Fossils and rock layers

The first skeleton of Archaeopteryx, a feathered animal with teeth and a bony tail, turned up in Germany in 1861, two years after the Origin was published.

Most living things vanish without a trace when they die. A fossil forms only when remains, or traces such as footprints, are buried quickly and preserved, usually in sediment that later hardens into rock. Hard parts like shells, bones and teeth survive far more often than soft bodies. Even so, fossils from all over the world give a record of what lived when, and they show that the organisms of the past were not the same as those of today. Burial helps a great deal: the Neanderthal fossil record is unusually rich partly because Neanderthals buried their dead.

Reading the layers

Sediment settles in layers, called beds or strata, with each new layer on top of the last. That gives the law of superposition: in an undisturbed stack, each bed is younger than the beds below it and older than the beds above it. A second rule handles rock that pushes through the layers. A dike, a sheet of once-molten rock that cuts across the beds, is younger than every bed it cuts and older than any bed lying across its top. Neither rule gives an age in years. They give an order, which is called relative dating.

Some fossils make especially good clocks. An index fossil belongs to a kind of organism that lived for only a limited stretch of geological time but was widespread. Finding it in a rock in one country and in another rock far away tells geologists that the two rocks formed at roughly the same time. Index fossils are tied to the relative time scale, with its named periods such as the Cambrian, Devonian and Jurassic.

Ages in years

Numbers in years come from radioactive atoms, which break down at a steady rate. The half-life is the time it takes for half of the parent atoms in a sample to decay. The U.S. Geological Survey lists uranium-238, which becomes lead-206 with a half-life of 4.5 billion years, and potassium-40, which becomes argon-40 with a half-life of 1.3 billion years. These slow clocks date old volcanic rock, such as a lava flow or a dike between fossil beds. Carbon-14 has a half-life of 5,730 years. Living things take it in while they are alive, so it dates wood, bone and shell from the more recent past. After one half-life half the parent atoms are left, after two a quarter, after three an eighth. Measured this way, the Earth is about 4.5 billion years old, and more than 80 percent of that time passed before complex life appears in the rocks.

Fossils that sit between groups

Darwin's theory predicts that, now and then, fossils should turn up with a mix of features from two groups. Archaeopteryx lived in the Late Jurassic, 149 to 145 million years ago, in what is now Germany. The first skeleton was found in 1861, and the Natural History Museum in London bought it in 1862; it is still on display there as the London specimen. It had broad feathered wings like a bird, but also sharp teeth and a long bony tail. Today scientists treat birds as one surviving branch of the dinosaurs. Tiktaalik roseae is another example. It is a 375-million-year-old fish found in 2004 on Ellesmere Island in the Canadian Arctic, with bones and joints inside its front fins. According to the University of Chicago team that found it, it lived about 12 million years before the first four-legged animals and shows tetrapod features appearing in a fish. The team looked for it on purpose, in rocks of the right age that had once been shallow streams.