The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

Isotopes, then, differ only in the number of neutrons they have, along with the fixed number of protons ...

some may have an unstable nucleus, meaning it has an occasional tendency to change at an unpredictable instant, though with predictable probability, into a different kind of nucleus. Other isotop...

Another word for unstable is radioactive. Lead has four stable isotopes and 25 known unstable ones. All isotopes of the very heavy metal uranium are unstable—all are radioactive. Radioactivity is the key to the absolute dating of rocks and ...

What actually happens when an unstable, radioactive element changes int...

the two best known are called alpha decay ...

alpha decay the parent nucleus loses an ‘alpha particle’, which is a pellet consisting of two protons and two neutrons stuck together. The mass number therefore drops by four units, but the atomic number drops by only two units (corresponding to the two protons lost). So the element ...

Uranium 238 (with 92 protons and 146 neutrons) decays into thorium 234 (with 90 p...

Beta decay is di...

One neutron in the parent nucleus turns into a proton, and it does so by ejecting a beta particle, which is a single unit of negative charge or one electron. The mass number of the nucleus remains the same because the total number of protons plus neutrons remains the same, and electrons are too small to bother with. But...

Sodium 24 transforms itself, by beta decay, into magnesium 24. The mass number has remained the same, 24. The atomic number has increased from 11, which is uniquely diagnostic of sodium...

A third kind of transformation is neutron-pro...

A fourth way in which one element can turn into another, which has the same effect on atomic number and mass number, is electron capture.

This is a kind of reversal of beta decay. Whereas in beta decay a neutron turns into a proton and expels an electron, electron capture transforms a proton into a neutron by neutralising its charge. So the atomic number drops by one, while the mass number remains the same. Potassium 40 (atomic number 19) decays to argon 40 (atomic number 18) by this means. And there are various other ways in which nuclei can be radioactively transformed into other nuclei.

One of the cardinal principles of quantum mechanics is that it is impossible to predict exactly when a particular nucleus...

But we can measure the statistical likelihood that it will happen. This measured likelihood turns out to be utterly characteristic of a given isotope...

To measure the half-life of a radioactive isotope, take a lump of the stuff and count how long it takes for exactly one half ...

The half-life of uranium 238 is nearly 4.5 billion years. This is approximately the age of the solar system. So, of all the uranium 238 that was present on Earth when it first formed, about half now remains.

It is a wonderful and very useful fact about radioactivity that half-lives of different elements span such a colossal range, from fractions of seconds to billions of years.

The fact that each radioactive isotope has a particular half-life offers an op...

Volcanic rocks often contain radioactive isotopes, suc...

Potassium 40 decays to argon 40 with a half-life of 1.3 billion years. Here, potentially, is an accurate clock. But it’s no use just measuring the amount of potassium 40 in a rock. You don’t know how much there was when it sta...

Fortunately, when potassium 40 in a rock crystal decays, the argon 40 (a gas) remains trapped in the crystal. If there are equal amounts of potassium 40 and argon 40 in the substance of the crystal, you k...

is therefore 1.3 billion years since the crystal was formed. If there’s, say, three times as much argon 40 as potassium 40, only one quarter (half of a half) of the original potassium 40 remains, so the ag...

The moment of crystallisation, which in the case of volcanic rocks is the moment when the molten lava solidified, is the...

Thereafter, the parent isotope steadily decays and the daughter isotope remains trapped in the crystal. All you have to do is measure the ratio of the two amounts, look up the half-life of the parent isotope in a physics book, and it is easy to calculate the age of the crystal. As I said earlier, fossils are usually found in sedimentary rocks, while dateable crystals are usually in volcanic rocks, s...

when uranium 238 decays it passes through a cascade of no fewer than 14 unstable intermediate stages, including nine alpha decays and seven beta decays, before it finally comes to rest as the stable isotope lead 206. By far the longest half-life of the cascade (4.5 billion years) belongs to the first transition, from uranium 238 to thorium 234.

The uranium/lead method and the potassium/argon method, with their half-lives measured in billions of years, are useful for dating fossils of great age. But

for dating younger rocks. For these, we need isotopes with shorter half-lives. Fortunately a range of clocks is available with a wide selection of isotopic half-lives. You choose your half-life to give best resolution for the rocks with which you are working. Better yet, the different clocks can be used as checks on each other.

The fastest radioactive clock in common use is the carbon 14 clock, and this brings us full circle to the teller of this tale, for wood is one of the main materials subjected to carbon 14 dating by archaeologists. Carbon 14 decays to nitrogen 14 with a half-life of 5,730 years. The carbon 14 clock is unusual in that it is used to date the actual dead tissues themselves, not volcanic rocks sandwiching them. Carbon 14 dating is so important for relatively recent history—much ...

Most of the carbon in the world consists of the stable isotope carbon 12. About one million-millionth part of the world’s carbon consists of the unstable isotope carbon 14. With a half-life measured in only thousands of years, all the carbon 14 on Earth would...

Fortunately, a few atoms of nitrogen 14, the most abundant gas in the atmosphere, are continually being transformed, by bombard...

The rate of creation of carbon 14 is approximately constant. Most of the carbon in the atmosphere, whether carbon 14 or the more usual carbon 12, is chemically combined with oxygen in the form of carbon dioxide. This gas is sucked in by plants, and the carbon atoms used to build their tissues. To plants, carbon 14 and carbon 12 look almost the same (plants are only ‘interested’ in chemistry, not the nuclear properties of atoms). The two varieties of carbon dioxide are imbibed approximately in proportion to their availability. Plants are eaten by animals, which may be eaten by yet other animals, so carbon 14 is dispersed in a known proportion relative to carbon 12 throughout the food chain during a time which...

All this changes at the moment of death. A dead predator is cut off from the food chain. A dead plant no longer takes in fresh supplies of carbon dioxide from the atmosphere.

dead herbivore no longer eats fresh plants. The carbon 14 in a dead animal or plant continues to decay to nitrogen 14. But it is not replenished by fresh supplies from the atmosphere. So the ratio of carbon 14 to carbon 12 in the dead tissues starts to drop. And it drops with a half-life of 5,730 years. The bottom line is that we can tell when an animal or plant died by measuring the ratio of carbon 14 to carbon 12.

is a wonderful tool for dating the relics of relatively recent history, but it is of no use for more ancient dating because almost all the carbon 14 has decayed to nitrogen 14, and the residue is too tiny to measure accurately.

The beauty of having so many methods is partly that they collectively span such an enormous range of timescales. It is also that they can be used as a cross-check on each other. It is extremely hard to argue against datings that are corroborated across different methods.

Since the 1970s, we have known that the amount of DNA in a creature’s genome, called the ‘C-value’, tells us very little about its overall complexity. This even has a name, the ‘C-value paradox’.

known as polyploidy, is fairly common in plants, especially crops like wheat and ornamental plants such as varieties of daffodil.

In these cases the duplication is recent, and it is very clear under the microscope that the chromosomes come in (say) four or six near-identical copies, rather than the more conventional two.

we use ‘gene’ in the technical sense to mean a protein-coding sequence.

humans, protein-coding regions account for only just over 1 per cent of the DNA, and we actually expect that figure to be less in onions.

By comparing known genomes, it is clear that the answer to the C-value paradox lies in the vast expanse of non-cod...

Coding DNA is so named because it uses the ‘genetic code’ (the three-letter mapping of DNA letters to amino acids in a protein). Non-coding DNA does not, but can still contain functional instructions.

the translation from DNA to protein is not really a code anyway. Technically it is a cipher: a method for converting one length of information into another. It would be better to axe the term ‘non-coding DNA’ from the genetic lexicon and use something less misleading, perhaps ‘untranslated DNA’.

these untranslated regions contain the vital switches that turn genes on and off. But as we also mentioned there, such controls account for less than a tenth of...

The function of a DNA sequence, inasmuch as we read one into it, is ultimately to make more copies of itself.

If it is reasonable to say that our eyes have evolved ‘for seeing’, then at the molecular level it is reasonable to say that red, green and blue opsin genes have evolved ‘for seeing’ too. We ascribe them a function: to produce proteins that absorb certain wavelengths of light, allowing us not just to see, but to see in colour

There is only one ultimate reason that both red and green opsin genes exist: because they have been copied more times than the bodies that they are in have died.

they confer a statistical benefit on the body as a whole, allowing an ancestral monkey to distinguish red from green, perhaps pick riper fruit, and have more children—children which themselves are likely to inherit copies of the opsin genes.

Most protein-coding genes exist for the same historical reason. They have co-operated with other genes to ensure that more copies are made of the entire genome—a succession of entire genomes through populations and through generations. That is what gives the illusion both of function and of coherent design.