Experiments showed that the stone tips inflicted more damage on the hunted animal. Not because they penetrated deeper but because they did wider damage, probably slowing the animal down.
From ScienceDaily:
“Putting a fragile stone tip on a spear is risky,” said Schoville, “but we show that there are serious rewards in terms of both the size and shape of the wound created that made this innovation extremely worthwhile during our evolution.”
The authors also posit that the manufacture of the stone-tipped spears may represent the origin of new cognitive and social development in our human ancestors. Working memory (the ability to hold attention to multiple tasks in order to collect, prepare and combine different kinds of raw materials into a weapon), and constructive memory (the ability to imagine and plan for future tasks), were mental capacities required for tipped spear construction. In addition, since a stone-tipped spear is a skill likely to have been learned by being passed from generation to generation through social or group learning, stone-tipped technology is evidence of the evolutionary impact of “cumulative culture.”
The paper is open access.
See also: What questions about evolution come down to is, “Who are we?” (How come chimps (we are the third ape, according to some views) never did this stuff, never tried to, never thought of it?)
Here’s the abstract:
Stone-tipped weapons were a significant innovation for Middle Pleistocene hominins. Hafted hunting technology represents the development of new cognitive and social learning mechanisms within the genus Homo, and may have provided a foraging advantage over simpler forms of hunting technology, such as a sharpened wooden spear. However, the nature of this foraging advantage has not been confirmed. Experimental studies and ethnographic reports provide conflicting results regarding the relative importance of the functional, economic, and social roles of hafted hunting technology. The controlled experiment reported here was designed to test the functional hypothesis for stone-tipped weapons using spears and ballistics gelatin. It differs from previous investigations of this type because it includes a quantitative analysis of wound track profiles and focuses specifically on hand-delivered spear technology. Our results do not support the hypothesis that tipped spears penetrate deeper than untipped spears. However, tipped spears create a significantly larger inner wound cavity that widens distally. This inner wound cavity is analogous to the permanent wound cavity in ballistics research, which is considered the key variable affecting the relative ‘stopping power’ or ‘killing power’ of a penetrating weapon. Tipped spears conferred a functional advantage to Middle Pleistocene hominins, potentially affecting the frequency and regularity of hunting success with important implications for human adaptation and life history. – Jayne Wilkins, Benjamin J. Schoville, Kyle S. Brown. An Experimental Investigation of the Functional Hypothesis and Evolutionary Advantage of Stone-Tipped Spears. PLoS ONE, 2014; 9 (8): e104514 DOI: 10.1371/journal.pone.0104514
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