The generally accepted theory of solar system formation begins with a giant cloud of gas and dust that collapses under its own gravity. As the cloud contracts, most of its mass collects in the center, forming a protostar (an early stage of star formation; "proto" indicates "first"). The rest of the matter forms a disk around the protostar. When the pressure becomes great enough in the center of the protostar, nuclear fusion begins, and a new star is born. The material in the disk accretes (comes together) into larger and larger bodies (to form asteroids, comets, moons, and planets), which are all in orbit around the central star.
Computer simulations indicate that it took about 100,000 years for the cloud to collapse to form the protosun and the disk. The gas and dust in the disk accreted to form small bodies (planetesimals) and larger bodies (protoplanets) over millions of years, which would eventually become the asteroids, comets, planets, and moons that we have today. Within about 10 million years after the initial collapse, the young Sun's solar wind swept away the remaining gas in the disk and the formation of the outer, gas-giant planets (Jupiter, Saturn, Uranus, and Neptune) was complete. Rocky protoplanets in the inner solar system continued to collide and merge, forming the terrestrial planets (Mercury, Venus, Earth, and Mars). Within about 100 million years, the eight planets of the solar system, a region of rocky asteroids between Mars and Saturn (called the asteroid belt), and millions of comets on the outer edge of the solar system (called the Kuiper Belt) were formed.
However, the solar system continued to evolve. It is hypothesized that about 4 billion years ago, 500 million years after the formation of the Sun, all the planets and moons in the solar system were pummeled by asteroids and comets. This period, called the Late Heavy Bombardment (LHB), would have been caused by an instability in the solar system. According to one popular computer model, the four gas giant planets and the icy objects of the Kuiper Belt used to be closer to the Sun than they are now. The orbits of Jupiter and Saturn changed so that Jupiter completed two orbits in the time that Saturn orbited the Sun once. The resulting interaction between Jupiter and Saturn created an enhanced gravitational force, which destabilized the orbits of Neptune and Uranus and flung them into the Kuiper Belt, causing the small objects in the belt to scatter. Some planetesimals were ejected from the solar system while others collided with other objects in their paths. Impact craters on the Moon, Earth, and other moons and planets may be evidence of this period of heavy bombardment.
Such a period of heavy impacts would have had a significant effect on the evolution of the planets and the development of life on Earth. It is possible that planetesimals delivered basic ingredients for life, such as amino acids and water, to Earth during this period of heavy bombardment. It is also possible that life had already developed on Earth before the LHB. If true, the LHB may have challenged the evolution of early life on Earth and altered its development.