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Another study questions human interbreeding theory

Did ancient human beings interbreed?

According to a new study, it is more difficult to prove than previously thought.

A newly published study from the University of Michigan examined two species of modern-day howler monkeys in Mexico. Researchers say the study likely sheds light on whether early humans interbreed thousands of years ago. Using genetic markers, from both mitochondrial and nuclear DNA, researchers examined shared traits between the the monkeys.  The use of molecular markers made it possible to approximate the relative genetic contributions of the parental species to each hybrid.

Between 1998 and 2008, the researchers sampled 135 adult howler monkeys from Tabasco, Mexico, along with 76 others from Veracruz, Campeche, Chiapas and Quintana Roo states in Mexico and Peten in Guatemala. The field team collected blood, hair and morphometric measurements from the anesthetized animals before releasing them in the same locations. Scientists discovered that the hybrid monkeys — defined as monkeys that have gone through generations of interbreeding — share a majority of their genome with one of the two species.

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

The study could allow scientists to better understand the evolution of humans. For years, anthropologists have attempted to examine hybridization among human ancestral species based on the fossil record, which represents only a snapshot in prehistory. Various studies have concluded that hybridization was extremely rare.

Given the utility of living primate models for understanding human evolution, the howler monkey study “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,” said  Mary Kelaita, a postdoctoral fellow at the University of Texas at San Antonio’s Department of Anthropology.

The study comes as generally accepted theories of hybridization— or the concept that modern humans and Neanderthals interbred at some point in the development of the current human race—were widely debunked by a study conducted by University of Cambridge researchers. Rather than a genetic difference based on hybridization, scientists say the new study shows that common ancestry was in fact the reason for genetic similarity.  The common ancestor shared by Africa and Eurasia approximately half a million years ago is the same population that produced both anatomically modern humans and Neanderthals, according to the study. Researchers cite numerous genetic and archaeological studies as saying populations at the time were structured according to the amount of exchange possible within the population—that is, those populations with limited traveling capabilities remained distinct in terms of genetics and morphology.

The study was published online in the American Journal of Physical Anthropology on December 7.

The study was supported by grants from the National Science Foundation, PROMEP-UVER Mexico, Universidad Veracruzana, and the U-M Office of the Vice President for Research, Museum of Zoology, Department of Anthropology and Rackham Graduate School.