For more about the Genographic Project, visit nationalgeographic.com.

Guest editor Bono as a toddler, circa 1961, with maps showing the migrations of his matrilineal (top) and patrilineal ancestors (middle), based on analysis of his DNA. His father's ancestors were among the first modern humans to enter Europe. Courtesy of the Hewson family.

Do you think you know who you are? Maybe Irish, Italian, Jewish, Chinese, or one of the dozens of other hyphenated Americans that make up the United States melting pot? Think deeper—beyond the past few hundred years. Back beyond genealogy, where everyone loses track of his or her ancestry—back in that dark, mysterious realm we call prehistory. What if I told you every single person in America—every single person on earth—is African? With a small scrape of cells from the inside of anyone's cheek, the science of genetics can even prove it.

Here's how it works. The human genome, the blueprint that describes how to make another version of you, is huge. It's composed of billions of sub-units called nucleotides, repeated in a long, linear code that contains all of your biological information. Skin color, hair type, the way you metabolize milk: it's all in there. You got your DNA from your parents, who got it from theirs, and so on, for millions of generations to the very beginning of life on earth. If you go far enough back, your genome connects you with bacteria, butterflies, and barracuda—the great chain of being linked together through DNA.

What about humanity, though? What about creatures you would recognize as being like you if they were peering over your shoulder right now? It turns out that every person alive today can trace his or her ancestry back to Africa. Everyone's DNA tells a story of a journey from an African homeland to wherever you live. You may be from Cambodia or County Cork, but you are carrying a map inside your genome that describes the wanderings of your ancestors as they moved from the savannas of Africa to wherever your family came from most recently. This is thanks to genetic markers—tiny changes that arise rarely and spontaneously as our DNA is copied and passed down through the generations—which serve to unite people on ever older branches of the human family tree. If you share a marker with someone, you share an ancestor with him or her at some point in the past: the person whose DNA first had the marker that defines your shared lineage. These markers can be traced to relatively specific times and places as humans moved across the globe. The farther back in time and the closer to Africa we get, the more markers we all share.

What set these migrations in motion? Climate change—today's big threat—seems to have had a long history of tormenting our species. Around 70,000 years ago it was getting very nippy in the northern part of the globe, with ice sheets bearing down on Seattle and New York; this was the last Ice Age. At that time, though, our species, Homo sapiens, was still limited to Africa; we were very much homebodies. But the encroaching Ice Age, perhaps coupled with the eruption of a super-volcano named Toba, in Sumatra, dried out the tropics and nearly decimated the early human population. While Homo sapiens can be traced to around 200,000 years ago in the fossil record, it is remarkably difficult to find an archaeological record of our species between 80,000 and 50,000 years ago, and genetic data suggest that the population eventually dwindled to as few as 2,000 individuals. Yes, 2,000—fewer than fit into many symphony halls. We were on the brink of extinction.