Earth: The Origins of Life

Discovery Science: The Origins of Life

In the beginning, Earth was a boiling, fiery ball. Conditions improved gradually as it cooled down and eventually the primordial soup of inorganic compounds generated complex, organic molecules-the source of all life.

A primordial atmosphere of hydrogen and helium existed even in the earliest phase of the Earth’s existence. However, due to the high temperature and low gravitational forces this atmosphere simply escaped back into space. Only once the meteorite showers reduced in intensity did temperatures drop and conditions begin to stabilize. In many places, molten material made its way from the core to the surface, through the still brittle crust.

During this process large amounts of water vapor containing hydrogen (H), carbon dioxide (CO,), ammonia (NH3), and methane (CH4) were released, forming a layer of gas around the Earth. This secondary atmosphere was still vastly different from today’s, particularly because it lacked oxygen (O,). The production of oxygen only occurred once water molecules (H ,0) began to be broken down by the ultraviolet (UV) radiation from the sun, which easily penetrated the thin, unprotected atmosphere.

The formation of today’s atmosphere

The formation of the atmosphere and oceans were closely connected. Earth’s early atmosphere was about 80 percent water vapor. At saturation point, this condensed and fell as heavy rain during massive thunderstorms. As Earth’s surface was very hot at this time, large amounts of acid rain simply vaporized immediately, only to condense and fall again in an ongoing cycle. This process gradually cooled the planet until water began to collect in depressions on the surface.

About 3.5 billion years ago, the evolution of life began. A well accepted theory for the origin of life is that energy from lightning and the sun caused inorganic chemicals to form into simple biomolecules. From these, more complex molecules and macromolecules, slowly increased. About 570 million years ago it made up 12 to 15 percent of the atmosphere, opening the way for the evolution of oxygen-respiring organisms.


PRISCOAN 4.6 to 3.8 billion years ago: Earth forms from the protoplanet and a shell-like structure develops.

ARCHEAN 3.8 to 2.5 billion years ago: The Earth’s crust hardens and the temperature sinks to below 212°F (100°C). The first organic molecules form.

PROTEROZOIC 2.5 billion to 542 million years ago: The oxygen content of the atmosphere increases due to photosynthesis and a protective ozone layer develops.