550 million years ago, as the planet celebrates its four billionth birthday, oxygen levels in the atmosphere have risen from next to nothing to as much as 13%.
550 ميليون سال قبل، زماني که زمين تولد 4 ميليارد سالگيش رو جشن ميگيره، ميزان اکسيژن در اتمسفر بالا ميره