Since the fossil’s discovery in 2002, paleoanthropologist Erik Trinkaus of Washington University in St. Louis has argued that it displays signs of Neandertal influence, including a wide jaw and large teeth that get bigger toward the back of the mouth. In other ways, such as a distinct chin and narrow, high-set nose, a skull later found in Oase Cave looks more like that of a late Stone Age human than a Neandertal.
Roughly 6 to 9 percent of DNA extracted from the Romanian jaw comes from Neandertals, the team found.
“That study gave me great happiness,” Ackermann says. Genetic evidence of hybridization finally appeared in a fossil that had already been proposed as an example of what happened when humans dallied with Neandertals.
Hybridization clues such as those seen in the Oase fossil may dot the skulls of living animals as well. Skull changes in mouse hybrids, for instance, parallel those observed on the Romanian fossil, Ackermann’s Cape Town colleague Kerryn Warren reported at the anthropology meeting in April. Warren and her colleagues arranged laboratory liaisons between three closely related house mouse species.
First-generation mouse hybrids generally displayed larger heads and jaws and a greater variety of skull shapes than their purebred parents. In later generations, differences between hybrid and purebred mice began to blur. More than 80 percent of second-generation hybrids had head sizes and shapes that fell in between those of their hybrid parents and purebred grandparents. Ensuing generations, including offspring of hybrid-purebred matches, sported skulls that generally looked like those of a purebred species with a few traits borrowed from another species or a hybrid line. Borrowed traits by themselves offered no clear road map for retracing an animal’s hybrid pedigree.
There’s a lesson here for hominid researchers, Ackermann warns: Assign fossils to one species or another at your own risk. Ancient individuals defined as H. sapiens or Neanderthals or anything else may pull an Oase and reveal a hybrid face.
Part of the reason for Ackermann’s caution stems from evidence that hybridization tends to loosen genetic constraints on how bodies develop. That’s the implication of studies among baboons, a primate viewed as a potential model for hybridization in human evolution.
Six species of African baboons currently interbreed in three known regions, or hybrid zones. These monkeys evolved over the last several million years in the same shifting habitats as African hominids. At least two baboon species have inherited nearly 25 percent of their DNA from a now-extinct baboon species that inhabited northern Africa, according to preliminary studies reported at the anthropology meeting by evolutionary biologist DietmarZinner of the German Primate Center in Göttingen.
Unusual arrangements of 32 bony landmarks on the braincase appear in second-generation baboon hybrids, Cape Town physical anthropologist Terrence Ritzman said in another meeting presentation. Such alterations indicate that interbreeding relaxes evolved biological limits on how skulls grow and take shape in baboon species, he concluded.
In line with that proposal, hybridization in baboons and many other animals results in smaller canine teeth and the rotation of other teeth in their sockets relative to parent species. Changes in the nasal cavity of baboons showed up as another telltale sign of hybridization in a recent study by Ackermann and Kaleigh Anne Eichel of the University of Waterloo, Canada.
The researchers examined 171 skulls from a captive population of yellow baboons, olive baboons and hybrid offspring of the two species. Skulls were collected when animals died of natural causes at a primate research center in San Antonio. Scientists there tracked the purebred or hybrid backgrounds of each animal.
First-generation hybrids from the Texas baboon facility, especially males, possessed larger nasal cavities with a greater variety of shapes, on average, than either parent species, Ackermann and Eichel reported in the May Journal of Human Evolution. Male hybrid baboons, in general, have large faces and boxy snouts.
Similarly, sizes and shapes of the mid-face vary greatly from one Eurasian fossil hominid group to another starting around 126,000 years ago, says paleoanthropologist Fred Smith of Illinois State University in Normal. Mating between humans and Neandertals could have produced at least some of those fossils, he says. One example: A shift toward smaller, humanlike facial features on Neandertal skulls from Croatia’s Vindija Cave. Neandertals lived there between 32,000 and 45,000 years ago. Smith has long argued that ancient humans interbred with Neandertals at Vindija Cave and elsewhere.
Ackermann agrees. Ancient human skulls with especially large nasal cavities and unusually shaped braincases actually represent human-Neandertal hybrids, she suggests. She points to fossils, dating to between 80,000 and 120,000 years ago, found at the Skhul and Qafzeh caves in Israel.
PURE AND MIXED Researchers classify the two Stone Age skulls at left as H. sapiens and Neandertal. Ancient DNA now indicates that Romania’s Oase skull, second from right, belonged to a H. sapiens with recent Neanderthal ancestry. Some scientists think an 80,000- to 120,000-year-old H. sapiens, whose remains were found at Israel’s Skhul Cave, skull at far right, also interbred with Neanderthals.
Eurasian Neandertals mated with members of much larger H. sapiens groups before getting swamped by the African newcomers’ overwhelming numbers, Smith suspects. He calls it “extinction by hybridization.” Despite disappearing physically, “Neandertals left a genetic and biological mark on humans,” he says.
Some Neanderthal genes eluded extinction, he suspects, because they were a help to humans. Several genetic studies suggest that present-day humans inherited genes from both Neanderthals and Denisovans that assist in fighting infections (SN: 3/5/16, p. 18).
One physical characteristic of hybridization in North American gray wolves is also a sign of interbreeding’s health benefits. Genetic exchanges with coyotes and dogs have helped wolves withstand diseases in new settings, says UCLA evolutionary biologist Robert Wayne.
“There are few examples of hybridization leading to new mammal species,” Wayne says. “It’s more common for hybridization to enhance a species’ ability to survive in certain environments.”
Despite their name, North American gray wolves often have black fur. Wayne and his colleagues reported in 2009 that black coat color in North American wolves stems from a gene variant that evolved in dogs. Interbreeding with Native American dogs led to the spread of that gene among gray wolves, the researchers proposed. The wolves kept their species identity, but their coats darkened with health benefits, the scientists suspect. Rather than offer camouflage in dark forests, the black-coat gene appears to come with resistance to disease, Wayne said at the anthropology meeting. Black wolves survive distemper and mange better than their gray-haired counterparts, he said.
Similarly, DNA comparisons indicate that Tibetan gray wolves acquired a gene that helps them survive at high altitudes by interbreeding with mastiffs that are native to lofty northern Asian locales. Intriguingly, genetic evidence also suggests that present-day Tibetans inherited a high-altitude gene from Denisovans or a closely related ancient population that lived in northeast Asia.
Hybridization of North American gray wolves with Native American dogs may have endowed some wolves with an immunity-boosting gene and a black coat.
Labeling gray wolf hybrids as separate wolf species is a mistake, Wayne and colleagues contend (SN: 9/3/16, p. 7). Hybrids smudge the lines that scientists like to draw between living species as well as fossil hominid species, Wayne says.
Like wolves, ancient hominids were medium-sized mammals that traveled great distances. It’s possible that an ability to roam enabled humans, Neanderthals and Denisovans to cross paths in more populated areas, resulting in hybrid zones, paleoanthropologist John Hawks of the University of Wisconsin–Madison suggests.
Hominids may have evolved traits suited to particular climates or regions. If so, populations may have rapidly dispersed when their home areas underwent dramatic temperature and habitat changes.
Instead of slowly moving across the landscape and stopping at many points along the way, hominid groups could have trekked a long way before establishing camps in areas where other hominids had long hunted and foraged. Perhaps these camps served as beachheads from which newcomers ventured out to meet and mate with the natives, Hawks says.
All ancient hominid populations were genetically alike enough, based on ancient DNA studies, to have been capable of interbreeding, Hawks said at the anthropology meeting. Specific parts of Asia and Europe could have periodically become contact areas for humans, Neandertals, Denisovans and other hominids. Beneficial genes would have passed back and forth, and then into future generations.
Ackermann sees merit in that proposal. Hominid hybrid territories would have hosted cultural as well as genetic exchanges among populations, she says, leading to new tool-making styles, social rituals and other innovations.
“These weren’t necessarily friendly exchanges,” Ackermann says. Many historical examples describe cultural exchange involving populations that succumb to invaders but end up transforming their conquerors’ way of life.
However genes, behaviors and beliefs got divvied up in the Stone Age, a mix of regional populations — including Neandertals and Denisovans — can be considered human ancestors, she theorizes. They all contributed to human evolution’s braided stream.
That’s a controversial view. Neandertals and Denisovans lived in relatively isolated areas where contact with other hominid populations was probably rare, says paleoanthropologist Matthew Tocheri of Lakehead University in Thunder Bay, Canada. Random DNA alterations, leading to the spread of genes that happened to promote survival in specific environments, played far more important roles in human evolution than occasional hybridization did, Tocheri predicts.
Neandertals and Denisovans can’t yet boast of being undisputed hybrid powers behind humankind’s rise. But a gallery of interbreeding animals could well help detect hybrid hominids hiding in plain sight in the fossil record.
Monkeys living in half-acre, fenced spaces at the California National Primate Research Center in Davis may have a lot to teach scientists about recognizing hybrid hominids in the fossil record.
Interbreeding between Indian macaques (left) and Chinese macaques (right) housed at a California primate facility may provide scientists with clues for recognizing hybrid hominid fossils.
Indian and Chinese rhesus macaques have been interbreeding at the West Coast facility since the mid-1980s. Most of the more than 5,000 monkeys housed at the center are rhesus macaques. To boost genetic diversity in what was an exclusively Indian rhesus colony, researchers introduced Chinese rhesus DNA into the mix. The two populations are classified as subspecies that occasionally mate in the wild and mate frequently when kept in common enclosures. Offspring of Indian-Chinese unions, and of hybrid macaques that mate with members of either parent population, have been tracked since the Chinese monkeys arrived in Davis.
UC Davis anthropologists David Katz and Timothy Weaver, anthropologist Rebecca Ackermann of the University of Cape Town in South Africa and geneticist Sree Kanthaswamy of Arizona State University in Tempe are studying the effects of different amounts of hybridization on the monkeys’ bodies.
So far, full-body CT scans and DNA have been obtained from nearly 70 animals. The scientists plan to study more than 200. The researchers will look closely for skeletal signs of hybridization in the animals, which might include changes in the shape of the nose and braincase.
The Davis macaque colony is a promising model for interbreeding between humans and Neandertals, Katz says, even if subspecies are not as genetically distinct as species. First, he says, roughly 14,000 generations passed between the two macaque subspecies diverging from a common ancestor and interbreeding in captivity. That’s not too far off from the approximately 18,000 generations that passed between H. sapiens and Neandertals splitting from a common ancestor and interbreeding in the wild.
Second, Indian macaques and present-day people display comparable levels of genetic diversity across individuals. And most monkey hybrids are essentially Indian macaques that carry a small DNA contribution from Chinese macaques, about 3 to 6 percent. That’s closely in line with the average amount of Neandertal DNA inherited by present-day non-Africans.
Article Credit: Science News