In order for the principles of mutation and natural selection in the theory of evolution to work, there have to be living things for them to work on. Life must exist before it can to start diversifying. Life had to come from somewhere, and the theory of evolution proposes that it arose spontaneously out of the inert chemicals of planet Earth perhaps 4 billion years ago.
Could life arise spontaneously? If you read How Cells Work, you can see that even a primitive cell like an E. coli bacteria -- one of the simplest life forms in existence today -- is amazingly complex. Following the E. coli model, a cell would have to contain at an absolute minimum:
- A cell wall of some sort to contain the cell
- A genetic blueprint for the cell (in the form of DNA)
- An enzyme capable of copying information out of the genetic blueprint to manufacture new proteins and enzymes
- An enzyme capable of manufacturing new enzymes, along with all of the building blocks for those enzymes
- An enzyme that can build cell walls
- An enzyme able to copy the genetic material in preparation for cell splitting (reproduction)
- An enzyme or enzymes able to take care of all of the other operations of splitting one cell into two to implement reproduction (For example, something has to get the second copy of the genetic material separated from the first, and then the cell wall has to split and seal over in the two new cells.)
- Enzymes able to manufacture energy molecules to power all of the previously mentioned enzymes
Obviously, the E. coli cell itself is the product of billions of years of evolution, so it is complex and intricate -- much more complex than the first living cells. Even so, the first living cells had to possess:
- A cell wall
- The ability to maintain and expand the cell wall (grow)
- The ability to process "food" (other molecules floating outside the cell) to create energy
- The ability to split itself to reproduce
Otherwise, it is not really a cell and it is not really alive. To try to imagine a primordial cell with these capabilities spontaneously creating itself, it is helpful to consider some simplifying assumptions. For example:
- Perhaps the original energy molecule was very different from the mechanism found in living cells today, and the energy molecules happened to be abundant and free-floating in the environment. Therefore, the original cell would not have had to manufacture them.
- Perhaps the chemical composition of the Earth was conducive to the spontaneous production of protein chains, so the oceans were filled with unimaginable numbers of random chains and enzymes.
- Perhaps the first cell walls were naturally forming lipid spheres, and these spheres randomly entrapped different combinations of chemicals.
- Perhaps the first genetic blueprint was something other than DNA.
These examples do simplify the requirements for the "original cell," but it is still a long way to spontaneous generation of life. Perhaps the first living cells were completely different from what we see today, and no one has yet imagined what they might have been like. Speaking in general terms, life can only have come from one of two possible places:
- Spontaneous creation - Random chemical processes created the first living cell.
- Supernatural creation - God or some other supernatural power created the first living cell.
And it doesn't really matter if aliens or meteorites brought the first living cell to earth, because the aliens would have come into existence through either spontaneous creation or supernatural creation at some point -- something had to create the first alien cells.
Most likely, it will be many years before research can completely answer any of the three questions mentioned here. Given that DNA was not discovered until the 1950s, the research on this complicated molecule is still in its infancy, and we have much to learn.