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DNA TRAVELOUGUE

Story by Ellen Ternes
Portrait by John T. Consoli
Photos courtesy of Sarah Tishoff

It is in the blood of the living that Sarah Tishkoff hopes to uncover the mystery of humankind's past.

In the African bush, from the Sandawe, Hazda, Maasai and other peoples whose footprints have dusted the pathways of ancient civilization, University of Maryland molecular anthropologist Tishkoff has been gathering thousands of DNA samples that may help confirm one theory of our beginnings.

"There's a lot of evidence that modern humans--Homo sapiens--had their origins in East Africa," says Tishkoff, "and that they moved from East Africa into Asia and Europe, possibly in multiple migrations and as recently as 100,000 years ago. That would imply that we have a recent common ancestry. I'm hoping the information I find in the DNA will give us more clues about who our ancestors were and how they moved around."

The fossil has long been the codebook of anthropologists and paleontologists who dig for evidence of human history. Fossils of bones and skulls can reveal things like the age, size and stage of evolution of the once-living being. But the DNA work of Tishkoff and others may prove to be the crucial link to answers about whole populations of people, about where and when modern humans originated and the routes they took into the modern world.

DNA is short for deoxyribonucleic acid, the chemical that composes genes and that is found in the cells of blood, skin and other body fluids. DNA taken from people alive today contains sequences of the genes that ancestors have contributed for hundreds and, for the oldest populations, even tens of thousands of years. By comparing the sequences in the DNA of people from different African groups, Tishkoff can see how far back their lineage goes, where and when gene sequences were the same and when they differed, even diverging forever, possibly because one group migrated out of a valley, out of a region, out of Africa altogether.

"We're studying variation in more than 40 ethnic groups," Tishkoff says. "We want to address questions such as when and where modern humans originated, the size and structure of the ancestral human populations and reconstructing recent population migrations and relationships."

• The evolution of color vision in humans
• The evolution of humans' ability to digest milk
• How immune response developed--In 2001 Tishkoff discovered that the genetic mutation that protects against malaria developed at about the same time malaria became a major human threat.
• The connection between populations who speak a click language, the language spoken by the Africans in the film "The Gods Must Be Crazy." The DNA shows that different peoples who use click language but live hundreds of miles apart, may be related. Says Tishkoff, "We have been able to show, for the first time, that the Sandawe of East Africa and bushmen from the south of Africa and the southern bushmen of Botswana, Namibia and South Africa share a recent common ancestry, within about 35,000 years. We speculate the southern bushmen originated in East Africa, and that they both are remnants of a very old group of hunter-gatherers, perhaps the earliest ancestors of modern humans."
• African genetic diversity, including DNA samples from 200 African Americans.

Over the past three years, Tishkoff's research group and her African collaborators have collected samples from more than 3,000 people in four countries--Nigeria, Cameroon, Tanzania and the Sudan. They've built what Tishkoff calls, "one of the largest repositories of genetic samples from Africa in the U.S."

And they're not stopping there. She's applied for permits to begin collecting in Ethiopia and Kenya in the next year.

Tishkoff's molecular approach is still a fairly new kid on the block in the anthropology neighborhood. Gene sequencing has been used in anthropological studies for only about 20 years. It wasn't until 2001 that the first major milestone in sequencing the human genome in the Human Genome Project was declared successful. That sequence was done on the genomes of just a handful of individuals. Says Tishkoff: "The next major milestone is to characterize sequence diversity across global populations, to identify mutations that may predispose some people towards disease and to reconstruct the history of populations and of our species."

Tishkoff's lab in the university's Biology-Psychology Building is a model of modern high bio-tech with computers and software that analyze gene sequences and genetic data to make inferences about genetic history of populations; thermocyclers to make copies of genes; an instrument that can do quick genotypes of specific mutations in large sets of individuals and high throughput automated capillary sequencers--the same as those used at the genome sequencing project--which does 96 samples of 500 nucleotides in less than three hours.

A biomedical research does lab work in the Nigerian Medical Research Institute in Lagos during a visit by Tishkoff.

But the fieldwork it takes to get the DNA into the lab is a different matter. It can be rudimentary at best, evoking anthropological expeditions of past decades. Many of the people from whom Tishkoff needs to get samples live in remote areas of the African bush. The night's meal for many of them still depends on hunting skills.

Unlike traditional archaeological digs that stay in one place for long periods, Tishkoff's team was on the road almost every day, navigating Land Rovers long distances, sometimes on rough, unpaved roads that throw up thick coats of dust. Housing wasn't much easier. Accommodations often meant a tent or foam bed and bucket showers in a local guesthouse.

Sarah Tishkoff with young Maasai warriors in Tanzania. Their village of Endulen was one of the sites where Tishkoff drew blood samples.

Arranging to collect blood and tissue samples is a complicated process, much of it begun on the phone from College Park, setting up connections in Africa and arranging for translators to be on hand to explain what Tishkoff is doing and help get informed consent from the volunteer subjects. In her first summer in Africa, in 2001, Tishkoff wasn't sure until she arrived at each location, if she would encounter people's reluctance to give blood or cells from interior cheek lining.

Turns out "that wasn't the case at all," says Tishkoff. "Almost everyone was willing to let us take blood samples. When I returned the second summer, everyone remembered me. They pulled out the photos we had given them on my first visit. They want to learn about their history too."

Once the samples were in hand, the white blood cells, which contain the DNA, had to be separated from the red cells, then preserved to survive African heat and rugged travel conditions. That called for a centrifuge, sometimes used in borrowed labs, often used right in the field, to whirl the white cells and a preservative buffer into hard white pellets that Tishkoff could hand-carry back to the United States. Because there was no electricity in many locations, the field centrifuge had to be run by a generator powered from the Land Rover's battery.

Back in the College Park lab, the work of analyzing genetic sequences has been painstaking. The pellets have to be broken back down to DNA in a slow, thorough way that produces a clean and stable sample. So far, the team has analyzed one loop of the maternally inherited, or mitochondrial, DNA (mtDNA) from about 500 samples and done whole genome mtDNA sequencing of about 40 people.

"The whole genome sequencing is a big chore," says Tishkoff. "The mitochondrial genome is only 16,000 nucleotides in size, compared to nuclear genome inherited from both Mom and Dad, which is about 3.3 billion nucleotides.

"We're isolating about an eighth of each person's DNA for this stage of the research and saving the rest for future studies, when new techniques become available," Tishkoff says. "I hope to be able to use this DNA for the rest of my career."

There is a long-raging debate in circles of anthropologists and paleontologists about modern human's arrival on the scene. One group holds that Homo sapiens emerged coincidentally in several different places around the world. Another large and growing group, including Tishkoff, believes that modern humans originated only in Africa and moved out from there, evolving and adapting to new environments.

Tishkoff is a multi-disciplinarian--she believes it takes several different kinds of puzzle pieces to complete the big picture. What do language similarities and differences of distant peoples say about human movement? What do the fossils tell us? What is the story in the DNA?

Bantu-speaking women carry firewood near a Pygmy village in Cameroon where Tishkoff and her team collected DNA samples.

DNA research like Tishkoff's has been one of the driving forces behind the "modern humans out of Africa" theory. Molecular research done in the 1980s predicted that the ancestors of modern humans lived in Africa between 100,000 and 200,000 years ago. But few fossils of the right age had been found to back up the DNA theories. Unearthed fossils were either too old, predating Homo sapiens, or too recent to be the missing link, the first true Homo sapiens. This past summer changed that.

Tim D. White, of the Laboratory for Human Evolutionary Studies of the University of California-Berkeley, published an analysis of his team's discovery of a skull in Ethopia. It is, he reported in the journal Nature, the oldest known Homo sapiens fossil, dating back about 160,000 years ago. The fossil predates classic Neanderthals, who were still populating Eurasia at the time, which, White wrote, "shows that modern human morphology emerged in Africa long before the Neanderthals vanished from Eurasia."

Tishkoff is preparing a paper to announce results of her analysis of mitochondrial DNA she collected from peoples in East and northeast Africa. They are breathtakingly close in date to White's discovery. Tishkoff's results show that the DNA donors alive in Africa today are possibly direct descendants of those first Homo sapiens.

"We found very old mtDNA lineages, among the oldest in Africa, in several of the groups we've sampled," says Tishkoff. "Our estimated time to most recent common ancestry of the mtDNA lineages is also about 170,000 years ago, which supports White's fossil finding. These data suggest that modern humans may have originated in East and northeast Africa. I have been predicting these old lineages, but this is a novel finding."

Tishkoff says, in spite of her findings, heated discussion of man's origins will continue. "Human evolution is always controversial! I'm sure there will be people who don't believe that modern humans originated in East Africa--some would argue for southern Africa. And I'm sure that the origins of the click speakers will raise some controversy among African linguists. It's still a big mystery, but I think we might have some exciting clues here."

Genes 101