Thursday, 13 June 2013

Lousy Creator

The thing about obligate parasites is that they are obliged to live on their hosts, so their host and parasite histories become inextricably linked. The parasite either co-evolves with its host or goes extinct. Just as the biblical story of Ruth has her saying, "...whither thou goest, I will go; and where thou lodgest, I will lodge...". And so it is for human lice, and lice of other species for that matter.

Our lice share our history and were forced to go where we went, to adapt to changes in our life-style.

A species-specific parasite becomes an isolated population so far as its related species are concerned and, as populations diverge and become genetically isolated so the obligate parasites become isolated too, and evolve in their own direction. So, if we look at our parasites and the corresponding obligate parasites on the species with which we share a common ancestor, we should see the same genetic and morphological relationships between them as between us and our ancestors.

And this is exactly what we see in the three human lice and those of our closest relatives:

So, where did we pick up these unwanted blood suckers in the first place? All signs point to a human–ape connection, and "connection" may mean something more tangible than an evolutionary link. Some studies suggest interaction between early Homo species and gorillas, and also between early Homo species and us.

The lice we carry around are sucking lice. That’s pretty self explanatory. Two subspecies we harbor, head lice and body lice, belong to the Pediculus (picture left) genus (Pediculus humanus capitis and Pediculus humanus humanus, respectively). The other species we harbor, you know, down there, is a member of the genus Phthirus. Our closest living relatives, the chimpanzees, harbor Pediculus, as well, while gorillas are home to another Phthirus species.

In other words, we share a genus with each of them.

Sucking lice have been sucking primate blood for at least 25 million years. The big story, though, is what happened about 6 to 7 million years ago, and in the case of the gorilla, even later. Humans and apes are supposed to have parted evolutionary ways at about the 6 million year mark. The Pediculus genus seems to have split at the same time, with Pediculus schaeffi hitching a ride with the chimp lineage, and Pediculus humanus sticking with what would become the Homo line. The gorillas split off a little earlier, maybe about 7 million years ago, and the Phthirus may have done the same, sending Phthirus gorillae with the gorillas (natch), while Phthirus pubis eventually became a human problem.

Things get a bit, well, sticky when it comes to the Phthirus line ..., however. The split between the gorilla and human lice seems to have happened around 3 to 4 million years ago, millions of years after the gorilla and human branches parted ways. That means that at the 3 to 4 million year point, human ancestors and gorillas must have had some kind of…contact.


This is supported by DNA analysis:

  1. Humans are infested with three types of lice, whereas modern primates are colonized by a single species. DNA analysis indicates the human head louse and chimp louse shared a common ancestor 6 million years ago. This independently agrees with the fossil record.
  2. Human pubic lice look very different from head lice and most resemble gorilla lice. DNA analysis indicates human pubic lice are most closely related to gorilla lice and shared a common ancestor 3.3 mya. Thus, our ancestors had lost most of their body hair at that time and were then infected by gorilla lice, inhabiting an unoccupied "hair-niche". We did not get our pubic lice from our ancestors but from gorillas.
  3. The loss of body hair was an adaptation for persistence hunting, where hominins hunted prey during the heat of the day. They could sweat and outlast fleeing prey that could only pant. This type of hunting can still be seen in African tribal hunters today.
  4. DNA analysis of head lice indicate at least two populations exist and the best explanation is infection from earlier hominins to Homo sapiens. One population is found world wide and the other is found only in North America. Thus, louse DNA studies predict that at one time H. erectus and H. sapiens came into contact with one another in Asia, picking up a second population of lice. It is unknown if this contact included interbreeding.
  5. Human louse DNA studies confirm the “out of Africa” theory that Homo sapiens  populations grew rapidly from a bottleneck population about 60 - 70,000 years ago when a small band left Africa. The bottleneck of Homo sapiens is supported by the louse data, which also was found independently to have suffered a severe reduction in population.
  6. Studies such as these show how evolutionary theory is predictive and can be confirmed by several lines of independent evidence. For example, the fossil record of human origins is confirmed by studying parasites that coevolved with our ancestors and carry with them a history of our evolution and origins recorded in their DNA. Only by using evolution can the various observations we see in nature be explained adequately.

The interesting thing here is the divergence into subspecies of the human head and body louse coincident with the period when humans lost body hair and then started wearing clothes. It was as though, from the louse's point of view, there were now two different environments into which the evolving species could radiate. And that's exactly what it was.

Also interesting, in view of the recent evidence of interbreeding outside Africa by Homo sapiens with descendents of an earlier common ancestor, (probably H. heidelbergensis rather than H. erectus), the Neanderthals and Denisovans and probably another as yet unidentified extinct Homo to form an effective incompletely evolved ring species, is the evidence of two different genetic lines of Pediculus humanus capitis showing how they had even begun to diverge having come out of Africa much earlier with archaic Homo hosts.

So, given that a map of the evolution of our lice can be overlaid almost exactly with similar map for our own evolution, how can this be explained by Creationism and the 'Intelligent Design' school of biblical literalism? Leaving aside the question of why an intelligent designer would design parasites like lice in the first place, why would one make it look exactly like they had co-evolved along with their human hosts by an evolutionary process best explained by Darwinian Natural Selection and descent with modification from a common ancestor?

References:
Origine - Human Lice, History & Archeogenetics.
Of lice and men: An itchy history. Emily Willingham, Scientific American Blogs, February 14, 2011.
DailyScience - Lice DNA Study Shows Humans First Wore Clothes 170,000 Years Ago.
Science Cases - A Tale of Three Lice: A Case Study on Phylogeny, Speciation, and Hominin Evolution.





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2 comments :

  1. Great work, Rosa! I hadn't looked into anything like this but, it's a perfect example of speciation. Thanks.

    ReplyDelete
  2. Very interesting and irrefutable, whether we like it or not.

    ReplyDelete

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