Just a crazy idea....
If we think of the lifestyle of our pre-sedentary ancestors as one that covered large distances in the annual cycle, and larger ones over a lifetime, with culture-bearing units having a lifespan of only several generations, and meetings with true strangers a regular fact of life, throughout life, then could the language acquisition faculty of young children been at that time a lifelong faculty of our species ... one rapidly lost among sedentary populations due to its high energetic costs?