A landmark study found that only 1.5% to 7% of the human genome contains uniquely (modern) human DNA. The rest is shared with relatives such as Neanderthals and Denisovans.
However, the DNA that is unique to us is pretty important, as it's related to brain development and function.
Researchers used DNA from fossils of our close relatives (Neanderthals and Denisovans) dating from around 40,000-50,000 years ago and compared them with the genome of 279 modern people from around the world. They used a new computational method that allowed them to disentangle the similarities and differences between different DNA with greater detail.
Many people around the world (all non-African populations) still contain genes from Neanderthals, a testament to past interbreeding between the two species. But the importance of this interbreeding may have been understated. The new study found that just 1.5% of humans' genome is both unique and shared among all people living now, and up to 7% of the human genome is more closely related to that of humans than to that of Neanderthals or Denisovans.
This doesn't mean that we're 93% Neanderthal. In fact, just 20% of Neanderthal DNA survives in modern humans, and non-African humans contain just around 1.5-2% Neanderthal DNA. But if you look at different people, they have bits of Neanderthal DNA in different places. So if you add all the parts where someone has Neanderthal DNA, that ends up covering most of the human genome, although it's not the same for everyone. This 1.5% to 7% uniquely human DNA refers to human-specific tweaks to DNA that are not present in any other species and are strictly unique to Homo sapiens.
In addition, this doesn't take into account the places where humans gained or lost DNA through other means such as duplication, which could have also played an important role in helping us evolve the way we are today.
What makes us human
The research team was surprised to see just how little DNA is ours and ours alone. But those small areas that make us unique may be crucial.
"We can tell those regions of the genome are highly enriched for genes that have to do with neural development and brain function," University of California, Santa Cruz computational biologist Richard Green, a co-author of the paper, told AP.
The exact biological function of those bits of DNA remains a major problem to disentangle. Our cells are filled with "junk DNA", which we don't really use (or we just don't understand how our bodies use it yet) -- but we still seem to need it. We're not even sure what the the non-junk DNA bits do. Understanding the full instructions and role that genes have is another massive challenge that's not yet solved.
What this study seems to suggest is that interbreeding played a much bigger role in our evolutionary history than we thought. Previous archaeological studies also suggest this: humans interbred with Neanderthals, Denisovans, and at least one other mysterious species we haven't discovered yet (but we carry its DNA). Researchers are finding more and more evidence that these interbreeding events weren't necessarily isolated exceptions but could have happened multiple times and over a longer period than initially thought. It's up for future studies to reconcile the archaeological and anthropological evidence with the genetic one.
The study also found that the human-specific mutations seemed to emerge in two distinct bursts: 600,000 years ago and 200,000 years ago, respectively. It's not clear what triggered these bursts; it could have been an environmental challenge or some other event, which at this point is unknown.
Researchers say that studying this 1.5-7% of our genome could help us better understand Neanderthals and other ancient populations, but it could also help us understand what truly makes us human. For instance, you could set up a laboratory dish experiment where you'd edit out the human-specific genes and revert them back to their Neanderthal function, and compare the molecular results of this change. It wouldn't exactly be like bringing back a Neanderthal, but it could help us deduct how Neanderthals would have been different from modern humans -- or, in counterpart, what makes humans stand out from our closest relatives.
The study "An ancestral recombination graph of human, Neanderthal, and Denisovan genomes" has been published in Science.