Starting about 3 million years ago, our brains got bigger, helping us master everything from cave painting to particle physics.

Now we’re in a modern-day growth spurt.

A new study by researchers at UC Davis Health found that the brains of people born in the 1970s had 6.6% larger volumes and almost 15% greater brain surface area than those of people born in the 1930s.

“We found that brains got larger with each 10 years,” said neurologist Dr. Charles DeCarli, principal investigator of the study, published in a recent issue of the journal JAMA Neurology. It was based on an analysis of thousands of volunteers in the famed Framingham Heart Study.

This doesn’t prove that we’re getting smarter, although other studies suggest that trend. In the human brain, size isn’t everything; wiring matters, too.

But bigger brains could increase so-called “mental reserve,” potentially reducing the overall risk of age-related dementia, according to DeCarli. Perhaps it explains a recent decrease in the percentage of people affected by Alzheimers disease, as reported in the 2016 New England Journal of Medicine.

“It may accommodate more of the connections that help organize the brain and make it more resilient, leading to a better ability to withstand aging,” he said.

The Framingham Heart Study, over seven decades old, keeps data on thousands of people living in Framingham, Massachusetts. It is the longest-running and most comprehensive project of its kind in medical history.

To study brain changes, the Davis scientists didn’t measure the squishy three-pound blob inside each human skull. Rather, they measured brain scans of 3,226 individuals obtained by MRI, which painlessly reveals the brain’s structure. The MRIs were conducted between 1999 and 2019 on more than 3,000 healthy Framingham residents born between the 1930s and 1970s, with an average age of about 57.

They then compared the images of people from three different eras: the Silent Generation of the 1930s, baby boomers of the 1950s, and Generation X, born in the 1970s.

Their analysis found that the brain’s volume and surface area were not the only regions to grow over time. Two other key areas — white matter, which contains connective fibers and delivers nerve signals, and the hippocampus, which processes memory – have also grown between 5.7% to 7.7%.

The increase in white matter suggests that brain cells are more interconnected, DeCarli said.

The increase isn’t explained by growth of the rest of the body. People born in the 1930s had a mean height of 66 inches, compared with 67.6 inches for those born in the 1970s. But even after adjusting for height, brains were bigger.

While there’s been an incremental upward drift in IQ test scores — approximately 3 points per decade — across generations, that may reflect improvement in education, life experiences and test-taking, not intelligence. It’s difficult to compare the intelligence of different generations, DeCarli said.

Scientists have long studied the evolutionary history of our brain to understand how and when it grew, allowing sophisticated skills to emerge. The human brain is gigantic relative to our body size. It’s three times as large as that of chimpanzees, our closest living relatives. It also has distinctive anatomical features.

At a pivotal time in human evolution, about 3 million to 4 million years ago, human brains dramatically increased in size. A set of three nearly identical genes seem to play a critical role in this development, according to a study led by David Haussler, professor of biomolecular engineering and scientific director of the UC Santa Cruz Genomics Institute. Other mutations may also turn out to be important.

Early hominins like Sahelanthropus and Australopithecus have relatively small brains. Fossils of the first Homo species show larger brains. The brains of Homo sapiens — us — are bigger still.

But the growth detected by the Davis team isn’t attributable to evolution, said DeCarli. It’s too recent.

Rather, improving lifestyles — especially during the crucial first 10 years of life, when the brain is developing — deserve credit. The 20th century delivered dramatic improvements in the standard of living, education, nutrition and health care for many Americans.

“This cannot be a genetic effect. It more likely to be environmental,” said DeCarli. “We don’t know what those things are, but I suspect they have to do with better prenatal care, better nutrition, better education, maybe a more ‘enhanced’ environment.”

Dr. Charles DeCarli, Professor of Neurology at UC Davis, used brain imaging to compare the sizes of brain structures of people born in the 1930s to those born in the 1970s.

Experts not involved in the project said the findings were provocative.

“Although these findings are new to our field, the substantial gains over four decades are intriguing,” wrote Prashanthi Vemuri, a neuroimaging scientist at the Mayo Clinic who wrote an editorial accompanying the study. If verified, it is important to study what’s driving this trend, she said.

“Replication of these results in other cohorts is essential,” she said. “If these results are confirmed by others, and the observed differences by decade are as large as those reported, it has important implications for aging and dementia studies.”

The study has limitations. Its participants are healthy, well-educated and middle class. They’re almost all non-Latino whites, not reflecting the diverse U.S. population.

Finally, it didn’t include people who had deprived childhoods. If a bigger brain is protective, such inequalities could put them at greater risk of dementia.

“We haven’t looked at populations that suffer from adversity,” said DeCarli. “And I suspect that they’re not experiencing, over time, the same kind of changes.”