For 200,000 years, Neanderthals thrived throughout Eurasia. They seem to have lived full and happy lives. Like us, they produced art, mourned their dead, and even used toothpicks to clean between their teeth. But 45,000 years ago, as Homo sapiens made a home in Europe for the first time, Neanderthals suddenly disappeared.

Now, new Japanese research, published in the journal Scientific Reports, gives some suggestions as to why—by looking at Neanderthals’ brains.

This is the first time they’ve been able to do so. Before this pioneering study, Neanderthal brains were inaccessible to researchers, with the soft tissue having long since perished. But a complicated technique called computational neuroanatomy allowed these scientists to produce detailed 3D models of Neanderthal brains using data from four Neanderthal skulls. Next, they compared them to brain models for early anatomically modern humans and an “average” modern human brain, using data from almost 1,200 MRI scans.