The Cell Theory

There are several theories about the origin of small molecules that could lead to life in an early Earth. One is that they came from meteorites Another is that they were created at deep-sea vents. A third is that they were synthesized by lightning in a reducing atmosphere; although it is not clear if Earth had such an atmosphere.

There are essentially no experimental data defining what the first self-replicating forms were. RNA is generally assumed the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions.
Cells emerged at least 4 until 4.3 billion years ago. The current belief is that these cells were heterotrophs. An important characteristic of cells is the cell membrane, composed of a bilayer of lipids. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid.

Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing organelles like the mitochondria and the chloroplasts are almost certainly what remains of ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, where the rest of the cell appears derived from an ancestral archaean prokaryote cell an idea called the endosymbiotic theory.

Source from: Wikipedia