random process. The only difference between this and Darwinian evolution is that the selection we have introduced is artificial – we are doing the selection. In nature, it is the environment that does the selection: natural selection. Perhaps regrettably, the ability to type lines of Shakespeare is unlikely to impress many female monkeys and would not cut much mustard in monkey society. A tropical forest environment is unlikely to favour a line of literary monkeys. The ability to see is, however, vital. A monkey equipped with sharper eyesight might be more successful at finding fruit with which to tempt prospective mates. It may more readily spot the attack of a rival. The monkey eye is thereby subject to Darwinian natural selection and it is this that shrinks the odds of developing the eye’s complex structure from essentially zero to something achievable within geological lengths of time.
The key to the feasibility of this evolutionary scenario is the existence of a selective advantage for each and every step from simple to complex. This point is crucial to Darwinian evolution. The eye could only evolve if all the precedents to its modern form were viable and each had an advantage over its predecessor. Creationists claim that this is the weak point of the argument, for before an eye can reach the complex structure of its modern form, it must evolve through a thousand intermediate stages. But what use is half or a tenth of an eye? Surely an eye is only useful when all its parts are present and functioning?
This is a surprising claim, since Darwin himself used the eye to illustrate how the evolution of complex structures was indeed feasible by natural selection. In his essay, Organs of extreme complication and perfection’, Darwin pointed out that far from half an eye or a tenth of an eye being of no value, there are many living animals with half an eye or a tenth of an eye which manage very nicely with their supposedly imperfect vision. Many microbes, including photosynthetic bacteria, possess the most rudimentary vision. These bacteria are able to swim towards bright light where their photosynthetic skills are most effectively deployed. They even have colour vision since they are able to concentrate where in the spectrum their chlorophyll absorbs the most light. Mutants can be isolated that lack this phototactic ability, demonstrating that the gene for some kind of photoreceptor is encoded in their DNA and thereby subject to mutation and natural selection. Whether bacterial vision represents a tenth or even one-hundredth of an eye is a matter of opinion but it certainly gives the bacteria a selective advantage over blind mutants. It is even possible that light sensitivity did not originally evolve with a role in vision at all. Many primitive organisms have light-sensitive proteins that are used to set their circadian (the biological rhythms that track night and day) clocks. It may be that this clock-setting function of primitive eyes arose well before their value for seeing was harnessed.
From its origins as a single photoreceptor protein in a bacterial cell wall, the next step towards the eye may be the patch of light-sensitive cells found on the body surface of some starfish, jellyfish, leeches and worms. These animals are unable to form an image but can respond to different levels of light and darkness, which may allow them to locate the brighter, and more productive, shallow waters and rock-pools. More complex light receptors are found in limpets, clams and flatworms in which the photosensitive cells form a shallow cup used to detect the direction of light. The obvious next step was to add some kind of focusing mechanism to form a simple image. Some molluscs achieve this by the pinhole camera principle – light is forced to travel through a narrow aperture that focuses the image onto a cup of light-sensitive cells, which we may now call the retina. Vertebrates, insects and octopuses instead incorporate a transparent lens that allows more light into the eye, yet focuses the image. Finally, a variable aperture pupil might be added to control the amount of light allowed in; thus we have the mammalian eye.
The key to this evolutionary scenario is its gradualism. There are however a group of eminent palaeontologists who challenge it. Stephen Jay Gould and Niles Eldridge point out that the fossil record does not actually record gradual changes in species. Instead most species, including most horses, appear abruptly in the fossil record, change very little over their entire history and then disappear just as unceremoniously. This pattern is well known to palaeontologists who have usually attributed it to the imperfection of the fossil record: the missing links between one species and another have all died without the decency to leave their remains as fossils. Yet recent exhaustive studies of well-preserved species, such as marine snails, tend to support the view that, generally, evolution seems to hop and jump, rather than crawl.
Gould and Eldridge claim that the punctuated pattern of change is a real phenomenon which reflects two rates of evolution. The first, stasis, is exemplified by living fossils like crocodiles that have changed very little or not at all for millions of years. The second pattern of evolution occurs more sporadically and is characterized by geologically instantaneous speciation events (sometimes called macroevolution) in which one or several new species are generated. The pattern of long periods of stasis interspersed with rapid spurts of evolutionary innovation, they term ‘punctuated equilibrium’. Evolution that goes at two different speeds clearly needs some kind of gearing mechanism to change from one to another. Gould and Eldridge suggest that when evolution makes a jump, natural selection may be acting at a higher level to select whole species or groups of species, rather than individuals. However, many other evolutionary biologists, such as Richard Dawkins, take great exception to the view that natural selection acts on any unit higher than that of an individual (or even a single gene).
Whether evolution proceeds by tiny steps or big leaps, by examining the fossil record we can trace the evolution of man and animals back to the emergence of the first animals in the Cambrian explosion five hundred and fifty million years ago. Rocks earlier than the Cambrian explosion have very few fossils – which are nearly all microbes. Unfortunately, microbial fossils are not very distinctive. They give few clues about the evolutionary changes that led to the emergence of animals. To go deeper into the history of life we need to dig into DNA, rather than rocks.
THE GENE CLOCK
The word for milk is lait in French, latte in Italian, leche in Spanish and leite in Portuguese but milk in English milch in German and mjölk in Swedish. French, Italian, Spanish and Portuguese are all Italic languages; whereas English, German and Swedish are Germanic languages. Other European language groups include Celtic, Hellenic and Slavic. In 1786, Sir William Jones, an English judge serving in India, first noticed similarities between the ancient Indian language, Sanskrit and various European languages. For instance, the word for king is rex in Latin, ri in Irish, raja in Sanskrit. The same root turns up in the English word ruler. Sir William considered that these similarities could not have arisen by chance but must reflect a common linguistic inheritance. The English scholar Thomas Young later coined the term Indo-European to describe these common languages.
Modern languages are thought to be derived from an ancestral proto-Indo-European language spoken by either a Bronze or Neolithic Age people. The original Indo-Europeans would have spoken a common proto-Indo-European but gradually as the people dispersed, their languages diverged to develop into the modern family of languages. Philologists (those who study language development) compare similar words in each language to derive a plausible ancestral word. For instance, a single word for milk, approximating to lakte, is thought to have been used by people who spoke proto-Italic, the ancestral language of modern French, Italian, Spanish and Portuguese. The patterns of divergence in each language group could then be estimated by counting the number of sound shifts required to change from the putative ancestral word to all its modern forms. Languages linked by few sound shifts, such as Spanish and Portuguese, are considered to have diverged relatively recently. Languages linked through more sound shifts, such as German and Portuguese, must have separated much further back. In this way, a family tree of languages can be suggested. By dating language divergence to a historical event (for instance, the settling of England by an Anglo-Saxon speaking people in about 550 ad that led to the separate development of English), philologists can provide a very rough calibration of the rate of divergence of languages.
People’s common inheritance
