Just 7% of Human DNA Is Unique, Says Latest Large-scale Genetic Study
Among all the species of man that ever existed, Homo sapiens (modern man) is the sole survivor. But that doesn’t make us quite as special as we thought. A genetic study carried out by researchers at the University of California Santa Cruz (UC Santa Cruz) found that modern humans only possess a small fraction of fully unique human DNA. The vast majority of our collective genetic inheritance is something we shared with other species of ancient man, specifically our long extinct “cousins” the Neanderthals and the Denisovans.
So how much of human DNA belongs exclusively to us, and was never carried by any other human species? Just seven percent, the researchers responsible for this research explain in their Science Advances journal study.
“That's a pretty small percentage,” UC Santa Cruz computational biologist and study co-sponsor Nathan Schaefer told the Associated Press. “This kind of finding is why scientists are turning away from thinking that we humans are so vastly different from Neanderthals.”
That seven percent is something we share with all modern humans who have lived and died over the past 200,000 years, the approximate time that has passed since Homo sapiens first evolved. Of this seven percent, the majority is present in some people but not others. Just 1.5 percent of their DNA is both unique to us and shared by everyone currently living on the planet.
Modern humans are not a singular or special creation of evolution. They are mostly a mixture of genetic materials taken from other ancient species, all of which developed long before Homo sapiens came into existence.
Fig. 2 from the Science Advances study: Performance of SARGE on SGDP and archaic hominin dataset: (A) Pairwise coalescence times for randomly sampled sets of up to 10 pairs of phased genome haplotypes per population in ka (thousands of years ago). Values are calibrated using a 13 million years ago (13 Ma) human-chimp divergence time. (B) Unweighted pair group method with arithmetic mean trees computed using nucleotide diversity from SNP data (top and left) against similarity matrix from shared recombination events inferred by SARGE (Speedy Ancestral Recombination Graph Estimator). (Science Advances)
Tracking The Shared Genetic Inheritance in Human DNA
To complete this study, the team of researchers from UC Santa Cruz examined genetic data collected from the fossilized remains of Neanderthals and Denisovans that lived between 40,000 and 50,000 years ago. They compared this ancient DNA to genetic material extracted from 279 people living today, searching for overlaps and differences.
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Since ancient genomes taken from human fossils aren’t always complete, this is a difficult comparison to make. But the researchers developed a method that allowed them to fill in the blanks when information was missing from ancient genomes. As a result, they were able to identify all the genetic material shared between modern humans and Neanderthals or Denisovans, which represented 93 percent of the total found in the Homo sapiens genome.
It is important to emphasize that Neanderthals and Denisovans are not the direct ancestors of Homo sapiens (we didn’t evolve from them, in other words). Instead, they share common ancestors with us, which is why they are often referred to as our cousins. The 93 percent of our DNA that we share with Neanderthals and Denisovans was inherited from those ancestors, who lived on earth millions of years ago.
This shows how close the genetic relationship between these three species actually was and explains how humans could interbreed with the others despite their differences.
It is known that interbreeding occurred, since a small part of the human genetic code contains DNA inherited from Neanderthals and Denisovans (interbreeding with Neanderthals was far more common). Modern humans began migrating out of Africa and spreading across the planet in significant numbers between 100,000 and 70,000 years ago. They would have lived side by side with the other two species for 50,000 years or so after that, until the Denisovans and Neanderthals became extinct (the exact dates when that happened are disputed).
It is possible to see the Neanderthal and Denisovan decision to interbreed with modern humans as a kind of survival strategy. By doing so, they guaranteed their species would never die out completely. They exist even now, as traces of DNA that are still shaping our development.
What is it then that makes humans unique? The answer is neural development and brain function. This modern woman's brain is "wired" differently than the brains of our prehistoric ancestors. (raisondtre / Adobe Stock)
Human Uniqueness? Neural Development and Brain Function!
Some of the most eye-opening information obtained in this new study emerged from an analysis of the 1.5 percent of modern human DNA that is shared by everyone. Further study of this genetic material may help scientists understand what separates Homo sapiens from other species of man, since it never existed in those other species but is universal in us.
"We can tell those regions of the genome are highly enriched for genes that have to do with neural development and brain function," explained UC Santa Cruz computational biologist Richard Green, a co-author of the Science Advances study.
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The primary goal of this study was to uncover what genetic factors make modern humans unique. Homo sapiens is the only hominin (the biological group made up of modern humans, extinct humans, and all our immediate ancestors) that survived into the modern era, meaning it likely possessed some qualities that the others lacked.
It is presumed the advantage had something to do with cognitive abilities. This would seem to make the discovery of unique genes in humans that affect mental development and brain function highly significant.
Of course, it is possible that Homo sapiens survived by luck rather than skill. As the populations of all three species grew, maybe Homo sapiens just happened to be living in the right places at the right times, enjoying more favorable weather and more access to food and water when resources started to become scarce.
Most likely, good luck and evolutionary advantages both played a role in deciding the final outcome.
Top image: Neanderthal skull (left) compared with a modern human skull (right). A recent study has revealed that only 7% of human DNA is unique. The other 93% of human DNA is shared with our ancient “cousins” the Neanderthals and Denisovans. Source: procy_ab / Adobe Stock
By Nathan Falde