great apes, than to other mammals. If not, then common descent is falsified. DNA analysis has shown that humans and chimpanzees share a large percentage of their DNA (between 95% to 99.4% depending on the measure). Also, the evolution of chimpanzees and humans from a common ancestor predicts a (geologically) recent common ancestor. Numerous transitional fossils have since been found. Hence, human evolution has passed several falsifiable tests. Many of Darwin's ideas and assertions of fact have been falsified as evolutionary science has developed, but these amendments and falsifications have uniformly confirmed his central concepts. In contrast, creationist explanations involving the direct intervention of the supernatural in the physical world are not falsifiable, because any result of an experiment or investigation could be the unpredictable action of an omnipotent deity. In 1976, the philosopher Karl Popper said that "Darwinism is not a testable scientific theory but a metaphysical research programme." He later changed his mind and argued that Darwin's "theory of natural selection is difficult to test" with respect to other areas of science. In his 1982 book, Abusing Science: The Case Against Creationism, philosopher of science Philip Kitcher specifically addresses the "falsifiability" question by taking into account notable philosophical critiques of Popper by Carl Gustav Hempel and Willard Van Orman Quine and provides a definition of theory other than as a set of falsifiable statements. As Kitcher points out, if one took a strictly Popperian view of "theory", observations of Uranus when it was first discovered in 1781 would have "falsified" Isaac Newton's celestial mechanics. Rather, people suggested that another planet influenced Uranus' orbit—and this prediction was indeed eventually confirmed. Kitcher agrees with Popper that "there is surely something right in the idea that a science can succeed only if it can fail." But he insists that we view scientific theories as consisting of an "elaborate collection of statements", some of which are not falsifiable, and others—what he calls "auxiliary hypotheses", which are. Tautological nature A related claim to the supposed unfalsifiability of evolution is that natural selection is tautological. Specifically, it is often argued that the phrase "survival of the fittest" is a tautology, in that fitness is defined as ability to survive and reproduce. This phrase was first used by Herbert Spencer in 1864 but is rarely used by biologists. Additionally, fitness is more accurately defined as the state of possessing traits that make survival more likely;