Ancient humans, Neanderthals, and Denisovans were genetically closer to each other than many modern species, such as polar bears and brown bears, according to a study led by the University of Oxford’s School of Archaeology. The study, published on June 3 in Proceedings of the Royal Society B, suggests that, like these bear species, our ancient relatives could easily produce fertile hybrids, challenging the traditional view of species as rigid, separate entities.
The research, spearheaded by Professor Greger Larson, Director of the Palaeogenomics & Bio-Archaeology Research Network (PalaeoBARN) at Oxford, used a novel genetic metric to measure the genetic distances between different species. The study showed that the genetic differences between humans, Neanderthals, and Denisovans were smaller than the genetic distances between pairs of species known to readily hybridize, such as polar bears and brown bears, and coyotes and wolves. This suggests that ancient humans and their relatives were capable of producing fertile hybrid offspring, similar to how different bear species or canids can interbreed.
Professor Larson explained that our desire to classify species into discrete boxes does not reflect the fluidity of genetic exchange in nature. “Biology does not care about these rigid definitions, and lots of species, even those that are far apart evolutionarily, swap genes all the time,” Larson said. The study provides a metric to predict the likelihood that two species can produce fertile hybrid offspring, based on their genetic distance. By analyzing the genetic sequences of species known to produce hybrids, the researchers found a clear correlation: the greater the genetic distance between two species, the less likely their offspring will be fertile.
The team’s findings also shed light on the long-standing question of hybrid fertility among ancient human species. While the possibility of interbreeding between Neanderthals, Denisovans, and early humans has been confirmed through genetic evidence, the fertility of these hybrid offspring has remained uncertain. Some scientists have even suggested that Neanderthals and humans were at the “edge of biological compatibility.” However, this new metric indicates that the genetic distances between these ancient humans were small enough that they could produce fertile offspring with ease.
The study’s predictive approach has broader applications, not only in understanding human evolution but also in conservation biology. By using genetic distance as a proxy for hybrid fertility, the metric can help assess the likelihood that different mammal species, including endangered ones, can successfully hybridize. This is especially important in conservation efforts, where hybridization programs might be considered as a strategy to preserve genetic diversity or as a last resort for species facing extinction.
Richard Benjamin Allen, joint first author of the study, noted that many conservation decisions have been based on the idea that related species producing hybrids in captivity should be avoided. However, this view has overlooked the role hybridization plays in natural evolution. He suggested that the study’s findings could inform future conservation strategies, particularly for species that are closely related but may benefit from hybridization or surrogacy programs to increase genetic diversity or boost population resilience.
Source: University of Oxford