Hookworms and the peopling of America -more data

Today's post continues my previous post on hookworms and the peopling of America.

One of the issues was if it was possible for these parasites to make it into America across the cold Beringian region during the lifespan of an adult hookworm since it seemed improbable that they could fulfill their early life phase in the area's frozen soil. An option could be a warmer period which allowed the eggs to hatch and larvae appear in time to re-infest the migrating humans but, which was not warm enought to melt the ice caps and flood the Beringian landbridge. It seems that this is could be the case:

Warm but not too warm

A paper by Rabassa and Ponce [1] looks into the very cold Heinrich (H) periods and warmer Dansgaard-Oeschger (D-O) climatic events that took place during MIS 3 (Marine Isotope Stage 3), a "relatively warm climatic period" between 60-50 and 30 ky ago (cal. ka BP).

During this period the "mean annual temperatures were ca. 5 - 8°C higher than those active at the Last Glacial Maximum (LGM)". The LGM took place some 25 kya. and marks the point of maximum ice coverage (and also the lowest temperatures).

What is surprising is that even though the climate was much warmer, the ice sheets did not melt completely, so sea level remained "low enough to allow the persistence of the Beringia land bridge between Siberia and North America, without any interruptions throughout the entire MIS 3.". [1]

These peculiar conditions lead the authors to conclude that:

"... thus both the hinterland path from Beringia southwards and the coastal route would have been open and enjoying moderate climate ecosystems, and thus available for humans. In this case, it is now possible to suggest possible moments for human penetration in North America, perhaps between ca. 50 to 28 cal. ka B.P...[1]

In other words we have a period during which the Beringian land bridge was high and dry and warm enough to allow humans to cross it 60 - 30 kya. Which also opens the possibility for a Neanderthal migration into America or a very early migration of modern humans. Furthermore, since the mechanisms that originted the H and D-O events are not unique, it also may mean that similar conditions could have existed during other even earlier interstadial stages, which could have been used by Neanderthals or even H. erectus to cross into America under warmer conditions.

Could these warmer temperatures have allowed the survival of hookworm eggs in the soil? Maybe. This would do away with the need for hypothetical transoceanic migrations that purportedly brought the worm to America before the oldest dated worm sample from this continent (7.2 kya).

On Hookworms and their diversity

A recent paper by Hasegawa H, Modry D, Kitagawa M, Shutt KA, Todd A, et al. (2014) [2] looked into different genetic markers of hookworms in Africa, both among humans and Great Apes and came up with some interesting information but fail to recognize it, because of the "old" references they used:

Below they mention the "ITS" or Internal transcribed spacer of nuclear ribosomal DNA:

"The ITS sequences of type I ( = N. americanus) were much closer to those reported in samples from human in Guatemala than those in samples from humans in China or Malaysia." [2]

This is very interesting: How come American and African samples are closer to each other than to the Asian ones which are located midway between them?

The authors don't find this relevant and play it down stating that:

"This close relationship is not unexpected, as N. americanus in the Americas might have been introduced from Africa by human migration in the early modern ages." [14]
[...]
[14] Looss A (1911) The anatomy and life history of Agchylostoma duodenale Dub. A monograph. Part II: The development in the free state. Rec Sch Med 4: 159–613." [2]

But, as I reported in my previous post, N. americanus was found in +7,200 year old fossil human excrement discovered in Brazil, long before any "early modern ages" exchange between Africa and America (i.e. slave trade). The authors did not consider this paper and quote a rather old one from 1911!

Of course, it is quite probable that the Guatemalan sample came from an African hookworm, that colonized the New World after the XV century, spreading across the continent and displacing the ancestral forms such as the Brazilian one.

The paper also found "the first molecular evidence that N. americanus parasitizes wild western lowland gorillas, but at a much lower prevalence than we reported in humans." [2]. This is a clear indication of an African origin of this variety of hookworm. We would not expect a human parasite to plague gorillas, rather, it should be the opposite.

The paper also reports that "Although humans are often regarded as the only natural host of N. americanus, other primates, including gorillas, chimpanzees, several Old and New World monkeys and other mammals, such as pangolins, have also been reported as hosts of this nematode" [2]. If this is the case, then the hookworm could have entered America via a small mammal millions of years ago during a very warm period. This would make any conjectures about human migrations pointless.

Nevertheless, the similarity between American and African lineages means that the split is much more recent.

Closing Comments

I believe that more data is needed, a comparison of the genome of Asian, African, American (contemporary and archaic) hookworms in humans and other mammals to settle the issue.

But even if the hookworms don't provide evidence for an early peopling of America, the fact that the Beringian land bridge may have existed during older periods and be warmer than the latest bridge (the one H. sapiens is supposed to have used to enter America), is encouraging and may ultimately support the theory of an initial peoping by Neanderthals (or even H. erectus).

Sources

[1] Raassa J., and Ponce, J. (2013)., The Heinrich and Dansgaard-Oeschger Climatic events during Marine Isotopic Stage 3: Searching for Appropriate Times for Human colonization of the Americas. Quaternary International, 2013 vol. 299 p. 94.
[2] Hasegawa H, Modrý D, Kitagawa M, Shutt KA, Todd A, et al. (2014). Humans and Great Apes Cohabiting the Forest Ecosystem in Central African Republic Harbour the Same Hookworms. PLoS Negl Trop Dis 8(3): e2715. doi:10.1371/journal.pntd.0002715

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