Where are you Gravity?

Earlier today, I was in my humanities class, enjoying a discussion entailing the thoughts of Edward O. Wilson. When arriving at Wilson's discovery of the theory of evolution, naturally controversy came knocking. I expect there to be people who argue against evolution, especially religious fanatics. What baffled me was the interjection by one student. He honestly claimed that man could not have orginated from apes becuase the earth has only sustained life for 20,000 years. He argued that 20,000 years ago, gravity did not exist on earth; therefore, no form of life was able to survive. I'm not even sure how to respond to a claim such as that. Following this, he decided that the idea of microevolution was acceptable, but macroevolution was absurd. This is a timeless view on evolution that is easily discredited, and was. Before I was able to state the obvious, atleat to me, another student in the class brought to his attention that the combination of many microevolutions is nothing more than macroevolution. After this information was brought to his attention, he had nothing more to say. In the end, I got a good laugh from it. I'm all for a listening to someone elses ideas, but atleast back yourself up when entering such a heated debate.