One of the qualities of the planet Earth that makes it habitable for life is its abundance of water. Although less than ten percent of the water on the planet is fresh (suitable for drinking), the water itself is very conducive to bacterial and plant life, and the water cycle of the planet ensures that the water is filtered and spread across the planet. The origin of the Earth's water is rooted in the development of the planet; as the dust of the evolving Solar System drew together to create a planet, the oxygen and hydrogen in the dust was locked together into the developing rock of the planet. This meant that there would be no liquid water on the surface, especially since the planet was very hot as it formed. Over time, the planet was struck by various planetary bodies such as comets and meteors; comets are formed largely of ice, so they would release their water into the atmosphere. The impacts on the surface freed hydrogen and oxygen into the atmosphere as well; once enough of those elements were released, water began to form in larger quantities, as well as existing as water vapor. Over time, the water that landed on the earth remained because of the planet's high gravity and thickening atmosphere; in comparison, the moon retained very little water, because of its low gravity and almost non-existent atmosphere. It took over three billion years for enough water to land on Earth to start the water cycle that renews and circulates water and minerals, and to start the evolution of life in stagnant ponds, where amino acids mutated and joined to create the first cellular life forms.