Evidence that the evolution of life forms has indeed taken place on earth has come from many quarters. Fossils are remains of hard parts of life-forms found in rocks. Rocks form sediments and a cross-section of the earth's crust indicates the arrangement of sediments one over the other during the long history of the earth. Different-aged rock sediments contain fossils of different life-forms who probably died during the formation of the particular sediment. A study of fossils in different sedimentary layers indicates the geological period in which they existed. The study showed that life-forms varied over time and certain life forms are restricted to certain geological timespans. Hence, new forms of life have arisen at different times in the history of the earth. All this is called paleontological evidence . Embryological support for evolution was also proposed by Ernst Heckel based upon the observation of certain features during the embryonic stage common to all vertebrates that are absent in adults. The embryos of all vertebrates including humans develop a row of vestigial gill slit just behind the head but it is a functional organ only in fish and not found in any other adult vertebrates. However, this proposal was disapproved of a careful study performed by Karl Ernst von Baer . He noted that embryos never pass through the adult stages of other animals. Comparative anatomy and morphology show similarities and differences between organisms of today and those that existed years ago. Such similarities can be interpreted to understand whether common ancestors were shared or not. Though these forelimbs perform different functions in these animals, they have similar anatomical structures – all of them have a humerus, radius, ulna, carpals, metacarpals, and phalanges in their forelimbs. Hence, in these animals, the same structure developed along with different directions due to adaptations to different needs. This is divergent evolution and these structures are homologous. Homology indicates common ancestry. In plants also, the thorn and tendrils of Bougainvillea and Cucurbita represent homology . Homology is based on divergent evolution whereas analogy refers to a situation exactly opposite. Wings of butterflies and birds look alike. They are not anatomically similar structures though they perform similar functions. Hence, analogous structures are a result of convergent evolution - different structures evolving for the same function and hence having similarity. Similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry. These biochemical similarities point to the same shared ancestry as structural similarities among diverse organisms.