What a Wonderful World (4 page)

Hydrogen when combined with oxygen – think rocket fuel, think respiration – liberates large amounts of energy. It could therefore be used, in fuel cells, to power all manner of machines from cars to computers. There are three main steps in the creation of artificial photosynthesis. First, light must be captured and its energy transferred to an electron, boosting its energy. Next, the electron must be freed from its parent atom. Finally, the super-energetic electron must be used to smash apart water to liberate the all-important hydrogen. Artificial photosynthesis, able to make hydrogen fuel from sunlight, would wean the human race from its dependence on fast-dwindling reserves of fossil fuels such as oil. It would be a game-changing technology. It could transform the world.

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.
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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.
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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
.
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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.
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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.
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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.
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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.
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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.