Our own genomes contain DNA from Neanderthals and Denisovans, two ancient human lineages that have been discovered via genetic study. And there may possibly be more to discover.

Humanity was much more diversified tens of thousands of years ago than it is today. Neanderthals and Denisovans were two of the many human lineages that made up the species Homo that formerly roamed the planet. The last surviving member of the group is Homo sapiens today. But our DNA still contain traces of those extinct humans.

We are aware of several occasions throughout our history when our ancestors interbred with both Neanderthals and Denisovans. In our modern genomes, DNA fragments from both groups were left over from those trysts. The percentages can reach up to a few percent of the human genome and vary depending on ethnicity. Neanderthal genes have been linked to a variety of modern biological features, including our immune systems, hair, and skin.

The Origins of Hominins

A more complex picture of the relationships between the two groups has been formed in recent years by studies into the linked histories of humans and our hominin relatives. Multiple instances of human interbreeding with both lineages suggest a complicated common past. (Anthropologists disagree as to whether Neanderthals and Denisovans should be considered different species; the term lineages is frequently used.)

Even more research has suggested that our genomes may contain genetic material from undiscovered human lineages. Although they can be seen as weak echoes in our genomes, so-called "ghost lineages" have not yet been connected to real fossil evidence.

Around ten years ago, the first genomes of Neanderthals and Denisovans were sequenced, giving ancient human research a significant boost. Anthropologists can now examine the genomes of H. sapiens and other ancient people to look for commonalities for the first time. The findings showed unmistakable evidence of interbreeding, as the human genome is capable of containing DNA segments that strikingly resemble both the Neanderthal and Denisovan genomes.

Humans' Neanderthal DNA

Although we are aware that both Neanderthals and Denisovans contributed DNA to humans, the proportions vary. About 2% of the DNA of people with European and Asian ancestry comes from Neanderthals. Contrarily, denisovan DNA is only found in individuals from Asia and can range from a few to about 5% of their genomes.

Africans were long believed to lack Neanderthal or Denisovan DNA because these ancient humans only acquired those genes after leaving Africa. Contrary to that long-held belief, a study from 2020 found trace quantities of Neanderthal DNA in people from Africa. According to the authors, it most likely resulted from people returning to Africa from Europe or other places.

The story has recently been expanded by the discovery of more fossils. Scientists revealed in 2018 that they had discovered the remains of a young girl who was a Neanderthal-Denisovan hybrid. The "Denisova 11" specimen, who lived more than 50,000 years ago, had a Neanderthal mother and a Denisovan father. According to a report published in 2021 that examined the oldest human DNA ever discovered, which was discovered in the Bacho Kiro cave in Bulgaria and dates to roughly 45,000 years ago, the humans had Neanderthal forebears only a few generations ago.

Modern humans and Denisovan DNA

Similar results from comparisons of contemporary human DNA with the Denisovan genome suggest that our ancestors interbred with prehistoric hominins. Denisovan DNA has been found in the genomes of persons of Asian and Pacific Islander heritage in recent studies.

Scientists believe that after arriving to the area tens of thousands of years ago, their ancestors eventually mixed with Denisovans. These genes have occasionally shown to be advantageous. By inheriting a crucial gene from Denisovans, current Tibetans are better able to withstand the damaging effects of altitude, according to research on a Denisovan jawbone discovered in a cave high on the Tibetan plateau.

Neanderthal genes have also been connected to various diseases like myocardial infarctions, depression, allergies, and how our skin responds to UV light. We'll probably learn more as we gain a better understanding of the DNA of both human beings and other hominins.

Ancient Genetic Ghosts' DNA

There are indications that certain genes in contemporary humans come from unidentified hominin lineages. One study combed through genomes using sophisticated modeling tools and identified genes from an unidentified hominid in the Denisovan genome. The scientists claim that some of these genes were later passed on to humans, albeit they are unable to identify these early forebears.

There is evidence from earlier studies that humans and other hominid lineages may have interbred in Africa at some point. Homo naledi and Homo heidelbergensis, two now-extinct lineages of humans, co-existed on the continent at one time, and it's probable that they mated with our own ancestors at some point.

Even though we are aware that humans and Denisovans interbred, current findings suggest that the group presently known as Denisovans may have actually included several distinct lineages. If true, more than one different group may have contributed to the Denisovan DNA in H. sapiens.

DNA from the past and human history

Even as we discover more about our distant past and the hominin subspecies that share our genetic makeup with us today, it's important to keep in mind that not everything is certain.

In recent years, our understanding of many aspects of human history, such as when we left Africa and how many diverse groups of prehistoric humans there were, has evolved. Some of our current beliefs might not be valid once new information becomes available. And additional investigation could reveal totally new facets of our past.

Therefore, even while we are aware that ancient human populations have left DNA behind in us, we are unable to determine exactly how much or how many lineages it originates from. There is still a lot to learn, and that is the only thing that is definite.