When and how did modern humans, Homo sapiens, spread out of Africa and around the world?

Homo sapiens first evolved in Africa more than 300,000 years ago.

For a long time, our species was exclusively African. But over hundreds of thousands of years, Homo sapiens started to push out into Arabia, Asia and eventually the rest of the world.

This journey was not straightforward and didn’t follow a neat path. It’s a story of false starts and ghost populations, of extinct human species and vast distances. It’s a tale that’s constantly rewritten as new fossils are discovered, changing our understanding of the human evolution timeline.

Ultimately, it’s a journey that changed the world.

Why did Homo sapiens migrate out of Africa?

Why some Homo sapiens left Africa in the first place is uncertain. Of the many theories that exist, it’s believed that changes in the climate offer the best explanation.

Professor Chris Stringer, our human evolution expert, explains how warmer weather could have encouraged early humans to migrate.

“It’s generally assumed that Homo sapiens emerging from Africa were warm adapted but not at home in extremely dry environments, so it may have been that the climate temporarily became slightly warmer and wetter in western Asia,” explains Chris. “Alternatively, parts of northern Africa might have grown more arid, pushing these populations out of the continent.”

“Either way, this could have drawn or driven small groups out of Africa. It’s important to remember that they’re not setting out to explore new lands at this stage but are simply following their food.”

“Changes in the climate would have caused trees and plants to migrate, and the herds of animals that fed on them would have followed. Ancient humans would have followed too, taking them into unfamiliar territory.”

While it used to be thought that our species only left Africa once in the distant past, more recent discoveries have torn up the Homo sapiens timeline. DNA analysis of early modern human remains shows that there have been many migrations out of Africa over hundreds of thousands of years.

Most of these migrations ultimately died out before passing on their genes. As a result, they aren’t directly related to anyone alive today. But they left behind fossil and archaeological remains that show that they covered great distances and survived for tens of thousands of years before disappearing.

While these used to be known as failed dispersions, scientists are increasingly recognising the role of these pioneering populations.

The first Homo sapiens out of Africa

One likely route for early human migration out of Africa is through the Sinai and Arabian Peninsulas. By taking the only land bridge between Africa and Eurasia, Homo sapiens could have migrated on foot without having to attempt a sea crossing.

While the Arabian Peninsula is hot and arid today, it’s wasn’t always like this. The remains of ancient lakes have been found, including a very interesting site known as Khall Amayshan 4 (KAM-4) in the Nefud Desert.

It shows that the conditions were right for permanent freshwater lakes to exist in the region at least five times between 400,000 and 55,000 years ago. Stone tools found at KAM-4 suggest that these lakes weren’t just used by Homo sapiens, they were also used by earlier relatives that had left Africa long before our species.

“These migrations may not have happened all in one go,” Chris says. “These groups could have moved in stages, living in one place for long enough that another change in the climate allowed them to move even further away from Africa.”

“This journey would have been incredibly challenging for these humans, as they would be facing conditions, animals and diseases unlike anything they had experienced in Africa. Their ability to adapt would also be reduced, as the small groups leaving Africa would have limited genetic diversity.”

As Homo sapiens populations travelled further out of Africa, they would have met another species of human known as Neanderthals. These encounters would change both species forever.

Homo sapiens and Neanderthals

Neanderthals, Homo neanderthalensis, are an extinct human species that lived in Europe and Asia between around 400,000 and 40,000 years ago. While Neanderthals and Homo sapiens are regarded as separate species, they are very closely related.

Even so, closely related species can still sometimes have offspring together. Sequencing of the Neanderthal genome has shown that our two species interbred in the distant past, with the first signs dating back hundreds of thousands of years.

“The human body has distinct sets of DNA,” explains Chris. “There is the nuclear DNA, which is inherited from both parents, and DNA in the energy-providing mitochondria, which comes only from the mother.”

“Analysis of a Neanderthal leg bone found in Germany indicates breeding between early Homo sapiens and Neanderthals around 270,000 years ago. It’s been suggested that this eventually led to Neanderthals losing their original mitochondrial DNA entirely, and inheriting that of Homo sapiens’ instead.”

Another set of DNA is found in the Y chromosome, which is only found in males. Neanderthals seem to have had their original Y chromosome replaced by those of early Homo sapiens at around the same time they lost their original mitochondrial DNA.

While it’s not known exactly when and where these interactions took place, southern Europe is a strong possibility. Apidima Cave in Greece, for example, contains the back part of a skull that’s more than 200,000 years old, and is currently the earliest Homo sapiens fossil found outside of Africa.

Some 40,000 years later, Neanderthals were living at the site instead. Even though we can’t be certain that both species interacted here, it’s certainly a possibility.

The influence of Neanderthals was an important part of the success of the main migration of Homo sapiens out of Africa, which occurred within the past 60,000 years. Interbreeding between this population and groups of Neanderthals around 50,000 years ago means that around 2% of the DNA of living people originating outside of Africa comes from Neanderthals, depending on their ancestry.

This period of interbreeding transferred genes controlling skin colour, immunity and other bodily functions that would have helped Homo sapiens to adapt to life outside Africa. These genes were particularly useful as our species entered Europe.

Early modern humans in Europe

While there had been excursions into Europe by Homo sapiens for more than 200,000 years, these had mostly been limited to the east and south of the continent. It was only within the past 60,000 years that fossils show our species spreading further.

A tooth found at Grotte Mandrin, southern France, shows that our species had arrived in western Europe by around 54,000 years ago. While this population was replaced by Neanderthals shortly afterwards, Homo sapiens returned again around 45,000 years ago.

At the same time, fossils from Ranis, Germany, show our species had arrived in northern Europe too. The first known evidence of Homo sapiens in Britain is from around 40,000 years ago and is based on a fragment of upper jaw found at Kent’s Cavern in Devon.

The next 20,000 years would see further changes in the predecessors of modern Europeans. One of the most significant is the Aurignacian industry, which saw new stone tools, sculptures carved out of mammoth ivory and delicate cave paintings produced about 41,000–35,000 years ago.

Towards the end of this time, European populations also met a long-isolated group of Homo sapiens known as the basal Eurasians. It’s believed that this group became isolated from the rest of the species shortly after the Out of Africa expansion, and only started mixing with other Homo sapiens again around 25,000 years ago.

“People with European heritage have less Neanderthal DNA than people with east Asian heritage, which is odd, as we know Neanderthals mainly from European sites,” explains Chris. “The best way to account for this is that the ancestors of modern Europeans bred with a group that hadn’t mixed with the Neanderthals.”

“It’s currently thought this ghost lineage might have been a population who lived in Arabia or Persia before spreading into western Asia and Europe. This would have diluted the proportion of Neanderthal DNA in the genome of Europeans.”

As the predecessors of today’s Europeans began to settle into Europe, another group of Homo sapiens were travelling deeper into Asia.

Encountering ancient human species in Asia

Many different human species have lived in Asia over the past two million years. The most successful was Homo erectus, which is thought to have migrated from Africa to southeast Asia around 1.8 million years ago and lived there for more than 1.5 million years.

Another group of more mysterious residents are the Denisovans. This species is currently only known from a handful of fossils – and the DNA and fossilised proteins they contain – discovered in Siberia and China.

Our species only arrived in eastern Asia much more recently, with teeth found in Fuyan Cave, southern China, suggesting that some members could have arrived there 120,000–80,000 years ago. While these dates have been challenged by other scientists, evidence from sites such as Lida Ajer in Indonesia, shows Homo sapiens was present in southeast Asia by at least 63,000 years ago.

These early Asian arrivals likely fed on a variety of different foods as they traversed southeast Asia’s grasslands and rainforests. Preserved teeth from Tam Pà Ling Cave in Laos suggest that our species was omnivorous, eating a range of fruits, meats and bone marrow.

The main migration of Homo sapiens made its way into Asia within the past 45,000 years, but this didn’t all happen in one go. Fossils from Tianyuan Cave, China, that are 40,000 years old, come from an ancient relative of modern Asians that’s more closely related to some ancient Europeans than others. This indicates that multiple groups of our species must have made their way across the continent.

These different waves of migration led to a complex population structure developing in the region. Some parts of the population remained on the mainland, while others headed down into what are now the islands of southeast Asia.

“At the time, the sea level was lower than it is today,” explains Chris. “Instead, there would have been more land with open grasslands that would have provided a rich environment to migrate through.”

“Homo sapiens would have been able to walk from the mainland to as far as Java in what is now Indonesia.”

At some points along the way, Homo sapiens interbred with the Denisovans. While the limited knowledge we have of this extinct species makes it hard to say exactly where, the genetics of living people suggests that there were at least three distinct breeding events.

One breeding event, found in people of Asian ancestry, appears to have happened on the mainland. Among other things, this passed on genes involved in adapting to high altitudes, which are found in many living Tibetans today.

The other breeding events appear to have taken place in what are today the islands of southeast Asia with a mysterious group only identified so far as the southern Denisovans. No fossil remains of these hominins have yet been identified, but as much as 5% of the DNA of the Indigenous inhabitants of Australia, New Guinea and the Philippines comes from them.

Our species’ arrival in Java would have also marked the eastern end of the known world for our species. Up until this point, the migration of Homo sapiens across the world could have happened solely by foot, either across the continents we know today or via land bridges that have since been lost.

But to reach New Guinea, Australia and beyond, Homo sapiens would have needed to take to the seas, perhaps for the first time.

Island hopping to Australia

There are two main routes that Homo sapiens could have taken to reach Australia. The first is a northern route that goes via New Guinea, while the southern route goes via islands such as Timor and Flores, where ancient species such as Homo floresiensis lived.

The northern route is generally considered to be more likely, as until 8,000 years ago New Guinea, Australia and Tasmania were joined into a supercontinent known as Sahul. This would have meant a shorter sea journey before our species continued into Australia on foot.

While there’s currently no direct evidence of how this crossing was made, it can be inferred that Homo sapiens must have had some knowledge of raft building to begin with. Their vessels would have needed to be solidly built to survive such a journey and packed with enough food and water for its passengers.

They would have needed to make these crossings either in large groups or in smaller groups in quick succession. If a single, small population would have arrived, it would have led to severe inbreeding, and there aren’t any genetic signs of this in the region’s Indigenous inhabitants today.

What continues to puzzle researchers is why our species made this journey in the first place. Many different theories have been suggested, but Chris believes that population pressures could be the cause.

“We know that sea levels rose and fell over the period, which would have made the islands of southeast Asia shrink and grow,” Chris explains. “As the islands grew smaller, there would have been less living space and food to go around, so it would make sense for populations to look for new lands.”

“Perhaps as they sailed in groups aboard rafts to an island they could see, they could have then been blown by winds and currents to new islands or followed birds over the horizon.”

Based on sites such as Madjedbebe 2 in Australia, our species potentially made this journey multiple times. The oldest signs of human occupation at the site have been dated to about 65,000 years old. If correct, this suggests that a population of Homo sapiens might have made it to the continent more than 15,000 years before the ancestors of Indigenous Australians did.

The first confirmed early modern humans in Australia are dated to around 42,000 years ago, based on Homo sapiens remains found at Lake Mungo. A wave of extinctions of Australian wildlife at around the same time have also been linked to our species, with animals including the ‘giga-goose’ and giant wombats possibly hunted to extinction.

First humans in North America

Thousands of years after our species arrived in Australia, Asian populations of Homo sapiens began to migrate across to the Americas. Lower sea levels meant that a land bridge existed between Asia and North America, allowing them to island-hop or walk across what’s now the Bering Strait between Russia and Alaska.

It’s generally believed that Homo sapiens were the first humans to reach the Americas, but exactly when this happened is still debated. The first known Homo sapiens fossils from the Americas are around 15,000 years old, but other evidence suggests our species could have arrived some 10,000 years before that.

Fossilised footprints found in White Sands National Park, New Mexico, USA, have been dated to around 23,000–21,000 years ago. While the original dating was contentious, reanalysis of the footprints supports them as the first evidence of humans in America.

In any event, the arrival of Homo sapiens in the Americas was much later than their arrival to many other parts of the world. Our species subsequently spread down through North America into Central and South America, which is estimated to have happened within the past 15,000 years.

While this might have marked the end of our major global migration, many questions remain about how Homo sapiens spread around the world. Scientists continue to investigate the evolution of humans by studying new and known archaeological sites as well DNA samples for insights into our origins.                                                                   

This article includes information from the book Our Human Story by our scientists Dr Louise Humphrey and Professor Chris Stringer.