Migration or Vacation? Humans Left Africa, Returned

At least one lineage of early humans migrated out of Africa into Asia and Europe, likely mated with Neanderthals during their travels, and then journeyed back to Africa for better weather, new DNA evidence suggests.

The findings, published in the journal Scientific Reports, help to explain why native North Africans today are genetically related to people from Europe and Asia, in addition to other Africans. It also offers strong evidence that "Out of Africa" was not a one-way trip for some human lineages that traveled back to North Africa starting around 45,000 years ago.

At the center of it all is a prehistoric individual named the Pestera Muierii Woman, or PM-1 for short. Her skull, dated to 35,000 years ago, was unearthed in the Pestera Muierii cave of Romania.

Photos: Faces of Our Ancestors

Her "mosaic of modern human and archaic Neanderthal morphological (physical) features may be the result of Neanderthal interbreeding," senior author Concepcion de-la-Rúa of the University of the Basque Country's Department of Genetics told Discovery News.

de-la-Rúa and her colleagues extracted DNA from two of the woman's teeth and determined the ancient female's mitochondrial genome (mitogenome), i.e., a complete set of particular genes that are passed down from mothers to their daughters. The researchers noted that one component of this set, named "U6 basal," had not previously been identified in any ancient or existent humans.

Investigating the U6 mystery further, the scientists discovered that an evolved version of U6 does indeed exist today.

Humans May Have Left Africa Earlier Than Thought

"U6 is found predominantly in present-day North African populations," de-la-Rúa said, adding that the form in Pestera Muierii Woman dates to a much earlier time and therefore is "basal," or at the root of U6's emergence.

Putting the clues together, the researchers believe that the woman's Homo sapiens ancestors migrated out of Africa and, at some point, mated with one or more Neanderthals, since she appears to have been part Neanderthal. This is actually true of all people of Asian and European heritage today, who retain Neanderthal DNA in their genomes.

Individuals from the woman's lineage then migrated back to Africa, landing in the northern region of the continent, where they mated with locals, thereby continuing the evolution of U6.

The woman then "represents an offshoot to South-East Europe, i.e. Romania, of this back migration starting in the Early Upper Paleolithic period about 40–45,000 years ago," de-la-Rúa said.

As for why Romanians today do not have U6 basal, the researchers think this early form of U6 could have gone extinct hundreds or even thousands of years after the woman lived. de-la-Rúa explained that "mitochondrial lineages may disappear when a woman does not have children or if she only has male descendants."

First Humans Out of Africa Were Small, Scrappy

The "Out of Africa" migrants clearly went to a lot of trouble to travel to Asia and then to Europe, so why would many of their not-too-distant later relatives have been so eager to return to Africa? The researchers offered a two-word answer: a "difficult climate."

De-la-Rúa explained that "between 50,000–20,000 years ago, it was (often) very cold due to climatic fluctuations" in Europe.

Vicente Cabrera, professor at the University of La Laguna in Tenerife, Spain, said that "a migration from Eurasia is the best explanation for the radiation of U6 in Africa."

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Several questions remain about early human migrations and the Pestera Muierii Woman, however. One concerns when and where her Homo sapiens ancestors mated with Neanderthals. de-la-Rúa believes the answer will be known after the woman's nuclear genome (another more revealing set of genes) is completely sequenced.

Future analysis of the woman's DNA could also reveal additional genetic links shared today among people from Asia, Europe and North Africa.

Modern day travelers crossing the Sahara Desert in North Africa on the backs of camels.

Back in the Beginning

To put a human face on our ancestors, scientists from the Senckenberg Research Institute used sophisticated methods to form 27 model heads based on tiny bone fragments, teeth and skulls collected from across the globe. The heads are on display for the first time together at the Senckenberg Natural History Museum in Frankfurt, Germany. This model is Sahelanthropus tchadensis, also nicknamed "Toumai," who lived 6.8 million years ago. Parts of its jaw bone and teeth were found nine years ago in the Djurab desert in Chad. It's one of the oldest hominid specimens ever found.

Australopithecus afarensis

With each new discovery, paleoanthropologists have to rewrite the origins of man's ancestors, adding on new branches and tracking when species split. This model was fashioned from pieces of a skull and jaw found among the remains of 17 pre-humans (nine adults, three adolescents and five children) which were discovered in the Afar Region of Ethiopia in 1975. The ape-man species, Australopithecus afarensis, is believed to have lived 3.2 million years ago. Several more bones from this species have been found in Ethiopia, including the famed "Lucy," a nearly complete A. afarensis skeleton found in Hadar.

Australopithecus africanus

Meet "Mrs. Ples," the popular nickname for the most complete skull of an Australopithecus africanus, unearthed in Sterkfontein, South Africa in 1947. It is believed she lived 2.5 million years ago (although the sex of the fossil is not entirely certain). Crystals found on her skull suggest that she died after falling into a chalk pit, which was later filled with sediment. A. africanus has long puzzled scientists because of its massive jaws and teeth, but they now believe the species' skull design was optimal for cracking nuts and seeds.

Paranthropus aethiopicus

The skull of this male adult was found on the western shore of Lake Turkana in Kenya in 1985. The shape of the mouth indicates that he had a strong bite and could chew plants. He is believed to have lived in 2.5 million years ago and is classified as Paranthropus aethiopicus. Much is still unknown about this species because so few reamins of P. aethiopicus have been found.

Paranthropus boisei

Researchers shaped this skull of "Zinj," found in 1959. The adult male lived 1.8 million years ago in the Olduvai Gorge of Tanzania. His scientific name is Paranthropus boisei, though he was originally called Zinjanthropus boisei -- hence the nickname. First discovered by anthropologist Mary Leakey, the well-preserved cranium has a small brain cavity. He would have eaten seeds, plants and roots which he probably dug with sticks or bones.

Homo rudolfensis

This model of a sub-human species -- Homo rudolfensis -- was made from bone fragments found in Koobi Fora, Kenya, in 1972. The adult male is believed to have lived about 1.8 million years ago. He used stone tools and ate meat and plants. H. Rudolfensis' distinctive features include a flatter, broader face and broader postcanine teeth, with more complex crowns and roots. He is also recognized as having a larger cranium than his contemporaries.

Homo ergaster

The almost perfectly preserved skeleton of the "Turkana Boy" is one of the most spectacular discoveries in paleoanthropology. Judging from his anatomy, scientists believe this Homo ergaster was a tall youth about 13 to 15 years old. According to research, the boy died beside a shallow river delta, where he was covered by alluvial sediments. Comparing the shape of the skull and teeth, H. ergaster had a similiar head structure to the Asian Homo erectus.

Homo heidelbergensis

This adult male, Homo heidelbergensis, was discovered in in Sima de los Huesos, Spain in 1993. Judging by the skull and cranium, scientists believe he probably died from a massive infection that caused a facial deformation. The model, shown here, does not include the deformity. This species is believed to be an ancestor of Neanderthals, as seen in the shape of his face. "Miquelon," the nickname of "Atapuerca 5", lived about 500,000 to 350,000 years ago and fossils of this species have been found in Italy, France and Greece.

Homo neanderthalensis

The "Old Man of La Chapelle" was recreated from the skull and jaw of a Homo neanderthalensis male found near La Chapelle-aux-Saints, in France in 1908. He lived 56,000 years ago. His relatively old age, thought to be between 40 to 50 years old, indicates he was well looked after by a clan. The old man's skeleton indicates he suffered from a number of afflictions, including arthritis, and had numerous broken bones. Scientists at first did not realize the age and afflicted state of this specimen when he was first discovered. This led them to incorrectly theorize that male Neanderthals were hunched over when they walked.

Homo floresiensis

The skull and jaw of this female "hobbit" was found in Liang Bua, Flores, Indonesia, in 2003. She was about 1 meter tall (about 3'3") and lived about 18,000 years ago. The discovery of her species, Homo floresiensis, brought into question the belief that Homo sapiens was the only form of mankind for the past 30,000 years. Scientists are still debating whether Homo floresiensis was its own species, or merely a group of diseased modern humans. Evidence is mounting that these small beings were, in fact, a distinct human species.

Homo sapiens

Bones can only tell us so much. Experts often assume or make educated guesses to fill in the gaps in mankind's family tree, and to develop a sense what our ancestors may have looked like. Judging from skull and mandible fragments found in a cave in Israel in 1969, this young female Homo sapien lived between 100,000 and 90,000 years ago. Her bones indicate she was about 20 years old. Her shattered skull was found among the remains of 20 others in a shallow grave.