Part of Hall of Human Origins.
Fossils arranged chronologically, for instance, can often suggest evolutionary relationships among species. And fossils of the same age demonstrate the distributions of ancient plants and animals around the world. Fortunately, paleontologists today have a number of techniques they can use to answer the question, "How old is it?"
One way of dating fossils relies on their relative positions in the ground. When paleontologists dig deeper in sedimentary rock they are, in effect, looking back in time. As sediments carried by wind and water accumulate, they bury older layers—so the bottom layers in a geological sequence are usually the oldest and the top layers the youngest. Paleontologists can thus put fossils in chronological order without knowing the actual age of each specimen.
When fossils are buried one on top of another, it is easy to arrange them in chronological order. But the same sedimentary layers can often be traced over wide areas, so even fossils from sites far apart can be arranged in a relative chronology.
Today, scientists use a variety of techniques to date rocks and fossils precisely. Most often, they measure the amounts of particular radioactive elements—often radiocarbon or potassium—present to determine when a rock was formed, or when an animal or plant died. Some techniques work best with materials millions or even billions of years old. Others only work for much younger materials. And each method only works for certain materials, ranging from volcanic rock to charcoal to bone.
Living plants and animals absorb carbon from the atmosphere, including carbon-14—a radioactive form of the element produced when cosmic rays from the sun interact with nitrogen in the upper atmosphere. But when organisms die, they no longer take in any carbon, and the carbon-14 in their bodies begins to decay at a known rate. Scientists use particle accelerators to measure the amount of carbon-14 in biological materials to determine when that organism died.
The cliff face at the Gran Dolina, a site in the Sierra de Atapuerca in northern Spain, is over 18 meters (59 feet) tall and encompasses 11 distinct layers formed over more than one million years. Here scientists are working in layer TD 6, where they found hominid fossils over 800,000 years old—some of the earliest Europeans.
Gray volcanic tuffs—produced when layers of hot ash are laid down after volcanic eruptions—are dateable and can help date fossils found in adjacent layers. As ash layers cool, radioactive potassium-40 contained within begins to break down into the rare gas argon at a known rate. By comparing the amounts of stable potassium and argon, paleontologists can estimate how much time has passed since the volcanic tuff was formed.