When animals or plants die, their remains are decomposed by microorganisms in the presence of oxygen, moisture, etc. The absence of oxygen, moisture, and other environmental factors can sometimes prevent the body of an organism from completely decaying. When we dig the earth, we find an organism’s body (or a part of its body) preserved as a fossil. In many instances, the soft parts of the animals dissolve, leaving behind a fossil skeleton of the hard parts (such as teeth, bones, etc.). Even the soft, quickly decomposing components of plants and animals can occasionally be retained as fossils in the form of their impressions in rocks.
For example, a dead leaf that gets caught in mud won’t decay right away. The mud will harden around the leaf, producing a mould that will ultimately become a rock while still retaining an impression of the leaf. After a considerable amount of time, the soil can be dug out by a fossil leaf.
The age of fossils can be determined in two ways as given below:
(i) By the relative method: When we dig into the earth, we find fossils at different depths. The fossils which we find in layers closer to the surface of the earth are more recent and those fossils which are found in deeper layers are older; whereas the fossils found in the deepest layers of the earth are the oldest ones.
(ii) Carbon dating method: When a living object dies and forms a fossil, its carbon-14(C14 ) radioactivity goes on decreasing gradually. By this method, the age of fossils is found by comparing the carbon-14 radioactivity left in fossils with the carbon-14 radioactivity present in living objects today.