Last year the 18,000-year-old remains of a 1-m-tall woman with a braincase the size of a chimpanzee were discovered at the same site.
That loss in muscle strength, they say, allowed the braincase and brain to grow larger.
Scientists determined the age of the skull, unearthed near Hofmeyr, South Africa, by testing the levels of radiation in sand that had filled the braincase.