The Pleistocene spans a period from around 2.5 million years ago to just over 12,000 years ago, and it was an epoch of enormous change on Earth, mainly characterized by climate changes involving fluctuations between periods of extreme heat and long periods of glaciation. This period is commonly known as the Ice Age despite the fact there were several separate periods of cold. Along with the climate challenges, this was also the period that saw the development of modern humans. The origin of our ancient ancestors is still a matter of debate amongst paleontologists, and classification systems for early hominoids are constantly being updated as discoveries are made. What is generally agreed upon is the species Homo sapiens belong to the order primates and the sub-order anthropoids. Within the anthropoids sub-order, humans belong to the family hominids, which also includes other animals such as the orangutan and the great apes. Drilling down even further, humans belong to a sub-group of hominids known as a hominin. The sub-group hominin includes humans, as well as chimpanzees, and gorillas. Discoveries have revealed more than twenty species of the genus Homo, all of which appeared during the Pleistocene Epoch, and all but Homo sapiens became extinct during the same period. The challenge is understanding which of these groups are predecessors to Homo sapiens and which are separate groups that died out leaving no current representation. Not knowing this information makes it difficult to determine neat classification and establish precisely when hominins are separated from the rest of the non-hominin primates. These new creatures could hardly have chosen a more difficult time to appear. In addition to facing the challenges of simply surviving in a generally hostile environment, the world was about to enter a period of convulsive climatic change. The new humans would face drought and extreme heat, as well as long periods of cooling where glaciers spread across the surface of the planet, but they survived, and by the time the Pleistocene Epoch ended around 12,000 years ago, Homo sapiens had become one of the most significant species on the planet.