Origins of Life

Earth began to form over 4.5 billion years ago from the same cloud of gas (mostly hydrogen and helium) and interstellar dust that formed our sun, the rest of the solar system and even our galaxy. In fact, Earth is still forming and cooling from the galactic implosion that created the other stars and planetary systems in our galaxy, a process which began about 16 billion years ago as the Milky Way began to form.

As our solar system began to come together some 6-7 billion years ago, the sun formed within a cloud of dust and gas that continued to shrink upon itself by its own gravitational forces. This caused it to undergo thefusion process and give off light, heat and other radiation. During this process, the remaining clouds of gas and dust that surrounded the sun began to form into smaller lumps called planetisimals,which eventually formed into the planets we know today.

The Earth went through a period of catastrophic and intense formation during its earliest beginnings about 4.5 - 4.6 billion years ago. By 3.8 to 4.1 billion years ago, Earth had become a planet with an atmosphere (not like our atmosphere today!) and an ocean. This period of time of Earth's formation is referred to as the pre-Cambrian Period. The pre-Cambrian is divided into three parts: the Hadean, Archean and Proterozoic Periods.

Pre-Cambrian Period

The Earth formed under so much heat and pressure that it formed as a molten planet. For nearly the first billion years of its formation -- called the Hadean Period (or "hellish" period) -- Earth was bombarded continuously by the remnants of the dust and debris -- like asteroids, meteors and comets -- until it formed into a solid sphere, fell into an orbit around the sun, and began to cool down.

As Earth began to take solid form, it had no free oxygen in its atmosphere. It was so hot that the water droplets in its atmosphere could not settle to form surface water or ice. Its atmosphere was also so poisonous that nothing would have been able to survive.

Earth's atmosphere was formed mostly from the outgassing of such volatile compounds as water vapor, carbon monoxide, methane, ammonia, nitrogen, carbon dioxide, nitrogen, hydrochloric acid and sulfur produced by the constant volcanic eruptions that besieged the Earth. It had no free oxygen.

About 4.1 billion years ago, the Earth's surface -- or crust -- began to cool and stabilize, creating the solid surface with its rocky terrain. Clouds formed as the Earth began to cool, producing enormous volumes of rain water that formed the oceans. For the next 1.3 billion years (3.8 to 2.5 billion years ago), called the Archean Period, first life began to appear (at least as far as our fossil records tell us... there may have been life before this!) and the world's land masses began to form. Earth's initial life forms were bacteria which could survive in the highly toxic atmosphere that existed during this time. In fact, all life was bacteria during the Archean Period.

Toward the end of the of the Archean Period and at the beginning of the Proterozoic Period, about 2.5 billion years ago, oxygen-forming photosynthesis began to occur. The first fossils, in fact, were a type of blue-green algae that could photosynthesize.

Some of the most exciting events in Earth's history and life occurred during this time which spanned about two billion years until about 550 million years ago. The continents began to form and stabilize, creating the supercontinent Rodinia about 1.1 billion years ago. (Rodinia is widely accepted as the first supercontinent, but there were probably others before it.) Although Rodinia is composed of some of the same land fragments as the more popular supercontinent, Pangea, they are two different supercontinents. Pangea formed some 225 million years ago and would evolve into the seven continents we know today.

Free oxygen began to build up around the middle of the Proterozoic Period -- around 1.8 billion years ago -- and made way for the emergence of life as we know it today. This event, of course, created conditions that would not allow most of the existing life to survive and thus made way for the more oxygen dependent life forms.

By the end of the Proterozoic Period, Earth was well along in its evolutionary processes leading to our current period, the Holocene Period, also known as the Age of Man. Thus, about 550 million years ago, the Cambrian Period began. During this period, life "exploded," developing almost all of the major groups of plants and animals in a relatively short time. It ended with the massive extinction of most of the existing species about 500 million years ago, making room for the future appearance and evolution of new plant and animal species.