Biggest Known Genetic Difference Between Humans and Neanderthals May Be Related to Autism

A recent study says that a genetic change suddenly arose in the primate family tree about about 280,000 years ago. The researchers claim it is responsible for the largest genetic difference between humans and Neanderthals. The unique genetic structure also is believed to predispose humans to autism.

The genetic alteration is said to have arose approximately 80,000 years before Homo sapiens showed up in the fossil record.

The Daily Mail reports that researchers from the University of Washington came to these results by comparing “the genome of modern humans with the genetic code for chimps, gorillas and orangutans, as well as Neanderthals and Denisovans.”

A cladogram showing one possible classification sequence of the living primates, with groups that use common (traditional) names shown on the right. (Public Domain)

The study, was recently published in the journal Nature. It says the “recurrent copy number variation (CNV) at chromosome 16p11.2 accounts for approximately 1% of cases of autism and is mediated by a complex set of segmental duplications […] All humans examined carried one or more copies of the duplication, which nearly fixed early in the human lineage—a pattern unlikely to have arisen so rapidly in the absence of selection.”

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As Medical Xpress explains, copy-number variants are stretches of duplicated DNA which “are common in the human genome and often contain multiple copies of genes. Although most copy-number variants seem to have no adverse effect on health, some have been linked to disease.” They explain that “when both strands of a segment of DNA are flanked by highly identical sequences, they can be susceptible to large copy-number differences, including deletion, duplication and other changes, during the process of cell division. In this case, deletion, which causes the loss of the segment's 28 genes, results in autism.”

Evan Eichler, co-author of the current study, a professor of genome sciences, and an investigator of the Howard Hughes Medical Institute says that "Most duplications in our genome are millions of years old, and the speed at which this structure transformed our genome is unprecedented. The wide and rapid distribution of these copy-number variants suggests the genes within the repetitive sections confer benefit that outweigh the disadvantages that come with the increased risk of autism in some offspring, should deletion occur."

The benefit in this case may be a gene called BOLA2, which forms a complex with another protein (glutaredoxin 3). The combination reportedly allows cells to capture iron more efficiently and make it accessible to proteins. “This ability to help humans to acquire and use this essential element early in life might confer a significant enough benefit to outweigh the risk of having some offspring with autism,” Eichler said.

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Eichler told the press that "If you took two chimps out of the wild, they would have twice as many genetic differences in their genomes than you would see between two humans. And orangutans have three times as many differences. Having these structures means we have a way to radically restructure our genome over a very short time frame, bringing about changes that might otherwise take hundreds of millions of years of evolution to acquire – but at the cost of an increased risk of autism and other neuropsychiatric disorders.”

Adult female and infant wild chimpanzees feeding on Ficus sur fruits in Kibale National Park, Uganda. (Alain Houle/CC BY 4.0)

Another recent study suggests that a genetic mutation could have made human ancestors more resistant to the harmful effects of smoke, helping them to survive better than their Neanderthal counterparts. Neanderthals appear lack the “aryl hydrocarbon receptor gene which plays a role in the breakdown of certain noxious substances.”

Comparison of Modern Human and Neanderthal skulls from the Cleveland Museum of Natural History. (Deriv) (CC BY SA 2.0)

The researchers believe that this could have made the Neanderthals more susceptible to smoke-related respiratory infections, fertility problems, and mortality. But this doesn’t mean that smoke was necessarily the undoing of the Neanderthals.

Gary Perdew, the author of the recent study, told The Guardian “We prospered because of this mutation,”. “I wouldn’t say Neanderthals died out because of it, but it could have been a contributing factor.”

Artist’s representation of a group of Neanderthals around a fire. (Public Domain)

Top Image: A reconstructed Neanderthal skeleton, right, is displayed next to a modern human skeleton at the Museum of Natural History in New York. Source: Museum of Natural History in New York

By Alicia McDermott