As the Earth Turns

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How long ago did eukaryotes develop? (And what are eukaryotes, anyway?) When and why was the first mass extinction? How many extinctions have there been? What is C4 photosynthesis, and when did plants start doing it? Here's a wonderful summary, presented in a simple timeline form, of the most important events in Earth's history, including links to related videos: http://www.bbc.com/earth/bespoke/story/20150123-earths-25-biggest-turning-points/index.html