- Just 1.5% to 7% of the human genome is unique to our species, a new study suggests.
- Neanderthals, Denisovans, and other ancestors share most of the same genes found in modern humans.
- Genes unique to humans are involved in brain development, which may be what sets our species apart.
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Humans like to think they're special, but our genes suggest that's far from the case.
No more than 7% of the human genome is unique to Homo sapiens, according to a study published Friday in the journal Science Advances.
We share the remaining chunks of our genetic material with other human ancestors, or hominins, including our Neanderthal cousins and the Denisovans first discovered in east Asia.
"The evolutionary family tree shows there are regions of our genome that make us uniquely human," Richard Green, director of the paleogenomics lab at the University of California, Santa Cruz and co-author of the new study, told Insider. "Now we have a catalog of those, and it's a surprisingly small fraction of the genome."
Anthropologists already knew that our hominin ancestors all interacted and interbred – exchanging genes and stone technologies that altered the course of our species' evolution. But these new findings further underscore just how frequently that intermingling happened in the last 300,000 years or so, since the first known population of modern humans emerged.
"More or less everywhere we look, admixture is not the exception at all, but rather the rule," Green said.
Genetic evidence suggests our ancestors interbred with mysterious hominins
To construct a hominin family tree, Green's team sequenced and compared genomes from 279 modern humans – sampled from people all over the world – to ancient genomes from one Denisovan and two Neanderthals. Then, the researchers used a computer algorithm to determine out how each of those individuals are related to each other.
The analysis tool, which Green said took years to develop, helped them distinguish what parts of the human genome are devoid of admixture – meaning these sets of genes aren't seen in Neanderthals or Denisovans.
The algorithm also highlighted what genes humans inherited from an even older ancestor, one that lived 500,000 years ago or so, that eventually gave rise to our species as well as Neanderthals and other hominins.
The study results suggest mysterious populations of human ancestors that scientists haven't even discovered yet may have interbred with Neanderthals and Denisovans before these species mixed with modern humans, Green added.
Genes unique to humans are related to our brain development
Researchers have already identified many of the human genes that resulted from cross-species trysts, but this is the first study to pinpoint what regions of genes were completely devoid of admixture, according to Green.
His group found these uniquely human regions of our genome were "incredibly enriched for genes that have to do with neural development," Green said.
While Neanderthals have similarly large, if not larger, heads than humans do, that cranium size tells us little about how well their brains work compared to ours.
"Now we know human-specific stuff has to do with brain function," Green said.
And most of these uniquely human genes came out during two distinct bursts of evolution – one that happened 600,000 years ago and another 200,000 years ago – the study found.
One of those evolutionary waves could've laid the genetic groundwork for human communication, Green said.
"It's extremely tempting to speculate that one or more of these bursts had something to do the incredibly social behavior humans have – mediated in large part by our expert control of speech and language," he said.