Enrico Fermi thought we should have heard from aliens by now…did they all suffer some disastrous fate? If so, how might we avoid it?
A fantastic overview of the Fermi paradox - http://waitbutwhy.com/2014/05/fermi-paradox.html
A quick calculation of the number of habitable planets in the Milky Way - http://www.universetoday.com/103379/60-billion-habitable-planets-in-the-milky-way-alone-astronomers-say-yes/
The creepy Wiki page about Global Catastrophic Risks - http://en.wikipedia.org/wiki/Global_catastrophic_risks
A fantastic report by the Future of Humanity Institute at Oxford projecting likelihood of human extinction via various events (thanks to Samuel Stringman!) - http://www.fhi.ox.ac.uk/gcr-report.pdf
The AAAS statement on human cloning - http://www.aaas.org/page/american-association-advancement-science-statement-human-cloning
Carl Sagan explains why aliens aren’t visiting us all the time - http://sploid.gizmodo.com/carl-sagan-explains-why-we-arent-being-visited-by-alien-1608579953/+caseychan