Study on Howler Monkeys Sheds Light on Interbreeding in Ancient Humans

Several genetic studies on modern humans and Neanderthals have suggested that both the groups interbred some thousands of years ago. However, fossil remains of the two species have not given conclusive evidence on hybridization.

Now, a new study on hybridization between two species of modern-day howler monkeys in Mexico sheds light on why it is difficult to trace evidence of interbreeding among primates, in particular, ancient humans by analyzing their fossil remains.

Researchers from University of Michigan studied more than 200 adult howler monkeys (mantled howler monkeys and black howler monkeys) that were captured and released in Mexico and Guatemala between 1998 and 2008. These two different species diverged from a common ancestor three million years ago. They differ in appearance, behavior and number of chromosomes they possess. Whilst both the groups live in separate habitats, howler monkeys in the Tabasco state of Mexico live in a hybrid zone, where they coexist and interbreed.

The research team collected blood samples, hair samples and morphometric measurements from the anesthetized animals before they were released in their habitats again. Based on nuclear and mitochondrial DNA analysis, experts detected 128 hybrid individuals that were most likely the product of several years of interbreeding between hybrids and pure individuals.

Statistical analyses on body measurements revealed large amounts of differences in the structure (morphology) of individual monkeys belonging to the mixed ancestry. But researchers found that the individuals of mixed ancestry who shared most of their genome with one of the two species were physically indistinguishable from the pure individuals of that species.

"The implications of these results are that physical features are not always reliable for identifying individuals of hybrid ancestry. Therefore, it is possible that hybridization has been underestimated in the human fossil record," Liliana Cortés-Ortiz, from University of Michigan, said in a statement.

For years, researchers have been attempting to find evidence of interbreeding among human ancestral species using fossil records. Mary Kelaita, now a postdoctoral fellow at the University of Texas at San Antonio, said that the study on monkey "suggests that the lack of strong evidence for hybridization in the fossil record does not negate the role it could have played in shaping early human lineage diversity."

Study authors conclude that more research work on the process of hybridization and other factors like the expression of morphology in hybrid individuals needs to be done.

The findings of the study are published in the American Journal of Physical Anthropology.