Indigenous peoples of the Americas make up 52 million people, of less than 1% of the world's population, according to Wikipedia. This number is in the hundreds of millions if including Mestizos. Before apocalyptic plagues erased Native American societies from the face of the planet, and New World crops increased the population of the Old World, what percent of humans were Native American?
It's impossible to REALLY know, but the answer would be between 1.9% and 11.8%. Estimates put the global population in 1500 between 425,000,000 and 540,000,000. Pre-Columbus native populations are estimated between 10,000,000 and 50,000,000. If take the extremities you will get the above numbers.
So a very rough estimate (which is the best we can get) is 6.9% of the world population lived in the Americas before the arrival of Columbus.
edit: I should also add that the CURRENT population of the Americas is around 13%, so the change is is not as dramatic as you may expect.