Up until about 2 billion years ago, the sun was 30 percent dimmer than it is now. If the sun were that dim now, our oceans would be completely frozen. According to fossil evidence, however, life and liquid water were both present as early as 3. 8 billion years ago.
The statement 'Our atmosphere currently holds in significantly less heat than it did 3. 8 billion years ago' most helps to resolve the apparent discrepancy described above.