Evolution

This combined with the fossil record, another observation supporting the evolutionary hypothesis, has produced an apparent sequence of life forms from most simple to most complex during the history of the planet. Fossils are any remains or traces of a once-living organism, which are formed by preservation, petrifaction, or sedimentation. Organisms can be preserved and protected against decay by being trapped in amber, tar, or frozen in ice. The hard parts of an organism, such as shells or bones, can be preserved when the flesh of an organism has decayed away. In other cases, materials of a dead organism may be gradually washed away and replaced by minerals from the water causing the organism to petrify, or harden. Imprints, molds, or casts left by an organism after it is enclosed in sedimentary rock and decomposed are also fossils, as well as footprints and tracks. The fossil record is the timetable of fossils found in within the earth's geologic record. Since the upper layers of sedimentary rocks are assumed to have been laid down over lower layers, the upper layers are younger than those deeper into the earth. Therefore, fossils found within the upper layers are also younger than those found within the lower layers. This combination of geologic and fossil records shows the progression of species as time also progressed. In reiterating the definition of evolution, we are reminded that gradual changes in one life form's anatomy, cytology, embryology, or biochemistry could cause a new species to originate. Similarities in these categories link species together and are therefore studied to support evolutionism. In comparing anatomy, the structures of different organisms often show unexpected similarities. Cell organelles, such as cell membranes, ribosomes, and other structures found within cells, are also similar in organisms of all kinds, showing that comparative cytology can also shows signs of evolution.