According to the stellar evolution theory, the young Sun radiated much less energy than it does today. It was only about one billion years ago that it warmed the Earth to above the freezing point of water. The Cambrian explosion followed 1/2 billion years later to initiate the diversification of multicellular life. However geological evidence has shown that unicellular organisms existed between 3.5 to 3.8 billion years ago even when there was not enough solar energy to liquefy the water. This is known as the “faint young sun paradox”