properties of DNA are due to the hydrogen bonding between protons and electrons in the DNA bases: the position of these particles determines which hydrogen bonds can form and thereby the base-pairing underlying the genetic code. Protons and electrons are fundamental particles and their position is subject to quantum mechanics. The genetic code thus becomes a quantum code.
One of the peculiar features of quantum mechanics is that in most circumstances we cannot know exactly where a particle is: its position is uncertain (an aspect of Heisenberg’s famous uncertainty principle that will be examined in more detail later). This uncertainty is the basis for a phenomenon called quantum tunnelling, whereby protons are said to tunnel from one position to another. In fact quantum particles don’t really tunnel anywhere. It is just that the inevitable (Heisenberg) uncertainty in their position means that they materialize in places you wouldn’t normally expect them.
The coding protons in DNA can (and must) quantum tunnel within the DNA molecule. This leads to tautomeric structures for DNA bases, with coding protons tunnelling from one atom to another to form modified chemical structures. Tautomeric forms of DNA bases can pair with the incorrect base: A can pair with G and T with C (rather than A with T and C with G). Watson and Crick proposed that if, during DNA replication, either the template DNA base or the incoming base is in the tautomeric form, then the wrong base may be inserted into the new strand, resulting in a mutation. Tautomeric forms of DNA bases account for about 0.01 per cent of all natural DNA bases, so incorporation of incorrect bases, due to tautomerization, is likely to be relatively common. However, our DNA replication machinery has proofreading enzymes able to recognize incorrectly inserted bases and clip them out of the growing strand. The inclusion of proof-reading into the system vastly reduces the error rate to only about one wrong base for every billion correct bases. Those errors that escape the correction machinery are the source of naturally occurring mutations; and their source is quantum-mechanical.
Watson and Crick’s structure was therefore the culmination of centuries of biological progress. The great mysteries were laid bare: how biological information is encoded, how it is inherited and how it is changed. But it also pointed in quite a surprising direction, towards the involvement of that other great triumph of twentieth century science – quantum mechanics – in the fundamental basis of life and the driving force of evolution.
We all need to place our lives in some kind of historical context, to know where we come from. Ancestor worship is one of the most ancient forms of religion and the same craving finds a modern expression in the current popularity of genealogy. A relative of mine, John McFadden, recently traced our family back to one Cornelius McFadden. Cornelius owes his fame to being caught (probably around 1760) stealing a sheep on the island of Arranmore1. Sheep stealers were hanged in eighteenth-century Ireland but the authorities could however show some mercy, since the sentence could be commuted for men with families. Fortunately for Cornelius, his wife Nancy was heavily pregnant so my great-great-great grandfather suffered the lesser punishment of having his ears cut off. His wounds bound, he and his young wife were placed on a raft and pushed out to sea on the ebb tide, with only a single oar. The pair rowed along the coast, finally beaching on the island of Innishirther – a rather bleak and inhospitable place, but uninhabited. They settled there and thrived, raising eleven children. Their descendants migrated to the mainland, giving rise to a long line of McFaddens, including eventually me. The punishment suffered by Cornelius is remembered in a Gaelic phrase, ‘Thug said Oidhe Concubhair air’ (roughly translated, the justice of Cornelius) that commemorates their cruel punishment.
The tale of Nancy and Cornelius gives me some connection with the past but a few hundred years are very shallow roots in a history of life that stretches back billions of years. To find more – truly ancient – roots we must dig deeper into the past.
THE HOOF-PRINT OF EVOLUTION
The standard textbook illustration of evolution is the development of the modern horse. Darwin’s friend and colleague Thomas Huxley worked out its sequence from more than two hundred species of fossil horses from Europe and America, using it to champion Darwin’s theory. Horses are members of the family perissodactyles (animals with odd-numbered toes) that also includes rhinos and tapirs. The first perissodactyles were dog-like browsing animals weighing only about twenty kilos that first appeared in the North American forests roughly sixty million years ago. The subsequent evolution of the horse is thought to have been in response to a changing environment as woodlands gave way to open savannah. Gradually, over millions of years, many new species appeared with fewer toes, longer legs adapted for fast running and stronger jaws with big teeth adapted for grazing. Each new species was only slightly modified from its likely progenitor, but over millions of years there was a gradual increase in size and parallel changes in bone structure. Most of the new species became extinct, particularly during the last great Ice Age, but seven species of modern horse survived, which include the domesticated horse, asses and zebras.
The standard interpretation of the fossil record of horses, and indeed other animals, is one of gradual evolution. At any point in time there would have been many individual horses, all slightly different. Natural selection would have favoured the more successful variants so that, over the course of many thousands and millions of years, there would have been a gradual shift in horse shape, size and toe bones, to suit their new environment.
TO BE OR NOT TO BE, WITH HALF AN EYE
A favourite argument of anti-evolutionists is that complex structures, such as the mammalian eye, could not possibly have evolved by random mutations. To make this point, a metaphorical monkey is often recruited to bang away at a typewriter, typing in random characters. The question is then asked: how long would it take our simian typist to type out the whole of Hamlet? The answer can be fairly easily calculated. He would have to type out about 2530000 words (24 letters plus the space key raised to the power of the number of words in the text) to have a reasonable chance of typing in the correct text. If the monkey were a fairly proficient typist, say, one hundred words a minute, it would take him 2530000, divided by one hundred, so approximately 1040000 minutes to hit the keyboard the requisite number of times. The number of minutes since the Big Bang are however a mere 1021, a number vastly smaller. If we had a cosmic army of monkeys, one for every single electron in the universe and they had all been typing merrily away ever since the Big Bang, they would not have had sufficient time to achieve a tiny fraction of this feat.
However, the odds are radically transformed if we move from an entirely random selection of keys to adding one extra ingredient, selection. Imagine that we start with a small army of monkeys (a few hundred) and allow each to hit the keyboard once, selecting for breeding only those that correctly typed the first letter of Hamlet, W. Now, suppose that the ability to type that one letter is inherited, so that the progeny of these W monkeys invariably type the letter W with their first bang on the keyboard. Their next attempt at literary creativity – the next letter – would again be random, but once more we select for breeding only those that type H, the play’s next letter. (Once again, saying that the ability to type the second character is inherited in the same manner as the first.) Continuing this breeding policy for just nine generations would breed monkeys that would competently type the first line: ‘WHO’S THERE’ (we will allow ourselves to add in the punctuation). If we continued with our breeding programme, allowing about ten years for each generation, then it would take a mere 300,000 years for us to breed a line of Shakespearean monkeys, able to type the entire text of Hamlet!
The odds are so much better because we have introduced selection
