Christopher Henshilwood / University of the Witwatersrand
Bifacial points recovered from Blombos Cave, South Africa. The tools were manufactured during the Middle Stone Age by anatomically modern humans and are made of silcrete and finished by pressure flaking. Scale bar = 1cm
Sophisticated stone tool-making, artistic symbolism and trade networks were all innovated during times in the Stone Age when the South African climate abruptly became warmer and wetter, according to a new study.
The research is the first to "show that there is a link between the occurrence of these cultural innovations and climate change," study leader Martin Ziegler, an earth science researcher at Cardiff University in Wales, told NBC News.
South Africa got warmer and wetter as the Northern Hemisphere became cold and dry during periodic Ice Age slowdowns in an ocean circulation that brought warm water from the tropics north, he added.
This allowed warm and wet conditions to prevail in South Africa for centuries to thousands of years at a time between 100,000 and 40,000 years ago, according to the study published Tuesday in Nature Communications.
The findings are based on analysis of marine sediments dumped into the ocean from rivers flowing off of South Africa, which Ziegler and colleagues used to reconstruct climate variability over the past 100,000 years.
"There is a very good fit between rapid climate change and the occurrence and disappearance of these first evidences of modern behavior in early humans," he said.
Abundance breeds innovation
Humans need water. Plants need water. So too do the animals that humans hunt and eat. These conditions thus are favorable for population growth, explained Chris Stringer, an authority on human origins at London's Natural History Museum, and a study co-author.
Modeling research from other scientists, Stringer noted, suggests that as human population density increases, people are able to network more readily, share ideas and invent technologies. The new findings, he said, fits with the idea that population density breeds cultural innovation.
"Those dense populations are forming networks over the landscape which is no longer huge patches of arid land that they cannot cross," he told NBC News. "They are connecting with other populations and lo and behold … we get these cultural innovations."
Innovations from the time include "an explosion of what seems to be symbolic behavior," Stringer noted, such as messages written in ochre, a type of pigment, and seashell jewelry perhaps used to establish social rank.
Sophisticated stone tools with adhesives that require complex processing of materials gathered over a broad area suggest trade networks existed.
"We find that stone tools raw materials are traveling sometimes hundreds of kilometers [making] it likely that there are trading networks between different groups passing these materials backwards and forwards," Stringer said.
The findings "support our view, which is that it is population density that is really driving innovation and connectedness," Mark Thomas, a geneticist at University College London, who led the earlier modeling work but was not involved with the new research, told NBC News.
Other proposed drivers for cultural innovation include genetic mutations that re-wired the human brain, and necessity driven by worsening environmental conditions, he noted.
"We say necessity is the mother of invention," Thomas said. "I'm not sure it is. I think the first response to necessity if you haven't got the invention is dying."
Lessons from the past
According to Ziegler, the finding of this link between climate and cultural change adds to a growing list of studies that indicate cultures from human ancestors to the Maya have been affected by shifts in the climate.
"So it is another hint for us that we should keep an eye on the climate because when it is changing abruptly and largely in the past it has always affected humans and so it may do so in the future as well," he said.
Stringer added that the human population today linked by global trade, social networks such as Facebook, and rapidly evolving technologies such as mobile phones are the fruits of a climate that has been relatively stable for 11,000 years.
"It is that stability of climate that has allowed our populations to thrive and grow and build on ideas and innovate in a way that is far ahead of anything that our ancestors were able to achieve with smaller numbers," he said.
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.