There are some major evolutionary jumps that seem to have occurred only once. Eukaryotic cells contain membrane-enclosed structures to perform different functions, and they comprise all forms of multicellular life on Earth. They arose from prokaryotes only once in four billion years, and no prokaryotic cells have been found that show intermediate levels of complexity.
Why only once? A recent "Hypothesis" paper in Nature posits that the answer lies in bioenergetics. The mitochondria that produce much of a eukaryotic cell's energy, which were once free living prokaryotes, and still carry their own genomes, now contain only genes essential for energy production. In order to get an equal dose of energy-producing genes, a prokaryote now has to make extra copies of its entire genome, a hurdle that keeps it from evolving a complex genome.