During this time period, two major tool-making cultures arose: the so-called Still Bay and Howieson's Poort industries.
For the Science study, Jacobs' team examined artifacts from both industries found at nine sites in South Africa.
The objects include two-sided spearheads and other tools, decorative motifs and symbols, shell necklaces, and apparent personal items—such as an engraved ostrich shell set into a water container.
These artifacts signify high-level communication, Jacobs said, because they show that early humans "were thinking beyond the functional and trying to portray a message," she said.
The researchers then dated the sites themselves, which cross several climatic and ecological zones.
The dates were compared with ice core data from Antarctica.
The team found no "unique correlation between migration and climate," Jacobs said, but localized environmental impacts may have played a role.
The findings also confirmed the fairly sudden appearance of innovative periods, which appear to have had relatively short life cycles—about 1,000 to 5,000 years.
Why and how innovation came in bursts "remains an enigma," Jacobs said.
"We don't understand how they came and even less about why they went. We don't even know what happened in between."
Chris Stringer, a paleontologist from the Natural History Museum in London, was not involved with the current research.
"It is certainly an intriguing puzzle that modern behavioral features seem to wax and wane for tens of millennia before they finally become fully established," he said.
"Small populations would have been prone to extinction or forced into relatively rapid movement or adaptation to survive, and this could have led to the regular loss of innovations," he said.
Curtis Marean, a paleoanthropologist at the Institute of Human Origins at Arizona State University, said the shortest period of innovation is very interesting.
"It suggests early modern [humans] in Africa had the ability to invent and pass on, very rapidly, highly specialized and efficient tool designs. Neanderthals did not seem to have that ability," Marean said.
This may have given modern humans an advantage over Neanderthals, which died out as Homo sapiens colonized the globe, Marean said.
Which Way Out?
In a separate study published last week in the Proceedings of the National Academy of Sciences (PNAS), researchers add to evidence for the possible routes early humans might have taken out of Africa.
Some scholars believe our ancestors took a northeastern path up the Nile Valley and into the eastern Mediterranean through the Sinai Peninsula.
Others suggest that early humans migrated from Ethiopia via the Red Sea—a southeastern route that would have been only a few miles wide when most of the water was locked up in glaciers.
The PNAS study suggests yet another possible route—north through the Sahara.
Using satellite data and geochemical analysis of fossil snails, the authors report what appear to be ancient river channels running from Libya in the Sahara to the Mediterranean Sea, indicating the area was once a lush savanna that may have provided a pathway to the Middle East.
"The work highlights the importance of trans-Sahara corridors during early human history," said Stringer, of the Natural History Museum in London.
"Further archaeological work along these corridors will be extremely important in tracking these ancient human movements."
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