That may be how Rutherford remembered that day, but Kelvin never publicly retracted his old estimate. Two years after hearing Rutherford’s talk, he was writing letters to the London Times maintaining there wasn’t enough radioactivity in the earth to keep it hot on the inside.
Rutherford realized that radioactivity not only showed that Earth was old, but could show how old it was. Any uranium that got trapped inside a cooling rock would decay gradually into lead. And because physicists knew uranium’s half-life with great precision, it was possible to use the remaining proportions of lead and uranium to calculate how old a rock was.
With this method, geologists were soon estimating the age of various rocks not in millions of years but in billions. They later learned how to make Rutherford’s clock even more accurate. Instead of taking a single measurement of the lead and uranium in a rock, they began to measure their levels in many different parts of it. That allowed them to compare the parts that originally contained very little uranium to others that initially had high levels. If the uranium throughout the rock decayed at a uniform rate, the different samples should all point to the same age. And in many cases they do.
Geologists also learned how to measure time with two clocks at once. In addition to uranium 238, some rocks also contain uranium 235, which decays into a different isotope of lead, lead 207. It also has a different half-life, of only 704 million years. With two independent tests for the age of a rock, geologists can often narrow down the margin of error even more.
They can also eliminate the uncertainty over whether any uranium or lead has crept into a rock after it was formed. As certain types of rocks form, atoms of zirconium and oxygen combine into crystals known as zircons. Zircons act like microscopic prisons: any uranium or lead atoms trapped inside a zircon have a very difficult time escaping, and few new atoms can enter it. Within its zircon cage, the uranium slowly breaks down into lead without any interference from the outside world. The geophysicists who put a date of 4.04 billion years on the rocks of Acasta did so by dating their zircons. They fired a beam of charged particles at the crystals, blasting out tiny clouds of isotopes that they then measured. Thanks to all the different crosschecks they performed, they were able to estimate its age within a margin of error of only 12 million years. Twelve million years may be a vast gulf of time for us, but for the Acasta rocks, it represents a margin of error of less than 0.3 percent.
The rocks at Acasta are the oldest known rocks on the planet, but they formed when Earth was already 500 million years old. Geologists needed a gift from space to find out the true age of the planet. In the 1940s they began studying the isotopes of lead in meteorites. Most meteorites are jumbles of space junk left behind from the formation of the solar system. In 1953 Claire Patterson, a geologist at the California Institute of Technology, measured the lead and uranium in the meteorite that had carved out the 1.2-kilometer-wide Meteor Crater in Arizona. It had practically no uranium left in it, because most of the atoms had turned into lead. This meteorite had formed at the dawn of the solar system and had circled the sun essentially unchanged ever since.
Meteorites and our planet all formed from the same primordial stuff, but each one ended up with different proportions of elements, including uranium and lead. By comparing the amount of uranium and lead isotopes in rocks from Earth and meteorites such as the one from Meteor Crater, Patterson determined Earth’s age. It is 4.55 billion years old.
Why is there a 500-million-year gap between the oldest rocks on Earth and its birth? Thanks in part to their ability to date rocks, geologists have discovered that Earth destroys its crust and creates new rock to take its place. The planet’s crust is actually a collection of drifting plates. Magma emerges from the depths of the earth and adds a fresh margin of rock to one side of a plate, and the other side becomes buried underneath its neighboring plate. As the sinking edge of the plate plunges into the planet, it warms up until it partially melts. Any fossils it might carry are destroyed with it.
Continents are floating islands of low-density rock that sit on top of the moving plates. When a plate slides under its neighbor, a continent does not get sucked down with it. If a rock is lucky enough to be nestled within a continent, it may be spared Earth’s fiery cycle—along with fossils and other clues to life’s history it may hold. The rocks of Acasta are geological freaks.
Uranium cannot tell the age of all rocks. Zircons, those exquisite clocks, form only in certain kinds of cooling lava. In sedimentary rocks, the uranium-lead system of telling time is almost useless. Another problem lies in the millions of years that uranium requires to turn into measurable levels of lead. On the scale of thousands of years—the scale of human history—uranium cannot tell time. Fortunately, geochemists are not limited to uranium and lead in their choice of clocks. They can turn to dozens of other radioactive elements, depending on the research at hand. To put absolute dates on human history, for example, scientists can turn to an isotope of carbon, carbon 14, which has a half-life of only 5,700 years, making it a good clock for telling time over the past 40,000 years.
Carbon 14 is born when the charged particles that are continually raining down from space slam into nitrogen atoms in the atmosphere. The transformation is only temporary; a carbon 14 atom eventually decays back into a nitrogen atom, shedding a handful of subatomic particles along the way. As long as plants are alive and absorbing fresh carbon dioxide carrying freshly made carbon 14 from the air, they maintain a steady level of the isotope in their tissues; so do the animals that eat them. But as soon as something dies, it can no longer take in any more carbon 14, and its supply starts to dwindle as the isotopes decay into nitrogen. By measuring the carbon 14 still remaining inside the dead tissue of a plant or an animal, you can calculate its age.
Isotopic clocks have allowed paleontologists to organize the history of life against an absolute calendar. Not only did Darwin not know how old Earth was, he didn’t know how old any fossils were. The best he and other scientists of his day could say was that a given fossil came from a certain geological period. The oldest period in which fossils had been found was called the Cambrian period, and all rocks that came from older layers were simply labeled Precambrian. For Darwin, the way those Cambrian fossils appeared without any predecessors posed a puzzle as deep as Kelvin’s warm Earth.
“If the theory be true,” he wrote of evolution by natural selection, “it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed… and that during these vast periods, the world swarmed with living creatures…. To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer. The case at present must remain inexplicable; and may be truly urged as a valid argument against the view here entertained.”
Paleontologists now know that the Precambrian actually did swarm with living creatures, and it was swarming more than 3.85 billion years ago. The earliest evidence of life comes from the southwestern coast of Greenland. There are no fossils to be found there, at least not in the conventional sense. An organism can leave behind a visible part of its body—a skull, a shell, the impression of a flower petal—but it also leaves behind a special chemistry, and scientists now have the means of detecting it.