Hmmm... Where to start....
IMHO, I feel you need some context to even begin to answer the topic of this thread in any
meaningful way.
As in, "natural" with-respect-to "what" ? Biology ???
As well as, what Criteria do you use to assess whether it is, or is not ?
As CraigD and Freezestar have so eloquently stated, you basically have no "rudder" to steer
this thread by.
You want my "opinion", it is free, no charge...

I would base my opinion on Biology (not my subject), Anthropology (interested, no expert),
and Logic (one of my subjects).
My criteria for success (being natural or not) is whether we behave similar to other
animals like ourselves ("being civilized") and would "civilization" cause extinction.
The former is studied by Behavioral Anthropologists. In every group studied, a population
group was fine until the population density got to much. Looking through history, you
see where either a population group of people either migrated elsewhere or disease
and death took over to thin the population.
As for Civilization possibly bringing about distinction, we have no evidence one way or
another to conclude.
Another way to consider, for any group of humanoid (bipedal mammalian creatures)
that learn enough to
1. communicate together
2. build a working group that extends beyond the immediate family
3. work together for a "common good".
Would this be enough to promote what becomes "civilization" ?
Look over history, you tell me.
maddog