Millions of Years ago:
Cyano bacteria has existed on earth for millions of years.  
It is a simple micro organism that grows in colonies.
Bacteria spores are carried by wind, birds, squirrels, etc.
These colonies of algae need three things to grow:  Heat, moisture, and nutrients.    
Typically, they are seen on fences, and close to the ground bricks where nutrients can be found.  

1890 - 1949
Asphalt was soaked onto a mat made from felts containing cotton or wool fibers.

1950 - 1977
Cellulose fibers derived from recycled paper or wood chips were used to make the asphalt shingle mat.

1977 - today
An Inorganic fiberglass mat is coated with asphalt that is then covered with a protective layer of U.V. resistant, colored ceramic granules. Limestone is used as a void filler between individual mats and also provides resistance to fire and weather.  

Unfortunately, limestone is a favorite food for "cyano bacteria gloeocapsa".  
Prior to 1977, cyano bacteria would starve to death when it landed on your roof - now it thrives on your roof.  
As the algae grows, it holds moisture allowing the growth to accelerate.