It is not known how the genes were transferred, but Dr. Klein theorizes that one form of transfer took place during the reproductive cycle, with the genes having been incorporated into either sperm or egg cells or incorporated shortly after fertilization. It's possible that the transfer could also represent a form of infection where genetic material is transferred into these reproductive cells and thereby into the entire genome of the recipient.

Bacteria do transfer genes to other bacteria, by means of a circular DNA molecule known as a plasmid. However, Dr. Klein said, bacteria are not believed to be capable of passing plasmids to animal cells.

The study's authors offered an alternative explanation for the fact that some genes are present only in bacteria and animals. According to this explanation, all living organisms once possessed these genes as well, and most lost them. However, the authors wrote that it is unlikely that such a large group of living organisms could have lost so many genes.

An understanding of how the enzymes function in bacteria may provide insight into how they function in animals, Dr. Klein said. All the enzymes may be important to bacteria because they provide a detoxification function ” they make chemical changes within the bacteria that eliminate potentially toxic substances. AANAT, he said, is present in both the pineal gland, located in the brain, and in the retina of human beings and other primates. In the pineal gland, AANAT plays a role in manufacturing melatonin. However, AANAT in the retina does not manufacture melatonin. Dr. Klein suspects that, in the retina, AANAT may have a role in neutralizing and eliminating toxic substances. He is currently investigating whether a disruption in AANAT function plays a role in the development of macular degeneration, a disease that impairs vision and that may result in blindness.

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