You may be wondering, since the cell theory says that all cells come from preexisting cells, where did the first cell come from? The first scientist to suggest a theory which explained the formation of life was the Russian scientist A. I. Oparin.
While certain elements of his theory are still not agreed upon by biologists, most scientists now accept Oparin's ideas regarding the environment of Earth when life first formed, about 3.5 billion years ago. They believe that, at that time, there was very little free oxygen available in the atmosphere (that means that oxygen may have been part of other molecules, like water, but not freely floating in the atmosphere as it is today), and that nitrogen, carbon, oxygen, and hydrogen, were all present in one form or another.
Scientists also generally agree that the Earth was a very violent place at the time when life was first created. Volcanoes, lightning, and ultraviolet rays from the Sun all provided energy that was to forge the first organic molecules (organic molecules are necessary for life and will be discussed in Chapter Two). However, this hypothesis was not tested until the 1950s by Stanley Miller, thirty years after Oparin had published his work.
Miller created an apparatus which simulated conditions that scientists believed existed on Earth when life formed. Remarkably, within a mere 24 hours after beginning the experiment, large amounts of amino acids (an important type of organic molecule) had formed. Subsequent experiments with slightly different initial conditions resulted in the formation of all sorts of other organic molecules. These experiments suggested that the molecules necessary for life could have formed in the primitive Earth. Over the course of time, more and more of them would be produced, until finally they began to chemically combine to form larger molecules. It is thought that these large, organic molecules eventually combined to create the first organisms, which changed and developed over time into the large and complex organisms we know of today.
