Sometimes science deals with incredibly large numbers, sometimes with great distances still other times with infinitely small particles. In science we must expand our conception of reality all the time. One of the very difficult concepts is the understanding of time. Everyone is conscious of the changes in the physical and biological world; they give us an awareness of time. The daily rhythm, the seasons, physical changes throughout a human lifetime are familiar concepts of time to us. Time is measured by change, but where change occurs over millions of years our own perception of time is on unfamiliar territory. To understand the rhythm of change of our planet and the effects it has on life on Earth we have to expand our perception of time. The geological processes that shape the surface of our planet, move the tectonic plates, build mountains and erode them again work over millions of years. These forces provide the ever changing conditions for life, which adapted to those changes. But those changes did not go undetected. Our understanding of Earth has expanded tremendously in the past 100 years, and new technologies have provided further insight into Earth's dynamic and history.Earth itself acts like a clock, rotating on its axis once every 24 hours. To read Earth's time it is necessary to look at the changes that have been recorded. To identify changes that occurred due to the geological processes of our planet we can look at rocks. They are key to both the past and the nature of processes. Life has managed to leave records of time and the changes it went through time as well. Fossils are the remains of ancient organisms. Some looked very similar to life forms that are still living today. Fossils can be bones, teeth, shells, impressions of plants and even imprints of animal tracks. Fossils within a rock are a type of organic clock that tick by systematic radioactive decay of certain chemical elements, which permit us to measure with remarkable accuracy the number of years that have passed since the minerals in a rock crystallized. Fossils are recorded in rocks much like your footprints are recorded on a beach. As you walk along the beach, if the sand is fine enough and soft enough, you will make footprints. If the wind and waves do not destroy your footprints, they may record your existence well after your passing.
Fossils are "footprints". Against the odds, these records of past life are preserved. In this section on Earth's history and evolution, we look at the story of Earth's geological and biological history that these "footprints" tell us.
