Fossils are evidence of ancient life forms or ancient habitats which have been preserved by natural processes. They can be the actual remains of a once living thing, such as bones or seeds, or even traces of past events such as dinosaur footprints, or the ripple marks on a prehistoric shore. Geologists can tell the age of a fossil through a variety of radiometric dating techniques. The breakdown of radioactive isotopes of certain elements, such as carbon, uranium and potassium takes place at a known rate, so the age of a rock or mineral containing these isotopes can be calculated.
People have been fascinated by fossils for thousands of years, and as long ago as ancient Greek times were correctly interpreting them as the remains of long dead creatures. Palaeontology began to be formalised and treated with scientific rigour from the 17th century onwards. At this time, people started to calculate the age of the Earth and get to grips with the fact that the extinction of a whole species was not only possible, but had occurred many times already. The publication of Darwin's 'On the Origin of Species' in the mid-19th century gave new impetus to palaeontology, as patterns and trends in evolution and extinction were eagerly sought and studied. Modern palaeontologists have an array of tools and processes at their fingertips, from sophisticated dating techniques to electron microscopes and medical scanners.
Body fossils are the preserved remains of the actual body parts of an animal or plant such as a skeleton or a pollen grain. Trace fossils are the remains of ancient activity, such as the burrow left by a worm or a stone tool made by a prehistoric person. Some fossils preserve original features in exquisite detail, while others are much cruder remnants.
The history of life on Earth began about 3.8 billion years ago, during the Archean era, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it's only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma. Mammals didn't evolve until 200Ma and our own species, Homo sapiens, only 200,000 years ago. So humans have been around for a mere 0.004% of the Earth's history.
The Tree of Life illustrates how different species arise from previous species via descent with modification, and that all of life is connected. The diagram below shows the relationship between the major biological groups. The centre represents the last universal ancestor of all life on earth, the outer branches the major biological groups.