The geological evidence for evolution is found in the fossil record. Fossils are the traces, imprints, or mineralized body parts of extinct organisms. The organic material of ancient living things can become trapped in sediment or volcanic ash, preventing its decomposition. Over time, this organic material is replaced with stone, a process called petrification. Petrification is very rare; many conditions must be met for the organic material of an organism to leave behind a trace of its existence.

How do fossils form? Imagine an animal drowns in a mighty river, and its body sinks to the bottom. Sediments from the land are constantly falling into the river. When a river reaches a lake or the sea, it slows down, and these sediments and minerals in the seawater get deposited on the corpse of our unfortunate animal. They begin to fill the gaps once occupied by living organic material, liquid, or gas. Over millions of years, the animal's entire body turns to stone preserving the animal's original anatomical shape. This marks the end of the fossilization process, but now we must locate our fossilized animal. For this to happen, the fossil must become visible. For example, the water surrounding the fossil must recede, or an earthquake must lift the rock layers above the water's surface. Or, receding glaciers can cause a drop in water levels and allow us to find the fossil. Because the Earth's dry surfaces are constantly eroded, fossilization on land is uncommon.

Paleontologists generally find the parts of an organism that contain hard minerals, such as bones, teeth, or shells. Although rare, ancient traces of softer tissue such as skin, hair, and feathers have also been found. Scientists in Mongolia found the fossil of a salamander eye dating back 160 million years.

Unfortunately, the fossil record does not reflect the gradual changes we should expect to see from evolution, a fact that concerned Charles Darwin. Darwin argued that most living things do not fossilize, and among those that do, even fewer are found by humans.

Only a tiny fraction of the species that have ever lived have been fossilized. In addition, many rock layers containing the remains of once-living organisms have been destroyed by the movement of tectonic plates. Other fossils have been bent, twisted, sunk, or changed beyond recognition, erasing all traces of their existence. Humans find very few of all the fossils that exist. Thankfully, even though most animals are eaten by scavengers or decompose, well-documented lineages of species do exist, as is the case with the evolution of whales and horses.

The age of a fossil can be calculated from the age of the rock in which it is found. Scientists calculate the age of the rock using radioactive isotopes. The magma from volcanoes, for example, can contain crystals with radioactive isotopes, such as potassium, uranium, or thorium. Each radioactive isotope decays at a specific rate, which physicists call its half-life. (When an isotope decays, the number of particles in the nuclei of its atoms changes). The half-life of a radioactive isotope is the amount of time it takes for one-half of its atoms to decay. For example, every 704 million years, half of the uranium-235 atoms in a sample become lead-207. (The number after the element refers to the number of protons and neutrons in each atom's nucleus). The age of a rock containing uranium and lead can be estimated by measuring the ratio of the two elements. A newly formed rock of uranium will have no atoms of lead; all of the atoms in the sample will be uranium atoms. After 704 million years, the rock will have the same number of uranium atoms as lead atoms since half of the uranium-235 atoms have decayed. In 1.4 billion years, half of the remaining uranium atoms become lead. The rock will have three lead atoms for every uranium atom.

Sedimentary rocks do not contain radioactive minerals. Scientists use several methods to determine the age of sedimentary rocks. In geology, the law of superposition states that newer rock layers are deposited over older rock layers. Scientists can determine the relative age of a rock by looking at the layers above and below it. Scientists also use index fossils. An index fossil is a well-documented fossil always found in a specific age of rock. Index fossils can identify the age of a sedimentary rock layer. Finally, sedimentary rock layers may contain particles of volcanic ash or be found near layers of volcanic ash. These can be used to determine the age of the sedimentary rock and the fossils trapped within.

It isn't easy to envision the vast amount of time required for evolution to take place. We live in a world where events occur at much smaller intervals, days, years, decades, or centuries at the most. Evolution has taken billions of years, time intervals impossible for the human mind to imagine. Because many geologic changes occur slowly, such as the drift of the continents or the relative motion of stars, we have the impression that the planet around us is standing still. Even the ground beneath our own feet has changed position. Our ancestors who lived 100,000 years ago saw other constellations; the stars have moved relative to each other. It is surprising to us that India was once an island that collided with Asia, causing the Himalayas to rise from the Earth. The subcontinent of India continues to press against the Asian continent, and the mountains continue to rise. These processes are inconceivably slow, but they do happen. Our brains evolved to help our ancestors survive on the grasslands of Africa, registering much shorter periods of time. This makes it difficult to perceive these staggeringly long periods, but that does not mean they did not occur.