Simplify the following text for a Grade 9 native English-speaking person.
Original: Between 400,000 years ago and the second interglacial period in the Middle Pleistocene , around 250,000 years ago , the trend in skull expansion and the elaboration of stone tool technologies developed , providing evidence for a transition from H. erectus to H. sapiens .
Simplified: Between 400,000 years ago and the second warm period in the Middle Pleistocene , around 250,000 years ago , its skull grew and more sophisticated technologies based on stone tools developed . One possibility is that a transition between H. erectus to H. sapiens occurred .