Fossils are the remains of plants and animals that are preserved in rock. The oldest evidence for life comes from carbon found in rocks on the island of Akilia in southern Greenland that have been dated as being 3,850 million years old. Some authorities have argued that this date is inflated, and that the rocks are in fact 150 to 200 million years younger. Worse still, they have been interpreted as being a banded ironstone, which would indicate that the carbon was not organic. The oldest undisputed fossils are thought to be blue-green algal filaments found in the Apex chert in Western Australia, which are 3,465 million years old. For many millennia, fossils have been the focus of curiosity, but only in the past 150 years have fossils been systematically studied and described. They are useful tools in dating the geological past and have also been used to correlate sequences of rock from area to area, locally or even from continent to continent. The discipline of studying fossils is called palaeontology.

Our understanding of what fossils are and what they can tell us is continually evolving. This is what makes the subject so interesting. Research on fossils is always changing and new methodologies appear every year to test new ideas or re-evaluate old hypotheses. Twenty years ago, it was unheard of to use sophisticated instruments such as mass spectrometers to measure the ratios of the various oxygen isotopes trapped within fossil skeletons, which yield information about the temperature of the sea water from which the skeleton was precipitated.