The largest of the tools are around 20cm long and have been chipped into shape on two sides, a hallmark of more sophisticated stone toolmaking techniques probably developed by Homo erectus, an ancestor of modern humans.
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Researchers dated the sediments where the tools were found to 1.76m years old. Until now, the earliest stone tools of this kind were estimated to be 1.4m years old and came from a haul in Konso, Ethiopia. Others found in India are dated more vaguely, between 1m and 1.5m years old.
Older, cruder stone tools have been found. The most ancient evidence of toolmaking by early humans and their relatives dates to 2.6m years ago and includes simple pebble-choppers for hacking and crushing. These Oldowan tools, named after the Olduvai gorge in Tanzania, were wielded by our predecessors for around a million years.
But the latest collection of stone tools from Kenya belong to a second, more advanced generation of toolmaking. Known as Acheulian tools after a prominent archaeological site in France, they are larger, heavier and have sharp cutting edges that are chipped from opposite sides into the familiar teardrop shape.
http://www.guardian.co.uk/science/2011/aug/31/hand-axes-oldest-advanced-stone-tools