The discovery of a megavirus virus is confirmation that giant viruses descended from complex cells that became simpler.
Since the discovery of the first giant virus, mimivirus, in 2003, researchers have debated how they evolved. Unlike conventional viruses, mimivirus carries many genes thought to be unique to cellular life, suggesting that it evolved from a cell. But it could also have stolen those genes from cells that it infected.
This most simple mode of life has proved to be extremely successful. As Carl Zimmer points out in his short book A Planet of Viruses (sounds a lot like Mike Davis' Planet of Slums), viruses "out number all other residents of the ocean by about 15 to one."