You think you have big travel plans for the Memorial Day weekend? I guarantee they're nothing like what the first humans managed as they walked all over the globe after leaving their African homeland.

The human genome project has been a veritable treasure trove for scientists trying to tell the story of humankind’s migrations out of Africa. A couple of terrific books have chronicled this, and the use of genetics to reconstruct human history was a focus of a cover story we did last year. The fun part is when genetics throws a wrench into supposedly settled accounts, and that’s what a paper posted today in the open-access journal PLoS Genetics does.

In it, scientists from the University of Oxford and University College Cork describe a new technique they developed. It analyzes not just the Y chromosome, as many studies using genetics to trace human history do, but parts of chromosomes across the entire human genome. The details are complicated, but the bottom line is an ability to probe further back in time and identify smaller genetic contributions.

The technique confirms the out-of-Africa model, in which all human populations outside that continent today are the descendants of a single pulse of wanderers who left Africa. Hominids who originally lived in the regions of Asia and Europe colonized by the migrants contributed nothing to the modern gene pool, which is a polite way of saying that our ancestors wiped them all out (or at least prevented them from mating). Or, as I wrote in the cover story:

“The first modern humans—and therefore, unlike the earlier wave of Homo erectus into Asia a million years ago, the ancestors of everyone today outside Africa—departed Africa about 66,000 years ago. These pilgrims were strikingly few.... The best estimate: 2,000 men. Assuming an equal number of women, only 4,000 brave souls ventured forth from Africa.”

Now the technique is throwing up surprises about what happened next. Among them:

*The most northerly East Asian population that the scientists analyzed, Siberians called the Yakut, carry genes of the most northerly European population, the Orcadians (whose descendants live in the Orkney Islands), suggesting that northern Europeans walked into north Asia and hooked up with native peoples there.

*Populations in Central Eurasia have genes from the Near East (Bedouins and Palestinians) and even Kenyan Bantus.

*In Europe, the most ancient populations are the French, followed by the Tuscans and then other Italians, all of whom trace their ancestry to north Africans called Mozabites, today called Berbers, and to several Near Eastern and Central Asian populations. Europeans have more genetic ancestors than any non-European population, making Europe the world’s true melting pot.

*The youngest Europeans are the Sardinians, Russians, Orcadians and Basques—which makes sense, since they are all at the geographic extremes of the continent. People arrived there last. All four have big genetic contributions from the Near East and Central Asia, suggesting multiple waves of migrants into Europe.

*In the Americas, the Colombians are the oldest population. They can trace 47 percent of their ancestry to the Hazara of East Asia but, oddly, they also have genetic contributions from the French. That probably reflects intermarriage after Europeans arrived in the New World.

*The Pima are the oldest people of North America. They trace their ancestry to the Colombians but also, surprisingly, to Mongolians, who are not ancestors of the Colombians. That suggests multiple distinct colonizations into North America from Asia.

*The Mayans have Bantu and Tuscan donors, presumably due to intermarriage after the Europeans arrived.

For two cool little movies of all this, scroll to the bottom of the paper and click on Movie 1 and Movie 2.

Although the Europeans got silver, gold, converts and tobacco out of their conquest of the New World in the 1500s, the Native Americans got nothing but genocide, as what UCLA biologist Jared Diamond called “guns, germs and steel” killed an estimated 90 to 95 percent of the Native Americans—a horrifying 20 million souls. Nothing was more one-sided than the direction that germs traveled. European conquistadors thoughtfully introduced smallpox, influenza and measles, against which the populations of the Americas had no immunity. Result: disease killed more of them than guns or steel.

Only one disease, scholars have long suspected, might have made the trip east to Europe: syphilis. Circumstantial evidence supported an America-to-Europe trajectory: the first recorded epidemic of syphilis occurred in Europe in 1495, upon Columbus’s return. Although some medical historians have argued that the syphilis pathogen (the bacterium Treponema pallidum) existed in Renaissance Europe long before Columbus returned from his voyage to the New World, the most sophisticated study to date, being published today in the journal PLoS Neglected Tropical Diseases, concludes otherwise: a genetic analysis of the treponeme bacteria supports the “Columbian theory” of syphilis’s origins in the Americas. Call it Montezuma’s revenge, squared.

To trace the origins of syphilis, scientists have mostly studied old bones, which preserve evidence of late-stage syphilis. But because it is tough to pinpoint the exact age of the bones, these studies have been inconclusive. Kristin Harper of Emory University and colleagues therefore studied 21 genetic regions in the genomes of 26 geographically disparate strains of treponemes. Based on how much the different strains had diverged from the basic genetic blueprint, the scientists were able to create a family tree for treponemes. It showed that the strains that cause venereal syphilis originated most recently. Their closest relatives were strains collected in South America that cause the disease yaws. Together, they say, the analyses supports the idea that syphilis originated in the Americas.

But wait. The syphilis that was present in the Americas when Columbus landed (there was a treponemal infection in the Dominican Republic when he arrived) might not have been venereal—that is, spread sexually. “Therefore, it is not clear whether venereal syphilis existed in the New World prior to Columbus’s arrival,” write the scientists. “While it is possible that Columbus and his crew imported venereal syphilis from the New World to Europe, it is also possible that the explorers imported a non-venereal progenitor that rapidly evolved into the pathogen we know today only after it was introduced into the Old World.” If so, then the Americas provided the ancestral germ, but that germ assumed its deadly venereal form only after it became ensconced in Europe.

Critics of the new study say the analysis compared too few DNA sites to reach the conclusions it did, arguing that “no evolutionary order” for the syphilis family of bacteria can be inferred, and urging “caution” in accepting Harper’s claim. Still, this study, combined with earlier work, presents the strongest evidence that the Native Americans got at least a modicum of revenge on their killers.