There are so many factors in the climate over the history of earth. Early earth was warmed by a sun that was 30% less intense than our present earth. But the climate was hotter because there was little oxygen in the air and a ton of greenhouse gasses. It wasn't until the evolution of plants and photosynthesis that oxygen was put into the air and the planet started to cool. The earth wobbles on its axis and that creates a cyclic climate pattern. Volcanic activity was much greater in our young earth and that created climate cycles. Solar activity increases and decreases and effects things. There are so many cycles over earth's history that contributed to so many factors.
Life survives and in some cases thrive on the change. But some life becomes extinct. There are far more extinct life than there is life present today. A great deal of those extinctions were the inability of life to adapt to the changes of climate and the evolutionary pressures associated with that change.
The thing that I notice is that from everything discovered about earth's climate history, the increase in greenhouse gasses in the atmosphere are followed by increases in temperature. And whether the atmosphere become loaded with greenhouse gasses because we had no plant life like early earth, volcanic activity, or the activity of man, the physics remain the same....it gets hotter. The getting hotter isn't going to eradicate life on earth. But it might drastically change which life remains. Melting ice and rising oceans affects us. Think of the population bases that would be displaced.
I'm not sure dismissing the activity of man is a solid enough reason to dismiss global warming just because it happened before we showed our faces on this planet. The fact remains that if the atmosphere gets pumped with a bunch of carbon dioxide and greenhouse gasses,the physics isn't going to change whether it was put there by man, a volcano, or the lack of green plants. It's going to get hotter. With that comes change.