Before we ever wrote on paper or even stone tablets, humanity was unwittingly writing its history in the DNA of each successive generation. Now, after tens of thousands of years, scientists reading the history written in—and omitted from—those genes are re-writing the story of how human culture began.
By combing through 100,000 years of genetic history, scientists say they have shown that modern human behaviors which arose about 50,000 years ago traveled from person to person through cultural, not genetic exchange. The genetic data also add new layers to the history of human migration out of Africa and provide a valuable medical resource. The findings were published last month in the journal Nature.
Ancient humans became a lot more like modern humanity around 50,000 years ago, developing innovations like art, fishing, and blades within a relatively short span of time, said Stanford University anthropologist Richard Klein, who noted this change in his 2002 book The Dawn of Human Culture. “[There] was a radical change in human innovative ability about 50,000 years ago,” Klein said, “and I speculated that genetic change was responsible.”
Without genetic evidence, though, others argued the change was not a sudden genetic leap but gradual cultural change. The “human revolution” was instead “a gradual assembling of the package of modern human behaviors in Africa,” argued anthropologists Sally McBrearty and Alison Brooks in a 2000 paper.
The DNA evidence was collected by sequencing the genomes of more than 300 people from around the world. To ensure that a full history of humanity was represented, scientists made a special effort to collect DNA from a diverse grouping of humans. Thus the project became known as the Simons Genome Diversity project.
With each generation, our DNA accumulates small, mostly harmless mutations, which scientists can use as a sort of marker to trace which populations the DNA has passed through.
“Our genome is not just ours,” said Shop Mallick, a computational biologist at Harvard Medical School, and a co-author on the paper. “Your genome contains signals from all of your ancestors.”
By measuring how different the DNA of people from disparate populations are, the scientists could answer one critical question: “How far back in time do you have to go before the populations that two samples came from are linked?” Mallick said. For many populations in the study, enough mutations had occurred since their last genetic exchange that their last common ancestor must have been at least 100,000 years ago.
This paints a picture of humanity 50,000 years ago as a species consisting of sparsely separated geographical groups with little genetic exchange between them. If the groups were genetically separate before the “human revolution” and before the migration out of Africa, it is extremely unlikely that all of humanity underwent the same genetic change at the same time.
The “human revolution,” the study says, was a cultural one.
Examining this history didn’t just lead to a picture of humanity 50,000 years ago during the “human revolution.” It also confirmed how humans left Africa—finding that everyone from Europeans to Australian Aboriginals shared DNA from a single migration. Though there may have been earlier migrations which left small traces on non-African humans’ DNA, “we can be quite sure that less than 2% of genomic DNA that may have come from an earlier expansion,” Mallick said.
Because of the diversity of the sampled populations, the data from the study could prove valuable for more modern medical purposes as well. The researchers found 5.8 million new mutations in human DNA that had never before been encountered; the list of these mutations is a valuable resource for scientists researching the genetics ailments from cancer to developmental diseases. To aid these studies, the authors have posted the data online for other scientists to use.
The resulting depository, Mallick said, “creates a snapshot of humanity as it is today.” In this DNA is a sort of family photo of the seven billion of us—one that both hints at our past and could aid in a healthier future.