The story of human origins is a mysterious and complex tale, and one full of shadows. These aren't just the shadows of a past that we can't fully recall, either through the study of the historical record or earlier anthropological evidence; modern science continues to discover the literal "shadow" of our past relatives in our modern DNA.
Human ancestry was far more diverse and complex than was once believed, and every year we are adding new members to our ancient family tree. But unlike in the past when such discoveries were made in the dusty floors of caves or in ancient burial sites, today we can learn almost as much about our complex origins from studying the genetics of modern human groups around the world.
And, rather strikingly, we continue to find traces of unknown hominid ancestors when doing so.
Science Alert recently reported on a study led by João Teixeira of the University of Adelaide, and his colleague, biologist Alan Cooper, who discovered evidence of what they believe to be “ghost” ancestors in modern DNA.
One such discovery, made in 2018 but which had not been widely reported at that time, indicated an unusual genetic trait that is discernible in the population of Flores, Indonesia, which the researchers believe to be “as divergent from modern human DNA as Neanderthal or Denisovan.”
Neanderthal fossils were first discovered at the Feldhofer Cave of the Neander Valley, near Düsseldorf, Germany, in 1856. Although different in appearance from modern humans, there is still some debate as to whether they should be categorized as an entirely different species from us, or as a subspecies of modern humans. The Denisovans weren’t discovered until fairly recently, and going off of little more than a fingertip fragment and teeth recovered from Denisova cave in Siberia. Since then, other fragmentary remains have been discovered, including one mysterious hominid jawbone found off the Taiwanese coast which some believe could have Denisovan origins.
In all of the aforementioned cases, the discovery of these archaic human groups involved physical remains recovered from what a type site (the location after which the species or group is named) or other location. There is now also genetic evidence of encounters between modern humans and these known, but now-extinct relatives; shortly after our exit from Africa, Homo sapiens encountered Neanderthals, and the results of these interactions can still be found in the DNA of as much as 2% of modern populations. Similar genetic traces of our interactions with the Denisovans can be found (albeit to a lesser degree than that of Neanderthals).
However, there are also traces of interactions between humans and species that cannot be identified. According to Science Alert:
The first unknown extinct hominid - named EH1 - was roughly genetically equidistant from Denisovans and Neanderthals. The ancestor of all Asian and Australo-Papuan populations bred with EH1, resulting in 2.6 to 3.4 percent shared EH1 ancestry.
The other "unknown" in this equation, as discussed earlier in relation to genetic traces found among the modern populations of Indonesia (dubbed EH2) "only appears in short-statured people that live near Liang Bua Cave," which, not surprisingly, is where evidence of Homo floriensis, a diminutive species that have been likened to real-life "Hobbits" of the ancient world, was discovered in 2004 (we should note that the unidentified EH2 is not taken to be one and the same as Homo floriensis, but rather some other extinct hominid type that remains undocumented).
As Teixeira and Cooper summarize, "At least 3 different hominin groups appear to have been involved in Asia, of which only the Denisovans are currently known. Several interbreeding events are inferred to have taken place east of Wallace’s Line, consistent with archaeological evidence of widespread and early hominin presence in the area."
Current archaeological evidence, as with that of fossil remains, provides no evidence that these archaic human groups made it as far south as Australia and Tasmania (both being parts of the ancient continent of Sahul, as it would have existed prior to 18,000 years ago). However, as the authors of the PNAS study have noted, genetic evidence in this region of the world "remains enigmatic," hinting at further possibilities with relation to the many mysteries in our ancient family tree... and the "ghosts" of others whom our ancestors occasionally encountered.