And there probably weren't any sentient beings around at that time either? Is it possible that there wasn't much diversification at the time?
Is it possible that there was , I don't really know, but I can agrue that there was about as much if not more
biodiversity as there is now. Most of what would be called life is very old. And in truth the anaerobic bacteria are still all over the place today, there domain is just less than it might have been had oxygen came into play. And I have met people who are into biology and atsronomy and they say that the oxtgen arrived at our planet and then the algae took advantage of it. But others like to talk about the possibility of the unintended consequence of releasing oxygen.
The unknowable truth as I see it is that humanity is just a tiny blip in the geologic record so far, humanity is likely to be 100,000 to 60,000 years old as a species.
What our species has done is the same as all species do, anthrocentrism is inherent in the nature of our perception. We as humans may feel as though we are out of balance, but a skyscraper is about as cool as a beaver damn or a bee hive.
And who denuded the landscape so much to where this eventually became a problem?
Problem, who said there was a problem? Elephants change thier enviroment, there are also places where elephants live in jungle.
As I recall, ten years ago the ethologists were suggesting that the elephants and the amount of rainfall are what made the savvanah that we see today. I will grant you that there are deserts, in certain areas of the world, that seem to have sprung up after the developement of agriculture.
There are most likely many reasons the savvanah exists, elephants effect the lanscape.
And yet we don't see weasels over-running the planet do we? ... unless of course you want to refer to people as weasels. 
There are plenty of species that overrun other species, humans are in no way any different in that regard.
Weasels are just an example like the woverine of an animal that 'kills without reason'. Probably foxes as well.
Would you say the problem is wide-spread?
Depends on which problem, worse things have happened than humankind's arrival on the scene. We can probably make it much worse if we chose to. I don't think, and hope , that we as a species do not reduce the planet to the point where we really, really , really screw things up.
And yet why does it all tend to balance itself out? ... that is until lately.
'skim milk masquerades as cream'... appearances are decieving, I think that there are things as a species that we need to consider as soon as possible.
The balance is there, I do believe in isotropy, while biodiversity is less than what it was pre Cambrian, the 'balance' is always there.
How did I get here? Well, I honestly can't say that I remember? At first I was completely unaware, but now I am. It's almost as if my mother had "nothing" to do with it.