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In Petri dishes at the University of California, San Diego, unassuming white clumps of cells grow into miniature replicas of the developing brain. They are brain organoids — simplified models of the brain consisting of hundreds of thousands to several million cells. Although tiny compared with even the fetal brain, and lacking blood vessels and certain cell types, brain organoids created from human cells have already proved valuable. For instance, during the 2015 outbreak of Zika virus in Brazil, researchers used brain organoids to help to show that the virus caused microcephaly1, a smaller than normal head size in children whose mothers had caught Zika during pregnancy. But the organoids growing in Alysson Muotri’s laboratory are different from those that have come before. In his quest to understand the brain’s evolution, Muotri engineered brain organoids to carry a variant of a gene that is found in our closest extinct kin — Neanderthals.