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The idea of pea-size brains might make you think of T-rex or your least favorite political figure, but the truth is, the human brain is getting smaller. And it has been for a long time, since the Stone Age according to anthropologists. If you have enough brain power left to wonder how that happened and why we’re just now learning about it, here’s the deal.

The Brain Size Trend That Began In The Stone Age

Just when you thought you understood your species, along comes the news that the human brain has actually been getting smaller the past 40,000 years or so. If you’d thought that brains had been growing as humans continued their ascent, that was only true for the first few million years of human existence. Discovery likes to call it a “well-known secret among anthropologists,” this finding that brains in home sapiens have shrunk by about 10 percent in the past 4,000 centuries. In fact, Macie J. Henneberg’s asserted the diminishing brain size in an article published in Human Biology in 1988!

“Increase in the size of the braincase, so characteristic for human evolution, stopped by the end of Pleistocene,” according to the author. “Several authors have noticed a decrease since then, while others at least agree there has been no increase.” The 1988 work was based on craniometric data collected on about 9,500 male crania and 3,300 female crania from the NW quadrant of the Old World including Europe and North Africa. The material was dated anywhere from the Upper Paleolithic Period, also known as the Old Stone Age, to recent history. The study also determined that brain size for both genders measured peaked in the Mesolithic Period and was at its smallest in modern times.

The way anthropologists determine brain size for comparison’ sake involves skeletons, fossils and something called “endocranial volume,” which is the space within a skull that presumably was once occupied by brain cells. Scientists don’t use actual brain tissue for the measurements when they’re working with fossils, because unlike bones, the tissue deteriorates relatively quickly after death. But this endocranial volume method helped scientists learn that the open space for brains in the skulls of the earliest humans had about a 1.5 cup capacity. It increased from there, adding about a half-cup of capacity in the days between 4 million and 2 million years ago. Then the brain bulk started growing by leaps and bounds, adding two more cups in the next million years and topping six cups around 130,000 years ago, during the time Neanderthals were in vogue. Modern human brains are about 10 percent smaller on average though.

No Need To Panic

As humans, we like to think we’re not only the smartest beings on earth but the most intelligent versions of humanity ever to roam the earth. Does this shrinking brain phenomena contradict our views? Scientists in the know are reassuring. Some say, for example, that humans may have evolved away from larger brains because they’re less efficient in some ways. One example: When the brain must compute, every added millimeter of connection pathways slows it down. Others point at smaller bodies. “Some of the shrinkage is very likely related to the decline in humans’ average body size during the past 10,000 years,” Natural History Museum in London paleoanthropologist Christopher Stringer told Scientific American. “Brain size is scaled to body size because a larger body requires a larger nervous system to service it. As bodies became smaller, so did brains. A smaller body also suggests a smaller pelvic size in females, so selection would have favored the delivery of smaller-headed babies.”

And Stringer, who is a lead researcher on human origin for the museum, further asserts that our modern lifestyle may make brains shrink further. “The fact that we increasingly store and process information externally—in books, computers and online—means that many of us can probably get by with smaller brains.”

Maybe We’re Just Getting More Domesticated

One more theory of the past few decades (while anthropologists were keeping this open secret about shrinking brain selves) involves the idea that humans followed dogs and other domesticated animals with a sort of natural selection process. According to a 2011 study from Brian Hare of Duke University’s Department of Evolutionary Anthropology and Center for Cognitive Neuroscience, cooperative and friendly humans have been the most likely to reproduce these many thousands of years. The kindly procreators were also likely to have slighter builds and hence, smaller brains. Humans who self-selected based on being kindly may not have been the most intelligent, but the fact that the human race was able to reproduce in such strong numbers probably increased collective intelligence. Or maybe humans only reduced their brain size, not their intelligence levels: Researchers haven’t made hard and fast conclusions in that area yet.

But there is one thing. If you feel a little weird knowing the human brain is getting smaller, comparing humans to T Rex and the rest won’t be the instant fix it once was. Even the dinosaurs, as it turns out, probably didn’t have tiny brains, or at least not all of them. A collector in England found a fossilized brain from a large herbivorous dinosaur a few years back and the preserved tissue indicated that dinosaurs might have much bigger brains than previously thought. As for those political foes, though, their actual brain size has not been analyzed.