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| Paranthropus boisei The species responsible for human genital herpes? Source: Wikipedia |
One of the best pieces of evidence for common descent is the way the genetic relationships between our obligatory parasites and those of their related species, almost exactly maps onto the relationship between our genome and those of our own relatives. I have previously written about how the evolution of our lice can be mapped onto our own evolution and divergence from the other apes.
Another parasite which, if anything, is even more obligatory that lice is the herpes virus, and like our lice there is not a close but not perfect match with our evolution. All primates are host to two versions of the herpes simplex virus, HSV1 which in humans causes cold sores and HSV2 which causes genital herpes in humans. The latter is normally regarded as a sexually transmitted disease.
Whereas we can make a direct link between two of our lice - the head louse (Pediculus humanus capitis) and the body louse (Pediculus humanus humanus) - and the body lice of chimpanzees (Pediculus schaeffi), there is no such clear link with the pubic louse and an immediate ancestor. We can see how our body louse became our head louse when we lost our body hair and it had nowhere else to go and how it then diverged into two subspecies when we started wearing clothes to become our body louse as well. However, the closest relative of our pubic louse (Phthirus pubis) is the gorilla body louse (Phthirus gorillae)! Somewhere in our evolution, the gorilla louse jumped the species barrier, otherwise it, or as descendent, would be present on chimpanzees too. The degree of divergence is enough to show that this wasn't a recent event, but somewhere, after we diverged from the chimpanzees, one of our ancestors got close enough to a gorilla to catch pubic lice from it!
Similarly, although we can trace the divergence of the herpes simplex HSV1 from that carried by ancestral chimpanzees, there is no such clear evolutionary pathway for HSV2. In fact, from the modelling work done by a team of scientists from Cambridge and Oxford Brookes universities, it seems our earliest ancestors manages to avoid taking HSV2 with them as they diverged from chimpanzees. The reason for this is that both were originally infections of the mouth and the early proto-humans may have had some protection from HSV2 because they had HSV1. It was not until later that a mutation in HSV2 in chimpanzees enabled it to exploit the mucous lining in the genital tracts and become a sexually transmitted virus.
So the question is, how did one of our remote ancestors acquire HSV2 from ancestral chimpanzees.
As the Cambridge University press release says:
Herpes infect everything from humans to coral, with each species having its own specific set of viruses,” said senior author Dr Charlotte Houldcroft, a virologist from Cambridge’s Department of Archaeology.
For these viruses to jump species barriers they need a lucky genetic mutation combined with significant fluid exchange. In the case of early hominins, this means through consumption or intercourse – or possibly both.
By modelling the available data, from fossil records to viral genetics, we believe that Paranthropus boisei was the species in the right place at the right time to both contract HSV2 from ancestral chimpanzees, and transmit it to our earliest ancestors, probably Homo erectus.
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| Map showing the distribution of extant chimpanzee (P.troglodytes) and bonobo (P.paniscus) populations [IUCN redlist http://maps.iucnredlist.org/map.html?id=15933, http://maps.iucnredlist.org/map.html?id=15932]; the locations of hominin fossils [Supplementary Table S1] are shown with markers. The colour of the marker indicates the hominin genus; the symbol represents the species. The map also shows the location of hominin fossils relative to ancient minimum and maximum rainforest distributions [Peel et al. 2007]. This figure is available interactively: https://wadhamite.github.io/hsv2-map/. |
Abstract
Herpes simplex virus 2 (HSV2) is a human herpesvirus found worldwide that causes genital lesions and more rarely causes encephalitis. This pathogen is most common in Africa, and particularly in central and east Africa, an area of particular significance for the evolution of modern humans. Unlike HSV1, HSV2 has not simply co-speciated with humans from their last common ancestor with primates. HSV2 jumped the species barrier between 1.4 and 3 MYA, most likely through intermediate but unknown hominin species. In this article, we use probability-based network analysis to determine the most probable transmission path between intermediate hosts of HSV2, from the ancestors of chimpanzees to the ancestors of modern humans, using paleo-environmental data on the distribution of African tropical rainforest over the last 3 million years and data on the age and distribution of fossil species of hominin present in Africa between 1.4 and 3 MYA. Our model identifies Paranthropus boisei as the most likely intermediate host of HSV2, while Homo habilis may also have played a role in the initial transmission of HSV2 from the ancestors of chimpanzees to P.boisei.
Simon J. Underdown Krishna Kumar Charlotte Houldcroft
Network analysis of the hominin origin of Herpes Simplex virus 2 from fossil data
Virus Evolution, Volume 3, Issue 2, 1 July 2017, vex026, https://doi.org/10.1093/ve/vex026
© The Author 2017. Published by Oxford University Press.
Published open access
Reproduced under the terms of a Creative Commons Attribution Non-Commercial License (CC BY-NC 4.0)
The team concluded that the most likely scenario was that P. boisei acquired HSV2 from ancestral chimpanzees with a slightly lower likelihood that the transmission was from chimpanzee to Homo habilis with a much lower probability of ancestral chimpanzee to H. erectus. They suggest that it was either active hunting or scavenging and the consumption of raw flesh that may have been the original mode of transmission to P. boisei (or H. habilis) and that sexual contact between either of these and H. erectus (and so to their descendants, H. sapiens). Hunting and consumption of P. boisei by H. habilis or H. erectus cannot be ruled out.
So here we have evidence not only of the common ancestry of humans and chimpanzees in the similarities and differences between chimpanzee and hominin HSV1 but evidence of probably interbreeding, or at least sexual activity, between archaic hominin species. All of this is recorded in the genomes of the obligate herpes viral parasite.
The team also believe their methodology can be used to examine other transmission mysteries such as that of the pubic lice from gorilla to humans, probably via an intermediate.
It would be interesting to know how a creationist would explain why the herpes virus was created in so many different versions across so many different species and why their genomes seem to form clades which so closely resemble the cladistic arrangement of their hosts. It's as though the putative creator was trying to create evidence that species evolve and were not created as is. Why would that be?
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