Less Than 7% of the Human Genome Is Actually Uniquely Human, Study Finds

It’s common knowledge that humans share a portion of our genome with Neanderthals, but a new study reports just how much of the human genome is unique to our species and was not inherited from our archaic ancestors, and it's surprisingly little.

Researchers have found that only 1.5 to 7 percent of the modern human genome is uniquely human. This surprisingly small amount accounts for our neural development and function which happened in “multiple bursts” of adaptive changes over the past 600,000 years, according to their findings. 

The study, which was published in Science Advances on Friday, used an algorithm to build what are known as ancestral recombination graphs of the human, Neanderthal, and Denisovan genomes that allowed them to pinpoint regions of DNA that are unique to modern humans. The researchers used genomes from 279 humans, two Neanderthal genomes, and one Denisovan genome. 

Looking closer at regions of the DNA that were human-specific, the researchers report that groups of alleles were found to be “heavily enriched” for genes related to cell adhesion, neuron growth, and synapse assembly. A number of mutations involved in neural cell migration and the clearing of toxic substances from the brain were also found. However, they note that this test did not suggest functional consequences for identified mutations. 

“Any given mutation could be inconsequential, or alter the regulation or one or more genes,” Lead author Nathan K. Schaefer, a postdoctoral biomolecular engineering student at the University of California in Santa Cruz, told Motherboard in an email. “Tying human-specific mutations in those regions to functional changes is an exciting next step that will require experiments.”

The study doesn’t say much about what characterizes the genome that is shared with archaic hominin, but Schaefer says this can be difficult to pinpoint. Instead, he referenced other studies that highlighted alleles of genes involved in the immune system that were favored by natural selection in cases of “adaptive introgression,” or interbreeding, in humans who inherited DNA from archaic hominins.

Though less than 7 percent seems like a very small amount of uniquely human DNA, that approximate percentage of the human genome could determine more about the human species than we may realize.

“Biological features specific to modern humans are under the control of a relatively small number of genetic changes that can be investigated in follow-up studies. It also means that some groups of archaic hominins–such as Neanderthals–were probably not as different from ourselves as was once believed,” Schaefer said.