I think that answer is very simple.
Purpose of life is to be happy.
More difficult question is how to achieve it.
I think that answer can be find in many spiritual books and teaching because lasting happiness we can't find only in material things.
We need also God in our lives.
The answer is not very simple. Depending from what aspect you're looking at. From the aspect of biology the meaning of humans is the same as of the animal, reproduce and keep the species alive, therefore there is no purpose in life besides that but the goals in your life and what purpose you want your life to have is up to you.
Actually, the biology of humans is different than that of animals. No animal has the ability to reason. There are differences in the biology of people that allow them to do this.
But if you look at it from the aspect of religion, it's pretty much be born, obey, die sinless. Depending of what religion you follow that's your "purpose". But since religions preach differently from one another, it's stupid to blindly follow one and deny the rest, but that's where faith comes in.
Actually, faith is the thing that everyone lives by (or dies in). Why? Because nobody knows the future. We think we know some of the future simply because God made life to be reasonably stable in most ways. But there have always been people who died in accidents that they never expected would hit them, or they would have lived their life in a different way to avoid the accident. Consider the person who dies in a terrorist bombing without even realizing that he is dead.
The faith of everyone is automatically placed in God, even though he may not realize it. There are no atheists... way down deep inside each person.
Your and everyone elses "purpose" on this planet is to reproduce and keep the human species alive, you decide what you want your own purpose and goal to be.
It was the command of God that caused people to reproduce. God is looking for people who believe in Him. And He is getting what He wants... even though he has to go through a bunch of unbelievers to get some believers. He doesn't want unbelievers to be unbelievers and die for it. But He offers freedom, so that it is the unbeliever's choice.