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In an exper­i­ment that could por­tend a real-life Plan­et of the Apes sit­u­a­tion, sci­en­tists spliced human genes into the fetus of a mon­key to sub­stan­tial­ly increase the size of the primate’s brain. And it worked.

Researchers from Germany’s Max Planck Insti­tute of Mol­e­c­u­lar Cell Biol­o­gy and Genet­ics in Ger­many and Japan’s Cen­tral Insti­tute for Exper­i­men­tal Ani­mals intro­duced a specif­i­cal­ly human gene, ARHGAP11B, into the fetus of a com­mon mar­moset mon­key, caus­ing the enlarge­ment of its brain’s neo­cor­tex. The sci­en­tists report­ed their find­ings in Science.

The neo­cor­tex is the newest part of the brain to evolve. It’s in the name—“neo” mean­ing new, and “cor­tex” mean­ing, well, the bark of a tree. This out­er shell makes up more than 75 per­cent of the human brain and is respon­si­ble for many of the perks and quirks that make us unique­ly human, includ­ing rea­son­ing and com­plex language.