Telling stories from stone tools

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Imagine if tens of thousands of years from now, archaeologists were to dig up a pile of wrecked, 20th century cars and try to figure out what people did with the strange-looking things.

After measuring soil-encrusted automobile shells and scattered engine innards, the researchers might well announce the discovery of ancient religious altars. Support for their interpretation would come from fragments of 20th century texts describing widespread car worship. Eminent scientists might propose that basic altars were made in a city called Detroit before being modified by their owners into objects suitable for worship. A flood of publications would sort the artifacts into categories of altars based on the presence or absence of tail fins and roof racks.

Archaeologist Harold Dibble of the University of Pennsylvania in Philadelphia likes to tell this futuristic farce when he gives lectures about the modern study of ancient stone tools. His point: Long-held assumptions about how human ancestors made and used stone tools are probably way off. It is easy to generate plausible explanations of behavior from way, way back. But rigorously testing those accounts is hard.

When scientists find stones that appear to have been pounded or chipped, they sort them into what they call “tool industries” based on the general shape and age of their finds. The various tool industries are thought to define particular ancient cultures or populations. Researchers also evaluate stone tools based on how they were made, breaking down the steps probably used to make tools for, say, chopping plants or slicing meat from animal carcasses. Taken together, this evidence is used to reconstruct how hominids, such as Homo sapiens and Neandertals, interacted and moved across Africa, Asia and Europe.

Current evolutionary scenarios based on that evidence inspire about as much confidence as calling a Ford Pinto a holy relic, Dibble says. Researchers unearth stone objects that, for the most part, were discarded by ancient individuals. No one can say for sure whether those items were final versions of tools that had specific uses, partly completed or worn out tools, or garbage produced during toolmaking. Even stone artifacts that show signs of extra sharpening may not be stellar examples of a finished, ready-to-use stone tool, as archaeologists have long assumed, Dibble holds.

And there’s no reason to conclude (as many in the field do) that stone tool industries define particular ancient cultures or populations, such as Neandertals, he adds. Worse, many pounded and sharpened stones discovered at archaeological sites are ignored because they don’t fit neatly into any of the traditional named tool industries.

Armed with only this partial view of the past, it becomes easy to assume, for instance, that a single tool industry monopolized Western Europe, South Asia, the Arabian Peninsula and North Africa between 200,000 and 40,000 years ago. But don’t bet on it.

“Archaeologists have built evolutionary stories on a foundation of sand,” Dibble says.

Unhappy with the stone tool status quo, Dibble and others are experimenting with new ways of squeezing insights about hominid evolution out of pieces of rock. They are looking for hidden clues to how ancient folk worked stone into tools, adapted those methods as they traveled in and out of Africa, and passed toolmaking knowledge from one group to another.

Some archaeologists doubt that all this fuss will clarify hominid evolution or generate testable predictions about a largely unknowable past. Others say stone tool industries still provide valuable insights when applied to large numbers of artifacts found in relatively limited areas.

If any of the new approaches pans out, it could reveal previously overlooked ties between toolmakers in different regions and dramatically change ideas about how and when humans left Africa and settled the globe. With a revamped understanding of how stone tools relate to hominid evolution, for example, evolutionary geneticists, who study ancient DNA, would have a framework for interpreting their latest molecular discoveries.

Banish the NASTIES

Purveyors of new methods for analyzing stone artifacts are bucking longstanding tradition in archaeology. Stone tool industries were formulated in the late 19th and early 20th centuries to describe discoveries at single European and African sites. Because there is no formal process for adding new industries or retiring tired ones, those categories have become bloated over time, says John Shea, an archaeologist at Stony Brook University in New York. As a result, each industry encompasses artifacts found across massive geographical expanses where two or more hominid species once lived at the same time.

Researchers are left wondering, for example, whether triangular stones with sharp points made between around 200,000 and 40,000 years ago and found at various Middle Eastern sites were shaped by H. sapiens or Neandertals. Or maybe each species invented the same tool on its own. If all the Middle Eastern finds are lumped into one category of tool industry, the question doesn’t even arise.

Shea has a pet name for the “named stone tool industries.” He calls them NASTIES because, in his view, they hide more than they reveal about the makers of ancient artifacts. Mobile hunter-gatherers probably made different kinds of tools as they traveled from, say, savannas to forests. NASTIES may lump together the tools of different hominids adapting to similar habitats in the same way.

NASTIES also don’t account for ingenuity among  Stone Age humans, who devised up to a dozen different ways of making teardrop-shaped hand axes and chiseling lumps of rock into cores from which sharp flakes could be removed, Shea argues.

“NASTIES are like archaeologists’ family heirlooms,” Shea says. “We don’t know what to do with them, but we don’t want to throw them away.”

One archaeological heirloom deserves an immediate heave-ho, Shea says. In the November 2014 Quaternary International, he calls for discarding the influential Mousterian stone tool industry. Named in 1883 for finds at France’s Le Moustier Cave, Mousterian artifacts are exemplified by triangular stone points and lumps of rock from which toolmakers pounded off sharp implements. Mousterian tools have since been found in sites across Europe, West Asia, the Arabian Peninsula, India and North Africa. Those discoveries date to between 200,000 and 40,000 years ago, when both H. sapiens and Neandertals occupied many of those regions.

But the Mousterian label has been stretched so thin over the last 130 years that the term has become meaningless, Shea contends. Consider that Middle Eastern Mousterian artifacts commonly include cores shaped like a tortoise shell, known as Levallois cores, and triangular flakes with reworked edges. French Mousterian tools, on the other hand, feature oval flakes with reworked edges and large, double-edged hand axes.

Middle Eastern Mousterian tools, which are generally divided into two or three types, have been unearthed at sites that have yielded Neandertal and ancient human fossils. Disagreement reigns about whether one hominid borrowed toolmaking techniques from the other or each species operated on its own.

Attempts to unravel hominid relationships in the Middle East with Mousterian tool types “are quite a mess,” says archaeologist Lawrence Straus of the University of New Mexico in Albuquerque.

Shea also gives the boot to the oldest tool industry, the 2.6-million-year-old Oldowan. And he shoots down the Acheulean industry, best known for teardrop-shaped stone hand axes dating to as early as 1.7 million years ago. In a college textbook to be published in 2016, he suggests a new system based on identifying core toolmaking techniques rather than looking for similarities in artifacts’ shapes or reconstructing steps presumed to have been needed to make tools. Stone tool use, he proposes, developed from 2.6 million years ago onward based on nine ways in which rock can be fractured and ground into tools. One example consists of cores and flakes that have been placed on hard surfaces and pounded with another stone to detach smaller pieces of rock.

Analyzing stone artifacts from more than 50 ancient sites using this approach, Shea concludes that new ways of making stone tools were added over time, but few were permanently abandoned. As Homo species evolved an upright stance capable of long-distance travel after 1.6 million years ago, tools got lighter and were made for easy transport and multitasking, much like Swiss army knives.

If old toolmaking methods hung on as new techniques emerged, then the influential idea that humans progressed from “primitive” to “modern” behavior, as reflected in their stone tool industries, is wrong. That idea has led some researchers to contend that H. sapiens didn’t evolve brains capable of “fully modern” behavior until around 50,000 years ago.

Nonsense, says Shea. Specialized tools didn’t require specialized brains. Unlike previous hominids, early H. sapiens devised tools that were increasingly tailored to survival, including grinding stones and arrowheads. These innovations added to what was already known about toolmaking in populations that were expanding in size and becoming better able to transmit knowledge quickly. “The easiest way to be wrong [in evolutionary research] is to underestimate our ancestors’ abilities,” Shea says.

Reconstructionists

Our ancestors’ knack for learning and sharing toolmaking techniques has been underexplored and underestimated, says anthropologist Gilbert Tostevin of the University of Minnesota in Minneapolis. Influences of Stone Age Middle Easterners on Central Europeans’ toolmaking practices are starting to come to light thanks to a new way of studying stone artifacts, he asserts.

Like Shea, Tostevin rejects stone tool industries. Instead, he proposes a new way of organizing artifacts that might help reveal more about the transmission of one tool manufacturing style from one group to another. Over the last 50 years, researchers, especially in Europe, have sought to reconstruct how stone tools found at different sites were made. After defining what they regard as the steps involved in transforming a piece of rock to a finished tool — known as an operational chain or reduction sequence — these investigators lump artifacts into general toolmaking categories. Like NASTIES, those categories say nothing about how toolmaking practices spread from one ancient population to another, Tostevin says.

As an avid flintknapper, or maker of stone tools, Tostevin consulted his own and others’ studies on how people today pound and chip rocks into spearpoints and other objects. Tostevin got his first flintknapping lesson as a Harvard graduate student from Shea, an experienced flintknapper who was hired to show Tostevin’s class the basics of stone toolmaking.

Tostevin divides the toolmaking process into five types of behaviors. Importantly for reconstructing the transmission of such methods in ancient times, these behaviors can be learned by someone standing near enough to watch and then practice the actions. Each action leaves telltale marks on stone artifacts, Tostevin says.

To create a spearpoint or any other stone tool, a seasoned flintknapper first orients a piece of stone, or core, in a desired direction and trims it in specific ways. Second, the toolmaker decides how to deliver a blow with a hammerstone — including the angle of the blow and the spot to strike — to remove a suitably shaped chunk. Third, a flintknapper determines the direction from which to remove a desired chunk from a core. One possibility: Strike from the center outward. Fourth, steps are taken to shape a detached chunk, or flake, to make it thinner or rectangular, for example. Finally, edges, points and other tool features are further shaped and sharpened.

In a 2012 book titled Seeing Lithics, Tostevin described how he measured and compared markers of such learnable toolmaking actions among stone artifacts from 18 sites in Central Europe, Eastern Europe and the Middle East. Finds dated to between 60,000 and 30,000 years ago, a period during which Neandertals gave way to H. sapiens.

Unexpectedly, Tostevin found stark differences in how Middle Eastern tools from early and later stages of that time period were made. Researchers have traditionally regarded those tools as part of a single stone tool industry.

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Instead, Tostevin concludes, a common cluster of toolmaking techniques unites Middle Eastern artifacts from around 47,000 years ago with those made in Central Europe 41,000 years ago and in Eastern Europe 38,000 years ago. These similarities, seen in triangularly shaped stones with sharp points and other tools, could have arisen only by direct contact between Middle Eastern and European toolmakers, Tostevin holds.

After 38,000 years ago, those Middle Eastern–inspired tools appear to have hit a dead end in Europe. Around that time, rectangular flakes that received extra sharpening became the popular tool among European H. sapiens. For more than a century, archaeologists have placed these finds in the Aurignacian tool industry.

Based on further analyses presented in Honolulu at the 2013 annual meeting of the Paleoanthropology Society, Tostevin suspects Middle Eastern humans or Neandertals made triangular stone darts for spear-throwers that for some reason later got replaced by rectangular Aurignacian stone darts. “We need to figure out why only the Aurignacian industry went viral in Europe,” he says.

Across North Africa

Tostevin’s division of stone toolmaking into learnable parts has inspired a new take on one of the most controversial issues in evolutionary studies — how and when humans spread out of Africa.

Various H. sapiens populations moved east across North Africa and into the Arabian Peninsula between 130,000 and 75,000 years ago, says archaeologist Eleanor Scerri of the University of Bordeaux in France. After analyzing ancient temperature and rainfall data, she and her colleagues found that four east-west corridors containing lakes and vegetation ran through North Africa’s vast Sahara Desert at that time.

Comparisons of 4,700 stone artifacts from 17 ancient North African sites, based on measures of Tostevin’s five learnable toolmaking behaviors, indicate that related ways of modifying rock spread eastward through each of those geographic corridors, Scerri and her colleagues reported in last October’s Quaternary Science Reviews. Migrants slightly modified how they made tools as they trekked from one habitable part of North Africa to the next, she says.

Humans fanning out across North Africa eventually left the continent, Scerri suspects. Stone artifacts excavated by her team at six sites on the Arabian Peninsula, all dating to at least 75,000 years ago, were manufactured much like those found in North Africa.

North African tools from that time usually get sorted into any of three stone tool industries: the Aterian, the Mousterian and the Nubian Complex. But the three industries lump together too many artifacts to detect distinct clusters of tools associated with the continent’s ancient geographic corridors, Scerri says. In her view, Tostevin’s approach combined with models of ancient climate and habitat change can track the spread of toolmaking techniques, and therefore, populations, across great distances.

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New School

John Shea’s approach to studying stone tools focuses on technique. Categories are based on nine toolmaking operations. Three are shown below. This proposed approach would replace the longstanding practice of sorting finds into stone tool industries.

Credit: Photos by J. Shea, J. Pargeter; illustrations: used with permission by J. Shea/copyright reserved, adapted By E. Otwell

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Her evidence challenges a 2013 proposal, published in the Proceedings of the National Academy of Sciences, that humans first left Africa in a big way around 60,000 years ago, via a coastal route from East Africa to Arabia and then South Asia. A team led by archaeologist Paul Mellars of the University of Cambridge found similarities linking stone tool industries in those regions.

“One migration out of Africa 60,000 years ago is an appealing story, but human dispersals were more complicated than that,” Scerri says. Instead, her linkage of ancient wet zones to particular stone tools suggests that more than 75,000 years ago human groups shuttled back and forth across habitable parts of North Africa and Arabia, adjusting toolmaking styles along the way.

On those journeys across sparsely populated landscapes, mating between different human groups would have been sporadic, Scerri contends. If so, restricted breeding among spread-out groups dampened genetic diversity. That’s something for paleogeneticists to keep in mind, she adds, since minimal differences in ancient Africans’ DNA could mistakenly be read as a sign of a dramatic population decline. Ancient DNA can illuminate evolutionary connections between populations but not the history of each population’s travels and encounters with outsiders, Scerri says. That’s where a revitalized archaeology comes in.

Alien tools

Like Scerri and her colleagues, Pennsylvania’s Dibble would love to see improvements in how archaeologists analyze stone tools. But he’s skeptical of both new and traditional approaches. People today have no idea what ancient humans or other hominids considered proper toolmaking techniques or finished tools, Dibble contends.

“The alien nature of stone artifacts makes them difficult to study,” he says.

Shaped rocks from Western European Neandertal sites provide a case in point. Analyses of these finds and their points of origin indicate that Neandertals carried resharpened triangular points, irregularly shaped lumps of rock and various other artifacts across distances sometimes exceeding 100 kilometers.  A team led by archaeologist Alain Turq of France’s National Museum of Prehistory reported the finding in 2013 in the Journal of Human Evolution. The team used microscopic evidence to trace artifacts from 27 Neandertal sites, mostly in France, to more than 1,000 rock sources in the general area.

Many of those items are not regarded as tools by archaeologists, the scientists found. Neandertals often transported small, irregularly shaped flakes that attract little scientific attention, as well as the hand axes and large, symmetrically shaped flakes long cherished by investigators.

Turq’s group also found that traveling Neandertals picked up naturally broken pieces of flint that display evidence of having been used for cutting and scraping. These close relatives of H. sapiens had no hard-and-fast rules for how stone tools were supposed to look, the researchers concluded.

To further complicate matters, laboratory experiments led by Dibble over the last two decades demonstrate that there are many ways to make the same stone tool. Making a tool the easy way might represent an advance over making it the hard way, in line with Shea’s argument that there was no inevitable progression toward mental complexity in human evolution. 

Dibble’s team attaches hammers made of synthetic bone and other material to a pneumatic cylinder. This device thrusts hammers into molded pieces of rocklike glass to detach flakes. The researchers can control for many factors that influence what a flake looks like, including core shape and the speed and angle of hammer strikes.

Dibble’s group finds that thin, elongated flakes — thought by some researchers to reflect surges in human intelligence and toolmaking skills starting about 50,000 years ago — can be produced fairly simply by adjusting the angle of a platform on which a core rests and striking the core with a relatively soft bone hammer. Those results appeared in the June Journal of Archaeological Science. 

Present-day flintknappers have worked out a number of possible ways in which hand axes, cores and other artifacts can be made. But ancient toolmakers probably employed strategies that people today can’t intuitively re-create, says Dibble, himself an experienced flintknapper.

“Flintknappers want good-quality rock and a good-looking product,” he says. “Ancient hominids made tools as quickly and as easily as possible to get the job done.”

Industry standards

Despite getting hammered by critics, traditional stone tool industries still find favor among some researchers.

When large numbers of artifacts with characteristic shapes are found in a restricted area, it makes sense to place those finds in a tool industry, says archaeologist Jeffrey Rose.

Rose, of the Ronin Institute in Montclair, N.J., and archaeologist Anthony Marks of Southern Methodist University in Dallas have identified five previously defined tool industries made by H. sapiens groups that intermittently occupied the Arabian Peninsula between about 106,000 and 50,000 years ago. Signature methods of working stone into long points and other objects appear among tens of thousands of artifacts excavated at more than 250 sites, the researchers conclude in the 2014 Quartär , an annual German archaeological journal.

The oldest Arabian artifacts resemble comparably ancient tools manufactured by hunter-gatherers in North Africa’s Nile Valley, Rose says. Echoing Scerri’s argument, Rose and Marks suspect nomadic human groups moved back and forth across Africa and Arabia for at least 50,000 years, as geographic corridors dotted with lakes and ponds expanded and receded in response to climate fluctuations.

Even the much maligned Mousterian industry can’t be dismissed, contends Stanford University anthropologist Richard Klein. No formula exists for determining whether similar tools from different sites were made by separate or intermingling populations, Klein says. A handful of Mousterian stone tool types defined 50 years ago have limitations, but, in his view, researchers such as Shea and Tostevin haven’t come up with anything better.

Meaningful ways of connecting stone tools to hominid evolution are desperately needed, says archaeologist Daniel Adler of the University of Connecticut in Storrs. Adler and his colleagues have found that hominids in West Asia as well as Africa added carefully prepared stone flakes to their toolmaking repertoire around 330,000 years ago (SN: 11/1/14, p. 8). If West Asian and African populations independently learned this Mousterian technique while still producing Acheulean hand axes in a process dating back 1.7 million years, then traditional stone tool categories and their presumed links to specific hominid species “are kind of meaningless,” Adler says.

With these questions at play, understanding of prehistoric life is still beyond reach. That doesn’t bode well for future researchers who will confront artifacts far more complex than pounded pieces of rock. It makes Dibble’s tale of archaeology’s future less far-fetched. Perhaps tens of thousands of years from now, archaeologists will assign excavated 20th century cars to religious-altar industries with names such as Chevrolettian and Toyotaterian.

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This article appeared in the April 4, 2015, issue of Science News under the headline, “Reading the stones.”