How are you defining "human existence?"  The remains of homo sapiens were recently discovered in North Africa that are 300,000 years old, 100,00 years older than previously known.  Our Pre-homo sapien ancestors go back 7.2 million years, per remains found in the Balkans.  Do they count?

The natural forces that affect climate change precede man.  The AGW theorists can't explain why we've had periods of "global warming" followed by increases in CO2 (to levels as high as 10,000 ppm) between each of the Ice Ages, long before man could have had anything to do with it.  During the time of the dinosaurs and other very large creatures, the Earth was covered with plant life growing prolifically during very high levels of CO2.  The Earth was a more accommodating place for life then than it is now.  Up to ten degrees warmer, too.

If we can't understand why it warmed back then under current AGW theory, why shoud be believe the theory applies now?