Ever since Darwin published his Origin of Species literate people have tended to regard the attributes of living things as outcomes of an evolutionary process, and to suppose that humans like other organisms are in some way derived through organic evolution. In terms of searches for explanation there are several meanings to this remark. Laboratory scientists, for example, may assume that common or similarly derived mechanisms underlie physiological phenomena observed even in widely different animals, and they may as a result use simpler or more easily studied organisms, such as rats, to help understand complex species, such as humans, which are difficult to study in the laboratory. Primatologists may assume that humans are similar to other primates because of genetic similarity or a recent common ancestry; this assumption may be used in developing and testing theories about human behavior and its history, or about the phylogenetic derivation of humans. Paleontologists assume that the phylogenetic patterns they are able to trace across geological time are the result of mutation, selection, and isolation, as observable today, even though they are hardly ever able to reconstruct the environments in which the changes occurred well enough to understand the adaptive significance of ancient trends.