Our brains grew tinier when the Earth warmed up in the past. Will global warming repeat things?

If you've been relying on the good ol' 'cave-brain' to round off your arsenal of insults, you might want to, very ironically, rethink things. The newest chapter to the epic of human evolution reveals that our brains might've actually been larger than usual during cold spells of the ancient past!

While we're undoubtedly much smarter off now (try inventing the internet, you cave-dwellers!), Jeff Morgan Stibel from California's National History Museum set off to assess if this was always the case.

Brain size doesn't always correspond to the level of cognitive function, but it could certainly have a massive effect on how we look and function. As Stibel argues in the paper, 'even a slight reduction in brain size across extant humans could materially impact our physiology in a manner that is not fully understood'.

Considering commercial cold freezers tragically weren't around thousands of years ago, we have to look at skull sizes to assess the brain size of the common hominids from the time. Stibel thus obtained data from 373 measurements from 298 human bones, some going as far back as 50,000 years!

Grouping the fossils into distinct groups helped account for dating errors. Based on when these early humans trod the Earth, the four groups’ births were thus dated back to 100 years, 5,000 years, 10,000 years, and 15,000 years.

Luckily, we have pretty accurate measurements of what the surface temperatures were like at different points in the Earth's history. Comparing the brain size against these climate records showed that our noggins shrunk by over 10% during the Holocene period (about 11,700 years ago to current time), when the climate was warming up. Some experts even compare this period to what's in store for us under current climate trajectories.

In general, our ancestors sported rather large brains during colder periods. There was also a correlation between drier times and slightly bigger volumes, although that was vastly overshadowed by the temperature connection.

"Brain size changes appear to take place thousands of years after changes to climate, and this is particularly pronounced after the last glacial maximum, approximately 17,000 years ago," the paper elaborates.

More than the doubtless stunning appeal of a massive forehead, Stibel thinks ecosystem factors such as predation and climate change-induced vegetation changes are to blame. Non-climate factors such as developing culture and technologies could've also contributed to the variation.

The other striking finding was that this evolution took place rather rapidly; much of the change occurred within just 5,000 to 17,000 years!

While the findings might have helped assess the patterns of the past, this surprisingly short span indicates what's to come for us in this harrowing period of climate change. Stibel thinks that global warming could have detrimental effects on our cognition in the coming years as well.

"More work will be needed to determine whether the impact of climate change on Homo physiology is a result specifically of temperature changes or an indirect effect from other elements of a changing environment," the paper remarks.

The findings of this study have been published in Brain, Behavior and Evolution, and can be accessed here.

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