The human family tree just got another — mysterious — branch, an African “sister species” to the heavy-browed Neanderthals that once roamed Europe.

While no fossilized bones have been found from these enigmatic people, they did leave a calling card in present-day Africans: snippets of foreign DNA.

There’s only one way that genetic material could have made it into modern human populations.

“Geneticists like euphemisms, but we’re talking about sex,” said Joshua Akey of the University of Washington in Seattle, whose lab identified the mystery DNA in three groups of modern Africans.

These genetic leftovers do not resemble DNA from any modern-day humans. The foreign DNA also does not resemble Neanderthal DNA, which shows up in the DNA of some modern-day Europeans, Akey said. That means the newly identified DNA came from an unknown group.

“We’re calling this a Neanderthal sibling species in Africa,” Akey said. He added that the interbreeding probably occurred 20,000 to 50,000 years ago, long after some modern humans had walked out of Africa to colonize Asia and Europe, and around the same time Neanderthals were waning in Europe.

The find offers more evidence that for thousands of years, modern-looking humans shared the Earth with evolutionary cousins that later died out. And whenever the groups met, whether in Africa or Europe, they did what came naturally — they bred. In fact, hominid hanky-panky seems to have occurred wherever humans met others who looked kind of like them — a controversial idea until recently.

In 2010, researchers from the Max Planck Institute for Evolutionary Anthropology in Germany announced finding Neanderthal DNA in the genomes of modern Europeans.

Barrel-chested people whose thick double brows, broad noses and flat faces set them apart from modern humans, Neanderthals disappeared about 25,000 or 30,000 years ago.

Another mysterious group of extinct people recently identified from a 30,000-year-old finger bone in Siberia — known as the Denisovans — also left some of their DNA in modern-day Pacific Islanders.

And while modern humans and the newly found “archaic” Africans might be classified as distinct species, they produced viable offspring. Likewise, donkeys and horses, lions and tigers.

One skull found in Nigeria with puzzling “primitive” features may represent a survivor of these mystery people — or a hybrid with anatomically modern humans — said Chris Stringer of the Natural History Museum in London, who was not involved in the new work.

“You can argue, are these really different species?” Stringer said.

Stringer added that he was not surprised to see genetic evidence for another humanlike group in Africa that interbred with anatomically modern people, who are thought to have emerged in Africa about 200,000 years ago.

Still, without a definitive fossil, it’s impossible to say what these people looked like. But one thing is clear: This enigmatic group left its DNA all across Africa. The researchers found it in the forest-dwelling pygmies of central Africa and in two groups of hunter-gatherers on the other side of the continent — the Hadza and Sandawe people of Tanzania.

Starting a decade ago, a team led by Sarah Tishkoff and Joseph Lachance of the University of Pennsylvania drew blood from five individuals in each of the three groups. Using the latest genetic technology, Tishkoff spent $150,000 to read, or sequence, the DNA of these 15 people. The research was reported Wednesday in the journal Cell.

In addition to finding evidence of the now-extinct humans, the team discovered a huge range of genetic diversity between the three groups. The human genome contains about 3 billion letters, or base pairs, of DNA. Before this study, scientists had found that about 40 million of these letters vary across human populations.

But in the 15 Africans, Tishkoff and Lachance found 3 million more genetic variants — a huge treasure trove of human diversity. Among this stunning variety, Tishkoff says they have pinpointed some of the genes responsible for the short stature of the pygmies, who average less than 5 feet in height. She also found that immune system genes and genes for taste and smell varied wildly between the three groups — confirming Africa as the seat of the widest range of human diversity.

The oldest modern human skull, found in Ethi­o­pia, dates to 195,000 years ago. For more than 150,000 years, then, humans shared the planet with cousin species.

Despite all the amorous advances, however, only one group survived: us.

Akey said: “As we were conquering the world, we also conquered similar human populations that were dying out.”