Many people living today have a small component of Neanderthal DNA in their genes, suggesting an important role for admixture with archaic human lineages in the evolution of our species. Paleogenetic evidence indicates that hybridization with Neanderthals and other ancient groups occurred repeatedly, with our species history being more like a network or braided stream than a tree. Obviously, the origin of mankind was more complex than previously thought.
It is essential to use several data sources to study the impact of such hybridization. Ancient DNA is rarely well preserved in fossil specimens, so scientists must recognize possible hybrids from their skeletons. This is vital to understanding our complex past and what makes us human. Professor Katerina Harvati from the Senckenberg Center for Human Evolution and Paleoenvironment at the University of Tübingen, Germany, and Professor Rebecca R. Ackermann from the Institute for Human Evolutionary Research at the University of Cape Town, in South Africa, studied the impact of hybridization using fossil skulls and individual potential hybrids identified in the past. Their work has been published in the journal Nature ecology and evolution.
Careful data analysis
To do this, the researchers studied a large number of fossil remains of ancient humans from the Upper Paleolithic of Eurasia, dating from around 40 to 20 thousand years ago. Several of these individuals produced ancient DNA showing a small component of Neanderthal ancestry in their genes, reflecting their recent admixture with this group. Their cranial bones were compared to (unmixed) samples of Neanderthals and ancient and recent modern humans from Africa.
The researchers examined three regions of the skull: the mandible, the braincase, and the face, looking for telltale signs of hybridization. “These can include, for example, an intermediate morphology compared to Neanderthals or modern humans, dental abnormalities or unusual sizes. These are characteristics we see in hybrids of various mammals, including primates,” explain Harvati and Ackermann. Their study showed that hybridization signals were evident in puzzles and jaws, but not in faces.
In individuals with known genetic backgrounds, the researchers also examined whether signs of hybridization on the skeleton matched the percentage of Neanderthal ancestry. The fact that he did not suggests that “the presence of particular genetic variants is likely more important than the overall proportion of Neanderthal ancestry,” the researchers say.
Harvati and Ackermann also identified some of the individuals studied as potential hybrids, including individuals from the Middle East – well known to be a contact region for the groups – but also beyond, in Western and Eastern Europe. However, “whenever possible, individual hybrid status should be confirmed using genetic data, and as such, we view these identifications as hypotheses to be tested,” says Harvati. This was the first study of its kind, she says, adding that “we hope it will encourage researchers to take a closer look at these fossils and combine multiple lines of evidence to identify hybridization in the fossil record.” .
Innovator of Evolution
In other organisms, from plants to large mammals, hybridization is known to produce evolutionary innovation, including both novel and diverse outcomes. “It is estimated that around 10% of animal species produce hybrids, including, for example, bovidae, bears, cats and canines,” says Ackermann. Hybrids are also known in primates, our close relatives, such as baboons, she says. “Because hybridization introduces new variation and creates new combinations of variation, it can facilitate particularly rapid evolution, especially in the face of new or changing environmental conditions.”
Hybridization, therefore, may have provided ancient humans with genetic and anatomical characteristics that gave them significant advantages as Africa spread across the globe, resulting in our physically diverse and evolutionarily resilient species, say the authors.
These baboons borrowed a third of their genes from their cousins
Katerina Harvati, Merging Morphological and Genetic Evidence to Assess Hybridization in Late Pleistocene Hominids of Western Eurasia, Nature ecology and evolution (2022). DOI: 10.1038/s41559-022-01875-z. www.nature.com/articles/s41559-022-01875-z
Provided by the University of Tübingen
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What fossils reveal about the hybridization of early humans