Today every human on earth is known scientifically as a Homo sapien. But if you dig a bit back into time, say around 700 to 200 thousand years ago, give or take a few thousand, you’ll find that there were a number of other Homo species. This time period is called the Middle Pleistocene, and during this time there may have been up to 7 different Homo species living simultaneously (Figure 1).
However, Paleoanthropologists differ on the number of species that overlap through time for a variety of reasons, which is just one small area of controversy within the story of human evolution.
To list a few:
- The confirmed range of a species (when it first and last appeared).
- The legitimacy of a species.
- Acceptance of reliable dating techniques.
Although there are contradictions to how many Homo species existed and if they interacted, we now know that there was at least sexual admixture between the Neanderthals, Denisovans, and modern humans (Wei-Haas, 2018). So, with such a large amount of time during the Middle Pleistocene, and with potentially 7 other Homo species, where were they meeting?
To give a little more backstory. In general, there is a simplified travel theory on how H. sapiens became the last Homo species, and I really mean simplified (Figure 2). Around 2 million years ago H. erectus emerged in Africa and quickly moved throughout Europe and Asia (Eurasia). Homo erectus that remained in Africa eventually evolved into H. sapiens, whereas H. erectus that traveled out of Africa evolved into the Neanderthals (H. neanderthalensis) in Europe and the Denisovans (H. denisova) in Asia.
For more detail, it seems likely that the evolutionary pathway from H. erectus towards modern humans (H. sapeins), Neanderthals, and Denisovans was through another species named H. Heidelbergensis, popularly known as the last common ancestor (Buck and Stringer, 2014). The evolution from H. erectus to H. heidelbergensis is complicated and controversial, so for simplicity’s sake, H. erectus evolved throughout Eurasia into H. heidelbergensis, which then evolved regionally into modern humans (in Africa), Neanderthals (in Europe), and Denisovans (in Asia).
Throughout our human evolution story, there seemed to be one well-known travel and hookup spot (Kuhlwilm et al., 2016; Hershkovits et al., 2018). This area is known today as the Fertile Crescent; an area that hugs the south-east end of the Mediterranean sea, spreading through Africa and the Middle East. Here there are a number of paleoanthropological discoveries that suggest the interaction of separate species. Another well-known spot for species mingling was in Siberia in the Denisova cave (Warren, 2019). Although there is material evidence that more than one species occupied the same space during the same time, that's all we can really take from it, everything else is speculation.
Recently, however, genetic technologies have advanced, which have been able to tell us many great things that artifacts and fossils alone can’t. We now know that modern humans contain percentages of Neanderthal (Figure 3) and Denisovan DNA. Today you can get yourself screened by 23andMe to see how much Neanderthal DNA you contain. Furthermore, we now have evidence of interbreeding among Neanderthals and Denisovans (Sloan et al., 2018). This means that the preconceived notions of these Homo populations being identified as separate species are questionable. Species are loosely defined as being limited to having sexual relations with only its species. There are caveats to this, of course, one being the phenomenon of hybridization. But in general, it questions at least the placement of modern humans, Neanderthals, and Denisovans as separate species.
As of yet, there isn't any genetic evidence to challenge or change the identification of older Homo species, such as H. erectus and H. heidelbergensis. However, there are arguments based on fossil evidence that suggests subsuming fossil specimens into one variable species rather than splitting them into a variety of species. One such case is based on a group of fossils found in Dmanisi, Georgia (Lordkipanidze et al., 2013). This assemblage of fossils is so variable that it begins to question the identification of early Homo species. From this evidence, it is possible that all early Homo, including H. habilis, can be subsumed into H. erectus.
Although you won't find a Paleoanthropologist today that will say that all Homo species belong to one single variable group. It seems likely that as genetic technology advances and more fossil material is discovered, it will point to this conclusion. We are finding that previously claimed separate species were interbreeding and that ancient fossil assemblages contain great variability that doesn't exceed modern human standards.
I sometimes like to think of a somewhat silly hypothetical situation where a mass extinction event happens on Earth that wipes out all of life. Some of our bones will be fossilized, and some will be lost. Thousands of years go by and an alien race lands on Earth. Now, if they identified species the same way we do today, I wonder how many Homo species they would identify from our modern population.