On early human migration, geography and culture

By Kathleen Holder

Our species, Homo sapiens, left Africa earlier than previously thought and our diverse cultures have been heavily influenced by geography, according to a recent review by Alexander (Sandy) Harcourt, professor emeritus of anthropology at the University of California, Davis.

The paper grew out of a keynote address to a National Academy of Sciences colloquium in Irvine earlier this year on comparative phylogeography, the study of the geographic distribution of species (Watch a video of Harcourt’s lecture below).

Social scientists and biologists use a wide array of methods to uncover the tracks left by early humans on their migration from Africa to the far reaches of the globe—dating fossils and artifacts, tracing the development of languages, cultures and technology, and analyzing DNA.

Harcourt sifted through more than 150 scholarly publications to get a big-picture view of how and when Homo sapiens spread around the planet. He concluded that humans appear to have begun migrating around the globe earlier than previously believed. And, he found, geography did more than guide their journey around mountains and across ocean. Environment also shaped a multitude of cultures of billions of people around the world today — all descended from a few hundred intrepid people who left Africa about 125,000 years ago.

“Environment influences cultural diversity in the same way as it influences biological diversity or taxonomic diversity,” Harcourt writes in “Human Phylogeography and Diversity,” published in the July 19 issue of the Proceedings of the National Academy of Sciences.

Harcourt, who spent most of his career researching gorillas and other nonhuman primates, has written two books on the spread of humans —Humankind: How Biology and Geography Shape Human Diversity, released in 2015 for a general audience, and Human Biogeography, a scholarly text published in 2012.

Earlier out of Africa

Until recently, scientists believed that the first wave of human migrants out of Africa — blocked or forced to retreat by an approaching ice age and a drying climate — got no farther than what is now southeast Arabia until 60,000 to 70,000 years ago.

However, Harcourt said that recent archeological finds could push dates for Homo sapiens’ arrivals to between 80,000 to 120,000 years ago in China and as early 60,000 years ago in Australia.

Harcourt reports that modern humans arrived in northeast Siberia 32,000 years ago and reached the Americas possibly 20,000 years ago. Arctic North America was occupied about 5,000 years ago, the mid and eastern Pacific Islands in the past 1,000 to 2,000 years, and New Zealand a mere 700 years ago.

Humans distributed themselves around the world unevenly, concentrating in fertile equatorial tropics where there were plenty of animals, plants and habitats to support a “dense packing of cultures.”

Genetics reveals trails of men and women

They also spread in sometimes surprising ways, with men and women occasionally leaving different trails in regional gene pools — “almost,” Harcourt said, “as if they were different species.”

For instance, Y-chromosome genes, which are passed from father to son, show the legacy of Mongol invaders across Asia and Eastern Europe. Similarly, yDNA in western Iceland, as well as the Icelandic language, reflects Viking roots. However, the female genetic profile in western Iceland indicates Gaelic British origins. “The first millennium’s Viking raids into western Britain and the abduction of women from there to western Iceland would explain that pattern,” Harcourt said.

In agricultural communities, however, land-owning males tended to stay put while females moved when they married.

Human history, in addition to environment, has left lasting imprints on the distribution of people, Harcourt noted. The rise of wealthy cities, for instance, has created some of the world’s most culturally diverse populations — in effect, replicating the productive conditions of the tropical regions where humans first flourished.

Conquests and the spread of disease, on the other hand, have led to the disappearance of entire cultures and languages.

One example, Harcourt said, provides a cautionary tale—tropical South America, which has fewer cultures than tropical Asia or Africa. “One reason might be the relatively short period that humans have been in the American tropics. However, another could be the devastating effect of the Old World diseases (smallpox, for example) brought to the New World by invading Europeans from the 15th century on.”

The mass mortality of Native Americans, he concludes, “provides a foretaste, perhaps,” of the global spread of tropical diseases today.

More information: Watch Harcourt’s lecture

Kathleen Holder writes about the Social Sciences for the UC Davis College of Letters and Science. Follow her at @kmholder

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