The rocks beneath our feet are mostly built up from layers. Each one resembles a page in a history book, recording conditions on Earth during its formation. In a sedimentary rock the grains size and shape can record the energy of the environment in which it was deposited. Coarse pebbles or rock fragments indicate rapid erosion of a nearby land mass by waves and weather. Fine clays and silts indicate slower accumulation in still waters. Angular grains may come from a desert, rounded ones from a river, chemical deposits, such as lime stones, imply accumulation away from a source of eroded sand and mud. Many rocks contain fossils, visible by eye or by microscope and these can suggest the conditions in which the creatures lived. There are chemical clues too, to the salinity of seas, and the temperature and even composition of the atmosphere at the time of rock formation.
If the theory of evolution is right, every living thing contains molecular clues to its past. Genes are living fossils. If there is a constant rate of random mutation, DNA is like a molecular clock; the genetic differences between species record the time since they shared an ancestor. The differences also reveal which species are related; humans are genetically only one percent different from chimpanzees.
Genes and proteins have been recovered from extinct species from museum specimens of the marsupial wolf, and from a woolly mammoth in Siberian ice. DNA has even been found in a 100 million year old weevil preserved in amber. Dinosaur DNA may survive in bone but not in sufficient quantities to create a Jurassic Park.
Fossils and most sediments contains from the atmosphere at the time they were laid down. These atoms carry massages. For example, oxygen comes in different form or isotopes- sea water contains O16 and O18. O16 is lighter and evaporates more easily to fall as snow; it thus becomes locked in polar ice caps, and proportion of O18 or O16 remaining in the shells of tiny fossils indicates the amount of polar ice. Carbon too, comes in different forms. Plankton take up C12 more readily than C13 , If that carbon continues down the chain to bottom dwelling species, the atmosphere is depleted in C12, as are subsequently plankton. Thus a comparison of C13, in surface and bottom dwelling microfossils reveals biological activity at the time.