A Nilotic-speaking pastoralist from Tanzania / Sarah A. Tishkoff

Genes are just one component that children inherit from their parents. Throughout much of human history, especially when populations consisted of small hunter-gatherer groups, the language and lifestyle of a people were also inherited from generation to generation. This is why genetic patterns and cultural traits are often correlated. So, when scientists see cultural similarities between two populations, they can ask whether there are genetic similarities between the two groups as well. For many cultural traits, such as pastoralism and agriculture there is still a debate: did people actually migrate into new regions, bringing their genes and culture with them, or did the language and lifestyle simply spread by word of mouth to new lands?

In this week’s Proceedings of the National Academy of Sciences, scientists from Stanford University (several of whom are also associated with 23andMe, including myself) have used the principle of genetic and cultural exchange to find the first genetic evidence of a prehistoric migration of people from Tanzania to southern Africa. We discovered a mutation (aka ‘SNP’) on the Y-chromosome that originated about 10,000 years ago in eastern Africa and is now most common among people from two regions: Tanzania and southern Africa.

Pastoralists (people who rely heavily on animal husbandry for food) such as the Datog and Burunge of northern Tanzania carry the newly discovered SNP. In fact, it is present among 30-40% of men from these populations. Unexpectedly, the click-speaking Kxoe of southern Africa carry the same SNP at similar levels to the Tanzanian populations, indicating that these people are closely related to the Tanzanian pastoralists. The evidence indicates that men from southern and eastern Africa shared very recent common ancestors between about 1,200 and 2,700 years ago.

With this genetic evidence in hand, we then turned to archaeologists to see if the fossil record indicated an ancient migration around this time.

As it turns out, the current thinking among archaeologists is slightly different than what this new genetic evidence has revealed. Archaeologists currently favor a model in which the cultural practice of pastoralism spread from an unknown eastern African group into southern Africa about 2,000 years ago, perhaps without any sort of movement of people (i.e. genetic exchange). Our new genetic study, while still supporting the archaeological record for the timing and place of the origins of pastoralism in sub-Saharan Africa, puts a new twist on the current thinking. It suggests that a small group of men actually migrated into southern Africa about 2,000 years ago. These men probably married into local hunter-gatherer populations, contributing their livestock and cultural knowledge of pastoralism. These migrants were probably closely related to the modern day Datog and Burunge groups of Tanzania.

A shift to pastoralism was a fundamental change for the hunter-gatherers of southern Africa during the last couple thousand years. It caused a dramatic change in the culture and belief systems of these people. As pastoralism became more widespread in southern Africa, so did the beginnings of a sense of ownership of animals and the emergence of chieftans. These changes can still be seen today in the practices of people throughout Namibia, Botswana and South Africa. For example, the Nama of Namibia began practicing pastoralism not long after its arrival in southern Africa and continue to do so today.

The question of whether the shift from hunting and gathering to agriculture was the result of cultural exchange or actual migrations between groups is one of the most important debates among archaeologists and geneticists. With this new genetic evidence, we think we have answered this question, at least in southern Africa. Future studies will further examine the relationhip between genes and culture, and how this relationship has influenced the genetic and cultural makeup of modern African populations.

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A lot, said population geneticist and Harvard junior fellow Sohini Ramachandran, who spoke at 23andMe this week.

Ramachandran focused on how genes spread from one continent to another, and how they vary within each region as well.

As an example, she referenced some of the research she did as a graduate student at Stanford, tracing how genetic diversity decreases as the populations sampled get geographically farther away from Africa, long considered the cradle of humanity. For the purposes of measuring genetic diversity, distance from Africa is measured along the same paths that people presumably took at they migrated around the world – over land, without crossing major oceans.

Genetic diversity in the Americas (the darker the color, the more diverse the populations).

As people migrated, Ramachandran said, genetic diversity decreased due to what she called “serial bottlenecks.” In these cases, though a fairly large number of people might reach one spot, only part of that group would move farther on. By chance, some of the genes in the population would not make it into the new territory.

For this reason, Ramachandran said, the researchers find more diversity in DNA samples from African populations than they do in samples from American populations.

Because genetic diversity varies based on who got how far, Ramachandran doesn’t just study the genetic variations across continents but between continents as well. One of the topics she looks at is how genetics could support the theories outlined by evolutionary biologist Jared Diamond in his 1998 book, Guns, Germs and Steel. One section of the book she focused on – and which she called her favorite part – was Chapter 10, “Spacious Skies and Tilted Axes.”

Diamond proposed that continental expansion was partly dependent on how the landmasses are oriented, and that humans brought crops and technology with them as they did so. For example, because Eurasia is on an east-west axis, it was easier for humans to move plants and animals along that axis because the climate was similar. Ramachandran noted that a 2005 study found that humans and domesticated crops for farming spread from east to west about 3,000-5,000 years ago in Europe.

On the other hand, Diamond said, the Americas and Africa are oriented on a north-south axis and as people traveled north and south, they would have found that plants and animals they brought along couldn’t always adapt to the seasonal changes. For example, Diamond noted, the Indians in North and Central America had no pack animals because the llamas and alpacas never moved out of the Andes. And the corn grown in Mexico needed thousands of years to adapt to the changes in growing season and day-length to make its way up north.

If this holds true on a larger scale as Diamond suggested, then there should be population bottlenecks and more distinct genetic differences between populations across the Americas latitudinally (north-south) than there are longitudinally (east-west). Ramachandran also said that the distinct genetic differences between populations across the Americas latitudinally should be greater than the distinct genetic diversity seen longitudinally (east-west) across Eurasia.

Ramachandran and her colleagues reported on the genetic diversity of the Americas last year, but she says more data is needed for this region. She also thinks that Diamond’s proposed north-south axis of orientation isn’t quite right, arguing that it should run northwest-southeast. To better study the genetic diversities of the populations across this tilted axis, she and her colleagues change the way they view the map of the world.

If the planet were a ball, the point at which it would be spinning on someone’s finger might be the South Pole. Using points such as the southernmost area of South America, South Korea or Sri Lanka as the axis of rotation on which to turn the map, Ramachandran demonstrated how she and her colleagues can look at the genetic diversities of populations in the Americas and Asia and find patterns in the genetic variations that might not be so easily seen from other perspectives.

Ramachandran said she hopes to publish her research on the populations in the Americas soon. In the meantime, her talk illustrated how studying genetics can support theories raised about human origins in other areas of study such as archeology, anthropology and history.

Image from PLoS Genetics: Wang et al, 2007

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There has been plenty of research in both genetics and archaeology recently trying to figure out how the New World was colonized. Was it by boat or via the frozen wasteland of the Bering Strait? Was it a fast trip down to South America, or did these first inhabitants take a more leisurely stroll? And when did all this happen anyway?

As each new study is published we are learning vital information on the peopling of the Americas. Ideas and theories continue to be retooled as new evidence comes to light.

This will certainly be the case with regards to a paper in the May 2008 issue of PLoS Genetics. In this article, researchers from Oxford and Cornell Universities report on a new computer model they have developed to trace prehistoric human migrations across the globe.

Using genetic information from various populations alive today, the authors estimated how those groups may be related to one another. Then they used those relationships to piece together the prehistoric movements of early humans.

As expected, their analysis showed a single migration out of Africa that eventually populated Eurasia and the Americas. However, the results for the Peopling of the Americas were more surprising.

The conventional wisdom states that the first inhabitants of the Americas came from Asia in a single wave more than 10,000 years ago. But when the authors compared the genetic data of two Native American groups (one in Colombia and one in the American Southwest) to groups in East Asia, what they found supported a two-wave migration.

The Colombian sample of Native Americans was actually more closely related to the East Asian sample than it was to the American Southwest sample. That suggests the two populations come from independent sources – and that there were at least two separate migrations of humans into the New World. Clearly, one of these migrations would have come from East Asia and made its way into South America. However, the data suggest a separate migration, probably from a different part of Asia or Siberia, came at a different time, and this time only made it to North America. This conclusion is significant, as it contradicts current theories on the topic, which argue a more constant flow of migrants from an original source somewhere in Asia.

There are still plenty of questions regarding this research, especially with regard to how it compares to the archaeological record and to previous genetic studies. The answers to these questions can only come with additional research, which, thankfully, is always forthcoming on the peopling of the Americas.

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