The Earth's appearance has transformed over the course of millions of years. Obviously, for much of that time scientists weren't around to track the Earth's shifting face. But with the advent of the space age in the second half of the 20th century, it became possible to view the planet from afar and get a sweeping perspective. Indeed, an updated NASA gallery published in conjunction with Earth Day 2012 features more than 160 comparison views demonstrating in sharp relief the very real changes that have taken place during the Earth's recent history.