Primate Interbreeding In The Fossil Record May Have Been Underestimated Research On Howler Monkeys Finds, Implications For Human Origin Theories

New genetic research done on two separate species of modern-day howler monkeys in Mexico has exposed the fact that two separate species can interbreed and exchange genetic material while outwardly still showing the traits of only one of the species. The finding suggests that instances of human interbreeding with other hominid ‘species’ may have been vastly underestimated, as almost all previous research has relied on morphology to categorize species. The research has brought to light one of the primary limitations of species categorization based solely on morphology. It’s very possible that ‘humans’ have interbred with other hominids a great deal throughout the ‘species’ history and simply never showed the genetic exchange in their morphology.


The new research is based on the analysis of varied genetic and morphological data gathered over the past ten years via the live-capture of monkeys. Morphology is the part of Biology that deals with the structure of animals and plants, and how they change over time.

“The two primate species in the study, mantled howler monkeys and black howler monkeys, diverged about 3 million years ago and differ in many respects, including behavior, appearance and the number of chromosomes they possess. Each occupies a unique geographical distribution except for the state of Tabasco in southeastern Mexico, where they coexist and interbreed in what’s known as a hybrid zone.”

The amazing thing about the research is that it confirmed that individuals of mixed ancestry are not always distinguishable physically from the ‘pure’ individuals of the species. What it truly confirms is that morphology is not always an accurate indicator of the genome of an animal. An individual animal could easily carry genes that were gained from interbreeding, without outwardly being distinguishable from similar looking individuals in a population. This has important implications for many theories.

“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.

It’s long been the practice of anthropologists to assume hybridization events only occurred when there changes in the morphology of the very limited numbers of fossil specimens available. This way of thinking has led to the commonly accepted conclusion that hybridization has been extremely rare. With how similar other primates are to humans, the new research “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,” Mary Kelaita, a postdoctoral fellow at the University of Texas at San Antonio’s Department of Anthropology, and the lead author of this study said.

“The authors conclude that the process of hybridization (defined as the production of offspring through the interbreeding between individuals of genetically distinct populations), the factors governing the expression of morphology in hybrid individuals, and the extent of reproductive isolation between species should be given further consideration in future research projects.”


“In their study, Kelaita and Cortés-Ortiz analyzed different types of genetic markers, from both mitochondrial and nuclear DNA, to trace the ancestry of each howler monkey they studied. The use of molecular markers made it possible to approximate the relative genetic contributions of the parental species to each hybrid.”

The researchers identified a total of 128 hybrid individuals. These individuals were most likely the result of several different generations of interbreeding and hybridization, between hybrids and pure individuals.

“Subsequently, they performed statistical analyses on body measurements and found a large amount of morphological variation in individuals of mixed ancestry. However, when individuals were classified according to the amount of their genome they shared with each parental species, it became clear that individuals of mixed ancestry that shared most of their genome with one of the species were physically indistinguishable from the pure individuals of that species. Even individuals that were more ‘intermediate’ in their genetic composition were not completely intermediate in their appearance.”

“The study is the first to assess genetic ancestry of primate hybrids inhabiting a natural hybrid zone using molecular data to explain morphological variation.”

“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. Sample collection from wild monkeys was carried out in accordance with U-M’s University Committee on Use and Care of Animals protocol #09319, and in collaboration with researchers at the Universidad Veracruzana in Mexico.”

“The animals were weighed, and 16 body-part measurements were made: trunk, tail, leg, foot, arm and hand length; chest and abdominal girth; head circumference and breadth; head, mandible and ear length; interorbital breadth; internasal distance; and testicular volume.”

“Howler monkeys are among the largest of New World monkeys, with male mantled howlers weighing up to 22 pounds. Fourteen species of howler monkeys are currently recognized. They are native to Central and South American forests, in addition to southeastern Mexico.”

This is very interesting research, and offers yet more of a rebuttal to the idea that ‘modern humans’ all descend from a single population within the past 100,000 years. While I think that there is a lot about current genetic research that is more or less just argued assumptions, even genetic research has been pointing towards human hybridization with other hominid species, such as Denisovans and Neanderthals.

The research was just published online December 7th in the American Journal of Physical Anthropology.

Source: University of Michigan

Image Credits: Milagros González; Black Howlervia Wikimedia Commons

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