If the history-deniers who doubt the fact of evolution are ignorant of biology, those who think the world began less than ten thousand years ago are worse than ignorant, they are deluded to the point of perversity. They are denying not only the facts of biology but those of physics, geology, cosmology, archaeology, history and chemistry as well. This chapter is about how we know the ages of rocks and the fossils embedded in them. It presents the evidence that the timescale on which life has operated on this planet is measured not in thousands of years but in thousands of millions of years.

Remember, evolutionary scientists are in the position of detectives who come late to the scene of a crime. To pinpoint when things happened, we depend upon traces left by time-dependent processes - clocks, in a broad sense. One of the first things a detective does when investigating a murder is ask a doctor or pathologist to estimate the time of death. Much follows from this information, and in detective fiction an almost mystical reverence is accorded to the pathologist's estimate. The 'time of death' is a baseline fact, an inerrant pivot around which more or less far-fetched speculations by the detective revolve. But that estimate is, of course, subject to error, an error that can be measured and can be quite large. The pathologist uses various time-dependent processes to estimate the time of death: the body cools at a characteristic rate, rigor mortis sets in at a particular time, and so on. These are the rather crude 'clocks' available to the investigator of a murder. The clocks available to the evolutionary scientist are potentially much more accurate - in proportion to the timescale involved, of course, not more accurate to the nearest hour! The analogy to a precision clock is more persuasive for a Jurassic rock in the hands of a geologist than it is for a cooling corpse in the hands of a pathologist.

Man-made clocks work on timescales that are very short by evolutionary standards - hours, minutes, seconds - and the time-dependent processes they use are fast: the swinging of a pendulum, the swivelling of a hairspring, the oscillation of a crystal, the burning of a candle, the draining of a water vessel or an hourglass, the rotation of the earth (registered by a sundial). All clocks exploit some process that occurs at a steady and known rate. A pendulum swings at a very constant rate, which depends upon its length but not, at least in theory, on the amplitude of the swing or the mass of the bob on the end. Grandfather clocks work by linking a pendulum to an escapement which advances a toothed wheel, step by step; the rotation is then geared down to the speed of rotation of an hour hand, a minute hand and a second hand. Watches with hairspring wheels work in a similar way. Digital watches exploit an electronic equivalent of a pendulum, the oscillation of certain kinds of crystals when supplied with energy from a battery. Water clocks and candle clocks are much less accurate, but they were useful before the invention of event-counting clocks. They depend not on counting things, as a pendulum clock or a digital watch does, but on measuring some quantity. Sundials are inaccurate ways of telling the time.* But the rotation of the earth, which is the time-dependent process on which they rely, is accurate on the timescale of the slower clock that we call the calendar. This is because on that timescale it is no longer a measuring clock (a sundial measures the continuously varying angle of the sun) but a counting clock (counting day/night cycles).

Both counting clocks and measuring clocks are available to us on the immensely slow timescale of evolution. But to investigate evolution we don't need just a clock that tells the present time, as a sundial does, or a watch. We need something more like a stopwatch that can be reset. Our evolutionary clock needs to be zeroed at some point, so that we can calculate the elapsed time since a starting point, to give us, for example, the absolute age of some object such as a rock. Radioactive clocks for dating igneous (volcanic) rocks are conveniently zeroed at the moment the rock is formed by the solidification of molten lava.

Fortunately, a variety of zero-able natural clocks is available. This variety is a good thing, because we can use some clocks to check the accuracy of other clocks. Even more fortunately, they sensitively cover an astonishingly wide range of timescales, and we need this too because evolutionary timescales span seven or eight orders of magnitude. It's worth spelling out what this means. An order of magnitude means something precise. A change of one order of magnitude is one multiplication (or division) by ten. Since we use a decimal system,* the order of magnitude of a number is a count of the number of zeroes, before or after the decimal point. So a range of eight orders of magnitude constitutes a hundred millionfold. The second hand of a watch rotates 60 times as fast as the minute hand and 720 times as fast as the hour hand, so the three hands cover a range which is less than three orders of magnitude. This is tiny compared to the eight orders of magnitude spanned by our repertoire of geological clocks. Radioactive decay clocks are available for short timescales as well, even down to fractions of a second; but for evolutionary purposes, clocks that can measure centuries or perhaps decades are about the fastest we need. This fast end of the spectrum of natural clocks - tree rings and carbon dating - is useful for archaeological purposes, and for dating specimens on the sort of timescale that covers the domestication of the dog or the cabbage. At the other end of the scale, we need natural clocks that can time hundreds of millions, even billions, of years. And, praise be, nature has provided us with just the wide range of clocks that we need. What's more, their ranges of sensitivity overlap with each other, so we can use them as checks on each other.

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