There is strong evidence to support the hypothesis that life has existed on Earth for more than 4 billion years. Initially, single-celled life-forms dominated the planet, and they gradually evolved into more complex forms. Scientists theorize that life could have originated on Earth in two possible ways: either as an indigenous form, created from the organic compounds and conditions present in the primordial Earth, or from organic compounds or even living bacteria that were transported to Earth by comets or meteorites. This later "panspermia" theory suggests that organic compounds and living bacteria may have come to Earth from space and served as the seeds for life.

Regardless of its origin, life has flourished on Earth for billions of years and has adapted to an ever-changing variety of environmental conditions. For over 2.5 billion years, cyanobacteria were the dominant life-forms and were responsible for the gradual buildup of an oxygen-rich atmosphere. In the Cambrian geological period, there was an explosion in the diversity of marine animal life. Various species seem to have come and gone as if they were the products of some biological experiment to see which could survive best. The survivors continued to evolve into more complex organisms that gradually found their way to land.

It seems that the end of one species' dominance and the beginning of another's is usually marked by a dramatic change in global climate. The best evidence to support such a theory occurs at the Cretaceous-Tertiary boundary, approximately 65 to 70 million years ago. An iridium-rich layer of sediment, attributed to the impact of an asteroid, marks the boundary between the two geological periods. Dinosaur fossils are found below this layer but are notably missing from the layers above. Scientists interpret this as evidence for global climate change that led to the mass extinction of a large number of species. An event such as this is not limited to the Cretaceous-Tertiary periods but may also be responsible for several other mass extinctions.

There are many scientific theories that suggest the Earth is usually a cold planet covered by vast amounts of ice. Periodically, warm periods emerge that last for several thousand years and eventually phase back into long ice ages. It is during these warm periods that land animals flourish and perhaps even speed up their evolutionary processes. There are no certainties as to when ice ages begin or end. Global climate change is the underlying reason, but what initiates this change? Scientists have suggested such possibilities as changes in the Earth's axial tilt, increased volcanic activity blocking incoming sunlight, shifts in the world's ocean currents, impact debris from collisions with comets or asteroids, and the effects of human pollution. No one is certain which of these possibilities holds the answer, and the truth is probably found in some combination of factors. What is certain is that ice ages and their interglacial warm periods are features inherent to planet Earth, and they will continue to occur with or without human influence.

Astronomers have presented evidence to support the existence of hundreds of other planets orbiting distant and even exotic stars. These planets range from massive gas giants to a few with nearly earthlike masses. All evidence tends to indicate, however, that Earth is a very unusual planet. It has evolved from a hot gaseous world to one that is dominated by liquid water. Its atmosphere changed from one that was poisonous to animal life to one that is oxygen-rich and supports uncountable species of life, both on land and in the oceans. In the geological past, the processes of plate tectonics may have been beneficial to the evolution of life, while at other times they may have caused mass extinctions. Perhaps the one constant throughout Earth's history has been that change is inevitable. Present-day humans are the result of the ever-changing conditions of the Earth's surface. Adaptation to climatic changes means survival for some species, while others that cannot change face extinction. Unique to this period in Earth history is the fact that a particular species, human beings, possesses the ability to have either a positive or a negative effect on the environment. One path may lead to a better world, while the other may lead to extinction.

References:

1)         Canup, R. M., and K. Righter, eds. Origin of the Earth and Moon. Tucson: University of Arizona Press, 2000.

2)         Russell, Michael. "First Life" American Scientist 94, no. 1 (2006): 32-39.

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