigsaw. Evolution by natural selection. ‘How extremely stupid not to have thought of that,’ said Darwin’s friend and champion, Thomas Huxley. But of course Darwin had to see past the dizzying complexity of the natural world to the mechanism ticking at its heart and quietly generating its complexity. And that was no mean feat.
Richard Dawkins has called evolution by natural selection the greatest idea in the history of science. And it certainly has phenomenal explanatory power. Modern biology is literally the story of evolution by natural selection. ‘Nothing in biology makes sense except in the light of evolution,’ wrote Theodosius Dobzhansky in 1937.
According to his biographers, Darwin made no effort to publicise his idea, realising full well that it flew in the face of the Church’s teaching that God created all living creatures in their final form. Only in 1858 – after twenty years of sitting on his explosive idea – was he galvanised into action. A letter arrived from a man called Alfred Russel Wallace, who, while observing nature in Indonesia and Malaysia, had hit on the exact same unifying idea of evolution by natural selection.2 Stunned, Darwin locked himself in his study and began writing furiously.
Darwin’s epochal work, published in 1859, is universally known as The Origin of Species, though it says essentially nothing about the ultimate origin of life, which to this day remains a deep mystery. More apposite is the book’s full, though considerably more long-winded, title: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life.3
According to Darwin, all life on Earth today has evolved over aeons of time from a common ancestral organism by the process of natural selection. The idea conflicted not only with the biblical account of creation as a one-off event but with the Church’s claim that humans were, uniquely, forged in the image of God. According to Darwin, humans were neither at the pinnacle of creation nor special in any other way. They were just another animal.
Just as, in the sixteenth century, the Polish astronomer Nicolaus Copernicus showed that the Earth was not at the centre of things and occupied no special place in the cosmos, Darwin showed that humans were not at the centre of things and occupied no special place in the living world.4
Darwin was courageous to present a theory that flew in the face of entrenched religious orthodoxy. But he was also very honest about the theory’s shortcomings, freely admitting it was incomplete. He asked people instead to judge the idea on its broad claims, which he was sure were correct, and not on the fine details, which he did not possess but which he was certain future generations of biologists would fill in.
Two things stood out as glaring omissions. The first was the mechanism of variation. People clearly inherit traits from both their mother and their father: it is possible to see a mother’s red hair in a child or a father’s square jaw. But what causes the appearance of new traits from which natural selection, well, selects?
The second thing missing from Darwin’s theory was the mechanism of inheritance. Darwin initially thought that information about traits was carried from generation to generation when some kind of fluid from each parent intermingled. However, just as red and yellow paint mix together to make orange paint, while losing red and yellow for ever, combining such biological fluids should blend together traits, losing some for ever. We should see people with eyes only a blend of blue and brown and never people with undiluted blue or brown eyes, something that flatly contradicts reality. Over time, the blending of such biological fluids should cause all creatures in a population to become similar, drastically reducing the variation needed for the operation of natural selection. When Darwin realised this flaw in his fluid idea, he was deeply depressed.
It was a monk called Gregor Mendel in Brno, in what is now the Czech Republic, who was the first to glimpse the elusive mechanism of inheritance. Between 1856 and 1863, Mendel bred together varieties of pea plants in their tens of thousands and listed a number of traits that were inherited in their entirety. For instance, when Mendel bred pea plants with purple flowers with ones with white flowers, the result was not pea plants with a pinkish flower but a certain predictable fraction of white pea plants and a certain predictable fraction of purple pea plants. Characteristics are inherited equally, one from each parent, with some traits more dominant than others, Mendel found. Crucially, however, they are inherited as particles that can never be subdivided, not as a fluid that can be blended. Mendel, though he did not know it, had discovered what we now call genes.
Mendel published his findings in Proceedings of the Natural History in Brünn in 1866. But the journal was so local and obscure that his work was not widely recognised until the twentieth century. There is a story, often repeated, that, of the 115 copies of Mendel’s pea paper, one found its way to Darwin himself. It was discovered in his library after his death, sealed and unread. It would have been a terrible tragedy if true. However, the story is mere romantic myth. Darwin had no work by Mendel in his vast collection. The two biological geniuses, each of whom possessed a crucial jigsaw piece the other lacked, missed each other not by a hair’s breadth but by a significant span of space and time.
Mendel’s work was rediscovered only in 1900, long after Darwin’s death. Shortly afterwards, the American biologist Thomas Hunt Morgan began breeding together fruit flies. He observed that they inherited characteristics in a pattern very similar to Mendel’s pea plants. He even established that the physical elements responsible for inherited traits – genes – lay on tiny stringy structures called chromosomes. It was the birth of a new science: genetics.
The full picture of inheritance was filled out only in the late twentieth century. The building blocks of all life are cells, tiny bags of chemicals, whirring with chemical nanomachinery.5 In the centre of every cell is a mini cell, or nucleus. And, in each nucleus, chromosomes made of DNA.
DNA is a molecule the shape of two spiral staircases intertwined. The core, or backbone, of this double helix is made of a sequence of just four molecules, or bases – adenine (A), guanine (G), cytosine (C) and thymine (T) – which are joined in pairs. A, G, C and T are the four letters of the genetic code.6 Each triplet of bases codes for a particular amino acid. And amino acids are the building blocks of proteins, miraculous molecules that can carry out all manner of biological tasks, from speeding up the chemical reactions of life to detecting sunlight in your eye to providing the scaffolding that keeps your body rigid enough not to collapse into a puddle of jelly and water.
A stretch of DNA that encodes a protein is called a gene. And herein lies the connection with Mendel. The traits he identified that were inherited were associated with genes. A particular gene, for instance, makes a protein that influences the development of a pea to be wrinkly or smooth.
There are about 3 billion letters in a strand of human DNA, accounting for about 23,000 genes. This seems a woefully inadequate number to create a human being, and biologists were truly shocked that there were not more. But they have had no choice but to live with it – 23,000 genes are all there are.
Some of the genes are involved in controlling other genes. They switch off or switch on their ability to make, or express, proteins at various times in a developing embryo. And they do this depending on factors such as the concentration of a particular chemical in the cell.7 Such control genes cause different sections of DNA to be read in different types of cell, explaining how, despite every cell in a human being containing a copy of exactly the same DNA, some cells develop as blood cells, others as liver cells or brain cells, and so on.
But DNA explains not only the mechanism of inheritance but the mechanism of variation too. If an offspring is to inherit traits from its parents, their DNA must be copied. With a whopping 3 billion letters to reproduce faithfully in the case of human DNA, the amazing thing is how good the copying process is.8 But it is not perfect. A mistake is made about once every 1 billion base pairs. Sometimes a letter is not copied correctly. Or a sequence of DNA is deleted or duplicated. There are a myriad possible transcription errors. In addition, changes in genes can be caused by cancer-causing chemicals, viruses, ultraviolet light and nuclear radiation.
The upshot is that over time genes gradually change.
There is a lot of redundancy built into DNA to minimise copying mistakes, so many of the individual changes make little difference – the protein encoded by the gene still works. Some changes are harmful, causing inherited diseases such as cystic fibrosis. But, very occasionally, a change in DNA turns out to make a beneficial change to an organism – for instance, conferring on it an increased resistance to malaria. Of course, the ultimate arbiter of what is beneficial to an organism is its environment. A change in a gene that results in a thick, warm coat is beneficial to an animal living in a world plunging into an ice age but not to one living in a tropical world.
It is worth pointing out that changes, or mutations, occur in the DNA of all organisms. But, whereas simple organisms such as bacteria merely create copies, or clones, of themselves when they reproduce, other creatures have sex, producing offspring with half their genes from each parent. Such a composite of different traits passed down the maternal and paternal line greatly boosts the novel gene combinations available for natural selection.9
Mutations explain the existence of species – groups of animals, which, broadly speaking, cannot interbreed. Species can arise in many ways. For instance, a geographical barrier such as a river or mountain range might split a population in two. Or, as in the case of the Galápagos, an ocean might divide creatures from their cousins on the mainland. Separated in this way and subjected to different survival pressures, the DNA of each group accumulates different mutations, so the populations gradually diverge. Eventually, the two groups can no longer interbreed.
There could be many reasons for this. It could be that a mixture of their genes simply does not lead to a working organism, in much the same way that putting a motorbike engine in a Rolls-Royce does not create a viable car. Or it could be that members of one group hang out on a particular type of fruit, waiting for a mate, whereas members of another group prefer another type of fruit entirely; though they could easily mate, they miss each other like ships in the night. In the case of insects, which have complex genitalia, two groups might no longer interbreed because one develops sex organs that, like a skeleton key and a Yale lock, physically do not fit each other.
Whatever the reasons for groups of creatures diverging from each other, natural selection has populated the world with a myriad distinct species, each with as little ability to breed with each other as humans and oak trees.
Darwin’s theory explains so many aspects of the world. For instance, it explains why life on Earth is so staggeringly diverse, boasting more than 5 million living species. It also explains why we share around 99 per cent of our DNA with chimpanzees – and even a third with mushrooms. This is exactly what would be expected if we evolved from a common ancestor. Since changes in genes accumulate over time, the DNA differences reflect the fact that the common ancestor of humans and chimpanzees lived relatively recently whereas the common ancestor of humans and mushrooms lived in the very remote past.
Arguably, the most remarkable DNA sequence on Earth is GTG CCA GCA GCC GCG GTA ATT CCA GCT CCA ATA GCG TAT ATT AAA GTT GCT GCA GTT AAA AAG.10 It is present in every single living organism – even in organisms not technically classed as alive such as giant mimiviruses. The reason the sequence is so widespread is that it existed in the common ancestor of all life. Carrying out a crucial process, it has remained unchanged for 3 billion years: the oldest fossil in your body.
Darwin’s theory also explains why our antibiotics become less and less effective with time. Initially, they may kill the overwhelming majority of bacteria infecting a person. However, genetic variation within a population of bacteria ensures that some, inevitably, will survive to reproduce. Each successive generation will, therefore, contain a higher proportion of antibiotic-resistant bacteria, until eventually the antibiotic is next to useless. ‘Evolution is … an infinitely long and tedious biologic game, with only the winners staying at the table,’ says Lewis Thomas.11
Most of all, however, Darwin’s theory explains the illusion of design – why organisms are so perfectly suited to their environments. The reason a finch on an island in the Galápagos has a beak perfect for cracking open the nuts it lives on is because its ancestors prospered, leaving more offspring than did finches with less effective beaks. The shape of a beak turns out to be controlled by a single gene, slight variants of which express different proteins in the growing jaw of a finch embryo.
The remarkable thing is that such an exquisite match between organism and environment is achieved without a designer. But, then, the natural process identified by Darwin is not random. ‘Mutation is random,’ says Richard Dawkins. ‘But natural selection is the very opposite of random.’12 It preferentially culls all the variants except those that confer on their host the ability to survive to reproduction. Incrementally, generation by generation, it accumulates advantageous changes, slowly but surely assembling machines far more exquisite and complex than any designed by humans. ‘The whole trend of life, the whole process of building up more and more diverse and complex structures, which we call evolution, is the very opposite of that which we might expect from the laws of chance,’ wrote American biologist Gilbert Newton Lewis.13
But evolution by natural selection has its limits. The only organisms that can arise are those that are the result of a long string of advantageous changes. ‘Evolution walks backwards into the future,’ says British biologist Steve Jones. ‘It doesn’t know what’s coming.’14 This has led some people to claim that Darwin’s theory cannot explain the existence of complex organs such as the eye, which consists of multiple components. Until all components are in place – a lens, a light-detecting surface, and so on – goes the argument, no advantage is conferred on an organism. What use is 50 per cent of an eye? Or 5 per cent of one?
However, it turns out that all the steps along the road to the eye were indeed advantageous. Examples of primitive eyes can be seen throughout the animal kingdom. Some creatures have only a patch of light-sensitive cells for sensing which way is up and which down. Others, like the pit viper, have light-sensitive – actually, heat-sensitive – cells at the bottom of a pit in their skin, so their ‘sight’ has a directional capability. From this, it is a short step to close over the pit with a transparent protein, creating a lens that can focus an image of an object.
In addition to having no foresight, evolution by natural selection does not necessarily result in more complex forms. It can, but it does not always do so. After the advent of the first cell, there really was nowhere to go but up in terms of size and complexity. But, as soon as larger creatures evolved, it was possible to evolve back down to simpler forms. This can be seen in the case of parasites, which live off their more complex hosts.
Darwin’s theory of evolution by natural selection – Dawkins’s ‘greatest idea in the history of science ’ – has passed every test. ‘It could so easily be disproved if just a single fossil turned up in the wrong date order,’ wrote Dawkins.15 All it would take would be the discovery of a rabbit in the pre-Cambrian period 500 million years ago. As yet, this has not happened.
1 Useful traits are not only those that boost a creature ’s chance of surviving long enough to reproduce but also those that boost a creature ’s chance of getting the opportunity to reproduce if it survives that long. Such sexually selected traits include the peacock’s tail – which makes a male attractive to a female – and a stag’s antlers – which enables a male to out-compete other males for a mate.
2 Alfred Russel Wallace exempted humans from the process of natural selection. He therefore avoided the controversy that surrounded Charles Darwin – and also the fame. Wallace ’s collected works – books, articles, manuscripts and illustrations – can be found at http://wallace-online.org.
3 The complete works of Charles Darwin can be found online at http://darwin-online.org.uk.
4 Actually, our Milky Way Galaxy turns out not to be at the centre of things but merely one among 100 billion or so others in our Universe. And there is a growing suspicion that our Universe itself is not special but merely one among countless others in a multiverse. So, seen in this context, Darwin is merely one of many scientists who have applied the Copernican principle, moving humans remorselessly from the centre of the world and revealing their insignificance in an indifferent, bewilderingly huge, and possibly infinite, cosmos. See Chapter 21, ‘The day without a yesterday: Cosmology’.
5 See Chapter 1, ‘I am a galaxy: Cells’.
6 The copying of DNA is made possible by a remarkable circumstance. A always pairs with T, and G with C. So, if a cell’s double helix of DNA is split down the middle, it forms two complementary strands. As floating about in solution automatically lock like jigsaw pieces to exposed Ts; Ts mesh with As; Gs with Cs; and Cs with Gs. The result is two identical copies of the original DNA. No wonder that, when Francis Crick and James Watson discovered this in 1953, they rushed into the Eagle pub in Cambridge, England, and declared they had found the secret of life. See James Watson, The Double Helix.
7 In the 1960s, British biologist Lewis Wolpert (‘Shaping Life ’, New Scientist, 1 September 2012) proposed that complex body plans of animals can be created by gradients in the concentration of chemicals across embryos. Depending on the local level of these compounds, known as morphogens, different genes get activated in different locations.
8 DNA stores information thousands upon thousands of times more compactly than the best current solid-state storage devices. In 2012, a team led by George Church of Harvard Medical School translated into DNA a non-fiction book consisting of 53,000 words and 11 images. The team encoded the book in binary, using the bases A or C to represent a ‘0’ and G or T to represent a ‘1’. The book was the size of a typical bacterium’s DNA. Cell division has already created 70 billion copies of the book – 10 for every man, woman and child on Earth. All of them would fit in a single drop of water.
9 No one is absolutely sure why sex evolved since so many organisms do perfectly well without it. But one possibility is that it wrong-foots parasites, which are in a never-ending arms race with their hosts. A parasite can never become perfectly adapted to its host and kill a population if new variants of the host are continually thrown up. See Chapter 4, ‘The big bang of sex: Sex’.
10 Michael Le Page, ‘A Brief History of the Genome’, New Scientist, 15 September 2012, p. 30.
11 Lewis Thomas, The Lives of a Cell.
12 Richard Dawkins, The Blind Watchmaker.
13 Gilbert Newton Lewis, The Anatomy of Science, pp. 158–9.
14 Author’s telephone interview with Steve Jones.
15 Richard Dawkins, The Greatest Show on Earth: The Evidence for Evolution.