One of the iconic weapons of the Paleolithic is the fire-hardened spear, its wooden tip carbonized by fire to a wicked point. Unfortunately, it turns out that our hunter ancestors were wrong about fire-hardening. Yes, the charring can make wood slightly harder, but it becomes so much more brittle and weak that there's little overall improvement of the weapon. After experimenting with their own fire-hardened spears, a group of British biomechanics researchers now believe our ancestors used fire not so much to make a more deadly weapon but to speed up the process of cutting wood into a point.

The oldest spear ever discovered, the Clacton spear (named after the region in England where it was discovered), dates back 450,000 years. Made by some unidentified ancestors of modern humans, its sharp wooden point was snapped off and buried in soil. There it was sealed away from the elements and preserved far longer than wood ordinarily can be. When the Clacton spear was discovered in the early twentieth century, archaeologists noticed that its tip had been fire-hardened, using a technique that some hunter gatherer groups still use. It has long been believed that the practice of heating a pointed spear tip in the fire was a way of making it sharper and harder. But a new paper published in Royal Society Biology Letters suggests otherwise.

Two bioscientists at the University of Hull, Roland Ennos and Tak Lok Chan, decided to find out for themselves whether fire really makes spears harder. So they harvested 20 rods from local hazel trees and spent weeks abusing them inside machines of very precise, codified destruction. First, each rod was divided in half. One half dried naturally over two weeks in the laboratory, and the other half were given a simulated fire-hardening treatment with an experimental rig known technically as a "disposable barbecue."

"Rods were laid out on top of a disposable barbecue holding glowing charcoal," the researchers write. "They were continually turned as the internal water was expelled, and subsequently heated further. The rods were removed once they had browned but before they had started to blacken, though two samples had started to char and were discarded."

Then they tested each rod for hardness, as well as how each responded to bending and sharp impacts. What they discovered was that the fired wood was indeed slightly harder, but it was also significantly weaker. Compared to the unfired control rods, the fire-treated hazel rods' strength was reduced by 30 percent, and its "work of fracture," or ability to resist being fractured along its grain after an impact, was reduced by 35 percent.

So it would seem that fire-hardening probably did not actually improve spears' effectiveness overall. Any gain in hardness would have been undermined by brittleness. In fact, this very brittleness may be why the Clacton spear tip was broken off in the first place. The researchers suggest that fire hardening may have begun as a time-saving device. Instead of having to dry and laboriously cut a green rod, an ancient human could slowly toast it in the fire, scraping off the ashes to make a sharp point at the tip. In their paper, the researchers conclude that firing cuts labor time dramatically. "It has been shown, for instance, that the Clacton spear point could have been produced shaving the end with a sharp ‘Clactonian notch’ flint blade, but that this process can be speeded up from 2 hours to 45 minutes by alternately charring the tip and removing the carbonized layer with the notch," they write. "Fire hardening of spears may, therefore, have originated as a by-product of their manufacture; the benefits of the process are equivocal."

So maybe our spear-throwing ancestors weren't dipping their weapons in the fire to become even more hardcore. They were just trying to be productive with their time.

Royal Society Biology Letters, 2016. DOI: 10.1098/rsbl.2016.0174