Three hypotheses account for the origin of life on Earth:
1. Life was generated from non living matter, or Abiogenesis.
2. Life is prevalent in the universe, known as Panspermia. On Earth it began by seeds of life arriving from outer space.
3. Life was created, as described in the first chapter of Genesis.
1a. The first hypothesis was tested experimentally. A “primitive soup” containing atoms molecules and water was treated with high voltage, or irradiated with U.V. or x-rays. In the following days, some basic organic molecules that form the building blocks of modern life aggregated spontaneously, yet did not replicate.
2a. The British astronomer Fred Hoyle was the proponent of Panspermia http://www.panspermia.org, He believed that life on earth came from outer space.
3a. According to Genesis, life requires a different creation act than the rest of the universe.
Already Aristotle favored abiogenesis basing his hypothesis on the observed fact that some animals arise from putrid matter, or that plant lice arise from dew. In the 19th century this brand of abiogenesis was finally refuted by Louis Pasteur, which led to the conclusion that Life comes from Life (omne vivum e vivo). Yet the concept Life is too broad, and Rudolph Virchow replaced it with: Cells always emerge from cells (omnis cellula ex cellula).
The cell is the atom of life. If disrupted it dies. It is extremely complex. This complexity is conserved as such and passed from generation to generation. The first cells which appeared on earth were the archaea. They lack a nucleus and are called also prokaryotes. Then some invaded other cells, continued living in their cytoplasm, and became cell organelles, like mitochondria. (v. Lynn Margulis http://www.bio.umass.edu/faculty/biog/margulis.html ). In the same way cell nuclei were formed in eukaryotes.
The rest of the evolution can be summarized as a gradual assembly of cells into different organisms. When a sperm meets an ovum, both contribute to each other their enormous complexity. The complexity of the ovum cytoplasm surpasses its genetic complexity. Without this indispensable complexity which the two provide, life cannot exist.
There is a smooth transition between the inorganic and organic world. Atoms assemble into molecules, and molecules into complex macro molecules. None of these intermediary states actually lives. Life’s smallest atom is the cell.
Now imagine that life on earth is a computer, and its processes are computations. This bio-computer does not generate its complexity from scratch, or from some simple programs. It starts its computations from a baseline, the complexity of a cell. We ought therefore to distinguish between two kinds of complexity: NKS complexity, which can be generated with simple programs, and that of a cell, which cannot.
In order to exist, life needs a certain amount of complexity which initially arrived from outer space, and may have been arriving ever since.