By Nicole Rojas | | @nrojas0131 ( | First Posted: Dec 08, 2012 12:19 AM EST

CORRECTION: Human beings living thousands of years ago likely had sex with Neanderthals and may have also interbreed with them, a new study on genetics found. According to LiveScience, a study on Mexican monkeys revealed to scientists why proving interbreeding between the two using fossil remains is so difficult. 

Scientists studied mantled howler monkeys and black howler monkeys that live separately in Mexico and Guatemala except for a "hybrid zone" in the southeastern Mexican state of Tabasco, where the two species coexist and interbreed, LiveScience reported.

By studying genetic markers, from both mitochondrial and nuclear DNA, researchers found 128 hybrid individual monkeys that are believed to be the product of several generations of interbreeding. Scientists discovered, however, that the hybrid monkeys share a majority of their genome with one of the two species and tended to physically look like the pure individuals of said species.

In a statement about the study, Liliana Cortés-Ortiz, an evolutionary biologist and primatologist with the University of Michigan, said, "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."

According to the University of Michigan, anthropologists have tried to assume hybridization among ancestral human species based on the fossil record and have concluded that hybridization is extremely rare.

University of Texas at San Antonio postdoctoral fellow Mary Kelaita, whose work on howler monkeys was part of the study, added that the report "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."

LiveScience reported that a Neanderthal genome sequencing in 2010 suggested that not only had humans had sex with Neanderthals, but also that they had acquired some genes and an immunity boost from them. However, some scientists said that producing human and Neanderthal offspring would have not been too successful.

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

*Correction Note: A previous version of this article stated that early human beings did not interbreed with Neanderthals and that the study on Mexican monkeys proved why interbreeding shouldn't be expected. In actuality, the study showed that proving interbreeding using fosil remains is difficult.

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