"Well guess what? People are using only around 10% of their brain, so this shatters the theory of evolution."
This is one of those statistics that sound good, but are misleading. About 10% of our neurons are the cells that form our thinking, correct. But the other 90% is in the form of neuroglial cells, like astrocytes, that give the brain formation and structure. It's not like 90% of our brain is neurons that have nothing better to do than sit around.
As far as the complexity of the human brain goes, I'll repost an example I wrote here a little while ago:
"An example would be the Apache helicopter. Without looking at all the developments that led to the creation of all the sytems of the Apache, some sketchings from Leonardo DeVinci, the use of gunpowder by the feudal Chinese, Bernoulli's principle, the development of sonar and more recently radar, the combustion of liquid fuel (not to mention all the mechanics of the engine itself), the use of plastic components derived from oil, and so on, one might conclude that the Apache was developed by an force far greater than any corporeal source could summon.
The complexity of life is parallel to this. It's easy to look at the high levels of intelligence and power that humans have and say that we couldn't have developed without outside aid. All I say is, why not?"
Now, it is true that the helicopter had an intelligent designer, but the example is valid with regard to evolution, too. It's an extremely gradual process of discrimination and weeding-out, and if you just look at the finished product and ignore the process, (early life was much more primitive and moved up in complexity throughout time) it's easy to apply it to some mysterious omnipotent God.