The notion that giant viruses represent a potential fourth domain of life is now closer to being disproven, researchers say. They have identified a group of giant viruses that harbor components of many other viruses and proteins, and their analyses suggest that these whopper viruses acquired the various components in an evolutionarily recent timeframe -- likely from and as an adaptation to their hosts. Since the discovery of giant viruses, important questions have been raised about whether these life forms are indeed viruses, or perhaps even a fourth domain of life. Here, researchers uncovered a clue in the wastewater of a treatment plant in Austria. In analyzing metagenomes within the sewage, Frederik Schulz and colleagues identified four new, related species of giant viruses, which they called Klosneuviruses. One of these giant viruses was so "immense" that it maintains the machinery to produce enzymes that interact with 19 amino acids, providing scientists with the best opportunity yet to run an in-depth comparison of these giants against cells and other viruses. The researchers determined that Klosneuviruses belong to a family called Mimiviridae, and analyzed differences among the family's three distinct lineages. They found that gene gain exceeded the amount of gene loss, leading to substantial genome size increase, in each of these three lineages independently. As well, a handful of enzymes appear to have emerged independently across Mimiviridae lineages. Based on their analysis, the authors suggest that Klosneuviruses did not evolve from a cellular ancestor, but rather are derived from a much smaller virus through extensive gain of host genes.