Humans and Neanderthals: Did They Interbreed?
Genetic Evidence
In 2010, groundbreaking research revealed that humans share 1-4% of their genes with Neanderthals. This discovery sparked a heated debate about whether our ancestors engaged in interbreeding.
The Interbreeding Hypothesis
Proponents of the interbreeding hypothesis argue that the presence of Neanderthal DNA in modern human genomes is evidence of hybridization. According to their models, a relatively small number of liaisons between humans and Neanderthals could account for the observed genetic overlap.
The Non-Interbreeding Hypothesis
However, other researchers contend that the genetic similarities between humans and Neanderthals can be explained by population structure. They propose that the Neanderthal genome carried a genetic signature that was also present in a group of pre-modern Africans. When this African population gave rise to modern humans, they inherited this signature, leading to the appearance of Neanderthal DNA in modern genomes without the need for interbreeding.
Contrasting Studies
Two recent studies have presented contrasting views on the interbreeding question. A paper published in PNAS suggests that humans and Neanderthals never mated, while another study scheduled for publication in PLoS ONE argues strongly for interbreeding.
The PNAS Study
The PNAS study built a model assuming that the African population had a structured genetic makeup. They found that this model could predict the current human genome without any interbreeding. However, the study acknowledges that some interbreeding may have occurred, but that the offspring were likely not viable.
The PLoS ONE Study
The PLoS ONE study, on the other hand, argues that interbreeding did occur, but that it was infrequent. Their model suggests that as few as 197-430 liaisons between humans and Neanderthals could have introduced Neanderthal DNA into modern Eurasian genomes.
Interpreting the Evidence
Interpreting the genetic evidence for human-Neanderthal interbreeding is challenging. Scientists are working with fragile and difficult-to-extract DNA, and they must rely on models to infer how the two species interacted.
Population Dynamics
Anthropologist Chris Stringer suggests that human-Neanderthal encounters occurred in waves. In the early waves, small groups of modern humans would have encountered large groups of Neanderthals. Later waves would have seen the situation reversed.
The Impact of Population Structure
Population structure can significantly impact genetic analysis. If different groups of people lived in isolation, they would accumulate unique genetic signatures. When these groups later came into contact, the genetic similarities between them could be misinterpreted as evidence of interbreeding.
Mitochondrial DNA
Mitochondrial DNA is inherited exclusively from the mother. The absence of Neanderthal mitochondrial DNA in modern human genomes suggests that any offspring resulting from human-Neanderthal interbreeding were likely not viable.
Future Research
More research is needed to fully understand the nature of human-Neanderthal interactions. Scientists require a better understanding of ancient population structures and how they have influenced the genetic makeup of modern humans.