Flakes of rock with chipped edges, used for chopping, and "pounded" pieces (like anvils), found in Ethiopia are something like 2.6 million years old This is depressing. It effectively extends by several hundred millennia the period in which our forefathers and mothers managed to avoid any technological innovation whatsoever. And there were already 2 million years in which they achieved very little indeed. How on earth can this be? We are, after all, talking about a very, very long time. It is only 10,000 years since crops were first grown, 5,000 years since humans smelted metal, 60 since TV was invented, 124 months since The Independent was first published - such achievements in such a short period! Yet in 2 million years our forebears failed to get beyond stone chips Even by accident. You might have thought that somewhere in those vast tracts of time, somebody falling off a mountain, and saved by her voluminous squirrel skins, would have passed on the secret of parachuting But no. How come?Were they just too busy? Hunting and gathering can be time-consuming; skinning a mammoth takes a lot of energy; grubbing around for roots, or blackberrying, are not conducive to watching carefully while Ug demonstrates gliding with two leaves and a long stick.

But in that case, how come things ever changed?Explanation two is that most humans, far from being the restless innovators of popular myth, are in fact deeply conservative. Having been tutored in a perfectly efficient flaking method which was good enough for their parents, why change? Anyone who doubts the joint power of nostalgia and inertia should consider this week's call for a return of cadet forces in schools, and the commissioning of a new royal yacht.But it takes relatively few innovators to change life completely for everyone else. So enter Dr Steven Mithen, proponent of cognitive archaeology and author of the recently published Prehistory of the Mind. Dr Mithen's view is that, until a sudden explosion of intelligence - somewhere between 100,000 and 30,000 years ago - our ancestors were just too stupid. They had "modular intelligences", in which technical intelligence was separated from social intelligence (i.e.

you could tie your shoe, if shown, but you couldn't understand knots). Then language itself became a vehicle for thought (before then, the best that humanoids could do was gossip - which is also, of course, the distinction between a tabloid and a broadsheet newspaper). Metaphor was loosed upon the world.There are still a couple of problems that Dr Mithen's explanation fails to iron out. The first is how his theory deals with the flaked tools in the first place.