Horizontal gene transfer (HGT) has long been recognized as a principal forc
e in the evolution of genomes(1). Genome sequences of Archaea and Bacteria
have revealed the existence of genes whose similarity to loci in distantly
related organisms is explained most parsimoniously by HGT events(2-4). In m
ost multicellular organisms, such genetic fixation can occur only in the ge
rm line. Therefore, it is notable that the publication of the human genome
reports 113 incidents of direct HGT between bacteria and vertebrates(5), wi
thout any apparent occurrence in evolutionary intermediates, that is, non-v
ertebrate eukaryotes. Phylogenetic analysis arguably provides the most obje
ctive approach for determining the occurrence and directionality of HGT(6,7
). Here we report a phylogenetic analysis of 28 proposed HGT genes, whose p
resence in the human genome had been confirmed by polymerase chain reaction
(PCR)(5). The results indicate that most putative HGT genes are present in
more anciently derived eukaryotes (many such sequences available in non-ve
rtebrate EST databases) and can be explained in terms of descent through co
mmon ancestry. They are, therefore, unlikely to be examples of direct HGT f
rom bacteria to vertebrates.