The timing of many evolutionary and geologic events on Earth has been determined through two complementary lines of evidence: relative dating and absolute (numerical or radiometric) dating. This evidence makes it clear that life is very old and places a timescale on the pace of evolutionary change, diversification, and extinction.

• Relative dating places fossils in a temporal sequence by noting their positions in layers of rocks, known as strata. As shown in the diagram, fossils found in lower strata were deposited earlier and thus must be older than fossils in higher strata (this principle is known as superposition). Sometimes this method doesn’t work because the layers were up-ended or overturned, or because older rock has been pushed up over younger rock, as can sometimes happen when great mountain belts are formed.  In such cases, we can date the fossil-bearing layers relative to one another using other lines of evidence and reasoning. For example, in the vast majority of strata studied, rock layers containing ammonite fossils are found beneath rock layers containing dinosaur fossils. Because of this, scientists are quite confident that ammonites died out before dinosaurs arose. In fact, ammonites are an example of an index fossil – a type of fossil that is restricted to a particular geologic age and so can be used to figure out the ages of the rocks and fossils relative to them. If a paleontologist were to find a rock formation in which ammonites are above dinosaur fossils, she could infer that the strata have been overturned sometime after their original deposition. This line of evidence and reasoning is called faunal succession. By studying and comparing strata and their fossils from all over the world, paleontologists have established the geologic timescale, which orders the events of life and Earth history. However, we need further evidence to figure out the numerical ages that calibrate this timescale and tell us exactly when different species lived.

  

• Numerical dating relies on radioactive elements, such as uranium, potassium, rubidium and carbon. Radioactive elements decay, or convert to a non-radioactive form, at rates that scientists have carefully observed. If you know how long it takes for a radioactive element to decay, and you measure how much of the element is left in a sample, you can work out how old the sample is. Fossils less than about 50,000 years old can be dated directly using their radioactive carbon content, but for the most part, fossils themselves cannot be dated directly based on radioactive elements. However, we can use this method to date volcanic ashes or other igneous rocks (rocks that form directly from the crystallization of molten rock) to constrain the age of older fossils. For example, by dating volcanic ash layers both above and below a fossil-bearing layer, as shown in the diagram at the top of the page, you can determine that the fossil trilobites are “older than 520, but younger than 545 million years.” Geologists have assembled the ages for the geological timescale on the basis of numerical dating of rocks from around the world.