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Scientists trying to reconstruct evolutionary history have been challenged by the fact that genes can sometimes transfer between distant branches on the tree of life. This movement of genes can occur through horizontal gene transfer , scrambling the information on which biologists relied to reconstruct the phylogeny of organisms. Conversely, HGT can also help scientists to reconstruct and date the tree of life. Indeed, a gene transfer can be used as a phylogenetic marker, or as the proof of contemporaneity of the donor and recipient organisms, and as a trace of extinct biodiversity.
HGT happens very infrequently – at the individual organism level, it is highly improbable for any such event to take place. However, on the grander scale of evolutionary history, these events occur with some regularity. On one hand, this forces biologists to abandon the use of individual genes as good markers for the history of life. On the other hand, this provides an almost unexploited large source of information about the past.