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Researchers: Interbreeding may have been underestimated in the human fossil record

Researchers at the University of Michigan have learned more about the role of interbreeding in human evolution. According to U-M, researchers have spent years trying to answer a rather complex question: Did different species of early humans interbreed and produce offspring of mixed ancestry? Unfortunately, answering this question is not a straight forward or easy process.

Geneticists believe, based on recent studies, that Neanderthals may have had sex with modern humans tens of thousands of years ago in the Middle East, adding to the modern human gene pool. According to LiveScience, when scientists finished sequencing the Neanderthal genome in 2010, they announced that some modern humans’ DNA came from the early humans.

This discovery, however, is not universally accepted, and the fossil record has not helped to make clear the role of interbreeding or hybridization in human evolution.

U-M researchers are confident that they may have discovered why it is so hard to confirm cases of interbreeding among primates and early humans by relying solely on fossil remains. They made this discovery while examining cases of interbreeding between two species of modern-day howler monkeys in Mexico. Their conclusions are based on analyses of genetic and morphological data gathered from live-captured monkeys over the past ten years.

Researchers studied mantled howler monkeys and black howler monkeys (two primate species that diverged approximately 3 million years ago). The two species of modern-day howler monkeys are different when it comes to behavior, appearance and the number of chromosomes they have. Besides the state of Tabasco in southeastern Mexico, each species lives in a different geographical region. In Tabasco, however, the two species live together and interbreed in what researchers call a hybrid zone.

The researchers discovered that individuals of mixed ancestry who share most of their genome with one of the two species are 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,” said Liliana Cortés-Ortiz, an evolutionary biologist and primatologist and an assistant research scientist at the U-M Department of Ecology and Evolutionary Biology and the Museum of Zoology, in a statement.

For many years, anthropologists have tried to identify interbreeding among human ancestral species based on the fossil record. They have come to the conclusion that hybridization is extremely rare, according to the U-M researchers. The howler monkey study, however, “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.

In the future, researchers argue, studies should revisit and more fully investigate the process of interbreeding, the factors determining the appearance of morphology in hybrid individuals and the magnitude of reproductive isolation between species.

The researchers examined various types of genetic markers to trace the ancestry of each howler monkey they live-captured. Molecular markers helped researchers determine the relative genetic contributions of the parental species to each hybrid.

The researchers concluded that the 128 hybrid individuals they found were likely the result of several generations of hybridization or sex between hybrids and pure individuals.

They also conducted statistical analyses on body measurements and discovered a lot of morphological variation in individuals of mixed ancestry. Researchers found that when individuals were classified based on the amount of their genome they shared with each parent species, it was clear that individuals of mixed ancestry that shared a large portion of their genome with one of the species were physically indistinguishable from the pure individuals of that species.

Researchers examined 135 adult howler monkeys from Tabasco and 76 others from Veracruz, Campeche, Chiapas, the Quintana Roo states in Mexico and Peten in Guatemala. They gathered blood, hair and morphometric measurements from the monkeys before releasing them in the locations they were captured. In addition to obtaining the animals’ weight, 16 body-part measurements were taken, including trunk, tail, leg, foot, arm and hand length.

The study’s findings were recently described in detail in the American Journal of Physical Anthropology.

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