The transition between the Middle and Later Stone Age is a controversial topic among archeologists. Recent findings offer a possible answer, suggesting that the shift occurred between 67,000 and 71,000 years ago, and has as its hallmark a sudden abundance of small, sharp, fine-grained stone tools. “There was no universally accepted definition of the Later Stone Age before this,” says lead author Ceri Shipton, an archaeologist at University College London in the United Kingdom.
Shipton’s conclusions, which appear in the Journal of Human Evolution, are based on the cave site Panga ya Saidi, in coastal Kenya. The cave’s domed limestone walls rise like an amphitheater’s toward an open ceiling that’s pierced by trees growing up from the dirt floor. Sediment layers reflect roughly 80,000 years of human occupation. Shipton’s research group scraped back sediment layers and sifted them on screens to pick out any bones, shells, and other anthropogenic material. The team has counted and classified a variety of stone artifacts over several years, including white quartz and greenish chert stone flakes. To date each layer, they used radiocarbon and optically stimulated luminescence, in which sediment samples are irradiated and the intensity of the resulting luminescence is correlated with their age.
Archaeologists have long tried to define the transition between the Middle Stone Age (now thought to be about 300,000 to 50,000 years ago) and the Later Stone Age (from 50,000 to about 2,500 years ago). In the 1920s, some researchers, working in South Africa, first suggested that a change in tool-making technologies marked the transition. Older sediment layers revealed technologies such as Levallois cores, small discs of stone flaked around their circumference with a flat island of unworked stone in the middle. “It’s the waste product left at the end once you’ve made several spear points or a large scraping tool,” Shipton explains.
A different technology called a prismatic blade was thought to mark the transition to the Later Stone Age. Prismatic blades are hunks of rock shaped something like a cocoa pod, with deep ridges in the sides. An artisan would hit the prismatic blade core with another rock to flake knives off of it.
Interestingly, at Panga ya Saidi Shipton found that Levallois cores and prismatic blades flipped back and forth in the sediment sequence over time. “There was no unidirectional change from one to the other,” he says. The authors also found crescent-shaped pieces of stone typically used as arrowheads. Believed to characterize the Later Stone Age, these were also scattered through different time points at Panga ya Saidi, further bucking expectations. The body of evidence suggests these tools and technologies may not be “a coherent package” appearing in a predictable sequence through sediment layers that would distinguish the transition between time periods, he says.
However, what did unite all the shards of rock, dated from roughly 67,000 years ago was their size, which was typically small, as well as their material, typically finer-grained stone.
“It’s a little hard to detect patterns from a single site, but it’s an exciting start,” says Paleolithic archaeologist Christian Tryon at the University of Connecticut in Storrs, who was not involved in the new study. Because Panga ya Saidi is coastal, which is rare among African archaeological sites, the site could be especially useful for inspecting changes in human societies and the ways that peoples interacted, Tryon says, noting that the extent of connections between coastal and interior communities remains largely unknown.
The switches between technology types at Panga ya Saidi might represent a snapshot of a society in transition, notes archaeologist Britt Bousman at Texas State University in San Marcos. But without earlier records, it’s hard to say for sure. “We’re not at the point we can say conclusively that miniaturization is the [only] factor we need to look at,” he says.
While the findings represent only one record from one site, Shipton notes that they are consistent with records elsewhere, such as Eastern and Southern Africa, where tools also became small, sharp, and fine-grained. And similar trends have been documented in other tropical forest settings, such as Sri Lanka and the islands of Southern Indonesia. Shipton suspects the trend is related to a shift in the utility and purpose of the tools, from a longer use-life to a more disposable way of making very sharp edges and keeping them sharp by continually replacing them.
Future microscopy studies could determine what exactly the smaller, sharper blades were used for, Shipton adds. Traces of blood would suggest hunting or weaponry, for example. And the findings could help address another major question in the field: When advanced modern human cognition and technology evolved. Moving back and forth between prismatic blades and Levallois cores could be evidence for increasing cognitive adaptability through time, adds archaeologist Stanley H. Ambrose at the University of Illinois, in Urbana—perhaps in response to environmental degradation during the last ice age. “Risky environments tend to promote cooperation,” he says.
Other recent papers recommended by Journal Club panelists: