The faint Sun paradox presumes that the Sun has maintained the same mass throughout its history. Astronomers observe, however, that solar-sized stars throughout our galaxy lose significant mass during their infancy and youth. They also note that old lunar rocks and old grains from meteorites show evidence of a much more intense solar wind (the mechanism for solar mass loss) three to four billion years ago. Therefore, a more reasonable model for the Sun—recently produced by astrophysicists Juliana Sackmann and Arnold Boothroyd—is one in which the Sun starts off with between 4 and 7 percent more mass than it has today. That extra mass translates into a brighter Sun. (The Sun’s luminosity increases with the fourth power of its mass.)
Specifically, from the time when the Sun was 40 million years old to when it was about 1.0 to 1.5 billion years old it would gradually grow dimmer as it loses mass. At 1.0 to 1.5 billion years old, the Sun stops shedding significant mass. Thus, from 1.0 to 1.5 billion years old onward the Sun gradually gets brighter and brighter.
In Sackmann and Boothroyd’s model the Sun at the time hydrogen fusion burning was first ignited (4.52 billion years ago) would have been between 90 and 105 percent of its current brightness (cf 71 percent for no solar mass loss). At the time of life’s origin on Earth (3.8 billion years ago) the Sun would have been between 14 and 16 percent of its current brightness (cf 75 percent for no solar mass loss). From 3.0 billion years ago to the present there would be no difference in the Sun’s luminosity between Sackmann and Boothroyd’s model and a solar model with no mass loss.
Compared to no mass loss, the Sun losing mass in its youth would not demand the quantity of powerful greenhouse gases such as methane and ammonia in Earth’s early atmosphere at the time of life’s origin. In fact, it may not require any methane or ammonia at all. This lack of dependence on ammonia and methane could be critical for life’s stability, abundance, and diversity on the early Earth since both ammonia and methane are not only very difficult to produce in Earth’s atmosphere but also extremely unstable. Lessening the dependence on greenhouse gases to sustain adequate temperatures for life permits much greater temperature variation over Earth’s surface. (The greater the quantity of greenhouse gases, the less temperature variation that is possible in Earth’s atmosphere.) A greater temperature variation allows for a much higher diversity of bacteria at the time of life’s origin. Increasing the diversity, abundance, and stability of life on Earth previous to three billion years ago shortens the time window needed to prepare Earth for humans and human civilization.
 I.-Juliana Sackmann and Arnold I. Boothroyd, “Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars,” Astrophysical Journal, 583 (2003), pp. 1024-1039.