Fossils are once-living organisms that have been turned into rock, in which the shape or form of the organism can still be seen. As one looks farther up, at younger and younger rock layers, the fossilized plants and animals become more and more familiar until they are a lot like organisms that are around now. The organisms also tend to become more and more complex. From this, Darwin concluded that organisms have not remained the same since earth's beginning, and that they have changed a lot, gradually becoming more and more complex. He also realized that as new species arise, other ones become extinct.
People look at fossils to discover which life forms evolved first and which later on.
Today scientists also have ways of dating the rocks, figuring out about how long ago each layer was deposited. This also helps us piece together the time scale of evolution and when certain events occurred. Similarities Among Living Organisms Layers of sedimentary rock.