We are the only human species left on Earth.

Once, we shared our planet with other human species, including Neanderthals and Denisovans. Today, they’re gone, leaving behind fossils, ancient tools and traces of their DNA within our own. So how did Homo sapiens end up as the last humans standing?

For decades, we believed our ancestors survived because they were smarter, stronger or better adapted. But what if the real secret to our success was something far more unexpected? What if we survived because we were friendlier?

Some researchers believe our ability to cooperate, build relationships and form larger social networks may have given us a crucial evolutionary edge, an advantage of just 2 per cent that, over thousands of years, could have changed the course of human history.

The story of how Homo sapiens left Africa is more complicated than we once thought.

Fossils suggest our species reached parts of Eurasia far earlier than the major migration traditionally dated to around 60,000 years ago. So who were these early pioneers and why did their populations disappear?

In this video, we explore the latest fossil, archaeological and genetic evidence reshaping the Out of Africa model. From a 210,000-year-old Homo sapiens skull found in Greece to evidence that our species may have evolved through interconnected populations across Africa, a new picture of human origins is emerging.

We also investigate a fascinating possibility: that stronger social networks, and the exchange of ideas and new technologies, helped Homo sapiens become adaptable enough to thrive in environments across the planet.

The story of humanity may not be of one great migration, but a long history of exploration, failure, connection and, eventually, survival.

Our brain may be a hybrid of two ancient nervous systems that were packaged together hundreds of millions of years ago. This is based on the finding that the front and back brain regions in people and several other species develop from two distinct cell types in embryos, instead of sharing the same developmental origin, as previously thought.

“Our research suggests that evolution took two existing neural systems and pushed them together spatially,” says at Stanford University in California. “Having the brain as one organ would probably be more efficient, but we rely on this primordial way to make the brain as two separate pieces.”

Loh and his colleagues studied early stages of mouse embryo development and found its brain develops from two types of early progenitor cells, proliferative cells with a limited capacity of self-renewal. One type expresses a gene called OTX2 and turns into the neurons found in the front part of the brain, comprising the forebrain and midbrain. The other expresses a gene called GBX2 and becomes the neurons in the back part of the brain, the hindbrain.

The researchers then conducted experiments using human cells in a dish and found that the neurons of the hindbrain and those of the forebrain and midbrain also develop from different progenitor cells. “We’ve shown for the first time that the front of the brain arises from a totally different progenitor cell than the back of the brain,” says Loh.

This explains why it has been so hard to grow human hindbrain tissue in a lab. “It was actually a summer student’s failed experiment that got us into this,” says Loh. Researchers have typically tried making hindbrain neurons from the progenitor cells that are destined to become forebrain and midbrain neurons, which doesn’t work, he says.

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Armed with this knowledge, the team was able to grow functional human hindbrain motor neurons in a dish for the first time by starting with the correct type of progenitor cell.  

The hindbrain coordinates fundamental, life-sustaining processes, such as breathing, sleeping, eating and the beating of the heart, whereas the forebrain is the centre of higher-level thought. Conditions like amyotrophic lateral sclerosis (ALS), the most common type of motor neuron disease, and spinal muscular atrophy cause speech and swallowing difficulties because they affect the hindbrain. Recently, scientists also discovered that GLP-1 drugs like Ozempic and Wegovy suppress appetite in mice by .

Now that it is easier to grow human hindbrain neurons in a dish, it should assist ALS and spinal muscular atrophy research, and allow us to investigate the precise mechanisms of how GLP-1 drugs work in people, says Loh.

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The researchers also studied early-stage embryos of chickens, zebrafish and acorn worms, and found their nervous systems are similarly derived from two different types of progenitor cells. This suggests our shared two-origin brain system arose at least 550 million years ago.

But jellyfish, which we diverged from around 600 to 700 million years ago, have two separate nervous systems. These may have joined up in our distant ancestors because it improved their overall processing power, says Loh. “You get more efficient communication when things are closer together,” he says.

Having our brains develop along two separate pathways might also have facilitated the evolution of complex thought, says Loh. While the hindbrain was taking care of all the basic functions for keeping us alive, “evolution could play around with the forebrain, and make mistakes and give rise to all the fancy things like memory and creativity”, he says.

A group of enigmatic ancient humans called the Denisovans thrived in what is now south-west China for more than 100,000 years. They lived in a conifer forest and hunted large animals like deer.

It is the first evidence of Denisovans in this region, and one of the only known sites with artefacts that can tell us about how they lived. For the first time, “we can know their technology and know their subsistence strategy”, says , an archaeologist at the Institute of Tibetan Plateau Research in Beijing.

The new site is only the third Denisovan locale “where we have everything”, says at the University of Toronto in Canada, who wasn’t involved in the study. “We have human remains, we have lithics [stone artefacts], we have fauna, we have the environmental context.” The other two sites are Denisova cave in Siberia and Baishiya Karst cave on the Tibetan plateau.

The Denisovans were hominins that lived in East Asia within the past few hundred thousand years. They were first identified 16 years ago from DNA preserved in a finger bone in Denisova cave. A smattering of other Denisovan remains have since been found elsewhere in East Asia, including Harbin in northern China.

However, for almost all of these remains, it is impossible to study their archaeological context – particularly in the case of . Because of this, we have little information about the tools and other artefacts they made.

The new finds come from Bianfu cave in Yunnan province. The area has been heavily quarried, destroying much of the cave, including the entrance, but in 2019 local workers spotted animal fossils and alerted the authorities. This led Li and his colleagues to conduct a rescue excavation that summer.

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They dug three trenches to a depth of 6.6 metres, revealing 18 layers of sediment – of which 10 contained artefacts made by hominins. One of these layers yielded four intact hominin teeth.

The excavation also unearthed over 64,000 fragments of bone. To find out whether any belonged to hominins, a team led by at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing first went through them by hand. The researchers identified 22 potential hominin remains, which they subjected to molecular analysis. Three proved to be from hominins: two fragments of skull and a piece of radius (an arm bone).

They tried extracting DNA from the three bones, but to no avail. Fu says the warm climate in the region and the age of the fossils were “unsuitable for DNA preservation”. However, the team did manage to obtain proteins from the three bones and two of the teeth. All five held protein variants that are unique to Denisovans, and none had any protein variants specific to H. sapiens or Neanderthals. Fu and her colleagues therefore determined that all the remains came from Denisovans.

The bones and teeth appear to be different ages, so Li’s team concluded that they come from at least four individuals. The two studied teeth had telltale protein variants showing they belonged to males.

The researchers dated the bones themselves and the surrounding sediments using multiple methods. They determined that the bones are between 134,000 and 167,000 years old. However, the artefacts cover a longer time frame: 70,000 to 190,000 years ago.

“We think this is one hominin species who occupied the site for a long time,” says Li. The excavators found a total of 1366 stone artefacts. These included 135 flakes and the cores from which they had been removed, along with simple tools such as scrapers and choppers. In general, the artefacts were relatively simple, compared with those made by Neanderthals living elsewhere in Eurasia at the same time.

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However, the Denisovans also seem to have been successful hunters. The team found 1654 identifiable animal bones, including large numbers of deer and bovids (hoofed animals that include cows and gazelles), plus rhinoceroses, hyenas and bamboo rats. Many of the bones had telltale marks suggesting they had been butchered by hominins; few showed any sign of being gnawed by non-human animals. Deer and bovids were the main prey, and the bones suggest the Denisovans generally hunted animals in prime condition.

Li says the Denisovans clearly had a “very skilful hunting strategy” to be able to bring down such prey. He argues they probably relied less on stone tools than other hominin groups, explaining why the stone artefacts they found were relatively crude. The team discovered seven bone tools, and Li suspects the Denisovans used these a lot. They may also have used tools made of wood or bamboo, which have since decayed.

This may reflect the comfortable environment in which these Denisovans lived. “It’s a different ecosystem from where we had them previously,” says Viola. Pollen evidence suggests the area was a mix of conifer forest and steppe, with a temperate climate and rich wildlife.

Denisovans elsewhere, such as those in Siberia or on the Tibetan plateau, faced greater environmental challenges like cold and low oxygen. They may well have developed different technologies and strategies to cope, says Li. Such places may have been at the limits of their range, while locations like Bianfu cave were in their heartland.

At Grotte Mandrin in France, a Neanderthal discovery has challenged everything we know about our human relatives. We travelled to the cave to meet palaeoanthropologist Ludovic Slimak, who discovered Thorin, one of the last known Neanderthals, reshaping our understanding of human history.

From the remarkable preservation of remains to the groundbreaking DNA evidence there, Slimak explores how an isolated Neanderthal population survived for tens of thousands of years, even interacting with early Homo sapiens, before disappearing. Thorin’s story challenges assumptions about intelligence, identity and survival, offering a new perspective on what it means to be human.

Scientists once believed they could pinpoint the moment when humans first appeared. But as new fossils have filled in our evolutionary history, that defining moment has begun to disappear – replaced by a far stranger picture of what it means to be human.

Lucy is one of the most famous fossils ever discovered, but in recent years her spotlight has dimmed. She was part of the species Australopithecus afarensis and for decades was considered one of our earliest known human ancestors. To understand Lucy is to understand the story of discovery, human evolution and, ultimately, the changing story of us.