It is well known that we are a bit Neanderthals: the traces of former mixtures between the human forms are known from the genome of present-day and fossil humans. Two researchers have now investigated the extent to which hybridization events are also reflected in anatomical features of fossils. They were able to identify signs in certain cranial structures and thus also gained clues to hybridization backgrounds in fossil humans. However, the research field is only at the beginning of its development, the scientists emphasize.
Skeletal features, skull shapes, tooth structures: In order to gain information about the members of the human family tree, scientists have long been limited to anatomical clues. But that has now changed: genetics has revolutionized anthropology. Because in some cases it is still possible to obtain DNA from fossils that are thousands of years old, which can be used for comparisons. As a result, researchers have already been able to prove that mixing with other lineages from earlier human forms played an important role in the evolution of our species. Specifically, it turned out that when Homo sapiens migrated from Africa to Europe and Asia, he mixed with the Neanderthals who were already living there. These crossings resulted in a few percent of “archaic” DNA in the genomes of many modern humans.
Target morphological signs
In addition, in some human fossils from the era of coexistence of modern man and Neanderthal, scientists found traces of recent interbreeding. When researching the history of hybridization, Katerina Harvati from the University of Tübingen and Rebecca Ackermann from the University of Cape Town are now calling for a renaissance of traditional anthropological methods: Traces of mixing could also be recorded using anatomical clues. Because, as they explain, many interesting fossil finds have not retained any usable DNA to provide insight into the ancestors of these people. It would therefore be helpful to be able to obtain indications of hybridization based on morphological characteristics.
To provide a basis for this approach, the two researchers examined a large number of fossil remains of early humans from Eurasia, which are dated to around 40,000 to 20,000 years ago. Traces of genetic material were found in several of these individuals, indicating that they had Neanderthal ancestry. In some cases, the paleogenetic evidence indicates that the interbreeding occurred only a few generations ago. The researchers compared the anatomical features of the skeletal remains of these individuals with those of “unamalgamated” Neanderthals and with early and younger representatives of Homo sapiens from Africa.
Possible indications of hybrids
Harvati and Ackermann examined the lower jaw, braincase and face particularly intensively for signs of hybridization: they looked for morphological transitional forms between Neanderthals and anatomically modern humans. They were also guided by familiar patterns known from hybrids of different mammals. “It is estimated that around ten percent of animal species produce hybrids, including, for example, cattle, bears, cats and canines,” says Ackermann. Hybrids are also known in primates, our close relatives, such as baboons.
As can be seen from the results, a hybridization background could apparently not be seen in the people of the late Paleolithic at first glance: the scientists did not find any clear patterns with an indicative character in the facial structures. However, traces were evident in more subtle features: As Harvati and Ackermann report, they found signs of hybridization in certain features of the cranium and mandible. Based on these findings, they also identified some fossil individuals, for which no genetic data was available, as potential hybrids. These include specimens from the Middle East – a region where the two groups are known to have met – but also some from western and eastern Europe.
The researchers emphasize, however, that so far these are only indications. They view their work as a pilot study designed to show the potential of combining clues. “When possible, an individual’s hybrid status should be confirmed using genetic data, so we consider our results as hypotheses to be tested,” says Harvati. “But we hope that they will also encourage other researchers to study fossils more closely and combine multiple lines of evidence to detect hybridizations,” says the anthropologist.
Source: Eberhard Karls University of Tübingen, specialist article: Nature Ecology & Evolution, doi: 10.1038/s41559-022-01875-z