The array of animals that use tools may keep getting larger, but humans still have a big lead, thanks in part to the fact our ancestors were carefully choosing the right rock for the job a million years before our species evolved.
Olduvai Gorge, Tanzania is one of the richest known sites for early human tools. Stone flakes and cores found there date to 1.8 million years ago. One reason humans spent so much time there was the gorge offered three suitable kinds of rock: lavas, quartzite, or chert.
Dr Alastair Key of Kent University tested samples of each rock type, measuring cutting capacity and edge angle. Key was cutting PVC pipes, not meat or wood like our ancestors, but he says this provides a more reliable indication of durability and mechanical sharpness. He found quartzite made for sharper but less durable blades than the others, while chert was nearly as sharp and also quite durable, but had only limited availability.
“Why Olduvai populations preferentially chose one raw material over another has puzzled archaeologists for more than 60 years. This has been made all the more intriguing given that some stone types, including lavas and quartzite, were always available,” Key said in a statement.
After examining the Early Stone Age tools at the site Key and co-authors report in the Journal of the Royal Society Interface that well before our species evolved, our ancestors could match the rock to the purpose.
Flake tools, which are used for cutting but discarded far more quickly than cores, were frequently made from quartzite. Their value is revealed in the fact the inhabitants carried quartzite stones 3 kilometers (2 miles) from the main local source to the campsite where the tools were recovered.
Chert was heavily used over a 200,000-year period but not at all at other times – apparently the oldest example of a mineral resource exploited until it ran out.
Lava stones were much more easily available – littering the beds of seasonal rivers within the Gorge, where they were deposited from the surrounding highlands. Although more similar to each other than they are to quartzite or chert, lava rocks come in many types, probably with their own trade-offs. Key only studied the properties of basalt for this paper, and estimates basalt, while initially blunter than quartzite or chert, would have surpassed them after hundreds of cutting strokes, making it a better raw material for where long-term use was anticipated.
“Although Pleistocene individuals may not have been aware of doing so, a series of mechanical principles routinely applied during the design of modern metal-cutting tools were being exploited to maximize each tool's functional potential and ease of use,” the paper notes. Life in the Stone Age may look simple to us, but required complex questions of engineering.