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).