“It is possible that the resultant loss of linguistic diversity has narrowed and channeled habits of perception in the modern world. For example, all Indo-European languages force the speaker to pay attention to tense and number when talking about an action: you must specify whether the action is past, present, or future; and you must specify whether the actor is singular or plural.”

The Horse, the Wheel, and Language
David W. Anthony

对了，既然印欧语的发音越变越容易，那么更早的印欧人是不是发音的器官更发达更灵活呢？我不是说因为他们舌头更灵活所以选择了困难的发音，而是想说肌肉和大脑都是用进废退，不管他们有多灵活或不灵活，他们的后代因为机灵因为懒，选择了更简单的发音，舌头（及一切发音用到的器官）肯定越来越笨了啊？
考虑到现在凯尔特语的发音难度，我真的不知道古印欧语难成什么样了...
（古希腊语确实比现代希腊语难发音....）