"Precambrian rabbits" or "fossil rabbits in the Precambrian" are reported to have been among responses given by the biologist J. B. S. Haldane when asked what evidence could destroy his confidence in the theory of evolution and the field of study. The answers became popular imagery in debates about evolution and the scientific field of evolutionary biology in the 1990s. Many of Haldane's statements about his scientific research were popularized in his lifetime. Some accounts use this response to rebut claims that the theory of evolution is not falsifiable by any empirical evidence. This follows an assertion by philosopher, Karl Popper, who had proposed that falsifiability is an essential feature of a scientific theory. Popper also expressed doubts about the scientific status of evolutionary theory, although he later concluded that the field of study was genuinely scientific. Rabbits are mammals. From the perspective of the philosophy of science, it is doubtful whether the genuine discovery of mammalian fossils in Precambrian rocks would overthrow the theory of evolution instantly, though if authentic, such a discovery would indicate serious errors in modern understanding about the evolutionary process. Mammals are a class of animals whose emergence in the geologic timescale is dated to much later than any found in Precambrian strata. Geological records indicate that although the first true mammals appeared in the Triassic period, modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period. Hundreds of millions of years separate this period from the Precambrian. Origin of the phrase Several authors have written that J. B. S. Haldane (1892–1964) said that the discovery of a fossil rabbit in Precambrian rocks would be enough to destroy his belief in evolution. However these references date from the 1990s or later. In 1996 Michael J. Benton cited the 1993 edition of Mark Ridley's book Evolution, Evolutionary biologist Richard Dawkins wrote in 2005 that Haldane was responding to a challenge by a "Popperian zealot". In 2004 Richa Arora wrote that the story was told by John Maynard Smith (1920–2004) in a television programme. John Maynard Smith attributed the phrase to Haldane in a conversation with Paul Harvey in the early 1970s. Theoretical background The philosopher Karl Popper held that any scientific proposition must be falsifiable, in other words it must at least be possible to imagine some reproducible experiment or observation whose outcome would disprove the hypothesis. Initially he thought that Charles