Well if everything was covered in snow and ice, I wouldn't have come out either.
“Complex life originated hundreds of million year earlier?”
Although the earth may have been covered with snow and ice, there would have been areas around volcanoes, hot springs, “mid-Atlantic rifts”, and the like which would have been hospitible to life and potential incubators for more complex forms. Refer to the fascinating life forms that are being found around vents (smokers) near the mid-Atlantic rift. Also I wonder if by “complex life” you are referring to bacteria as opposed to viruses, or to critters like those found in the Burgess Shales as opposed to bacteria and protozoans.
Life first occurred on earth about 3.5 billion years ago (ya). The complex forms in the Burgess Shales in the early Cambrian would suggest a very long period of evolution, although some scientists refer to the Cambrian Explosion, personally I think that it must have been a much longer period of development. Here is an interesting link suggesting that life began about 3.85 billion ya and that by 2.5 billion ya there were simple amoeba like forms:
http://www.pbs.org/wgbh/evolution/library/03/3/text_pop/l_033_28.html
Here a Burgess Shale link: http://paleobiology.si.edu/burgess/
Some time ago I read about 3 major precambrian impact events. This site lists major North American impact craters including the giant Sudbury, which was originally 250 K in diameter and occurred about 1.8 billion ya:
http://www.thelivingmoon.com/43ancients/02files/Earth_Images_09.html