Final answer:
The earliest hominin species date back to the Pliocene epoch, with species like Ardipithecus dating to about 4.4 MYA and demonstrating bipedalism. The genus Homo, including Homo habilis and Homo erectus, appeared later, with Homo habilis dating to between 2.5 and 3 million years ago.
Step-by-step explanation:
The earliest hominin species are known to date back to the Pliocene epoch, which extended from about 5 million years ago (MYA) to 1.8 MYA. Some of the very early hominids that made headline discoveries include Ardipithecus, Sahelanthropus, and Orrorin. The species Ardipithecus, discovered in the 1990s, dates to about 4.4 MYA, with bipedal characteristics confirmed by subsequent fossil discoveries. Additionally, two species of Ardipithecus have been identified: A. ramidus and A. kadabba, with older specimens dating to 5.6 MYA. The Pliocene hominins exhibited bipedalism and evidence of primitive cultural behavior amidst changing climatic conditions.
As for the genus Homo, the first species such as Homo habilis appeared between 2.5 and 3 million years ago, with fossils indicating a larger brain size and other features more similar to modern humans than its predecessors. Homo erectus first evolved approximately 1.9 million years ago and was the first hominin species to migrate out of Africa and use fire, hunt, and maintain a home base.