For starters, a gene has to migrate somehow to the nucleus of the 'germ' cells that give rise to sperm and eggs. Otherwise it will not be passed on. How a gene successfully infiltrates the human genome is not clear, say researchers. Further, a transferred gene must be in or get in a format that allows long-term maintenance and replication. The claim that a pathogen infected a vertebrate early in vertebrate evolution means both that a bacterial gene entered the nucleus of a host cell, and that the infected cell was part of the reproductive system.
Many evolutionary biologists regard lateral gene transfer as an extraordinary event.
Proving that no bacterial genes have been transferred to vertebrates, on the other hand, is at present impossible. Instead, the TIGR researchers attempted to show that the existing genomic data do not support the original statement. Proteome sizes and number of genes shared with each of the human protein sets, using a Blast cutoff of 10 -10 . A gene that shows up in a particular species through lateral gene transfer in theory should be identifiable by its presence or absence in multiple species. For example, a gene may be present in certain bacteria and all plants but not in other bacteria or vertebrates.