It is thought viruses played a central role in early evolution before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth. Viruses are still one of the largest areas of unexplored genetic diversity on Earth. Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains. They are found wherever there is life and have probably existed since living cells first evolved. The origins of viruses in the evolutionary history of life are unclear because they do not form fossils. Molecular techniques are used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose. Some viruses may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity. Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through natural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators and as "organisms at the edge of life". The existence of viruses in the ocean was discovered through electron microscopy and epifluorescence microscopy of ecological water samples, and later through metagenomic sampling of uncultured viral samples. Marine viruses, although microscopic and essentially unnoticed by scientists until recently, are the most abundant and diverse biological entities in the ocean. Viruses have an estimated abundance of 1030 in the ocean, or between 1 and 100,000x106 per millilitre. Quantification of marine viruses was originally performed using transmission electron microscopy but has been replaced by epifluorescence or flow cytometry. Bacteriophages, often just called "phages", are viruses that parasite bacteria. Marine phages parasite marine bacteria such as cyanobacteria. They are a diverse group of viruses which are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. There are up to ten times more phages in the oceans than there are bacteria, reaching levels of 250 million bacteriophages per millilitre of seawater.