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The succession of fossil forms is compatible with what is known from other types of evidence about the major branches of descent in the tree of life. For instance, evidence from biochemistry, molecular biology, and cell biology places prokaryotes as the ancestors of all life and predicts that prokaryotes should precede all eukaryotic life in the fossil record. Indeed, the oldest known fossils are prokaryotes. Another example is the chronological appearance of the different classes of vertebrate animals in the fossil record. Fossil fishes predate all other vertebrates, with amphibians next, followed by reptiles, then mammals and birds. This sequence is consistent with the history of vertebrate descent as revealed by many other types of evidence. In contrast, the idea that all species were individually created at about the same time predicts that all vertebrate classes would make their first appearance in the fossil record in rocks of the same age, a prediction at odds with what paleontologists actually observe.

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Most fossils are found in sedimentary rocks formed from the sand and mud that settle to the bottom of seas, lakes, and marshes.

New layers of sediment cover older ones and compress them into superimposed layers of rock called strata.

Later, erosion may scrape or carve through upper (younger) strata and reveal more ancient strata that had been buried.

Fossils within the layers show that a succession of organisms has populated Earth throughout time.

Therefore, fossils are only formed under certain conditions and then have to be uncovered. The chance that a body will be fossilized is rare and the chance that it will be discovered is even rarer. Therefore the fossil record is far from complete. This may account for the ‘missing links’ and for apparently restricted distribution of many species.

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For example, all vertebrate embryos have structures called pharyngeal pouches in their throat regions at some stage in their development. These embryonic structures develop into homologous structures with very different functions, such as the gills of fish or the Eustachian tubes that connect the middle ear with the throat in humans and other mammals.