How Human Are Humans? New Study Suggests About 7 Percent

Our oldest ancestors came from Africa. Current models suggest that anatomically modern humans radiated out from the Great Rift Valley, which runs through modern-day Kenya, Ethiopia, Uganda, Tanzania and Sudan, some 200,000 years ago.

Neanderthals were an ancient group of hominins — human ancestors — that lived alongside early modern humans until about 40,000 years ago. They were shorter and stockier on average than humans, with broad noses and a prominent brow ridge. Like us, they made use of fire, created paintings and jewelry, and lived in shelters (which they apparently kept quite tidy). Some paleoanthropologist even believe that Neanderthals buried their dead.

The first Neanderthal fossil was identified in 1856 in the Germany's Neander Valley (although an earlier 1829 find was subsequently recognized as belonging to Neanderthal). They were found throughout Europe, where they apparently interbred with humans regularly. Today, most people of European descent have some Neanderthal genes.

The Denisovans are a less well-recorded group compared to Neanderthals. First found in 2008, these hominins were also contemporaries of early modern humans, disappearing sometime between 30,000 and 15,000 years ago. Not much is known about the Denisovans — except, of course, for their entire genome, which was sequenced from a single pinky bone discovered in a Siberian cave.

Scientists do have evidence that the Denisovans occupied much of the area that is now east Asia, Siberia, Indonesia and New Guinea. People with ancestry in these areas are likely to carry both Denisovan and Neanderthal DNA.

But how do we know what's in our DNA — or for that matter, where it came from?

A genome is a complete genetic map of an organism's DNA — every single gene, functional or not. Before the early 2000s, nobody had recorded the entire genome from a human being; all scientists had were snippets of individual gene sequences, like displaced puzzle pieces. That changed in 2003 when the Human Genome Project, a 13-year multinational effort to map all 3.2 billion base pairs in human DNA, was finally completed.

Genetic sequencing technology has undergone a Renaissance since then. Today, one lab can sequence hundreds of individual human genomes in a year. And scientists been able to map the genes of other species, including Neanderthals and Denisovans.

"Genomes are very useful for two reasons," says Omer Gokcumen, an evolutionary anthropologist with the University at Buffalo.

One reason is that genomes record ancestry. Every human inherits half of their genes from each of their parents in the form of tightly coiled chromosomes. Each parent, in turn, inherited half of their genes from their parents, and so on back down the line. "So you are actually carrying a population of genomes," Gokcumen says. That means that a particular version of a gene can be traced all the way back to the ancestor who first carried it.

The other thing that makes genomes interesting is mutations. DNA naturally accumulates tiny mutations over time. Not all of them get passed down to the next generation, but they do build up at a roughly steady rate. This allows scientists to measure the percent difference between two genomes to determine when they diverged from one another — a technique called "DNA dating," or "molecular clocks."

Some of those clocks are easy to spot when experts compare two genomes. Scientists sometimes find a chunk of genetic sequence, Schaefer says, and it becomes clear that "it's just a linked set of mutations that were all inherited together from Neanderthals."

"The idea of what it means to be human is kind of complicated given how much mixing has happened between us and these other species," Schaefer says.

But to tackle that complicated question, Schaefer and his co-authors did something interesting. A lot of contemporary research has looked at the places where human DNA aligns with the DNA of Neanderthals and Denisovans. "And we flipped it around and said, 'Well, where in the genome do you see neither of those?'" he says.

Shaefer and the study authors narrowed it down to a handful of genes, which could be traced back over 600,000 years, before our very earliest modern ancestors. "Even though this is a relatively small amount of the genome, it statistically contains a lot of genes and sequences that might be functional," Schaefer says. Even more interestingly, most of these genes seem to have something to do with brain development.

At the end of the day, we are beautiful puzzles made up of all of these pieces: Neanderthal, Denisovan and distinctly human. And our differences are just as important as our similarities.

"Biological variation is part of what makes us human," says Gokcumen, "and that is actually kind of cool."