Scientists have a new clue in the long quest to decipher what makes us uniquely human: tiny changes in brain chemistry that set us apart from our closest hominin cousins .

In a study published Monday in the Proceedings of the National Academy of Sciences, an international team of researchers scrutinized a version of a gene, ubiquitous in humans today, that is not present in Neanderthals or Denisovans — the hominins that lived alongside our ancestors.