About the SA Blog Network

Talking back

Talking back

A science blog, sans blague
Talking back Home

Out of Africa: Startling New Genetics of Human Origins

The views expressed are those of the author and are not necessarily those of Scientific American.

Email   PrintPrint

Western Pygmies

I love population genetics for its ability to peer back into human history through the medium of DNA’s ATCGs.

One of the stars of this discipline is Sarah Tishkoff, a standout in African genetics, someone who will readily haul a centrifuge into the bush in Cameroon.

Tishkoff of the University of Pennsylvania is lead author on a paper published online July 26 in Cell that details  whole-genome sequencing of five individuals each from three extant hunter-gatherer groups—the Pygmies of Cameroon as well as the Hadza and the Sandawe of Tanzania. The results reveal millions of newly discovered genetic variants—differences in single genetic letters, the ATCGs—and indicate that early modern humans may have interbred long ago in Africa with another species of hominid (although the fossil record does not provide much support for the latter finding).

Tishkoff answered a few questions for us about this paper, co-authored with Joseph Lachance and 11 other researchers. An edited version of the interview appears below:

Please describe the research that led to the paper that was published today:

We’re the first ones to look at these diverse groups of hunter-gathers in Africa who descend from some of the most ancestral lineages in the world. They’re  interesting because they have very unique and distinct lifestyles There are few  populations that maintain this active hunter-gatherer lifestyle.

This is the most extensive study in Africa using high-coverage deeply detailed sequence data. We focused on three groups because they’re  anthropologically interesting. They’re thought to be descended from groups that are ancestral to all modern humans. We wanted to understand the genetic basis of adaptation to their local environment  including, for instance the short stature trait in Pygmies.

So what did you find?

We discovered 13 million variants and, of those variants, greater than 3 million are completely novel, meaning that they have not been reported in any database. The current public database has 40 million variants. So we found 3 million novel variants by simply sequencing 15 individuals. That increases by about 8 percent all known human genetic variation. It also demonstrates that we’re missing a lot of really important variation that’s out there, particularly in Africa, which is the  homeland of modern humans and a place where there’s been a lot of time for differentiation to have occurred in very diverse environments. What this means is that there’s s probably a lot of regional or population-specific variation out there that has not been that well characterized, some of which is functionally very important.

What about natural selection?

Natural selection seems to be operating more on the non-coding genome [the regulatory portion that does not contain genes] than the coding region. A lot of people are doing exome sequencing [looking only at genes]. I think they’re missing a lot of important variation.

In our study, we looked at what regions of these groups’ genomes were uniquely differentiated to their local environments. There wasn’t a huge amount of overlap between the groups—or between them and other non-hunter-gatherer groups from Africa.  Due to natural selection, we found there were distinctive adaptations for  immunity, taste and smell.

In the Pygmies, we discovered genes involved with thermal regulation, immunity and stature, all likely to be adaptive to a tropical environment. We pinpointed genes related to pituitary and thyroid function, the latter perhaps an adaptation to a low-iodine environment.

In the Sandawe, we found a variant for  melanin, a gene involved in skin color.  The Sandawe are among the most fair-skinned groups in Africa. When I went to work with them, they said, ‘We’re like brothers and sisters because you look like us.’ This is not because of any European admixture; they look like the San [a hunter-gatherer group from southern Africa]. When I said: ‘Where do you come from, they pointed to a mountain in the distance. When I said ‘Can you take me there?” we went but there was no road. We went through the bush and they showed me cave paintings. Having lived in South Africa, I’ve seen the cave paintings of the San.

What about interbreeding with other human species?

A number of studies have shown a low amount of interbreeding between early modern humans outside of Africa and archaic species outside of Africa including Neandertals and, in Asia, with the species they call Denisova.They’ve never found any evidence of Neandertal DNA in Africa. The problem is that you just don’t get good preservation of fossils in Africa.  So what we did was collaborate with Josh Akey and Ben Vernot at the University of Washington and used a statistic they developed to recognize regions of genome that appear to be of archaic origin.

The first thing we did is to test this statistic by applying it to non-Africans and we found a very strong enrichment for Neandertal DNA in those genomes. But we didn’t see that in the Africans. They had no Neandertal DNA. When we applied the statistic to Africans, though, we still saw a lot of evidence for interbreeding from a hominid who diverged from a common ancestor that we shared about 1.2 million years ago, about the time that Neandertals split off as well. This suggests that there could have been a sister species in Africa. What it was nobody knows. But it seems to show that modern humans have been interbreeding and it’s not unique to non-African species.

Why are African genetics so exciting?

Africa was the site of origin of all modern humans and if you want to learn about when, where and how we evolved, you want to look at this continent. It has a long history of population subdivision and adaptation of those populations to very distinct environments and a broad range of phenotypes, ranging from the short stature of the Pygmies to the  very tall stature of the pastoralists in the east. It also has very different disease exposure and very different disease prevalence throughout.

What’s next?

We want to expand our genome-wide analysis to other populations, and we want to do so with larger sample sizes. We’re going to continue to try to correlate genetic variants with different phenotypic traits. We’d love to do functional studies of these genes to see, for instance, how they are regulating pituitary development. Is there some totally novel mechanism involved. We’re going to look at the Pygmies and other groups with a systems approach. You can’t look at height, as an example, by itself. You have to look at it in relation to metabolism and immunity and see how everything interacts.

Image source: Sarah Tishkoff


















Gary Stix About the Author: Gary Stix, a senior editor, commissions, writes, and edits features, news articles and Web blogs for SCIENTIFIC AMERICAN. His area of coverage is neuroscience. He also has frequently been the issue or section editor for special issues or reports on topics ranging from nanotechnology to obesity. He has worked for more than 20 years at SCIENTIFIC AMERICAN, following three years as a science journalist at IEEE Spectrum, the flagship publication for the Institute of Electrical and Electronics Engineers. He has an undergraduate degree in journalism from New York University. With his wife, Miriam Lacob, he wrote a general primer on technology called Who Gives a Gigabyte? Follow on Twitter @@gstix1.

The views expressed are those of the author and are not necessarily those of Scientific American.

Rights & Permissions

Comments 4 Comments

Add Comment
  1. 1. Descarreaux 8:50 pm 07/27/2012

    Mr. Stix

    It is an interesting article.

    It reminds me this question.

    Is it possible that interbreeding between Neanderthalensis and Sapiens were only between neanderthal males and sapiens females?

    It would explain why the mitochondrial eve does’nt have any neanderthal heritage.

    Thank you

    Link to this
  2. 2. Ahcuah 9:23 pm 07/28/2012

    We focused on three groups because they’re anthropologically interesting. They’re thought to be descended from groups that are ancestral to all modern humans.

    Um, aren’t we all “descended from groups that are ancestral to all modern humans”?

    Link to this
  3. 3. voyager 9:53 pm 07/28/2012

    English major here, amazed there’s been no comment. I wanted to learn more, especially about this: “Natural selection seems to be operating more on the non-coding genome [the regulatory portion that does not contain genes] than the coding region. A lot of people are doing exome sequencing [looking only at genes]. I think they’re missing a lot of important variation.”

    I realize it’s not the central point of the thesis, but it’s another bit of input to the conundrum that the much-vilified but hardly dismissed Dr. Behe put forward: that the multiple recorded studies of the rates of mutation, given the limited time life has existed here, can’t account for the complexities that life has achieved through purely random mutation. The virulence and ad hominem nature of the vilification isn’t matched by its substance, far as a layman can see.
    Dr. Behe is explicit in not positing a divine intelligence, but he does posit a guided process, an external force, at least at the outset of the Universe.
    But isn’t there a third option? For example, that some early and purely selective adaptations themselves, not yet observed as such, play a ‘guiding’ hand that takes much of the randomness out of ‘random?’

    Junk DNA, previously dismissed as a puzzling inefficient remnant, seems now to play not merely a significant but a major role in genetic reproduction (or is it coding?). Is the material referenced in the quote above junk DNA, or is it yet another source of possible feedback?

    Epigenetics looks like a potential bridge between individual experience and genetic coding, another short-cut to success.

    Can things like this amount to a possible justification of both Dr. Behe’s math and his critics’ defense of uninspired mutation? Or is the idea just the rambling of a non-scientist.

    Link to this
  4. 4. Donzzz 10:18 pm 08/1/2012

    Anthropology: What happened to our earlier ancestors?

    All modern humans “Homo Imaginative Sapiens” have descended from a single tribe living in Africa. For some mysterious reason even though they had remained on the same intellectual level for thousands of centuries between seventy and one hundred thousand years ago the children of this little tribe, began to become imaginative. The children in this little tribe began being “transformed” into imaginative beings. These special children began to become smarter then their parents. This never happened before!

    The “transformation” was a very gradual process. The children as they grew up began to become more curious. They began to try to solve problems of things that bothered them. They were never quite satisfied with their status quo and always wanted to improve their conditions, especially if they were challenged. Better tools to hunt with and better weapons to fight with.
    This is still going on today. As adults they began to create and improve their tools, trinkets, etc. they began to progress. It happened very slowly over a period of many generations. A new powerful dimension, “Human Imagination” was being intoduced to the world. This new dimension, mankind’s “Special Gift” has enabled mankind to adapt to conditions that had been overwhelming to our distant cousins. “Human Imagination” is mankind’s defining trait.

    Link to this

Add a Comment
You must sign in or register as a member to submit a comment.

More from Scientific American

Email this Article