I preferred the 1980s theories as I've always thought that we need an intergalactic enemy to unite the people on this planet and it seemed like Nemesis could've been that enemy 
http://www.dailymail.co.uk/sciencetech/article-1294372/Life-Earth-wiped-27-million-years.html
http://www.dailymail.co.uk/sciencetech/article-1294372/Life-Earth-wiped-27-million-years.html
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