Why we humans are worried about this period of climate change and not the others that were millions of years ago.

Why should we be more worried about our current period of climate change than the many big climate changes that have already happened on earth over millions and billions of years?

We weren't living on earth back then so we weren't around to worry about them. This period of climate change, that we are playing a big role in causing, is the one that's effecting us.