A full-scale replica of the skull belonging to Lucy, the 3.2 million-year-old member of the species Australopithecus afarensis, is displayed at the Houston Museum of Natural Science. (Craig Hartley/Bloomberg News)

We know where the human story started: In Africa, millions of years ago, with diminutive people whose brains were just a third of the size of ours.

And we know where it ended: with us.

Yet a lot of what happened in between is still debated, including the question of how humans' bodies and noggins got so much bigger than our ancestors'.

The traditional thinking is that the growth of both was spurred by the process of natural selection. The evolutionary advantages of a big body and a big brain are plentiful, so it seems reasonable to think that each developed independent of the other in response to the demands of survival in a hostile world.

But a new study in the journal Current Anthropology suggests that, while our brains are certainly an advantageous adaptation, our imposing physiques (such as they are) are more of an evolutionary fluke. That's because the genes that determine brain and body size are the same, argues Mark Grabowski, a fellow at the American Museum of Natural History. So as humans evolved bigger and bigger brains, our bodies "just got pulled along."

Grabowski acknowledges that it may seem like a counterintuitive conclusion — most of us learned in high school biology that evolution is about adapting to circumstances and that only the fittest survive. We're not used to thinking of traits as a product of happenstance. But evolutionary scientists know that lots of traits — even ultimately beneficial ones — are just the luck of the draw.

Even so, he added, "This was not what I was expecting to find at all."

Grabowski began by looking at the relationship between brain and body size among humans and other primate species. They were clearly correlated, both within species and between them. For example, on average, bigger chimps had bigger brains than smaller chimps. And species with bigger bodies had bigger brains than those with slighter builds.

Modern humans, who weigh an average of 130 pounds and have roughly a 3-pound brain, would probably get the better of our distant relative Australopithecus afarensis (whom you might recognize as the 3 million-year-old fossil Lucy) — those guys were about two-thirds our weight, and their brains were a measly 1 pound. For what little it's worth, humans are the heavyweight champions of the hominin line.

Then he applied formulas from the field of evolutionary quantitative genetics — "basically using math to try and sort out the processes behind things you see in the fossil record," Grabowski explained — to data on brain and body size for more than half a dozen species of ancient hominin.

That let him reconstruct several million years of evolution to figure out which came first — the body or the brain.

"When I first saw the data I said to my partner, 'I’ve got some crazy results, I need to recheck them,'" Grabowski said.

He did. And his conclusions were the same: "You really had selection on the brain leading the body instead of the other way around."

Mark Grabowksi, the author of a recent study on the evolutionary link between human bodies and brains, examines fossils. (Courtesy of Mark Grabowksi)

This happens in biology, Grabowski said, more often than you'd think. Traits that seem essential turn out to be a fluke. For example, most paleontologists believe that feathers evolved on Earth-bound dinosaurs, a variation on the more traditional scales. Later on, they finally came in handy when birds took flight.

There are some hitches in Grabowski's theory. Robert Martin, a biological anthropologist at the Field Museum in Chicago who was not involved in the study, noted that the recently discovered Homo naledi breaks the mold: Its body is much bigger than those of earlier Australopithecus species, but its brain size is roughly the same.

"But the main thing is that it does draw our attention back to the link between brain and body size," he added.

That's important because scientists believe that absolute brain size isn't a good arbiter of intelligence in a species. Elephants have brains four times larger than humans do, but that doesn't mean they're four times as smart as us. Considering the two traits in tandem — and understanding how the growth of one can "drag along" the other — gives a more nuanced picture of how we evolved, Martin said.

Bernard Wood, a professor of human origins at George Washington University who was not involved in the study (though he's worked with Grabowski in the past), found another lesson for evolutionary biologists:

"There is a tendency to think that [for] everything that happens in evolution, there has to be a reason for it," he said. "We think, 'It's got to be adaptive, it's got to improve our Darwinian fitness.' But what is clear from biology is sometimes things come along for the ride."

This might be one case where those big brains of ours led us astray.

"We humans, we're hardwired to find reasons. And sometimes there are no reasons," Wood said with a laugh. "Sometimes stuff happens."

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