At about this time (3.4 mill BP) we also find the first incontrovertible evidence of stone tools, though the assumption is that simpler tools were put to use several million years before this.

The climate continues to cool and dry, and at this magnification we begin to discern some of the internal climatic complexity of the Quaternary Ice Age (QIA). In the lower right-hand corner of the present timeline, I have suggested the periodic fluctuations between glaciation and interglacials that characterize the QIA. I have limited myself to the last eight glaciation-interglacial cycles, but must remind you that the cycle continues back in time for at least another 1.5 million years.

At approximately this time (2.5 - 3 million BP) a successor species to a. afarensis appears - perhaps a direct descendent, but evidence of this has yet to be unearthed. The new species - homo habilis - is the first of the new genus homo. It has a brain size of approximately 640 cubic centimeters (ccm), while afarensis had a mere 380-430 ccm. More complex tools (known as Oldowan, after the famous East African archaeological site) now appear, though they may still look crude to us.

H. habilis, in turn, evolves into homo erectus (brain size 850 ccm), the longest-surviving species of homo in history. With homo erectus, still more complex (acheulean) tools appear. It is likely that erectus were the first to tame fire. More fundamentally, this is when hunter-gatherer society first starts to find its form, based on small bands following animal herds over large territories, practicing innovative hunting techniques and coordinated hunting strategies, and possessing exhaustive knowledge of all food sources in their biotope. It is also likely that erectus mastered language in some form or another - perhaps a relatively simple proto-language.

The further details are somewhat obscure, but roughly speaking, homo erectus, during its nearly 2-million-year long residence on Earth, was constantly evolving and diversifying, spreading throughout Asia and Europe, and producing, among other things, the successor species homo heidelbergensis, which is, most likely, the direct ancestor of homo sapiens sapiens.