An article published in the journal “Science Advances” reports a genetic research that offers evidence that some West African populations inherited a portion of their genes from an unknown species of hominins. Arun Durvasula and Sriram Sankararaman of UCLA made a genetic comparison of the DNA of 405 African individuals with those available obtained from Neanderthal and Denisovan bones, discovering genetic characteristics that can be attributed to a species other than these.

Genetic studies carried out in recent years highlighted the interbreedings between Homo sapiens and the most famous species of hominins, the Neanderthals, but also with the Denisovans, another species of which few bones have been found so far that lugkily were in Siberia, preserved in such conditions that DNA fragments could be extracted. Genetic comparisons showed ancient introgressions, biological phenomena resulting from the interbreeding between two different species. The hybrids interbreed again with at least one of the two original species forming other hybrids. Over time, these interbreedings introduce various combinations of genes into the species from the other species.

Thanks to these studies, we now know which populations of European and Asian origin inherited the genes of Neanderthal and Denisovans but Arun Durvasula and Sriram Sankararaman of UCLA (University of California at Los Angeles) tried to understand if there are also African populations that in ancient times interbred with these hominins. The results offered surprises for four West African populations: Yoruba, Esan, Mende and some Gambians. In fact, they discovered genes different from those present in Homo sapiens but also from those of Neanderthals and Denisovans for a total ranging from 2% to 19% of their DNA.

The most likely explanation for the genetic discoveries made in these African populations is an introgression deriving from the interbreeding with a species of hominins of which we don’t have DNA fragments. The image (Courtesy Science Advances 2020. All rights reserved) shows demographic models of the populations of the best known hominins. Section B adds an introgression that includes a ghost species of hominins that separated from modern humans before the separation between modern humans and the ancestors of Neanderthals and Denisovans.

Unfortunately, the hot and humid conditions existing in Africa cause DNA to degrade rapidly in bones so it’s almost impossible to find any more of it in the bones of African hominins. For example, attempts to extract DNA from Homo naledi bones so far failed but the significant physical differences with Homo sapiens suggest that it’s not the ghost species whose genetic traces remained in West Africans.

An interesting candidate is the population of the individual whose skull was discovered in Iwo-Eleru, Nigeria, in 2011. Again, it wasn’t possible to extract DNA but its hybrid characteristics suggest a possible interbreeding between Homo sapiens and Homo heidelbergensis, a possible common ancestor of Homo sapiens, Neanderthals and Denisovans.

This is not a complete surprise because an article published in July 2017 in the journal “Molecular Biology and Evolution” described a research that from a protein present in human saliva led the authors to deduce that in ancient times there were interbreedings between Homo sapiens and another unknown species of hominins. In this new research published in “Science Advances” the authors haven’t found high frequencies of the gene called MUC7, which encodes a mucin, a saliva protein that was associated with the interbreeding with unknown hominins. This means that in Africa there may have been interbreedings with two species of hominins of which we don’t have DNA but we don’t have enough information to provide conclusive answers.

Drawing conclusions from these different results about different African populations is really difficult, but perhaps it indicates new paths for genetic research on the various possible interbreedings between Homo sapiens and other hominins. It remains difficult to understand how many species of hominins coexisted with Homo sapiens and how many of them interbred with Homo sapiens, but scientists are slowly reconstructing this complex story.