In a previous post, I mentioned the strange virus that infected all African great apes, but didn't affect humans, it is known as PtERV 1.
PtERV 1 is an endogenous retrovirus found in chimpanzees (Pan troglodyte). And it gets its name from the initials of Pan troglodyte endogenous retrovirus (PtERV).
Let's first explain what it is.
Retroviruses are a type of virus (an example is HIV, that causes AIDS) that instead of store their genetic information as RNA. They differ from other virus because when a retrovirus infects a cell, it uses a special enzyme called reverse transcriptase to convert its RNA into DNA (hence its name "retro", from this reverse transcription). This DNA is then inserted into the host's DNA strand, and the cell reads it as if it was its own. In this way the cell makes copies of the virus, propagating it.
When a retrovirus is inserted into a cell that later becomes an egg or sperm, it can be passed on, within the cell's DNA, into the next generation, who inherit it as part of their genetic material. This is known as an "endogenous retrovirus" because it comes from within, inherited. The acronym ERV applies to Endogenous Retrovirus.
Humans carry ERVs in their genes, the outcome of ancient infections in our distant ancestral monkey relatives around 25 million years ago. But we don't carry the PtERV 1 variant. Neither do orangutans. The insertions are only found in African apes like gorillas, baboons, macaques, and chimpanzees.
Research (see Yohn, et al., 2005) reported that an ancient retrovirus infected "ancestral gorilla and chimpanzee lineages 3–4 million years ago", after humans had split from the lineage of chimpanzees. The infection took place at the same time, leading to very similar insertions into the genomes of gorillas and chimpanzees. PtERV 1 is also found in baboons and macaques, but there are differences between their genetics, suggesting a later infection ~1.5 million years ago, and by slightly mutated retrovirus.
We don't know which retrovirus caused this infection but it is likely a gammaretrovirus (there is no known gammaretrovirus that infects humans) and it shares roughly half of its genome with virus linked to leukemia in cats and rats (feline leukemia and murine leukemia viruses). The authors suggest that the infection was caused by a "retrovirus [that] was introduced into the great ape lineages by horizontal transmission, perhaps from contact with an ancient Old World monkey species."
Why didn't humans, or our Homo ancestors (H. habilis, H. erectus, etc.) become infected within Africa, as the virus spread among all other African apes and many Asian ones? Why were the orangutan and other Asian apes preserved from the infection?
- Not infected: humans, orangutan, siamang, and white-handed gibbon.
- Infected: African. Gorilla, bonobo, chimpanzee, Abyssinian black-and-white colobus monkey, olive baboon, gelada baboon, Hamadryas baboon, drill, mona, Black crested mangabey.
Asian monkeys: rhesus macaque (South, Central, and SE Asia), Japanese macaque (Snow monkey), Sichuan snub-nosed monkey (Central China), Long-tailed macaque (SE Asia), Tibetan macaque, gelada baboon, , lion tailed monkey (India), Celebes macaque (Indonesia), Myanmar snub-nosed monkey (Myanmar and South China)
The following image, from Abu Eed, et al., 2025 (Fig. 7) shows in red, the monkey species infected with PtERV-1, in black, those that are not infected.
The paper speculates that the gorillas and chimpanzees were probably more susceptible, or that our Homo ancestors were resistant or immune (so were the Asian orangutans). Though, the orangutans lived far from Africa and distance could have preserved them, isolated from infection.
An interesting option is that "the lineage that ultimately gave rise to humans did not occupy the same habitat as the ancestral chimpanzee and gorilla lineages. An excursion by early hominids to Eurasia during the time that PTERV1 infected African great apes and then a return to Africa would explain this phylogenetic inconsistency." Or, may I add, if Homo originated in Asia, and evolved there, it may have entered Africa hundreds of thousands of years later, even millions, once the retroviral infection and its source had vanished. This would have protected them from infection. But, it does not explain why equally distant Asian monkeys caught it!
Since Into Africa or Out of Eurasia theories are not favored by mainstream scholars, the paper also suggests that population structure could have somehow preserved our ancestors from infection. But this explanation would require very large populations of humans and orangutans in comparison to those of gorillas and chimpanzees, which is unlikely.
Another option is considered briefly but quickly dismissed "the PTERV1 phylogenetic tree is inconsistent with the generally accepted species tree for primates, suggesting a horizontal transmission as opposed to a vertical transmission from a common ape ancestor.An alternative explanation may be that the primate phylogeny is grossly incorrect, as has been proposed by a minority of anthropologists [29]. This seems unlikely in light of the extensive molecular evolutionary data that have been collected over the last few years [5,25] that clearly place orangutan as the outgroup species to the human–chimpanzee–gorilla clade and Old World monkeys as an outgroup to the human/ape lineage."
The "antropologist" cited as [29] is: Schwartz JH. The evolutionary relationships of man and orang-utans. Nature. 1984;308:501–505. doi: 10.1038/308501a0. [DOI] [PubMed] [Google Scholar]
Comments
The most parsimonious explanation is that humans and our Homo ancestors evolved in Asia, in a region that was isolated from the infection. Orangutans may be closer to us, as suggested by Schwartz than chimpanzees. They too lived isolated from the other apes that caught it, or, maybe they share with humans some protecting factor.
My next post will look into Schwartz and his orangutan-human link.
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