Well, this is the part where things become a bit more philosophical. Using what we know about early life (not FIRST life), let's try and put together a picture of what it might have been like.
But first, we must be in agreement about one thing: What constitutes life?
A biologist might begin with very broad characteristics. A living organism utilizes energy (metabolism), is capable of reproduction (increase in biomass), and undergoes evolution.
Sounds good! But immediately we begin to run into problems:
Unfortunately, any definition of life will have exceptions, counter-examples. Yet, most of us instinctively know what is and isn't life! So let's move on and define: What's required for life? (This will look a bit like what we defined above):
The Biogenic Elements: C, N, O, P, S, and H are considered contingent for life as we know it. Luckily, All of these elements are highly abundant.
Carbon based life is all we know. However, scientist argue that Carbon is like no other element: it forms the widest variety of Bonds than any other element, and thus a major constituent of all biological chemicals. It's likely life elsewhere would also be Carbon-based. (We'll discuss this much more later in the quarter)
Reproduction, while may not be required to define life (recall the Mule example), is vital for a species to maintain its presence in the universe. Complex DNA molecules would not have been available early on. The earliest life must have used very simple, short chains of organic molecules.
Energy Source. The first life may have been heterotrophic - requiring a constant source of organic compounds from which to derive energy for growth and reproduction. They probably were not photo-autotrophic (i.e. photosynthetic), which requires complex structures and reactions. However, the first life may have been chemo-autotrophic, deriving energy from inorganic chemicals, as many bacteria and archaea currently do.