Over 3.5 billion years ago, the Earth was much different than it is today. Scientists believe that early Earth’s atmosphere was made up of nitrogen, water vapor, carbon dioxide, and methane. All of these gases are still on Earth today, but in different quantities. The gases in Earth's atmosphere include: Nitrogen - 78 percent, Oxygen - 21 percent, Argon - 0.93 percent, Carbon dioxide - 0.04 percent, Trace amounts of neon, helium, methane, krypton and hydrogen, as well as water vapor. Read More...