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sixty-forth, a one-hundred-twenty-eighth, a two-hundred-fifty-sixth, etc.
    The word chemistry is the key in this business of geochemistry and radiochemistry. Rocks in Earth’s crust typically contain a certain amount of rubidium and a certain amount of strontium, along with other elements like calcium and potassium. The chemical behavior of rubidium is a lot like the chemical behavior of potassium, and the chemical behavior of strontium is a lot like the chemical behavior of calcium. (In chemistry we see that they reside in the same columns on the periodic table of the elements.) When the rocks are liquid, rubidium tends to remain free and unattached, but as the rock cools, rubidium sometimes takes the place of potassium in the rock crystals. In the same way, strontium substitutes for calcium. So by closely examining crystals that we know contain potassium and comparing the relative abundances of rubidium and strontium that are also in the crystal, we can determine the age of the rocks relative to other rocks. We can work our way into the past, pulling our date reckoning back in time by our bootstraps.
    There are several other geochemical clocks that radiochemists use to reckon the age of Earth, besides rubidium-strontium. There’s uranium-lead, there’s potassium-argon, and there’s samarium-neodymium. Each clock reckons time using different chemical elements and each provides us with incontrovertible evidence of Earth’s age. You may have heard of carbon dating or carbon-14 dating. That is a related technique that is well suited to measuring shorter timescales. It lets us work backward in time to determine when a living thing stopped transpiring (plants) or stopped breathing (animals). Carbon dating only goes back a few tens of thousands of years, because the half-life of this type of carbon is only 5,730 years. Compare that with rubidium-strontium; this radiochemical clock goes back into the past almost a million times further. Carbon dating is important for studying human history, but it’s not well suited to reckoning deep time.
    Evolution snaps into focus when you realize how fantastically old our planet is. To imagine it, try this. Look at a map of North America. To readers from other parts of the world, it’s interesting to note that what we often call the continental U.S. extends from the Atlantic to the Pacific oceans (as do Canada and Mexico). By means of the U.S. Interstate Highway System, one can drive a car from coast to coast. If we were to go from around San Diego on the Southwestern shore of the U.S. to, let’s say, Boston on the Northeastern shore, we would go about 4,500 kilometers or 2,800 miles. That much driving would take you from Lisbon, Portugal to Moscow, Russia. And along the way, you would have passed through eight different countries.
    Imagine a time line running from coast to coast. Let’s say that for every kilometer of travel (every ten football fields for U.S. readers), you pass through one million years of time. Well then, every meter (about every yard) represents one thousand years of time. For this charming thought-model, the distance from your chin to your outstretched arm represents a thousand years. A Thousand Years!
    For the next step, imagine yourself walking through time from San Diego to Boston. When you start, Earth is still a big orange-hot ball of molten rock. After two hundred kilometers, six or seven days into your hike, look up as you come across a marker reminding you that the Moon is forming. Another two days of walking, and a marker tells you that enough rain has fallen on a surface cool enough to have the ocean form; that was 4.4 billion years ago. After a month on foot, you’ll be coming upon the first signs of life, about 3.5 billion years ago. Two thousand kilometers from your embarkation point, somewhere near Broken Arrow, Oklahoma, you’ll find tiny microbes, blue-green bacteria. Before that, by the way, you would have