1. Humanity discovered the first marker hundreds of years ago on Earth, which was known as the black marker. The government fooled around with it, unleashed the necromorphs, and tried to wipe out the happenings. Altman, a scientist that worked on the project, and I believe the only survivor, tried to let the world know, and was assassinated. Unitology spawned from what he managed to get out to the world, and they misinterpreted his warnings. Red markers, [spoiler]and the gold giant one in Dead space 2[/spoiler] are the results of human engineering. The black marker gave instructions on how to build red markers. 2. The hive mind came from an outbreak on aegis 7 several hundred years ago, and was the result of a red marker. It was locked away in the planet and aegis 7 was red flagged as an off limits world. 3. They were stupid and misinterpreted altmans warnings. 4. Haven't gotten around to it yet, really should get dead space 3 at some point.

It's a bit lengthy, but [url=http://deadspace.wikia.com/wiki/Brethren_Moons]this[/url] pretty much sums up everything about the series. Haven't played the third one, and refuse to, so I just read all 'bout it.

Surprisingly the "Previously on dead space" in Dead Space 3 answers a lot of things I didn't even know. 1. The first black marker was found on Earth. 2."The Hive Mind was unwittingly created as a result of military testing of the experimental Red Marker over two centuries prior to the events of Dead Space, on the remote planet Aegis VII. When the Marker proved to be too dangerous to wield as a weapon, further experimentation with it was aborted and it was used one last time in order to suppress the Hive Mind - a massive Necromorph that was the source of the Necromorph outbreak on Aegis VII and, subsequently, the USG Ishimura." It also makes more sense at the end of Dead Space 3. 3. Because they believe it grants them special power and a life after death. 4. More is answered at the end of Dead Space 3.

1. Space 2. Space 3. Because they can 4. Nothing