We don't know for sure, but there are contenders. In 2013, researchers at the University of Reading in England looked at a variety of words that sound similar across different Indo-European languages. By analyzing the rate of change, they arrived at 23 “ultra-conserved” words that have likely sounded the same for the last 15,000 years. The oldest? I, who, two, three, and five. Other ultra-conserved words include man, mother, ashes, and worm.