The theory of evolution

Darwin introduced the theory of evolution in 1859 in his seminal work On

the Origin of Species. The idea of evolution can be traced back in history much

further, however, as far back as Ancient Greece. Anaximander introduced, in the sixth century BCE, the idea that humans evolved from different kinds of animals. An ancient testimony describes Anaximander’s theory as follows:

[Anaximander] declares that in the beginning humans were born from other kinds of animals, since other animals quickly manage on their own, and humans alone require lengthy nursing. For this reason, in the beginning they would not have been preserved if they had been like this.61

So Anaximander cites the much longer infancy of human babies, compared with that of other animals he had observed, as evidence that humans had existed in a different state in the past. The following testimonies further explain Anaximander’s theory:

Anaximander says that the first animals were produced in moisture, enclosed in thorny barks. When their age increased they came out onto the drier part, their bark broke off, and they lived a different mode of life for a short time.62

Anaximander … believed that there arose from heated water and earth either fish or animals very like fish. In these humans grew and were kept inside as embryos up to puberty. Then finally they burst and men and women came forth already able to nourish themselves.63

It is remarkable that Anaximander anticipated, however crudely, Darwin’s ideas that species evolve and that simple forms precede complex forms. In On the Origin of Species Darwin produced a familiar evolutionary tree, which is also called a ‘phylogenetic tree’. The tree is a way of illustrating evolu- tionary relationships among distinct species in a way that identifies their common ancestors. We can, for example, see on such a tree the evolutionary relationships and common ancestors of birds and mice. Some point out that this way of positioning species in a hierarchical order has its roots in the medieval concept of the ‘great chain of being’.64

The great chain of being is a pictorial illustration of the relationships that connect all beings, including inorganic substance, plants, animals, human beings, angels, and God. All beings on the chain are ranked in accordance with their attributes. God sits atop the chain because He exemplifies the highest or best possible instance of every attribute. In the chain God is followed by angels, human beings, other animals, plants, and inorganic substance. Just like the phylogenetic tree, the great chain of being displays relationships among distinct beings. Such a hierarchical ordering of indi- vidual beings or species was common in religious, philosophical, and polit- ical discourse in the medieval period.

As we have seen, the activity of finding connections between beings and presenting their relationships pictorially is nearly as old as human history. In this sense, the basic ideas behind evolution were conceived long ago. Yet these ideas had not been discussed seriously by biologists until the eighteenth century. In the eighteenth century several scientists, including Erasmus Darwin, Charles Darwin’s grandfather, tried to develop a proper scientific theory from the idea of evolution. In Zoonomia Erasmus Darwin writes as follows:

From thus meditating on the great similarity of the structure of the warm-blooded animals, and at the same time of the great changes they undergo both before and after their nativity; and by considering in how minute a portion of time many of the changes of animals above described have been produced; would it be too bold to imagine, that in

the great length of time, since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament, which THe GReAT FIRST CAUSe endued with animality, with the power of acquiring new parts attended with new propensities, directed by irritations, sensations, volitions, and associa- tions; and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end?65

Erasmus Darwin focuses here only on warm-blooded animals rather than on all living organisms, but the claim that they might have come from the same ancestor strongly anticipates Charles Darwin’s theory of evolu- tion. Erasmus Darwin does not, however, precisely identify a mechanism for evolution. He talks about ‘irritations, sensations, volitions and associa- tions’ as key factors in evolution but he fails to explain exactly how these phenomena might contribute to the evolution of animals.

Coming up with the basic idea of evolution per se is not so difficult. Observations of similarities and dissimilarities between species and individ- uals across generations give us the intuitive idea that they have evolved into their current forms from common ancestors. However, in order to estab- lish evolution as a proper scientific theory, we must answer the following question: what mechanism preserves specific features of animals and plants while producing changes in their other features?

In 1809, seven years after the publication of Paley’s Natural Theology, a French biologist, Jean-Baptiste Lamarck, attempted to answer this question. The core idea of Lamarck’s theory, which he introduced in his book Philosophie

zoologique, is the ‘inheritance of acquired characters’. According to this idea,

individual animals and plants develop physiological characters as a result of disease, use/disuse, and accidents and this could eventually contribute to the transformation of their own species. Consider, for example, giraffes’ necks. According to Lamarck, their necks are long because their ancestors had stretched their necks so that they could reach leaves of tall trees. Their attempt to stretch their necks throughout generations gradually extended their necks over time and resulted in the long necks that we see today.

Paley mentions, in his Natural Theology, a view that is similar to Lamarck’s:

Another system, which has lately been brought forward, and with much ingenuity, is that of appetencies. The principle, and the short account,

of the theory, is this: Pieces of soft, ductile matter, being endued with propensities or appetencies for particular actions, would, by continual endeavours, carried on through a long series of generations, work themselves gradually into suitable forms: and, at length, acquire, though perhaps by obscure and almost imperceptible improvements, an organization fitted to the action which their respective propensi- ties led them to exert. A piece of animated matter, for example, that was endued with a propensity to fly, though ever so shapeless, though no other we will suppose than a round ball, to begin with, would, in a course of ages, if not in a million of years, perhaps in a hundred millions of years (for our theorists, having eternity to dispose of, are never sparing in time), acquire wings. The same tendency to loco- motion in an aquatic animal, or rather in an animated lump which might happen to be surrounded by water, would end in the production of fins: in a living substance, confined to the solid earth, would put out legs and feet; or, if it took a different turn, would break the body into ringlets, and conclude by crawling upon the ground.66

Paley rejects the above theory saying, ‘The scheme under consideration is open to the same objection with other conjectures of a similar tendency, viz. a total defect of evidence. No changes, like those which the theory requires, have ever been observed.’67 _{Yet Lamarck’s theory of the inherit-}

ance of acquired characters is intuitively plausible and was widely accepted in his time. Even today laypeople often misconstrue Darwin’s theory of evolution as advocating something akin to Lamarck’s theory. The theory was eventually rejected by biologists, however, because it does not explain how individuals can convey their physiological characters, which are caused by disease, use/disuse, and accidents, to their descendants and ultimately trans- form their species over time. Darwin’s achievement is significant because he finally managed to provide a truly satisfactory solution to this problem by introducing a very simple theory that any intelligent person can understand. It explains, for example, exactly how giraffes’ necks could extend little by little over generations.

Charles Darwin was born in 1809 into a wealthy family in Shrewsbury, Shropshire, in England. His father, Robert, and his paternal grandfather, Erasmus (whom we have already encountered), were both physicians. His maternal grandfather, Josiah Wedgwood, was the founder of the renowned Wedgwood pottery firm. Following his father, Darwin studied medicine at

the University of Edinburgh, but he found the subject uninteresting. He then switched to study divinity at Christ’s College at the University of Cambridge, hoping (somewhat ironically in retrospect) to be trained for the ministry. At Cambridge he deepened his knowledge of zoology and geography. He was then invited in 1831 to join a two-year scientific expedition on HMS

Beagle. Robert Darwin was unhappy at the prospect of his son’s long absence,

but Charles Darwin’s uncle, Josiah Wedgwood II, persuaded the father to let him go on the trip. Later that year the Beagle sailed from Devonport, England. During his trip Darwin was amazed by the rich geographical and biological diversity in nature. He was particularly fascinated by observing distinct forms of finches on the various Galápagos Islands. Darwin noticed that they exhibit variously sized and shaped beaks and that each type is highly adapted to its own environment. This observation inspired him to construct the theory of evolution. Darwin returned to England in 1836 – five rather than two years after his departure.

The British political economist Thomas Malthus maintained in his book An

Essay on the Principle of Population (first published in 1798) that with unlimited

environmental resources the human population would grow exponentially and the world would soon become overpopulated. However, he continued, the propensity of populations to produce more offspring than can possibly survive with the limited resources available to them causes war, famine, and disease, which effectively reduce the size of populations. In other words, war, famine, and disease constitute a ‘survival game’, which helps in preventing overpopulation.

Darwin read Malthus’s book only for amusement. He soon realized, however, that it was relevant to his work because Malthus’s theory was also helpful in explaining the diversity of animals and plants. It struck him that given that animals and plants are also involved in similar survival games with limited resources, minute differences among them could be of life- and-death importance. For example, a certain finch might win its survival game because it has a beak of a slightly more advantageous shape than those of others.

Darwin spent more than 20 years developing his ideas and finally announced the theory of evolution in 1858 with Alfred Russel Wallace. Wallace was another biologist, and he arrived at the theory of evolution independently through his fieldwork in the Malay archipelago. In science we occasionally see two or more people reach the same idea at the same time independently. Darwin’s and Wallace’s paths to evolution represent a

particularly remarkable example of this phenomenon. While there are some differences in Darwin’s and Wallace’s theories, they agreed that their core theses were essentially the same.

Darwin’s theory of evolution is based on the idea that all life has descended from a common ancestor. Thus, despite radical differences in appearances, humans, mice, insects, fish, plants, and so on share the same ancestor. Organisms can acquire new traits from mutations in genes or from the transfer of such genes from one population or species to another. Among species that reproduce sexually, organisms can also acquire new traits through genetic recombination. Darwin called the process of preserving beneficial mutations and passing them on to succeeding genera- tions ‘natural selection’. (Darwin did not, of course, formulate his theory in terms of genes because On the Origin of Species was published before the birth of genetics.)

Natural selection allows organisms to preserve and accumulate minor genetic mutations that are advantageous for survival. Consider giraffes’ necks again. The distant ancestors of modern giraffes had quite short necks. However, by chance some individuals had relatively longer necks. Giraffes with longer necks tended to survive and produce new generations more successfully than shorter-necked giraffes because it was easier for them to reach abundant food in treetops. With each new generation, those surviving offspring with relatively longer necks tended to increase the proportion of long-necked giraffes in the population. Through this gradual process, over time, giraffes acquired very long necks. (The evolutionary process that elongated giraffes’ necks is not actually as simple as the description here suggests. For instance, it is not the case that giraffes always eat from tree- tops. In the dry season they eat from low bushes. In addition, there are biologists who defend alternative variant evolutionary accounts of giraffes’ necks. For example, some contend that giraffes developed long necks because it increases the surface areas of their bodies, which is a useful trait in tropical climates. To take another example, some others claim that long necks evolved for fighting because male giraffes use them to club one other. Nevertheless, Darwin’s account of the long necks is useful in illustrating the idea of natural selection and, in any case, almost all biologists agree that there is some evolutionary reason that giraffes’ necks are so long.)

The theory of evolution explains the magnificent diversity of the natural world with such a simple and elegant idea. When the biologist Thomas Henry Huxley learned the theory of evolution from Darwin, he exclaimed, ‘How extremely stupid not to have thought of that!’ He also remarked, ‘I

suppose that Columbus’ companions said much the same when he made the egg stand on end.’68 _{Julian Huxley, a biologist at Oxford and a grandson}

of Thomas Henry Huxley, says that Darwin’s theory ‘is the most powerful and most comprehensive idea that has ever arisen on earth’.69

The very last sentence in the first edition of On the Origin of Species read as follows:

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.

Curiously enough, Darwin inserted a new phrase in this passage in the second edition, which was published one year later:

There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator 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. (emphasis added.)

Some believe that this insertion shows how much Darwin was pressed by religious believers in response to his theory of evolution. In fact, he once said in a letter that proposing the theory of evolution ‘is like confessing a murder’.70 _{However, it is not obvious that the above insertion proves that}

Darwin harbored such a fear. The insertion might merely represent the widely known fact that Darwin himself was always undecided about the existence of God. In a letter of 1879 he wrote as follows:

It seems to me absurd to doubt that a man may be an ardent Theist & an evolutionist … But as you ask, I may state that my judgment often fluctuates. Moreover whether a man deserves to be called a theist depends on the definition of the term: which is much too large a subject for a note. In my most extreme fluctuations I have never been an atheist in the sense of denying the existence of a God. – I think that generally (& more and more so as I grow older) but not always, that an agnostic would be the most correct description of my state of mind.71

Although, as we have noted, Darwin studied to be an Anglican parson in his youth, he did not seem to be convinced by Christianity. In 1880 he wrote in a letter that, ‘I am sorry to have to inform you that I do not believe in the Bible as a divine revelation, & therefore not in Jesus Christ as the Son of God.’72 _{It is probably reasonable to assume that Darwin was an agnostic}

who was sometimes attracted to a non-religious form of theism, such as deism or pantheism.

Darwin suffered from endless pain and illness throughout his life. His health was generally poor and he often had to stay in bed for many months without being able to work. No one knows what caused his illness. Some believe that it had a psychological cause because Darwin himself observed that his condition often worsened following a stimulating event such as a meeting with other people. Others believe that Darwin caught a trop- ical disease during the Beagle expedition. Darwin recalls that he was indeed bitten by insects several times. He tried all sorts of remedies and consulted over 20 doctors but none of them was helpful. He suffered a heart attack and died in 1882 at the age of 73. His last words, spoken to his wife and cousin Emma, were, ‘I am not in the least afraid to die.’

As noted above, although Hume provided a number of powerful reasons for rejecting the design argument he failed to provide a compelling posi- tive naturalistic account of the origin of biological organisms. That is why until the mid-nineteenth century most thinkers had not abandoned Paley’s theistic account. Paley’s account was prima facie plausible and also compatible with known scientific facts. Thus even Richard Dawkins, one of the fiercest contemporary critics of the design argument, remarks, ‘Paley’s argument is made with passionate sincerity and is informed by the best biological scholarship of his day.’73 _{It was reasonable for people at the time to accept}

Paley’s account as the best available hypothesis concerning the origin of biological organisms.

However, with the publication of On the Origin of Species Paley’s theory was no longer tenable, because Darwin finally succeeded in establishing a viable non-theistic account of the origin of biological organisms. Darwin declares his triumph by saying, ‘The old argument from design in Nature, as given by Paley, which formerly seemed to be so conclusive, fails, now that the law of natural selection has been discovered.’ He continues:

We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the

hinge of a door by man. There seems to be no more design in the vari- ability of organic beings, and in the action of natural selection, than in