Neanderthals and early humans knew how to make spears but they didn't know how to throw them. Instead, they had a limited hunting strategy, and used their spears merely to stab animals they had already trapped or ambushed. This finding by a team of anthropologists provides an important insight into a defining moment in our ancestors' development, when early humans evolved from hunters who killed at close-quarters to sophisticated killers capable of bringing down large beasts from a distance.

The first direct evidence of thrown spears dates back to about 19,000 years ago. That's the age of the first known atlatl, or spear thrower- a device that allows a long, flexible dart to be thrown accurately at a range of 35 metres or more (New Scientist, 15 May 1999, p 40). Stone points that look like they were designed to be used with thrown spears date back to about 35,000 years ago.

But other evidence seemed to support the idea that spear throwing evolved much earlier. Analysis of the arm bones of Neanderthals, who lived between 230,000 and 30,000 years ago, and early humans living at the same time show that both were much stronger in one arm than the other; the difference is as great as that seen in professional tennis players today. That suggests they threw spears, rather than using both arms to thrust them.

But Steven Churchill at Duke University in Chapel Hill, North Carolina, thinks this reasoning is flawed. A two-handed spear thrust will put far more stress on the dominant arm holding the back end of the spear than the front arm, he says. This would explain the differences in strength found in fossil bones.

To test this idea, Churchill and his colleagues Daniel Schmitt and William Hylander initially measured the dimensions of a number of Neanderthal humerus bones. This showed they are thicker front to back than side to side, which is what would be expected if the bones had adapted to cope with an asymmetric force. Later humans who were known to have used spears had rounder humeri, which suggests that throwing a spear distributes force relatively evenly along the bones.

To find out exactly what forces are involved in thrusting a spear - rather than throwing one - the researchers fitted an aluminium pole with two sets of sensors, one at the front and another at the back. They then asked student volunteers to thrust the pole into a pad, and measured the forces that this generated (Journal of Archaeological Science, vol 30, p 103).

On average, the volunteers generated 70 per cent more force with the dominant back arm than with the front arm. In extreme cases, the force on one arm was six times that on the other. Each volunteer also oriented their humerus in a way consistent with the thickening measured in Neanderthal arms.

Churchill says that using the thrusting technique just once a week would probably be enough to produce the bone deformities seen in Neanderthals. Some models suggest that Neanderthals would have had to kill their favoured prey- reindeer, elk, horse and bison- several times a week to support a family.

Contact: Claire Bowles, claire.bowles@rbi.co.uk, 44-207-331-2751, New Scientist

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