Young earth creationism includes two basic tenets. The first tenet is that creation happened exactly according to the Biblical account. This means that species were created separately and no macroevolution took place. The second tenet is that the time-line of the natural history of the universe should be derived from the genealogies given in the Bible and the assumption that the days described in Genesis 1 are natural days, i.e., 24 hours. Consequently, the total history of the universe based on biblical genealogies is about six thousand years (though some proponents of YEC allow up to ten thousand). The contemporary form of YEC was crystallized when some Christian groups challenged Darwinian evolution in the early 20th century. Later, in the 1960s a modified version of YEC was proposed. It is typically referred to as “scientific creationism”. Scientific creationism differs from the original young earth creationism by proposing the congruence of scientific data with the young earth perspective derived from the Bible. The proponents of YEC often claim that all of Christendom before the geological revolution (18th and early 19th centuries) believed in a young earth. According to them, allowing an old earth is a betrayal of the uninterrupted Christian tradition and an offense to the Bible. They, however, fail to add that Christians never believed in young earth as a matter of faith. Christians of the pre-scientific era did not have any reason to adopt “deep time” (billions instead of thousands years of history) because geology did not exist at the time.