The Pleistocene spans a period from around 2.5 million years ago (mya) 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 actually a number of 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 among paleontologists, and classification systems for early hominoids are constantly being updated as new discoveries are made.

What is generally agreed upon is the species Homo sapiens belong to the order primates and the suborder anthropoids. Within the anthropoids suborder, 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 subgroup of hominids known as hominin. The subgroup hominin includes humans as well as chimpanzees and gorillas.

Discoveries have revealed more than 20 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 in 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 separated from the rest of the non-hominin primates.

It is generally accepted that hominoids and the first hominins evolved in what is now Africa. Somewhere around seven mya, the common hominoid lineage split into two distinct evolutionary lines: the ancestors of modern chimpanzees and those of modern humans. Around 2.5 mya, a new genus of hominin appeared. Homo had larger brains than their predecessors as well as smaller jaws and teeth. The very first stone tools date to this period, when there were a number of different hominin species. The very first true humans, Homo erectus, appeared around two mya.

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.

The Pleistocene Era: The History of the Ice Age and the Dawn of Modern Humans looks at the development of the era, what life on Earth was like, and the origins of archaic humans.