Horizontal gene transfer
Horizontal gene transfer poses a possible challenge to the concept of the last universal common ancestor (LUCA) at the root of the tree of life first formulated by Carl Woese, which led him to propose the Archaea as a third domain of life. Indeed, it was while examining the new three-domain view of life that horizontal gene transfer arose as a complicating issue: "Archaeoglobus fulgidus" was seen as an anomaly with respect to a phylogenetic tree based upon the encoding for the enzyme HMGCoA reductase—the organism in question is a definite Archaean, with all the cell lipids and transcription machinery that are expected of an Archaean, but whose HMGCoA genes are of bacterial origin. Scientists are broadly agreed on symbiogenesis, that mitochondria in eukaryotes derived from alpha-proteobacterial cells and that chloroplasts came from ingested cyanobacteria, and other gene transfers may have affected early eukaryotes. (In contrast, multicellular eukaryotes have mechanisms to prevent horizontal gene transfer, including separated germ cells.) If there had been continued and extensive gene transfer, there would be a complex network with many ancestors, instead of a tree of life with sharply delineated lineages leading back to a LUCA. However, a LUCA can be identified, so horizontal transfers must have been relatively limited.