Passage II

Life on Earth contains organic compounds (compounds containing carbon and other elements). The earliest compounds on Earth were inorganic. Scientists discuss 2 hypotheses about Earth's first organic compounds.

Hydrothermal Vent Hypothesis

Organic molecules were first formed several billion years ago in numerous hydrothermal vents on the deep seafloor. These vents were openings in the seafloor that spew magma-heated water. Scientists estimated that water temperatures ranged from 700° C in the deepest part of a vent to 10° C at the top of a vent (the seafloor opening). Laboratory experiments have shown that temperature gradients (increases or decreases in temperature in relationship to distance) promote chemical reactions. This natural temperature gradient provided the optimum conditions for the following events. Inorganic molecules were heated in the deep part of the vents, creating organic molecules. Seawater circulating through the vents cooled and preserved these organic molecules, which were then deposited onto clay lining the top of the vents. The clay particles provided irregular surfaces where the organic molecules became embedded. The organic molecules eventually combined to first form polymers (long chains of organic molecules) and, later, living, self-replicating cells.

Outer Space Hypothesis

Organic molecules were brought to Earth by bodies from space. Craters on other planets show that the solar system has been bombarded by asteroids, comets, and meteoroids since its formation. A meteorite (a meteoroid that strikes Earth) was found to contain amino acids (components of protein) and parts of DNA (carbon-based molecules found in living cells). Some meteorites are up to 5% organic material. Studies have found that comets contain organic material. Microscopic interplanetary dust particles fall to Earth from space in large numbers and are up to 10% organic material. These 3 types of bodies brought organic molecules to Earth. In the deep oceans, several billion years ago, conditions were right for those organic molecules to combine to form polymers and, eventually, living, self-replicating cells.