Fossils are the 'remains' of organisms from millions of years ago, which are found in rocks. Fossils may be formed: from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent; when parts of the organism are replaced by minerals as they decay; as preserved traces of organisms, such as footprints, burrows and rootlet traces. • Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth. • We can learn from fossils how much or how little different organisms have changed as life developed on Earth.