Unique among scientific theories, evolutionary biology finds its roots in a popular book by a single author. The grey-bearded genius presented a new and radical view of existence: that life has changed over time and space, in part through a simple process called natural selection.

Charles Darwin called his work “one long argument”. To a 21st-century reader it seems lengthy indeed, with only a single illustration to enliven its 150,000 words. But Darwin was a clear thinker and the book is an impressive piece of advocacy, moving from the familiar - how animals on farms have changed - to the less so, embryos and instinct included.

DARWIN’S MASTERPIECE REVISITED

Unique among scientific theories, evolutionary biology finds its roots in a popular book by a single author. Darwin presented a new and radical view of existence: that life has changed over time and space, in part through a simple process called natural selection.

To a modern reader Origin of Species seems lengthy indeed, with only a single illustration to enliven its 150,000 words. But Darwin was a clear thinker and the book is an impressive piece of advocacy.

In just a fraction the length of its archetype, this account sketches out how Darwin might make his case today, a century-and-a-half on.

Darwin also shows how what might seem to be problems for his argument, such as the uncanny perfection of complex structures like the eye, are in fact part of the solution, and how apparent weaknesses in his case - the incomplete nature of the fossil record included - can easily be explained. Now and again he was wrong, as when, unaware of Gregor Mendel’s work on genetics, he claimed that inheritance is based on the mixing of bloods, but mostly he was right.

Darwin described the process of evolution as “descent with modification”. Today that might be rephrased as “genetics plus time”. Offspring resemble their parents because they inherit DNA from them, but the copying process is not precise. Every round has errors, or mutations, and although they are individually rare - with perhaps one or two mutations in working genes each generation in humans - they can soon build up vast diversity. A copy of a copy is always imperfect, and for that reason alone, evolution is inevitable.

Darwin had a second insight. He saw that if a certain variant allows its carriers to survive, to mate and to pass on their heritage more successfully than others, in later generations it will spread. Such inherited differences in the chances of reproduction allow creatures to adapt to changing circumstances and can, in time, give rise to new forms of life. Natural selection, as he termed it, is a factory for making almost impossible things.

The Origin was written in something of a rush. When Darwin discovered that Alfred Russel Wallace had hit upon the very idea he had been cultivating since soon after his return from the Beagle voyage, he condensed and refined his plan for a much longer book and set out to bring his theory to a wide audience. The book is far shorter than that first scheme, but as a result it is much clearer - which is, no doubt, why it made such an immediate impact. Darwin apologised again and again for leaving so much out and spent much of the rest of his life filling in the gaps.

If The Origin was a hasty letter to its readers, this account is no more than a postcard. But I hope that in just a fraction the length of its archetype it sketches out how Darwin might make his case today, a century-and-a-half on.

CHAPTER 1 — Variation under domestication
In which Darwin uses examples from domestication to explore the causes of variability and the principles of selection

Farmers have been unwitting evolutionists since they began, as they have shaped the characteristics of domesticated species. Nowhere is the power of human selection more clearly seen than by the fireside. Dogs were domesticated around 16,000 years ago in China, perhaps for meat. Their ancestors were wolves - and the two still share the same scientific name, Canis lupus - but dogs have changed mightily since then. The breeders were ruthless, killing off the animals of which they did not approve. This selective death, combined with selective sex, soon subdivided dogs into a huge variety of forms.

Some kinds have been distinct for centuries but most are less than 400 years old. In Darwin’s childhood there were no more than 15 designated breeds. By the time The Origin was published, the number had risen to 50. It is now around 400. Many of today’s varieties have gained a distinct identity in no more than 30 or so canine generations. Sometimes a single mutation sparks a new variety.

The Irish wolfhound stands a metre high and weighs as much as 30 chihuahuas, but the difference in size between the two is due to a single gene, which comes in one form in the large animal and another in the small. Most divergence between breeds, however, involves many genes, unwittingly selected by breeders aiming to refine the characteristics of particular lineages. Were the various breeds of dog to run wild, they might be classified by an enthusiastic but naive naturalist as separate species. How could a dobermann possibly belong in the same category as a chihuahua?

Even so, in spite of some logistical difficulties with sex, crosses among dog breeds produce fertile offspring, which is one of the several definitions of what a species is. In the same way, the European grey wolf crosses freely with domestic dogs, which is why it shares its name. The pedigree dog clubs have laid out the case for Darwinism in a few short decades.

CHAPTER 2 — Variation under nature
In which Darwin considers individual differences and highlights the wide degree of variability within species upon which natural selection works

The natural world is full of variation. The number of animal species identified so far is 1.8 million and there are no doubt many more yet to be discovered. Of other kingdoms of life even less is known. Many habitats are almost unexplored. A scheme to classify waterborne microbes sampled from the Atlantic, Pacific, Baltic, Mediterranean and Black Sea revealed thousands of new families of genes. They are proof of the presence of vast numbers of as-yet-unknown forms of life in a habitat that comprises 99 per cent of the biosphere.

Darwin was fascinated by the diversity of existence. He realised that new species are usually generated from the variation within existing forms which, genetics has revealed, is astonishing in its extent. Intra-species variability is scrutinised by natural selection, which promotes particular traits in particular environments. Should a creature occupy two distinct habitats separated by space or time it may become subdivided into two forms unable to exchange genes: that is, into two species.

Sometimes, the boundary between species is shifting and unstable, in testament to the dynamic and ever-changing pressures brought by a changing environment. In North America, for example, the red wolf and the coyote - distinct as they are - have lived in the same kinds of places for many generations. Each has maintained its identity. In recent years, though, they have been forced closer together by human activity, and have begun to produce fertile hybrids. For them, the barriers to gene-sharing were not quite complete, and what became for a time two is sliding back to become one.

CHAPTER 3 — Struggle for existence
In which Darwin describes the competition in nature for limited resources

All species have the potential to increase vastly in number, given time - but they do not, because of lack of food or because of disease, predation or for want of a home. In Darwin’s day, the romantic poets and much of liberal society were in denial about this struggle for existence. Now we accept it, but we often forget quite how unforgiving it can be.

For example, around 400 million pet dogs live easy lives in human households. In the many cultures where dogs are despised, several million more roam in feral packs, forced to scavenge. Their days are chaotic, unhealthy and short. Meanwhile wolves, which once roamed throughout the world’s northern forests, have been squeezed out by humans. Europe now has only a few thousand left and there would soon be none if they were not protected. Both wolves and feral dogs were unlucky to come up against the most intractable enemy that nature has yet produced; and both are paying the price in the struggle for existence.

CHAPTERS 4 AND 5 — Natural selection and laws of variation
In which Darwin explains how inherited differences in the ability to survive and reproduce have shaped nature; in which he explores the forces influencing the variation upon which natural selection works

Ten thousand years ago, much of North America was covered with ice. As the glaciers retreated they scooped out an intricate landscape of low hills, small lakes and rushing streams. Slowly, sticklebacks moved in from the sea. They found two distinct kinds of place to live in and adopted two ways of life. In lakes, where most of the food was available in open water, they became slim and active swimmers. In streams, where the shallow bottom was a better place to forage, they evolved to be robust and tough so as to grub around for food. Although they are not yet classified as distinct species, each form now prefers to stay in its own home and to mate with its own kind. Natural selection has worked to adapt the fish to the challenges they face, with only those individuals best suited to passing the test of survival getting the chance to hand on their genes.

As Darwin recognised, the entrance examination to the next generation has two papers. The first involves staying alive, but there is a second paper that turns on successful reproduction. It leads to what he called “sexual selection”. Males of many species are forced to compete for the attention of females, and it pays a female to choose the best mate to father her offspring, the vessel of her own precious genes. This struggle for sex can lead to the evolution of bizarre structures such as the peacock’s tail. Far from helping their bearers to stay alive, these sexually attractive traits are often a handicap. That may be why they work, for they indicate that those who bear them have what it takes to survive despite their costly sexual signal.

That trade-off between sex and death is hard at work in a certain Trinidad guppy. In some streams the males have bright orange spots that entrance the females. In others there lurks a nasty predator that spots the spots and eats those who carry them. Here, the males have evolved to be dull. But move them to a stream without enemies and in just a few generations the bright spots reappear, testimony to the power of sex.

CHAPTER 6 — Difficulties on theory
In which Darwin considers organs of extreme perfection and other apparent stumbling blocks for his theory

The mammalian ear is a wonderful example of how evolution can craft what at first glance looks like the work of an engineer. Fish can pick up sound waves with nothing more than simple pressure sensors on their bodies because water is an excellent conductor. When animals moved onto land, though, they needed to amplify sound waves, feeble as they are in comparison. Reptiles and birds have a lever - a tiny bone - between the outside world and the sensory cells of the ear that does the job, but in mammals the lever is comprised of three small, interlocking bones that work far more effectively.

The tale of the mammalian ear is one of make do and mend, and of structures modified for a new and different end. The earliest fish had no jaws. These developed later from bony arches that once supported the gills. Another gill arch became the single amplifying lever found in reptiles and their avian relatives. As the ancestors of mammals appeared, the ear began to hijack other structures. First, the position of the hinge between the upper and lower jaw shifted, freeing one bone of the upper jaw and one of the lower. These became the other two bones of the middle ear, so we hear, in part, with what our ancestors chewed with. Fossils reveal the whole process with a succession of creatures with more and more complete middle ears as proof of natural selection’s ability to build on a series of successful mistakes and to craft complicated organs of apparent perfection.

Darwin knew nothing of genes, and his chapter on inheritance is the weakest in the book, but we now have molecular probes which reveal that the same genes are active in the fish sensory system as in the human ear - proof that genetics and evolution have become branches of the same science.

CHAPTER 7 — Instinct
In which Darwin faces the issue of how behaviour might evolve

Dogs have long been used to hunt. Those with particular abilities - to track, to run, to recover corpses - were chosen as parents for the next generation. That history lives on in the instincts of today’s breeds. Herding dogs such as border collies stalk sheep and do not bite them, but dogs bred to control larger animals, like corgis with cattle, go further and snap at their charges. Pit bull terriers are vicious creatures that can hold a bull - or a baby - by the nose. Can such psychological characteristics really evolve by selection?

Russian researchers in the 1950s showed that with strict selective breeding for tameness, once-wild silver foxes within a few generations began to wag their tails, bark and enjoy human company (New Scientist, 3 October, p 40Movie Camera). Even their appearance altered, with new coat markings and floppy ears. After 30 generations they were completely tame. The brain, it seems, can evolve just as quickly as the rest of the body.

CHAPTER 8 — Hybridism
In which Darwin examines what keeps species apart

Species retain their identity because they live in different places, fail to have sex with each other or, if they do, produce infertile offspring. Hybrid sterility is due to genes, but because most species cannot be crossed it is hard to find out which ones are involved. Experiments with two kinds of Mexican fish show how little change is needed to keep species apart.

The platyfish lives in streams and is covered with elegant dark spots. In other streams nearby lives the swordtail, which looks much the same, although the males lack spots and have a long tail. The two species never cross in the wild, but can be persuaded to do so in the laboratory. Hybrids are spotty and survive reasonably well, at least in an aquarium, but if they are crossed in their turn to one of their own parents the next generation of offspring suffers a sinister fate. The small spots turn into lethal black tumours - and, in a macabre twist, males with these cancerous growths are much fancied by platyfish females.

CHAPTERS 9 AND 10 — On the imperfection of the geological record and the geological succession of organic beings
In which Darwin considers why our palaeontological collections are full of gaps, and describes how his theory can account for the pattern of succession from fossils to living forms

The geological record is like having just a few lines of a few pages of the history of the world. Much of it is incomplete, and because soft-bodied creatures are rarely preserved in the rocks some parts may be fragmentary at best. Since Darwin’s day, however, vast numbers of fossils have been found across the world, and his concern that the geological record fails to support his theory now seems unduly pessimistic.

Darwin could only guess at the age of each stratum by estimating the rate at which rock was worn away. Today we can date fossils directly by examining how their chemical elements break down with time. The first life has been traced back more than 3 billion years and the death of the dinosaurs to 65 million years ago. Some of the records of that immense period are impressively complete.

The Himalayas are full of fossils - not of mountain creatures but of those of the sea, for long ago their peaks made up the floor of the Tethys Ocean. The fossils include the antecedents of great whales. The bones of the earliest ancestor of all cetaceans are found in beds some 50 million years old. They belong to a creature that had four legs and a tail, lived on the shore, and looked a little like a seal. Its ears, though, had a unique structure, now found only in whales.

The next prominent player, a million or so years later, “the swimming-walking whale” in Latin, looked like a 3-metre-long otter. Another million years on and the animal’s nostrils had migrated up the snout and the pelvis moved away from the backbone. A further 5 million years saw the oceans inhabited by a long mammal with tiny limbs.

Then came a great split. The ancestors of the blue whale and its relatives - those that filter tiny creatures from the water - began to develop gigantic sieves within their mouths, while others retained the sharp teeth found in earlier whales and in today’s killers. More recent deposits reveal the splits between dolphins and whales. A record that was once little more than an enormous gap has, with infinite labour and some luck, provided a complete history of the evolution of the largest animals that ever lived.

It has also given whales their rightful place - until not long ago quite obscure - within the family of mammals. Their earliest ancestors were close to those that gave rise to the hippopotamus. Whales, unique as they may seem, are hence members of a larger group that contains hippos, pigs, giraffes and cattle. DNA analysis backs up the record of the rocks. The whale’s whole story has been revealed within less than half a century.

CHAPTERS 11 AND 12 — Geographical distribution
In which Darwin demonstrates the importance of geographical barriers and climate change to explain the distribution of life as we see it today

Penguins are charming birds with between 17 and 20 species, depending on which classification is used. They take a variety of forms, ranging from the statuesque emperor found in the Antarctic, to the fairy penguin of Australia, just 1/20th its weight. Other penguins live in New Zealand, South Africa, South America and the Galapagos. They cannot fly (although the DNA suggests that albatrosses are among their closest relatives), so how did they reach such scattered places?

The oldest penguin fossils appear shortly after the demise of the dinosaurs around 65 million years ago. This ancestor of all living species lived in southern New Zealand and Marie Byrd Land, Antarctica, which were at that time separated by less than 1500 kilometres. Already the ancestral penguins had almost lost their wings. Fossils and DNA each show that the penguins’ spread matches the advance and retreat of the ice. Starting around 35 million years ago, a series of ice ages made the Antarctic uninhabitable, and the birds retreated northwards as the glacier spread. In addition, icy currents swept some westwards, eastwards and closer to the equator.

After 10 million chilly years, the world warmed, cooled and warmed again, and the birds followed the retreating edge of the ice sheet into the far south. They left behind colonies scattered on cold-water shores across the globe. In the years since then, isolation and the challenges faced by each separate group have led to the diversity of penguins seen today.

CHAPTER 13 — Mutual affinities of organic beings: Morphology: Embryology: Rudimentary organs
In which Darwin considers classification and shows how his theory can be used to organise the living world along evolutionary lines

As the past is the key to the present, so is the infant the key to the adult. Darwin realised that animals adopt the form which is adapted to their own evolved way of life as they develop. As a result, the deep relatedness between organisms might more readily be seen by comparing embryonic forms.

Darwin spent eight years working on barnacles, then an entirely obscure group but known to be related to crabs, insects and other jointed-limbed creatures. Adult barnacles vary in form from the familiar seashore kind to a sinister version that is an internal parasite of crabs and resembles a giant fungus. Whatever the divergence among the adults, though, the embryos are remarkably similar. They also resemble, to a lesser extent, the embryos of lobsters and crabs, and other jointed-legged creatures of the seas. All those in turn show affinities to the embryos of insects, a hint that butterflies have relatives on wave-battered shores, while that great group is in turn united in its earliest development with tapeworms and their relatives - creatures entirely different in their adult form. This shared identity, lost as the animals grow, shows how the embryo can reveal deep patterns of descent with modification, hidden as development goes on.

Darwin’s own classification of life - of groups within groups as evidence of a shared hierarchy of descent that encompasses not just barnacles and butterflies but birds and bananas - turned only on what he could see with the naked eye, or down the microscope. Now genes - the units of evolution - have come to the rescue, and they reveal whole kingdoms of existence quite unknown only a few decades ago.

The new tree of life is a direct descendant of the single sketch of shared descent that appears in The Origin. It is based on comparing the billions of DNA sequences now available from across the whole of existence. The affinity of whales with giraffes is a minor surprise when compared with discovery of the relatively close affinity of all animals to mushrooms, and the emergence of a whole new kingdom of single-celled creatures, the Archaea, which have a structure and a way of life entirely different from the bacteria that they superficially resemble. They may even have cooperated with them, in the early days of life, to provide the nucleated cells that build all plants and animals of today.

CHAPTER 14 — Recapitulation and conclusion
In which Darwin expounds his “long argument” and addresses the “mystery of mysteries”: why there are so many different species

A century and a half after The Origin, Darwin can be seen to have been triumphantly right about almost everything. Evolution is now no more “just a theory” than is chemistry and, like all other sciences, it provides a logical way of looking at the world. As he dared only to hope in that great book, light has been cast upon not just the worlds of plants and animals, but on ourselves and our origins.

Darwinism makes sense of what was once no more than a jumble of unconnected facts, and in so doing unifies biology. Modern psychology, ecology and more find their birthplace in the pages of his greatest work. A century-and-a-half on, evolution is as central to our understanding of life as gravity is to the study of the universe. The closing words of The Origin say it all:

“There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

Editorial: Darwin’s long argument is won

“THIS whole volume is one long argument.” That was among Charles Darwin’s concluding comments in arguably the most revolutionary science book of all, The Origin of Species.

In the 150 years since its publication, The Origin has been subjected to intense argument, not all of it edifying. Opponents of evolution often make the ludicrous claim that the book is treated like a bible, a sacred text of science.

That is why copies of Darwin’s great work, adulterated with a creationist introduction, are about to be distributed on college campuses. This is part of the same disinformation campaign that brought us intelligent design - invented to provide a “scientific” alternative - and teachers now fear to teach evolution because of a creationist backlash.

One reason creationists make this argument is that they can then claim that if the scientific bible can’t provide all the answers (and, of course, it never will), then turn to Christianity because the real one can. By pointing out the fallacy of this argument, Richard Dawkins was accused of fascism by an American TV host last month. A cardinal warmed to this repellent theme by comparing Dawkins to Hitler, explaining that both of them see humans only as vehicles for good genes.

Here we should invoke Godwin’s law (whoever makes a comparison with Nazism loses the debate) and remember what was really the substance of Darwin’s argument as he wrote The Origin. Although aware of the struggle between faith and reason, this does not come into the book at all. Instead, Darwin was preoccupied by the science.

Those who label The Origin as a sacrosanct text display a profound ignorance of how science works. Scientific understanding develops, changes and evolves all the time. Here Dawkins should be applauded for his diligent efforts to show how, as the great scientific enterprise rolls on, it has consistently confirmed Darwin’s ideas.

What is remarkable is that, though it is 150 years to the very month since John Murray published Darwin’s landmark work, its key ideas are still compatible with what we know from modern research, notably genetics. To underline this, Steve Jones of University College London has condensed and updated The Origin for this magazine.

In the heart of this issue is a 21st-century version of the founding text of evolutionary thought. Use it to peer inside the brilliant mind of Darwin and to read modern examples of the evidence that demonstrates the argument put forward in The Origin is no longer just a theory (in the colloquial sense). The veracity of Darwin’s thinking shines out with clarity. The long argument is over and, as Darwin himself aptly put it: “There is grandeur in this view of life.”