 | Evidence has emerged that an ancient virus previously known only from fossil evidence has persistently infected some humans at very low levels for hundreds of thousands or even millions of years. This ancient retrovirus is a kind of living fossil, and the discovery of an intact copy of it within the human genome poses questions as to how it has survived, and suggests others from the distant evolutionary past may lie dormant in the DNA of many species. A retrovirus replicates by inserting its genome into that of an infected cell. Occasionally, retroviruses infect germ line cells – those found in eggs and sperm – and if these cells survive and go on to create a new organism, that new organism will contain the retrovirus as an inherent part of its genome. |
 | The genomes of many mammals, birds and other vertebrates, have accumulated many DNA sequences derived from retroviruses, known as endogenous retroviruses (ERVs). About 8% of the human genome is comprised of ERVs. |
 | Almost all ERVs in humans (known as HERVs) appear to be non-functional remnants of extinct retroviruses. The only exception is one group, called HERV-K, which is potentially capable of replication despite being many millions of years old. Studies of HERV-K sequences in the human genome have indicated that it has been recently active in humans, and that it could even still circulate through infection. One of these new discoveries was an intact virus without any of the mutations that would degrade its function. The discovery of an intact virus lurking in the human population strengthens the possibility that this HERV-K retrovirus has remained “alive” within humans, and could still be circulating somewhere even today. HERV-K is believed to be one cause of breast cancer. |  |
 Particles of human retrovirus HERV K | Scientists believe HERV-K could be “reawakening” in patients, causing ALS, a disease that robs patients of the ability to walk, talk, eat, and eventually breathe. Researchers examined ALS patients and found elevated HERV-K levels in brain samples. Additional tests proved that the retrovirus had come alive in those individuals. It’s not clear what activates the genes, but physical trauma and exposure to certain chemicals has been linked to ALS. |  |
