A hammerstone (or hammer stone) is the archaeological term used for one of the oldest and simplest stone tools humans ever made: a rock used as a prehistoric hammer, to create percussion fractures on another rock. The end result is the creation of sharp-edged stone flakes from the second rock. Those flakes can then be used as ad hoc tools, or reworked into stone tools, depending on the technical skill and knowledge of the prehistoric flint knapper.
Using a Hammerstone
Hammerstones are typically made from a rounded cobble of medium-grained stone, such as quartzite or granite, weighing between 400 and 1000 grams (14-35 ounces or .8-2.2 pounds). The rock which is being fractured is typically of a finer-grained material, rocks such as flint, chert or obsidian. A right-handed flintknapper holds a hammerstone in her right (dominant) hand and bangs the stone on the flint core in her left, making thin flattish stone flakes come off the core. This process is sometimes called "systematic flaking". A related technique called "bipolar" involves placing the flint core on a flat surface (called an anvil) and then using a hammerstone to smashing the top of the core into the anvil's surface.
Stones aren't the only tool used to turn stone flakes into tools: bone or antler hammers (called batons) were used to complete the fine details. Using a hammerstone is called "hard hammer percussion"; using bone or antler batons is called "soft hammer percussion". And, microscopic evidence of residues on hammerstones indicates that hammerstones were also used to butcher animals, in particular, to break animal bones to get at the marrow.
Evidence of Hammerstone Use
Archaeologists recognize rocks as hammerstones by the evidence of battering damage, pits and dimples on the original surface. They aren't typically long-lived, either: an extensive study on hard hammer flake production (Moore et al. 2016) found that stone hammers used to strike flakes from large stone cobbles cause significant hammerstone attrition after a few blows and eventually they crack into several pieces.
Archaeological and paleontological evidence proves that we've been using hammerstones for a very long time. The oldest stone flakes were made by African hominins 3.3 million years ago, and by 2.7 mya (at least), we were using those flakes to butcher animal carcasses (and probably wood-working as well).
Technical Difficulty and Human Evolution
Hammerstones are tools made not just by humans and our ancestors. Stone hammers are used by wild chimpanzees to crack nuts. When chimps use the same hammerstone more than once, the stones show the same kind of shallow dimpled and pitted surfaces as on human hammerstones. However, the bipolar technique is not used by chimpanzees, and that appears to be restricted to the hominins (humans and their ancestors). Wild chimpanzees do not systematically produce sharp-edged flakes: they can be taught to make flakes but they do not make or use stone-cutting tools in the wild.
Hammerstones are part of the earliest identified human technology, called the Oldowan and found in hominin sites in the Ethiopian Rift valley. There, 2.5 million years ago, early hominins used hammerstones to butcher animals and extract marrow. Hammerstones used to deliberately produce flakes for other uses are also in the Oldowan technology, including evidence for the bipolar technique.
There has not been a lot of scholarly research specifically on hammerstones: most lithic studies are on the process and results of hard-hammer percussion, the flakes and tools made with the hammers. Faisal and colleagues (2010) asked people to make stone flakes using Lower Paleolithic methods (Oldowan and Acheulean) while wearing a data glove and electromagnetic position markers on their skulls. They found that the later Acheulean techniques use more diverse stable and dynamic left-hand grips on hammerstones and fire up different parts of the brain, including areas associated with language.
Faisal and colleagues suggest this is evidence of the process of evolution of motor control of the hand-arm system by the Early Stone Age, with additional demands for the cognitive control of action by the Late Acheulean.
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