The idea that organisms are able to change, or evolve, from one form to another over time is most frequently associated with Charles Darwin. His ideas have indeed won the arguments over evolution, but they were by no means the first. The Greek philosopher Aristotle believed that all natural things strove to fulfil a role in the universe, and this sowed the seed of an idea that living things were able to change their form in pursuit of that goal. As modern scientific methods developed in the 18th century, two opposing views took shape among the naturalists who catalogued living things. Some said that every organism belonged to a certain type of species, and that this was an unchanging aspect of nature. Others pointed to the vast age of Earth (which was becoming better understood by this time) as evidence that today’s life forms could have lived in other ways in the past. Erasmus Darwin (grandfather of Charles – see here) suggested that large animals had all descended from a microscopic ancestor. The challenge was to find a mechanism by which this could have happened.
Birds, dinosaurs and reptiles all share a common ancestry, but owe their different forms to a variety of evolutionary pressures and processes.
Inherent to early ideas of how organisms evolved was the idea that new life was constantly forming out of non-living things. This concept of ‘spontaneous generation’ stated that microorganisms (still to be studied in any great detail) arose from the putrid remains of dead organisms and their waste. This was still many decades before cell theory was formulated (see here), and so spontaneous generation was still seen as the best explanation for observations that saw moulds and other germs appear as if from nowhere on rotting matter. Even relatively complex life, such as fly maggots and beetle grubs that wriggled out of dung were supposed to have formed directly from inorganic material. Spontaneous generation was thought to provide the source material for evolution, as each primitive life form worked to move up the biological scale. This ‘teleological imperative’, in which organisms actively sought to improve, came straight out of Aristotelian philosophy and was seen as the driving force behind evolution. It is an idea that is hard to shake even today.
The modern science of biology grew out of the work of naturalists. These were 18th-century nature lovers – frequently men of the cloth – who began to document and above all catalogue the natural world. Each organism fell into a specific type, or species, whose members all shared a particular set of characteristics. Such ‘specific’ characteristics were deemed to be unchanging, a summation of the unique essence of that organism. This concept was hard to argue against in the years before inheritance was understood.
So when it came to looking for a way that species could differ from each other and effect an evolutionary change, naturalists turned to ‘acquired characteristics’ rather than specific ones. These are changes the body undergoes in response to its activity and environment – anything from a bodybuilder’s muscles or the way skin on the hands becomes thicker with manual labour. Early theories of evolution suggested such acquired traits could be inherited, making it possible for species to evolve.
The first fully conceived theory of evolution was put forward by French naturalist Jean-Baptiste Lamarck in 1809. He used the term transmutation to describe the way a species could change over many generations. The theory was superseded by Darwinism, but its central idea is now being revived with the discovery of epigenetics (see here).
Lamarck believed that evolution had a direction: nature strove to improve and progress towards more complex forms. However, he also introduced the idea that evolution worked to adapt an organism so that it was better suited to the environment than its forebears. The agents of change in ‘Larmarckism’ were characteristics acquired in life. According to the most famous erroneous example, each generation of giraffe reached up to the freshest leaves, stretching their necks. Their offspring inherited the longer neck, and continued the process, resulting in ever taller generations. Today’s giraffes are deemed to be ‘tall enough’, and so the upward trend has ceased.
Charles Darwin (1809–82) is among the most famous scientists of all time, and his theory of evolution by natural selection marks one of the greatest shifts in scientific thinking. Darwin was somewhat reticent about his role as scientific behemoth, and mostly left others to support his theory against those with opposing beliefs – church leaders among them.
Darwin was born in Shropshire – his father was a doctor and his mother an heiress to the Wedgwood porcelain fortune. Charles initially studied medicine at Edinburgh but did not thrive, and so his father moved him to Cambridge, ironically in preparation for taking holy orders. There, Darwin further developed his enthusiasm for natural history, making detailed studies of beetles found in the surrounding countryside. He also studied the work of William Paley, a theologian who had used natural history as evidence of the work of God. Upon graduation, Darwin’s eclectic education left him well placed to think the unthinkable.
Many of the most powerful observations that caused Charles Darwin to form his theory of evolution took place during a circumnavigation of the globe aboard HMS Beagle. The Beagle’s mission was a peaceful one, with orders to survey the coast of South America and Pacific islands. Captain Robert FitzRoy (later founder of the Met Office, Britain’s national weather forecaster) called for a scientific-minded civilian to join the crew as his companion. Darwin, who had graduated a few months before, accepted the offer – despite the fact that he would have to pay his own way.
The voyage took Darwin to Africa, South America, New Zealand, Australia and numerous islands, including the Galápagos on the mid-Pacific equator. The journey lasted five years, most of which Darwin spent ashore, collecting and comparing the organisms he found from place to place. The similarities he found between apparently unrelated species on separate continents were the starting point for his theory of evolution.
Darwin was not the only mid-19th century naturalist thinking about how evolution could shape the bodies of animals and plants. While Darwin lived in relative solitude pondering the immensity of his theory in private, Welsh explorer Alfred Russel Wallace (1823–1913) was making his own journey of discovery through the islands of Malaysia and Indonesia. This region of the world marks a boundary between many animals that were ancestral to ancient Australia on one side, and those of Asia on the other. Today, that boundary, running through the Celebes Sea and Lombok Strait, is known as the Wallace Line.
Wallace saw how the most closely related species were found in neighbouring areas, and the differences between them appeared to follow a gradual sequence, as if each species arose from its neighbour. Wallace wrote to Darwin in the 1850s asking for comments on his ideas about how this evolution occurred. As a result, Darwin was spurred into finally going public with his own long-gestating theory.
Upon his return to England from HMS Beagle, Charles Darwin married his cousin Emma Wedgwood and settled down to a comfortable life in the country. However, the deaths of three of their ten children in infancy weighed heavily on Darwin, as did the immense implications of the theory of evolution, which he developed over many years. Only a few colleagues got to hear about it, but Darwin planned to eventually present it in a huge opus entitled Natural Selection.
In 1858, however, Darwin received a letter from Alfred Russel Wallace outlining a similar theory of evolution. The pair presented joint papers to the Linnaean Society that same year, while Darwin paused Natural Selection to dash off a shorter work on his ideas. The result was On the Origin of Species, first published in 1859. Amidst copious examples, it explained how all organisms – including humans – are related to a common ancestor in the distant past. Few other books have had such a dramatic effect on the way humanity views itself.
Charles Darwin’s theory of evolution invokes the principle of ‘natural selection’ – an idea with one of its roots in the 1798 Essay on the Principle of Population, written by Thomas Malthus (1766–1834) and read by both Darwin and Wallace. Malthus’s work warned that growth in the human population was destined to outstrip the ability to grow food, leading to global famine.
Technological advances have prevented this ‘Malthusian’ catastrophe so far, but to Darwin’s naturalist mind it posed the question of how non-human populations were controlled. He reasoned that a wild population had a finite set of resources available to them – food, space, etc – which could only support a finite population. Only some of the population would live, and the rest would die, but the battle for survival would not be random. Nature selected the winners: those that were best able to command the resources they needed would survive, and those unable to do so died. Darwin’s masterstroke was to recognize the power of this ‘natural selection’ to create changes in species.
To paraphrase the philosopher Thomas Hobbes (1588–1679), life is ‘nasty, brutish and short’. This is especially so for populations of wildlife, where a long life followed by a death from old age is a rarity indeed. While earlier theories of evolution supposed that the process was underwritten by some supernatural goal of improvement, Darwin saw that the only thing needed to power evolution was the competition for survival.
All life is competing to survive, battling for a supply of energy, nutrition, oxygen and water – and the space to use it. The strongest competition of all is between members of the same species, which share the same requirements and use the same means to achieve them. In addition, the drive for survival is only a means to an end. The purpose of survival is to reproduce, and individuals compete to maximize their opportunities to do so. The natural selection of competition not only acts to kill weaker individuals, but also prevents them from breeding, blocking them from passing on their genes to the next generation.
Natural selection needs something to work with: if a population of animals were all identical then none would have an advantage over the others. However, nature is not like that – every population contains a degree of variation, and it is these differences that can make one individual a success and another a failure by comparison.
Lamarck (see here) suggested that the long necks of giraffes were due to stretching to reach leaves and became incrementally taller every generation (though how exactly, he could not say). Darwin’s explanation chimed better with the known facts. Some giraffes are taller than others; their height gives them an advantage, so they eat more – and breed more – than their smaller neighbours. Short giraffes are more likely to starve and not have young. Darwin understood that tall giraffes have tall offspring, so natural selection results in more tall giraffes being born – and giraffes as a species evolve to be taller. But they are never all identically tall; there is always some variation.
Darwin understood that for his theory to work, offspring would have to inherit some kind of ‘genetic’ material from their parents. This material was the means by which the advantages of the parents – those traits selected by nature – would be passed to their young. Thanks to the discovery of DNA and the central dogma (see here and here), we now understand a lot more about what that genetic material is and how it works. This also shows us where the variation that feeds evolution ultimately comes from: a population’s variation is due to the different alleles in its gene pool. These alternative versions of genes arise randomly due to mutations – errors made during DNA replication. Without such mistakes life would not have evolved at all. If a mutation occurs in an intron, it has no impact. If it appears in an exon, it will alter the structure of the protein coded by the gene. The chances are that this will create a disadvantage, and natural selection will soon wipe it from the gene pool. But occasionally a mutation creates a new kind of advantage – and evolution occurs.
Jacob sheep have four horns, not the usual two, thanks to a genetic mutation.
Although he did not coin it, Charles Darwin readily adopted the term ‘survival of the fittest’ as a description for his theory of evolution by natural selection. In this context, the term ‘fitness’ sums up the balance of advantages and disadvantages inherited by an individual. If advantageous traits outweigh the deleterious ones, an individual is ‘fit’, and would succeed in the battle for survival against less fit competitors. Natural selection ensures that the fittest survive and have the most offspring. Those offspring are also likely to be fit, having inherited advantageous traits – or genes – from their parents.
If a mutant allele arises that provides an advantage over earlier forms, its carrier will be fitter than his or her neighbours. Over the generations, natural selection will result in this mutant gene spreading through the population, while less fit alleles become rare or disappear. The change is small, perhaps imperceptible, but given a great expanse of time and many generations, tiny accrued changes like this can alter species entirely.
Evolution is in some ways a refinement process. Natural selection filters out the unfit genes and ensures that the population as a whole is better suited for survival. However, there is another side to the equation. An individual’s fitness can only be measured by the environment it finds itself in. A trout is well suited to life in a river, but it cannot compete among a herd of camels crossing a desert (or vice versa).
The environment in which a population of organisms finds itself is not constant. It can change its character, sometimes very quickly, and this throws up new challenges for survival. Any change will alter an individual’s fitness – the genes that once brought success are no longer enough. Natural selection simply carries on, promoting different alleles that provide an advantage in the new conditions. The result is that the organisms can adapt to their new habitat. It is evolution’s ability to create adaptations for different environments that has driven life divergence into a multitude of species.
The dark form of peppered moths has become more common than the pale variety as the species has adapted to hiding on trees blackened by industrial pollution.
Natural selection moulds organisms to their environments. Over millions of years and many small changes, a group of animals recognized as belonging to one species can change so much that they form an entirely new group. This process of change is called speciation, and there are two main ways it can occur.
The simplest mechanism involves a single-species population becoming divided by a physical barrier. Perhaps an exceptional summer has cleared ice from an alpine pass allowing a herd of goats to pass into a neighbouring valley – but the ice returns to block the route. The two groups of goats now live in different environments, with different foods and predators. As a result they evolve in different ways and become separate species. The second form of speciation occurs within a population: a mutant goat is able to stomach foods that are toxic to the rest of the herd, and so a subpopulation of mutants develops to exploit a different food source to the others. They stop breeding with the non-mutant herd, splitting the population into two species.
Extinction is perhaps well understood because of the dinosaurs and other fascinating species that are known to have lived in the past. Until the dodo of Mauritius was hunted to extinction around 1688, the idea that a species could die out was barely considered. In 1813, French anatomist Georges Cuvier (1769–1832) revealed that extinction was not just an unnatural act committed by humans. He showed that the fossil remains of mastodons were not the same species as living elephants: as time passes, species become extinct and are replaced by new ones.
According to an oft-quoted statistic, some 99.9 per cent of species that have ever evolved are now extinct, but this requires a little clarification. The dodo and the mastodon are truly extinct, meaning none of their species survive, and neither do any ‘daughter’ species that evolved from them. However, on a larger scale we can say that dinosaurs are only pseudoextinct: today’s extant bird species evolved from dinosaurs, and so still carry at least some dinosaur genes.
One of the strongest indicators of natural selection at work is convergent evolution. This is the observation that animals with very different ancestries tend to evolve in similar ways when they adapt to the same environment. A good example is the convergent evolution of large pelagic predators – animals that hunt in the open oceans. Sharks are the most ancient: they typically have a streamlined body with fins for stabilizing the body and a wide tail for propulsion. The icthyosaurs, marine reptiles that hunted in the oceans until about 90 million years ago, had the same body shape with fins and tail matching a shark’s. Today, dolphins occupy a similar place in the environment – known as an ecological niche – and they too have a similar body plan. These fish, reptile and mammal species all evolved independently but ended up looking very much the same, due to a phenomenon called selection pressure. The blind hand of natural selection tends to push evolution in the same direction, so unrelated organisms develop the same adaptations to survive in a particular niche.
The field of population genetics considers how the frequency of alleles in a gene pool can change. One of the biggest drivers of change is natural selection, but this is not the only thing that can alter the gene pool. Mutation is another factor. The rate at which viable mutations (ones that do not die out rapidly) appear is very slow, but over a long enough period they can be seen to produce regular changes in the gene pool. More rapid changes are introduced by phenomena called ‘genetic drift’ and ‘gene flow’.
Genetic drift is caused by the element of chance. A freakish catastrophe may wipe out a significant proportion of a population and certain alleles may disappear along with it. A more mundane possibility is that alleles are simply not passed on, not because of selection, but merely through all the random aspects of the inheritance process. Gene flow, meanwhile, is the result of novel genes entering the gene pool with the arrival of individuals from another, hitherto isolated population of the same species.
Natural selection is driven by the need for reproductive success, but males and females have different ways of achieving it. This come down to a difference between the sex cells known as anisogamy. Sperm contains only a half set of genes and they are easy to produce in copious amounts. A male’s best option is to spread them as far as possible, playing the numbers to produce many offspring. The female’s options are very different. Her eggs are primed with the energy needed to produce an embryo, and so are produced in much smaller numbers than sperm. After fertilization, the female must devote considerable resources to giving her offspring the best chances of survival – and she cannot rely on the male for help. Therefore, females make use of the shortage of eggs compared to the supply of sperm through the phenomenon of female choice: a female must choose, and choose carefully, how she wants to use her valuable reproductive resources. This creates a new element of competition among males that has had far-reaching effects on their evolution.
Two black grouse cocks compete for the best display position. Their mating success depends on being chosen by a female.
In 1871, Charles Darwin published The Descent of Man and Selection in Relation to Sex, in which he expanded on his concept of ‘sexual selection’. This form of selection, driven by mate choice (and largely female-led), does not necessarily lead to adaptations that aid survival. In fact, it can do quite the opposite.
Many of the impressive adornments seen in the animal kingdom, such as the antlers of a moose or the tail feathers of a peacock, are the result of this process. Darwin saw that sexual selection could outpace natural selection to create features that hindered survival. Taking antlers as an example, a female chooses a mate because he has large antlers. Any male offspring will grow large antlers as well, and any female offspring will chose mates with large antlers. This creates positive feedback that drives antlers to get bigger and bigger – far beyond their practical application as weapons. The result is that the sexes frequently evolve in different ways, creating marked differences known as ‘sexual dichotomy’.
Mate choice in animals frequently involves signals such as antlers, bright tails or some other adornment. Such signals are driven to extremes by sexual selection, but are nevertheless ‘honest’. There is a high cost to developing large, symmetrical antlers, and that cost signals that the stag’s genes as a whole are able to tackle the everyday requirements of survival and still have spare energy for growing large, often unhelpful antlers. A wonky-antlered rival’s genes are less suited to survival.
But there is another factor at play that is keeping the antler signal honest. The population of deer (or any species) is constantly under attack from parasites and pathogens that are evolving unseen to get around an animal’s defences. The deer evolve to counter these attacks – and those that succeed show it with their antlers. Although the species appears to remain unchanged, evolution is running all the time in the form of the ‘Red Queen Effect’, so-named for an Alice in Wonderland character who runs fast but always stays in the same place.
Darwin’s theory of evolution met with many opponents, and the most controversial aspect of his thesis was that humans were produced by the same mechanism of change as all other life forms. This clash of ideas was epitomized in the 1860 debate between Darwin’s ardent supporter Thomas Huxley and Samuel Wilberforce, Bishop of Oxford, when Wilberforce asked his opponent, ‘Is it on your grandfather’s or your grandmother’s side that you claim descent from a monkey?’
Darwin had proposed that the anatomical similarities between humans and other primates, most notably the apes, showed that these species were our closest relatives. DNA evidence has since proved that humans share 98.8 per cent of our genes with chimpanzees and bonobos, while fossil evidence suggests that humans and chimps share a common ancestor that lived around 8 million years ago. Chimps remained as forest creatures, while humans evolved in a different direction as they became adapted to live on open grasslands.
The earliest hominine (humanlike) fossil is Sahelanthropus tchadensis, a chimplike ape living in the forests of what is now Chad. Remains suggest it could stand on its back legs, but there is no evidence that this animal was a direct ancestor of modern humans. The earliest proven ancestor is ‘Lucy’ (Australopithecus afarensis), a bipedal ape specimen that lived in East Africa 3.2 million years ago (mya). Lucy was little more than a metre tall when she walked on two legs. However, her arms and fingers were considerably longer in proportion than ours, suggesting that Lucy and her fellow australopithecines (‘southern apes’) were able climbers, probably living in open savannah woodlands.
The lineage from Lucy to you and me is not clear but involves Homo habilis (c.2.5 mya – an omnivore that made simple stone cutting tools), and Homo ergaster (1.9 mya – thought to be the first species to control fire and spread beyond Africa). Several other hominin spread across Asia and Europe before Homo sapiens appeared about 150,000 years ago in Africa.
Dark fragments of Lucy’s skull are formed into a completed model.