3.7 billion year old mineral deposits in Greenland appear to hold signs of fairly complex, photosynthesizing life. Allowing for the preceding evolution needed to eventually produce such organisms, it seems that life on Earth appeared at almost the first opportunity, under conditions far different from what we think of as Earth-like, today. If life appeared so readily on Earth, it seems reasonable that it could have done so on a multitude other planets in space.

Link to article, with further details, below:

http://www.washingtonpost.com/news/speaking-of-science/wp/2016/08/31/3-7-billion-year-old-fossils-may-be-the-oldest-signs-of-life-on-earth/