Neanderthals and humans may belong to the same species, say scientists. It could rewrite the history of our evolution.

• Up until recently, the consensus was that Neanderthals and Homo sapiens were separate species.
• But most humans carry around 2% of Neanderthal DNA, challenging the view that we are different. 
• Other studies suggest Neanderthals weren't inferior to Homo sapiens and should be considered human.

Neanderthals have long been portrayed as dim-witted, brutish monsters who were genetically inferior to our direct ancestors, early modern humans.

These ape-like creatures spoke in grunts, were beset with illnesses, and died out 40,000 years ago after losing the evolutionary battle against Homo sapiens.

Or at least, that's what we've been told. Recent discoveries, however, are upending that view and reigniting a debate among scientists about whether Neanderthals should be considered to be the same species as early modern humans.

If Neanderthals belonged to our species, it could reshape the history of human evolution and challenge how we define what makes us human.

Most of us have some Neanderthal DNA

The first fossils of Neanderthals were identified almost 200 years ago. By now, you would think scientists would have made up their minds about whether they should be defined as a separate species from Homo sapiens.

But it turns out this is a matter of fierce debate, Antoine Balzeau, a paleontologist from the Museum National d'Histoire Naturelle in France, told Business Insider.

"When we were at first discussing the fossils in the 19th century, there was no real debate about specific species or not, simply because at the time, humans were seen as a species but by default," he said.

As more fossils emerged, scientists started questioning the strict separation between the species.

Still, up to recently, the consensus was mostly that Neanderthals should be seen as separate. The hominins, who roamed Europe as early as 430,000 years ago, only briefly interacted with Homo sapiens emerging from Africa, who reached Europe about 50,000 years ago.

The lineages separated about 500,000 years ago — relatively recently in the story of human evolution, but long enough ago that they looked significantly different. For many, that evidence was enough to close the debate: Neanderthals and Homo sapiens were separate species.

That view started to change in 2008 when Swedish geneticist Svante Pääbo achieved something that was thought to be impossible: he sequenced the genome of a Neanderthal by extracting DNA from ancient bones.

Through his research, Pääbo was able to show that there's a little Neanderthal in most of us. In fact, he showed most living humans carry around 2% of Neanderthal DNA.

The evidence also suggests that human ancestors and Neanderthals likely had children together when they cohabitated about 50,000 years ago.

This news created a dogmatic rift, opening up the possibility, once again, that Neanderthals and humans should be considered to be the same species.

After all, according to the strict biological definition of species, animals from different species shouldn't be able to produce fertile offspring.

"It definitely was a big game changer at that point," Laura Buck, an evolutionary anthropologist studying hybridization between hominin species, told BI.

"I think it has sort of brought that discussion to the forefront again," she said.

Were Neanderthals more than just our distant cousins?

The idea that species can't reproduce is "intuitively attractive because it's sort of clear cut," Buck said, "but biology isn't clear cut."

She points to several examples of mammals that have been known to interbreed and have fertile offspring, like wolves and dogs, despite being clearly defined as separate species.

For her, a better definition of Neanderthals, the most scientifically tried and tested, is the first one: the characteristics of their bones separate them from modern humans and their direct ancestors.

"I know there are various different papers saying if you shaved a Neanderthal, put him in a suit, and put him on the tube or the subway in New York, no one would notice, I don't think that's true," she said.

"I think we'd definitely think they looked a bit weird," she said.

Balzeau agrees. "There may be some discussion between specialists about how we define the different groups but from a paleontological point of view, Homo neanderthalensis and almost Homo sapiens are very clear anatomical differences," said Balzeau.

For others, however, the genomic information should be another argument to liberate the Neanderthal from its knuckle-dragging stereotype.

That's the case for Paul Pettitt, an archaeologist who specializes in the Paleolythic at Durham University in the UK.

"It would be guesswork to use that evolutionary divergence to assume that there are different species," he told BI.

Can culture define a species?

Over the past two decades, digs started emerging showing Neanderthals may have been much more sophisticated than had been previously thought.

Pettitt counts himself among those who, until recently, were skeptical that Neanderthals could have any sense of sophistication.

"Until say 20 years ago, Neanderthal behavior was looked on as fairly stupid, or at least fairly limited, and Homo sapiens were, by contrast, seen as quoting Shakespeare, as they dance across Europe," he joked.

"Which is, of course, nonsense, but it's a very entrenched view," he said.

After all, relative to their size, some studies suggest Neanderthals had a brain at least the same size, if not bigger, than our ancestors, indicating that they may have been very cerebral, Pettitt said

"You don't buy a top-of-the-range computer simply to use it as an alarm clock. There's got to be an evolutionary reason why Neanderthals had selected for this remarkably, metabolically expensive tissue," he said.

Studies have suggested that Neanderthals were skilled hunters, and hide workers, created rudimentary jewelry, had a complex lythic industry, and even worked with pigment.

Some scientists even believe they may have had some form of spiritualism, and would bury their dead, revere lions, and may have even created cave paintings — though that evidence is still a matter of debate.

For Pettitt, this suggests that as Neanderthals and humans lived alongside each other in Europe, there's a good chance they shared a culture or learned from observing each other.

If they spoke, he said, "we can assume they probably spoke different languages. But that similarity suggests there was in fact a shared meaning, however simple."

Could Neanderthals be called human?

There's a bigger picture question at stake here: should Neanderthals be considered humans?

"What humanity is very much depends on which group of people you're talking to," Buck said.

"It's something that is culturally defined, but it's also something that has sort of value judgments. We talk about inhumanity. We talked about humanity. It's not something that just refers to different types of organisms," she said.

With the sheer number of people who are alive today, there is arguably more Neanderthal DNA on Earth than there ever was before.

Angela Saini, author of "Superior: the Return of Race Science," argues that there is a real risk of getting this wrong. Those who are thought to have more Neanderthal DNA today could be wrongly thought of as inferior.

Early studies have linked these Neanderthal genes to modern health effects like autoimmune diseases, diabetes, and some cancer— though how these genes exactly affect the health of the person who carries them is still mostly unclear. Neanderthal genes have also notably been associated with catching COVID-19.

Because East Asian populations have been found to carry slightly more Neanderthal DNA on average, there is a real danger that this information will be used for discrimination.

The flip side is that our interpretation of Neanderthal culture has changed dramatically in recent years. Saini notes that the Neanderthal image was rehabilitated just as people started to draw them closer to populations in Europe, and genetic information started suggesting they were fairer skinned with red hair.

"That's what I find particularly galling. A hundred or so years ago, the supposed similarity between Neanderthals and Aboriginal Australians was used as a justification to draw living modern humans out of the circle of humanity," she told WNYC.

"Now, because we see that Neanderthals have some relationship to modern-day Europeans, Neanderthals themselves an extinct species has been thrown into that circle of humanity."

Rewriting our history

We are still in the process of understanding Neanderthals and our relationship to them. As we begin to unpick the history of human and Neanderthal evolution, new scrutiny is being placed on the decision of scholars to separate the two and depict one as superior.

When he revealed his research, Pääbo reflected on how humans living on the Earth today are rather exceptional — not necessarily because Homo sapiens are intrinsically better, but because there is very little time in the history of human evolution when Homo sapiens were the only hominin or human on the planet.

"Had Neanderthals and Denisovans survived, how would we deal with that today?" Pääbo said.

"Would we experience even worse racism against them than what we experience among us today — because they were in some respects really different — or could we think differently and say if we had them here today we would not just have one type of humans?" he added.

"I think both things are possible and it sort of reflects our view of humans and how we speculate about that."