Human evolution may be responsible for autism rates

Autism spectrum disorder (ASD) may be the result of millions of years of evolution. Rapid neuronal evolution in humans is likely ASD’s genetic cause, new research suggests. Though autism can cause certain developmental difficulties, the evolution of genes may have contributed to humans’ complex minds today.

Your brain on genes

Autism’s “exceptionally high prevalence” in humans may be a “direct result of natural selection,” said a study published in the journal Molecular Biology and Evolution. The development of ASD in humans is likely a result of the “peculiar bargains evolution struck to make us human,” said ZME Science.

The study found that the “most abundant type of outer-layer brain neurons, called L2/3 IT neurons, evolved exceptionally quickly in the human lineage compared to other apes,” said a news release about the study. This “coincided with major alterations in genes linked to autism — likely shaped by natural selection factors unique to the human species.” These genomic elements remained “relatively stable throughout the rest of mammalian history,” said Newsweek.

The Week

Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.

SUBSCRIBE & SAVE

Sign up for The Week's Free Newsletters

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

Sign up

Scientists also agree that neurological conditions like autism and schizophrenia are probably a uniquely human condition. It is “very rare to find behaviors associated with the disorders in non-human primates,” said Neuroscience News. Also, “behaviors associated with those disorders generally involve cognitive traits like speech production and comprehension that are either unique to or much more sophisticated in humans.” The evolution of autism “isn’t about one rogue mutation,” said ZME Science. Instead, it is “about dozens of small changes acting together, nudging the baseline wiring of human brains.” It is likely that “natural selection actively favored these changes, even though they came with trade-offs.”

Cause and effect

Approximately one in every 31 children is diagnosed with autism, according to the Centers for Disease Control and Prevention. Scientists have known for a while that genetics is likely the largest contributing factor to being diagnosed with ASD. “Several genes seem to be involved in autism spectrum disorder,” said the Mayo Clinic. Many of these genes are linked to developmental delays, which raises questions as to why evolution may take this path.

Although the definitive reason is unclear, researchers have ideas. For one, the “evolution of autism-related genes slowed early brain development or expanded language capacity, extending the time window for learning and complex thought in childhood,” said Newsweek. This “extended development may have offered an evolutionary advantage by fostering more advanced reasoning skills.” Essentially, “some of the same genetic changes that make the human brain unique also made humans more neurodiverse,” Alexander L. Starr, the paper’s lead author, said in the release.

There have also been many theories about the factors that increase the risk of autism. Researchers have been looking into various environmental factors like “infections, medicines, complications during pregnancy or air pollutants,” said the Mayo Clinic. Some have also claimed that vaccines play a role in causing autism spectrum disorder, though “many well-done research studies show no link between autism spectrum disorder and any vaccines.”