Our species made its debut some 300,000 years ago. During the preceding millenniums, our continent of origin underwent environmental shifts that very likely influenced the trajectory of human evolution. Archaeologists working in Kenya have uncovered new clues to support this assertion, showing the surprising extent to which climate change influenced the behaviour of early humans and their approach to technology.
On the left, old-timey stone tools of the Acheulian, and on the right, the sexy new compact tools of the Middle Stone Age. Image: Human Origins Program, Smithsonian
Dramatic climate instability in east Africa, starting around 360,000 years ago, had a pronounced effect on human evolution, but also on human culture, according to three new studies published this week in Science. As the African landscape changed, so too did the animals who lived upon it. This forced early humans to spread out, establish trade routes and construct innovative new tools in order to adapt and survive. Ancient climate change, according to this view, changed both our biology and our behaviour. What's more, these cultural shifts happened tens of thousands of years earlier than what was suggested by earlier archaeological discoveries.
Anatomically modern humans, otherwise known as Homo sapiens, emerged just prior to the onset of the Middle Stone Age, a period that lasted from about 280,000 to 40,000 years ago. Before the Middle Stone Age, humans lived in the Early Stone Age, an era characterised by the popular Acheulean stone handaxe. The new studies explore the environmental, ecological and technological changes that happened in East Africa during this critically important transitionary time in human evolution. All three studies focused on excavations done at the Olorgesailie Basin in southern Kenya, a region that, for the past 75 years, has yielded artefacts dating as far back as 1.2 million years ago.
The first study, led by Richard Potts from the Smithsonian Institute's Human Origins Program, looked at well-preserved sediments pulled from Olorgesailie, a rift that extends for 65 square km. Around 360,000 years ago, this region started to change, converting from a floodplain to a region that fluctuated wildly between very wet and very dry conditions. Eventually, the basin transformed into a gigantic grassland. It was this environmental shift, says Potts, that set the stage for the Middle Stone Age and the emergence of our species.
The shift from floodplain to grasslands had a profound effect on the flora and fauna that lived there, and thus the humans who depended on both. Climate change resulted in a changing of the guard, especially among the grazing animals. A number of elephant and horse species went extinct, replaced by animals that were significantly smaller in size, such as the springbok antelope.
For early humans, this represented an inconvenient truth: Access to food was becoming highly unpredictable. To adapt, the hunter-gathering clans had to disperse and spread out, gather information, and, in the words of the researchers, make an "investment in social resource exchange networks" - in other words, they had to start trading goods with other clans. These adjustments may have been disruptive, but together, they promoted foraging efficiency, reduced risk, and improved the overall "fitness" of the species, according to the research.
"This change to a very sophisticated set of behaviours that involved greater mental abilities and more complex social lives may have been the leading edge that distinguished our lineage from other early humans," said Potts in a statement.
Archaeological evidence collected in Kenya bears this out. Prior to the environmental shifts and the onset of the Middle Stone Age, humans living in the Olorgesailie Basin made stone tools from rocks collected in the area; some 98 per cent of these tools, namely Acheulean handaxes, were made from stones found no further than 5km away. But this changed entirely by about 320,000 years ago, as evidenced by the presence of tools and other artefacts made from obsidian, chert (a coloured stone) and quartzite - materials that came from far away. This means trade and long-distance travel had likely become an established part of human life.
"This represents a significant revision in African hominin behaviour at or near the time of origin of Homo sapiens," write the researchers in the study.
The second study, led by Alison Brooks from George Washington University, is an extension of the first, providing additional details about the artifacts found buried in the Olorgesailie Basin. Brooks' team analysed stone tools, weapons and pigments found at five different sites dated to between 500,000 and 298,000 years ago, looking for signs of technological development and trade networks. This approach allowed the researchers to see how the tools changed over time.
At the older sites, the tools were larger and bulkier, with handaxes manufactured from volcanic rock found in the region; for the Acheulian culture, this was the way of things for hundreds of thousands of years. But at the younger sites, dated to between 320,000 to 305,000 years old, the tools suddenly became smaller - more compact - and featured entirely new designs. Unlike the single-use handaxes, the new tools of the Middle Stone Age were highly specialised and crafted with care. The stones were knapped to a fine point, and likely attached to the end of a spear for use as a projectile weapon to hunt both large and small game. Other tools were forged into scrapers or awls.
Revealingly, slightly less than half of the artefacts found at the younger sites were made from obsidian, for which there was no local source. Obsidian is a hard and brittle volcanic glass that produces very sharp edges when fractured. This was a highly valued tool, and quite literally the "killer app" of its day. The closest sources of obsidian to Olorgesailie were located more than 25-50km away, suggesting trade or long-distance travel. The researchers also found no less than 46,000 obsidian flakes at the site. This means that obsidian was delivered to Olorgesailie as a raw material, and not imported as a finished product; the humans who lived at Olorgesailie knapped the obsidian themselves.
In addition to the obsidian, another commonly imported resource was brown or white chert. In a really cool discovery, the researchers uncovered a lump of red ochre pigment with a pair of holes punched into it, and it's now one of the oldest ochre artefacts ever found. The archaeologists speculate that colouring pigments were used to denote identity or status, among other possibilities.
"We don't know what the colouring was used on, but colouring is often taken by archeologists as the root of complex symbolic communication," Potts said. "Just as colour is used today in clothing or flags to express identity, these pigments may have helped people communicate membership in alliances and maintain ties with distant groups."
Taken together, "this evidence indicates that distinctive technological features of the African Middle Stone Age reflecting innovation, standardization, and new cognitive abilities were already developed in eastern Africa before 300,000 years ago," write the authors in the new study. Also, the establishment of trade suggests a new behaviour in the human repertoire: "the formation of networks of exchange or procurement over a significant area."
The third study, headed by Alan L. Deino from the Berkeley Geochronology Center, tied everything together by dating the samples and artifacts found at the Olorgesailie sites. Using argon and uranium dating techniques, Deino's team confirmed that the larger tools belonged to the older Acheulean era, and that Acheulean tech started to disappear around 320,000 years ago, replaced by Middle Stone Age tools and weapons. "These results establish the currently oldest repository of [Middle Stone Age artifacts] in eastern Africa," write the researchers in their study.
Jean-Jacques Hublin, a professor at the Max Planck Institute for Evolutionary Anthropology who's not affiliated with the studies, says these new papers confirm a new picture that's emerging, one displaying a "synchronicity" between the evolution of physical traits in humans and the onset of modern behaviours, beginning more than 300,000 years ago in Africa.
"Long it was believed that 'anatomical modernity' developed well before 'behavioural modernity,' Hublin told Gizmodo. "I find it striking that the gradual changes that my team described between [ancient] Homo sapiens and recent modern populations, especially regarding brain evolution, are now chronologically well matched by the changes observed within the African [Middle Stone Age]."
Hublin is referring to a Nature study he co-authored last year showing that Homo sapiens made their debut 300,000 years ago, and not 200,000 years ago as previously assumed. As Hublin points out, these new studies mesh rather nicely, showing the emergence of not just a new kind of hominid, but an entirely new approach to technological development and social organisation - one triggered by climate change.