Simplify the following text for a Grade 6 native English-speaking person.
Original: When the Archean began , the Earth 's heat flow was nearly three times higher than it is today , and it was still twice the current level at the transition from the Archean to the Proterozoic ( 2,500 ) .
Simplified: When the Archean began , the Earth 's heat flow was nearly three times higher than it is today . It was still twice the current level at the transition from the Archean to the Proterozoic ( 2,500 ) .