That is true, and is an excellent reason for us to watch ourselves carefully, and maybe listen to ecologists more closely when they predict the outcomes of our actions. We have no idea of the full ramifications of what we do, though what we CAN see does not bode well for what we cannot.

From what I've read, prior to humans' entrance on the scene, and excluding the mass extinctions, the normal extinction rate is about 1 species every year for every million species on the planet. This is of course an estimation, since we do not know every species on the planet. The problem is that recently (in the past few hundred decades, say, since the start of what we would call "civilization"), extinction rates have skyrocketed (I could not find an actual number here, I apologize. However, everything I've read agrees with this). I believe most of the causes of this increase can be quite easily traced back to human activity. So no, humans have never been the only thing causing extinctions. From what we can see, though, humans are very likely behind the start of this most recent bunch of mass extinctions.