Origin of Life on Earth
Life is enormously complex.
How did this come about? The key is again evolution.
By the process of natural selection, a species can evolve. Generally, adaptation through natural
selection may be expected to produce more complex organisms since (up to a point) more complex organisms
have better developed ways to respond to their environment. Over a few generations, the changes are likely to
be slight. But over the timescales of hundreds of millions of years a species can pass through many millions of
generations and small changes incremented millions of times can lead to very complex life forms.
We cannot know for sure, but it is likely that life began as a complex chemical process that crossed the boundary
to the simplest kind of life, and that this life then gradually became more complex as it was altered by
evolution through millions of generations.
Life Began in the Oceans
The oldest Earth fossils indicate that life began in the oceans on the Earth.
They suggest that simple bacteria and algae were already present in the Earth's
oceans by about 3.0-3.6 billion years ago, a mere billion years after the Earth
formed.
The fossil record indicates that life moved from the oceans to land much more recently, only about 400 million
years ago.
Chemical Evolution
Laboratory experiments using gases that might be found in a primitive atmosphere and a source of energy
such as electrical discharges to simulate lighting bolts or UV radiation to simulate sunlight
produce the chemicals of life rather easily.
For example, in a celebrated 1952 experiment, Stanley Miller and Harold Urey showed that such a simple experiment was
capable of producing amino acids, fatty acids, and urea, all essential building blocks of the chemistry of life.
But the chemicals of life are not the same thing as life. There is a considerable difference between a squirrel jumping
from limb to limb in a tree and a flask of amino acids. The process by which simple building blocks like amino acids
linked to form larger organic molecules in the primitive oceans of the Earth is sometimes called chemical
evolution. We may envision that over long periods of time the amino acids dissolved in the oceans linked to form the
more complex molecules called proteins. These molecules linked and broke apart at random, but some linkages were more
stable than others, and these tended to survive. Thus, over hundreds of millions of years the broth of precursors to life
in the oceans became enriched in complex molecules of particular stability.
From Chemical to Biological Evolution
But this is still not life because these molecules did not reproduce with purpose, one of our defining characteristics for
life; they were complex, but still produced by random events.
At some monumental point, out of all of these possibilities, a molecule appeared that had the ability to reproduce
itself. This we may envision as the first primitive life form, and the critical juncture
where chemical evolution transformed into
biological evolution. But as we discuss in the right panel, the transition from chemistry to biology may
not have been so clear cut.
It may have been more like the Escher illustration shown above: we know that it started with nonliving
chemicals and ended with living organisms, but it may not be possible to define
a single, unambiguous transition point.