genes may also simply lack true homologues and in such cases have an independent origins via de novo gene birth, which tends to be a more recent event. These processes may act at different rates in insects, primates, and plants. Despite their relatively recent origin, orphan genes may encode functionally important proteins. Characteristics of orphan genes include AT richness, relatively recent origins, taxonomic restriction to a single genome, elevated evolution rates, and shorter sequences. Some approaches characterize all microbial genes as part of one of two classes of genes. One class is characterized by conservation or partial conservation across lineages, whereas the other (represented by orphan genes) is characterized by evolutionarily instantaneous rates of gene turnover/replacement with a negligible effect on fitness when such genes are either gained or lost. These orphan genes primarily derive from mobile genetic elements and tend to be 'passively selfish', often devoid of cellular functions (which is why they experience little selective pressure in their gain or loss from genomes) but persist in the biosphere due to their transient movement across genomes. History Orphan genes were first discovered when the yeast genome-sequencing project began in 1996. Orphan genes accounted for an estimated 26% of the yeast genome, but it was believed that these genes could be classified with homologues when more genomes were sequenced. At the time, gene duplication was considered the only serious model of gene evolution and there were few sequenced genomes for comparison, so a lack of detectable homologues was thought to be most likely due to a lack of sequencing data and not due to a true lack of homology. However, orphan genes continued to persist as the quantity of sequenced genomes grew, eventually leading to the conclusion that orphan genes are ubiquitous to all genomes. Estimates of the percentage of genes which are orphans varies enormously between species and between studies; 10-30% is a commonly cited figure. The study of orphan genes emerged largely after the turn of the century. In 2003, a study of Caenorhabditis briggsae and related species compared over 2000 genes. They proposed that these genes must be evolving too quickly to be detected and are consequently sites of very rapid evolution. In 2005, Wilson examined 122 bacterial species to try to examine whether the large number of orphan genes in many species was legitimate. The study found that it was legitimate and played a role in