<P> The history of the Earth can be organized chronologically according to the geologic time scale, which is split into intervals based on stratigraphic analysis . The following four timelines show the geologic time scale . The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon . Therefore, the second timeline shows an expanded view of the most recent eon . In a similar way, the most recent era is expanded in the third timeline, and the most recent period is expanded in the fourth timeline . </P> <P> The standard model for the formation of the Solar System (including the Earth) is the solar nebula hypothesis . In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called the solar nebula . It was composed of hydrogen and helium created shortly after the Big Bang 13.8 Ga (billion years ago) and heavier elements ejected by supernovae . About 4.5 Ga, the nebula began a contraction that may have been triggered by the shock wave from a nearby supernova . A shock wave would have also made the nebula rotate . As the cloud began to accelerate, its angular momentum, gravity, and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation . Small perturbations due to collisions and the angular momentum of other large debris created the means by which kilometer - sized protoplanets began to form, orbiting the nebular center . </P> <P> The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began . After more contraction, a T Tauri star ignited and evolved into the Sun . Meanwhile, in the outer part of the nebula gravity caused matter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings . In a process known as runaway accretion, successively larger fragments of dust and debris clumped together to form planets . Earth formed in this manner about 4.54 billion years ago (with an uncertainty of 1%) and was largely completed within 10--20 million years . The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies . The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets . </P> <P> The proto - Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile metals . Having higher densities than the silicates, these metals sank . This so - called iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of Earth's magnetic field . J.A. Jacobs was the first to suggest that the inner core--a solid center distinct from the liquid outer core--is freezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years). </P>

Where were the first organic compounds on earth formed