The last "exit" from Africa, 70,000 to 60,000 years ago, was the right one. That of a small group of humans who, crossing the Red Sea, marks the start of an odyssey.
Prehistory, Upper Palaeolithic. A group of Homo sapiens grouped around a fire are carving flint tools. Painting by Francisco Fonollosa.
This article is taken from n°204 of Indispensables de Sciences et Avenir, dated January/March 2021.
A molar and a few bone fragments… This seems very little and yet, the human remains discovered in the Bacho Kiro cave, in Bulgaria, by the team of Jean-Jacques Hublin, of the Max-Planck Institute of Anthropology evolutionary from Leipzig (Germany), have a historical significance. Indeed, the study of the morphology of the tooth as well as the analysis of the mitochondrial DNA bone remains leave little doubt:we are dealing here with a Homo sapiens . The surprise comes from the age of these remains, published in the journal Nature in May 2020:46,000 years! They therefore attest to the presence of our ancestor in Europe 6,000 years earlier than previously estimated, extending the period of time during which it and Neanderthal coexisted on the continent... The excavations undertaken by the team since 2015 on this very rich site promise other surprises, not yet published. But one thing is already certain:the inhabitants of Bacho Kiro, if they are not our direct ancestors, are representatives of the first wave of modern humans to reach Europe.
A journey that began 70,000 to 60,000 years ago on the east coast of Africa, somewhere between present-day Djibouti and Egypt. At that time, in all likelihood, small groups of Homo sapiens walk towards Sinai, which they will cross to reach the Levant. Others, more adventurous, embark on board rafts of branches on a much shallower Red Sea than today and, progressing from island to island, pass the strait of Bab-el-Mandeb before joining the coasts of Yemen. This is the beginning of the global expansion of Homo sapiens , according to a theory dubbed "Out of Africa" by scientists.
It was in 1988 that British paleoanthropologists Peter Andrews and Christopher Stringer laid the foundations. Compiling all the data from fossils and genetics then available, they conclude that it is indeed in Africa, and only in Africa, that the origin of the humans populating all the continents must be located. The two researchers thus counter the "multiregionalist" theses which defended a simultaneous appearance of Homo sapiens in all regions of the world. "We postulated that Sapiens emerged from Africa and then replaced all other existing species in the world, notably Neanderthals" , recalls Christopher Stringer.
Strictly speaking, this theory should also be called "Out of Africa II". For many millennia before, old Homo had already left the African cradle:about two million years ago, Erectus had tried their luck, leaving in particular at Dmanisi, in Georgia, fossils dated 1.8 million years, as well as tools at Shangchen , in China (2.1 million years). Possible descendant of Homo erectus , another species, Homo heidelbergensis , has reached not only Germany, where a fossil dated around 600,000 years old was unearthed in 1907, but also Spain, Greece, Ethiopia… Is Heidelbergensis the ancestor of two new species? like Homo , Neanderthal in Europe and Denisova in Asia? Is it also that of Sapiens in Africa? These questions divide scientists.
The record for the oldest European Sapiens would go to a fossil found in Greece
This article is taken from n°204 of Indispensables de Sciences et Avenir, dated January/March 2021.
A molar and a few bone fragments… This seems very little and yet, the human remains discovered in the Bacho Kiro cave, in Bulgaria, by the team of Jean-Jacques Hublin, of the Max-Planck Institute of Anthropology evolutionary from Leipzig (Germany), have a historical significance. Indeed, the study of the morphology of the tooth as well as the analysis of the mitochondrial DNA bone remains leave little doubt:we are dealing here with a Homo sapiens . The surprise comes from the age of these remains, published in the journal Nature in May 2020:46,000 years! They therefore attest to the presence of our ancestor in Europe 6,000 years earlier than previously estimated, extending the period of time during which it and Neanderthal coexisted on the continent... The excavations undertaken by the team since 2015 on this very rich site promise other surprises, not yet published. But one thing is already certain:the inhabitants of Bacho Kiro, if they are not our direct ancestors, are representatives of the first wave of modern humans to reach Europe.
A journey that began 70,000 to 60,000 years ago on the east coast of Africa, somewhere between present-day Djibouti and Egypt. At that time, in all likelihood, small groups of Homo sapiens walk towards Sinai, which they will cross to reach the Levant. Others, more adventurous, embark on board rafts of branches on a much shallower Red Sea than today and, progressing from island to island, pass the strait of Bab-el-Mandeb before joining the coasts of Yemen. This is the beginning of the global expansion of Homo sapiens , according to a theory dubbed "Out of Africa" by scientists.
It was in 1988 that British paleoanthropologists Peter Andrews and Christopher Stringer laid the foundations. Compiling all the data from fossils and genetics then available, they conclude that it is indeed in Africa, and only in Africa, that the origin of the humans populating all the continents must be located. The two researchers thus counter the "multiregionalist" theses which defended a simultaneous appearance of Homo sapiens in all regions of the world. "We postulated that Sapiens emerged from Africa and then replaced all other existing species in the world, notably Neanderthals" , recalls Christopher Stringer.
Strictly speaking, this theory should also be called "Out of Africa II". For many millennia before, old Homo had already left the African cradle:about two million years ago, Erectus had tried their luck, leaving in particular at Dmanisi, in Georgia, fossils dated 1.8 million years, as well as tools at Shangchen , in China (2.1 million years). Possible descendant of Homo erectus , another species, Homo heidelbergensis , has reached not only Germany, where a fossil dated around 600,000 years old was unearthed in 1907, but also Spain, Greece, Ethiopia… Is Heidelbergensis the ancestor of two new species? like Homo , Neanderthal in Europe and Denisova in Asia? Is it also that of Sapiens in Africa? These questions divide scientists.
READ. 45,000-year-old human remains reveal ancient migration to Europe.
The record for the oldest European Sapiens would go to a fossil found in Greece
"It is likely that Sapiens left Africa several times", emphasizes Christopher Stringer. Indeed, the first attempts, almost 200,000 years ago, have left traces. The record for the oldest European Sapiens would thus go to a fossil found in Apidima, Greece, in the 1970s, and recently re-studied:it would be 210,000 years old. A hypothesis formulated on the basis of a skull fragment which allowed a 3D reconstruction, revealing a rounded morphology characteristic of Sapiens. However, it does not convince all paleoanthropologists. Other fossils, in particular a jaw with several teeth, discovered in 2018 in Israel in the cave of Misliya near Haifa, display nearly 190,000 years. The morphology of the dentition reveals characteristics found in modern humans.
And Sapiens didn't stop there. In Saudi Arabia, the team of Mathew Stewart and Michael Petraglia, from the Max-Planck Institute in Jena (Germany), was surprised to discover this year seven footprints belonging to three individuals, imprinted in the fossilized mud of an ancient lake. Dating of the sediments revealed that these humans took this path 120,000 years ago. However, Neanderthal never frequented the region. And the footprints are unlike those he may have left in other places.
It can therefore only be those of Homo sapiens , conclude the researchers. Did these steps lead the descendants of the little troop to China? Still, teeth attributed to Sapiens were also discovered in 2011 in Hunan, in the south of the country. Their dating, between 120,000 and 80,000 years, is however controversial. What has become of these first explorers of the world? What have they bequeathed to us? Nobody knows… "We don't see any trace in our genome today of these first releases, except perhaps in some inhabitants of Papua New Guinea, but this remains debated" , emphasizes Lluis Quintana-Murci, professor at the College de France. In fact, these first attempts did not allow the spread of the species on a global scale. But they are proof that Sapiens was able to venture very far from his native environment.
"To tell the truth, we should not speak of an exit from Africa, but rather of an extension of the ecosystem in which Homo sapiens, explains Silvana Condemi, who leads the paleoanthropology and bioarchaeology team at the University of Aix-Marseille. The first attempts are made according to the expansion of temperate regions, rich in meadows and forests, where game abounds. At a time when Europe, on the other hand, is experiencing glacial episodes. It will take time for Sapiens to adapt to the cold."
The "real" exit from Africa, the one that results in a global expansion of Homo sapiens , takes place about 70,000 to 60,000 years ago. Only a small group of individuals leaves the African continent. What population of Sapiens are we talking about? Still a mystery... "The genetic diversity of contemporary populations outside Africa is largely due to this single small group" , emphasizes Lluis Quintana-Murci. The work published by his team in 1999, based on the analysis of the mitochondrial DNA of current populations, then later confirmed on the DNA of the chromosomes, indicates that Homo sapiens quickly conquered Asia from East Africa, following a coastal route. The dating of the fossils discovered in these regions, between 60,000 and 40,000 years old, agrees with that obtained thanks to genetics.
First encounters with Neanderthals in the Caucasus, Iran and the Arabian Peninsula
But what has stung Sapiens so that he leaves Africa again? "It is possible that it was driven out of its territory because of constraints related to the climate or the availability of food, or that it colonized a new particularly favorable environment" , explains Christopher Stringer. Among the theories put forward are climatic changes induced by gigantic volcanic eruptions, such as that of Toba, in Indonesia, about 75,000 years ago. The millions of tons of ash projected into the atmosphere would have considerably cooled the climate, dried up Africa and caused an exodus. A hypothesis that remains controversial.
Impossible, therefore, to know the precise reasons for the departure, but the progression of Homo sapiens is very fast. 50,000 years ago, it already arrived in Australia. How could he accomplish such an expedition? "His arrival was a coincidence. It is likely that he used boats or rafts, although no remains have been found, imagine Christopher Stringer. The groups had to move from island to island, depending on the sea level." At the same time, heading west, via the Caucasus, other groups take the road to Europe.
And in these new territories that he discovers, Homo sapiens will soon see:he is not alone. In 2010, the team of geneticist Svante Paäbo, at the Max-Planck Institute in Leipzig, Germany, published in the journal Science an article which reveals the result of the sequencing of the Neanderthal genome. Surprise:we have in our own genome about 2% of the latter's DNA.
Sapiens has therefore hybridized with him! "This discovery did not challenge our theory, but it added a dimension of interbreeding:Sapiens is no longer 100% Out of Africa, but 95%", summarizes Christopher Stringer. Where and when does this meeting take place? "Very quickly, on the Arabian Peninsula, in Iran, in the Caucasus", emphasizes Lluis Quintana-Murci. The fossils found at Ust'-Ishim (Siberia) have made it possible to place these first intimate encounters between 52,000 and 58,000 years before our era. But this is only the beginning… Sapiens-Neanderthals one-on-ones will happen again, especially in Europe. Thus, in 2003, bones including a mandible were discovered in the cave of Pestera cu Oase, in Romania, a site occupied 34,000 to 36,000 years ago. Sequenced, the DNA extracted from the mandible was found to contain nearly 9% Neanderthal genes! An unusually high proportion, which indicates recent hybridization.
However, these meetings remain limited. "According to the models carried out taking into account the habitability of the territories, from the Atlantic to Siberia, the population of Neanderthals should not have exceeded 70,000 individuals, emphasizes Silvana Condemi. It was to be made up of groups of about twenty people." How is the contact? Contrary to what we readily imagine, not necessarily in an aggressive way:"They were very well able to exchange techniques, information on the game" , imagine Silvana Condemi. And by bringing it a small piece of its genome, Neanderthal gave Sapiens selective advantages. "When Sapiens arrives in Europe, it is not adapted to the climate, food or diseases. Interbreeding with Neanderthals, who have lived there for 300,000 years, allows them in particular to acquire genes for resistance to cold and immune response to pathogenic viruses" , explains Lluis Quintana-Murci.
A "genetic bottleneck" when leaving the African continent
Walking east and southeast, Sapiens makes another encounter:that of Denisova's man, named after the Siberian cave where, in 2008, a phalanx was unearthed. Two years after this discovery, the team of Svante Paäbo and Johannes Krause, at the Max-Planck Institute in Leipzig, sequenced the DNA. With once again an unexpected result:the unknown human is not related to either Sapiens or Neanderthal. This mysterious species has delivered only one other fossil, a mandible discovered in China in 1980, which has since been attributed to it. But contemporary population genetics shows that Denisova hybridized several times with Sapiens, notably in East Asia (for 1% to 2% of the genome) and in Papua New Guinea (3% to 4 % of the genome). Finally, continuing eastward, modern humans will set foot on the American continent, via the Bering Strait.
Sapiens' exit from Africa therefore seems increasingly complex and ramified in space and time. "The Out of Africa theory is solid today , says Lluis Quintana-Murci. It remains to refine the dating and measure the importance of what can be called the genetic 'bottleneck' that formed during the exit from the continent." Because by separating from the rest of the Sapiens, the starters only took with them a part of the multiple variants of the group's genes. How much exactly? This is unknown to date, and more genetic studies of contemporary populations will be needed to see this more clearly.
Some paleoanthropologists also plead for a thorough examination of older releases and the traces they may have left in the genetic heritage of populations. All agree to designate the regions still little explored which could provide valuable information in the future:India, where only one fossil is known, and the Arabian Peninsula, whose richness in remains of occupation human seems exceptional. The puzzle of modern humans is still far from being pieced together.