Like the UPC system of barcodes employed to uniquely identify manufactured goods, a single genetic marker could provide an universal barcode system that uniquely identifies all animals on Earth (Hebert et al. 2003). Cytochrome Oxidase c subunit I (COI) has been adopted by the Census of Marine Life (COML) as the preferred genetic barcode. COML is recommending the small subunit nuclear ribosomal RNA (SSU rRNA, also known as 18S), which forms the basis of the Tree of Life project, as a second elective barcode
Although there are limitations to the use of genetic barcodes—for example COI and 18S may not reflect important ecological, morphological, or adaptive genetic variation, and COI is not useful for phylogenetic reconstruction of anciently diverged species—a standardized genetic toolkit for helping to identify species is an important step toward a comprehensive global inventory of biodiversity.