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There were no people on Earth except Africans until 55,000 years ago. The evidence is very clear.

The human faces of Asia. First published in the first edition (1876–1899) of Nordisk familjebok. [Wikimedia Commons, CC BY-SA]

Before humans were falsely divided into races and ethnicities that exist today, we were all Africans. Africans were the only ones living on Earth until 55,000 years ago.

When the study of human origins intensified in the 20th century, two main theories emerged to explain the archaeological and fossil record:

The first theory known as the multi-regional hypothesis suggested that a species of human ancestor dispersed throughout the globe, and modern humans evolved from this predecessor in several different locations.

The other one, known as out-of-Africa theory, held that modern humans evolved in Africa for many thousands of years before they spread throughout the rest of the world.

In the 1980s, new tools completely changed the kinds of questions that scientists could answer about the past. By analyzing a shorter strand of DNA contained in the mitochondria in living human populations, geneticists could trace lineages backward in time. Mitochondrial DNA is inherited only from the mother. These analyses provided key support for the out-of-Africa theory. This new evidence showed that Homo sapiens ( the first modern humans ) evolved in Africa, probably around 200,000 years ago.

At that point in human history, which scientists have calculated to be about 200,000 years ago, a woman existed whose mitochondrial DNA was the source of the mitochondrial DNA in every person alive today. But she is neither our only ancestor nor our oldest ancestor. She is, instead, simply our "most recent common ancestor," at least when it comes to mitochondria. And her mitochondrial DNA backtracking showed she lived in Africa. In 2008, more sophisticated analyses using DNA from the cell's nucleus—chromosomes inherited from both father and mother have confirmed these findings, comparing nuclear DNA from 938 people from 51 parts of the world. This research, the most comprehensive to date, traced our common ancestor to Africa.

While DNA studies have revolutionized the field of paleoanthropology, the story "is not as straightforward as people might think. To piece together humankind's great migration, scientists blend DNA analysis with archaeological and fossil evidence to try to create a coherent whole.

In 2003, a team of anthropologists reported the discovery of three unusual skulls—two adults and a child—at Herto, near the site of an ancient freshwater lake in northeast Ethiopia. The skulls were between 154,000 and 160,000 years old and had modern characteristics. The Herto skulls fit with the DNA analysis suggesting that modern humans evolved some 200,000 years ago.

One of the most intriguing discoveries was reported in 2007. In a cave at Pinnacle Point in South Africa, a team led by Arizona State University paleoanthropologist Curtis Marean found evidence that humans 164,000 years ago were eating shellfish, making complex tools, and using red ocher pigment—all modern human behaviors. The shellfish remains—of mussels, periwinkles, barnacles, and other mollusks—indicated that humans were exploiting the sea as a food source at least 40,000 years earlier than previously thought.

As the gaps are filled, based on archaeological and anthropological evidence, today's scientists believe that from their beginnings in Africa, modern humans went first to Asia between 80,000 and 60,000 years ago. By 45,000 years ago, or possibly earlier, they had settled Indonesia, Papua New Guinea, and Australia. They entered Europe around 40,000 years ago, probably via two routes: from Turkey along the Danube corridor into eastern Europe, and along the Mediterranean coast.

By 35,000 years ago, they were firmly established in most of the Old World (Africa, Asia, and Europe). Finally, around 15,000 years ago, humans crossed from Asia to North America and from there to South America, which they reached by the Bering Land Bridge—or possibly by boat.

Spread of Homo sapiens. Image courtesy Wikimedia Commons.

DNA evidence suggests the original exodus involved anywhere from 1,000 to 50,000 people. Scientists do not agree on the time of the departure—sometime more recently than 80,000 years ago—or the departure point, but most now appear to be leaning away from the Sinai, once the favored location, and toward a land bridge crossing what today is the Bab el Mandeb Strait separating Djibouti from the Arabian Peninsula at the southern end of the Red Sea. From there, the thinking goes, migrants could have followed a southern route eastward along the coast of the Indian Ocean.

For many years paleontologists still had one gap in their story of how humans conquered the world. They had no human fossils from sub-Saharan Africa from between 15,000 and 70,000 years ago. Because the epoch of the great migration was a blank slate, they could not say for sure that the modern humans who invaded Europe were functionally identical to those who stayed behind in Africa.

But one day in 1999, anthropologist Alan Morris of South Africa's University of Cape Town showed Frederick Grine, a visiting colleague from Stony Brook University, an unusual-looking skull on his bookcase. Morris told Grine that the skull had been discovered in the 1950s at Hofmeyr, in South Africa. No other bones had been found near it, and its original resting place had been befouled by river sediment. Any archaeological evidence from the site had been destroyed—the skull was a seemingly useless artifact.

But Grine noticed that the braincase was filled with a carbonate sand matrix. Using a technique unavailable in the 1950s, Grine, Morris, and an Oxford University-led team of analysts measured radioactive particles in the matrix. The skull, they learned, was 36,000 years old. Comparing it with skulls from Neanderthals (archaic humans who lived in Eurasia until about 40,000 years ago), early modern Europeans and contemporary humans, they discovered it had nothing in common with Neanderthal skulls and only peripheral similarities with any of today's populations. But it matched the early modern Europeans elegantly.

The evidence was clear. Thirty-six thousand years ago, says Morris, before the world's human population differentiated into the mishmash of races and ethnicities that exist today, "We were all Africans."

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