Exponential growth using multiplication

Hi guys,

I'm finding something difficult to understand and perhaps someone could tell me the logic behind this.

I have two parents, each one of my parents have two parents and so forth, so.

I have 2 parents

I have 4 grand parents

I have 8 great grand parents

I have 16 great great grand parents

and 32 great great great grand parents ........

With this exponential growth, if I carry on to just 37 generations of humans, it turns out that I should have just over 63 Billion great great (37 times...) grand parents. How can this be possible as I'm pretty sure that there have been far more than 37 generations of humans, but at no time have there ever been 63 Billion people living on the planet at the same time as a single generation.

Can someone explain this?

thanks,