Many Clocks, One Story

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.

The ratio of carbon 13 to carbon 12 is lower in organic carbon, such as wood or hair, compared to inorganic carbon that escapes out of a volcano as carbon dioxide. This makes it possible to tell if the carbon in a rock had ever been inside a living thing. Consider, for instance, a leaf growing on an elm tree. It builds up a low ratio of C‑13 to C‑12. A caterpillar that nibbles that leaf will incorporate the carbon in its prey into its own tissue, and it will take on a low C‑13 ratio as well, as will the bird that eats the caterpillar. Birds, caterpillars, and leaves all die sooner or later, and when they do, they all become part of soil, which eventually washes out to the ocean and becomes sedimentary rock. And even those rocks, made partly from carbon that has cycled through life’s metabolism, will bear life’s low C‑13 ratio. Any sedimentary rocks that formed before life appeared on Earth would have the high C‑13 ratio of a volcanic origin.

In 1996 a team of American and Australian scientists traveled to the twisted fjords and bare islands of southwestern Greenland, where the oldest sedimentary rocks on Earth can be found. A layer of volcanic rock cuts through them, and the scientists used the uranium‑lead clock inside its zircons to date it to 3.85 billion years. They then sifted through the surrounding rock. Over its lifetime, it has been cooked, compressed, and otherwise ravaged almost beyond recognition. But the researchers found microscopic bits of carbon in a mineral known as apatite in the sedimentary rocks. They brought these samples back to their labs and blasted off bits of the apatite with a beam of ions and counted up the carbon isotopes it contained. They found that carbon in the apatite had the same low C‑13 ratio as biological carbon today a ratio that could only have come from life.

Scientists cannot say just how long life existed on Earth before it left this mark in the rocks of Greenland, because no rocks older than 4 billion years have survived. But it’s safe to say that life must have had a hellish birth. Giant asteroids and miniature planets pummeled Earth for its first 600 million years. Some of them were big enough to boil off the top few meters of the oceans and kill any life it held. Perhaps life survived these cataclysms hidden around the thermal springs at the ocean floor, where bacteria can be found today. When rains filled the seas again, the microbes were able to emerge from their refuges.

However life got started, it had to have been in full swing by the time it left its mark in the Greenland rocks. At the time, the oceans were teeming with bacteria generating their own food as they do today, either from sunlight or from the energy contained in the chemistry of hot springs. These self‑sustaining microbes were probably food for predatory bacteria, as well as hosts for viruses.

The oldest actual fossils of bacteria date back 3.5 billion years, about 350 million years after the earliest chemical signs of life. These fossils, discovered in the 1970s in western Australia, consist of delicate chains of microbes that look exactly like living blue‑green algae (otherwise known as cyanobacteria). For billions of years, these bacteria formed vast slimy carpets in shallow coastal waters; by 2.6 billion years ago they had also formed a thin crust on land.

Of course, life is not limited to bacteria. We humans belong to an enormous group of organisms called eukaryotes, which include animals, plants, fungi, and protozoans. The evidence for the oldest eukaryotes doesn’t come from traditional fossils, which date back only about 1.2 billion years. It comes, once again, from molecular fossils. Among the many things that distinguish eukaryotes from bacteria and other life‑forms is how their cell membranes are constructed. Eukaryotes stiffen them with a family of fatty acids known as sterols. (Cholesterol belongs to the sterol family; while it may be dangerous when too much of it gets into the bloodstream, you couldn’t live without it. Your cells would simply disintegrate.)

In the mid‑1990s, a group of geologists led by Jochen Brocks of Australian National University drilled 700 meters down into the ancient shales of northwest Australia to formations that have been dated with uranium and lead to 2.7 billion years ago. Inside the shale, the geologists found microscopic traces of oil that contained sterols. Because eukaryotes are the only organisms on Earth that can make these molecules, Brocks’s team concluded that eukaryotes–probably simple, amoeba‑like creatures–must have evolved by 2.7 billion years ago.