As usual I am always reading papers and articles on the antiquity of man in America trying to see beyond the orthodox assumptions of an early peopling of America seeking clues that may prove the opposite.
In this context I have read a paper (Montenegro et al., 2006) which deals with a parasite which plagues humans around the world, and which is also found in America, and could only have reached here by travelling piggy-back inside a human host.
The paper finds that "The introduction of the hookworm into the Americas by a land migration at around 13,000 years BP could have happened only under extraordinary circumstances..." [1]. This is very interesting since that is the orthodox point of view regarding the migrants that peopled the continent.
On Hookworms
Hookworm is a disease caused by any of two parasites that live in the human intestine (Ancylostoma duodenale or Necator americanus). See the photo below of the latter species:
Hookworms are a serious health concern and they infect approximately 700 million people across the world, in poor countries in tropical and subtropical regions (see map below for the N. Americanus).
The map above shows that its current habitat spans the equatorial regions between the Tropics of Cancer and Capricorn.
It is interesting to point out that according to Montenegro et al. the "N. americanus has been associated with humans longer than A. duodenale" [1], a clear indication of an archaic parasite. Could it have infested our ancestors H. erectus or Neanderthal?
Even more interesting is that the oldest recorded date for any kind of hookworm infection was recorded in South America, in human coprolites found in Brazil, 7,230 years old.
N. americanus, named so because it was first discovered in Brazil, and later in Texas, and was considered for a time as a New World species.
But since it is also found in Africa, India, Southeast Asia, and the southwest Pacific Islands, to be able to justify that vast range, it is now said to have originated in Africa and moved out of there with its human hosts, reaching America last. At first the African slave trade was assumed to be the culprit of infecting America with this worm, but the ancient, pre-slavery Brazilian coprolites prove that it was not the case.
Its genome was decoded recently [2], and some comparative studies have been undertaken (they are quite old - see [3][4] below), but these do not address the issue of its origin: Is this hookworm African? or, the opposite, is it American and from there spread across the globe? The studies do however show considerable differences between worms sampled in different parts of the world:
One study from 1998 compares rDNA sequences of worms from Togo (Africa) and Sarawak (Malaysia). As expected they differ and the authors conclude that :
"These findings suggest that there is either population variation in the sequence of N. americanus, or that N. americanus from the two countries may represent genetically distinct but morphologically similar (i.e. cryptic) species, however, comparison of the sequence differences among other hookworm species supports the latter conclusion". [3] In other words, they are genetically different species which look alike.
The other study, from 2003, compares the mtDNA of a worm from Togo with another from china. They also differ ("at both the nucleotide (3-7%) and amino acid (1-7%) levels" and also in regions of the mitochondrial RNA). The authors find this consistent with previous studies proving "evidence for substantial genetic variation within N. americanus".[4]
So until a study (like that done with the human mtDNA) settles the issue, as far as I am concerned, the New World worm may have originated there or in Africa or in South East Asia and dispersed from any of those points to the others.
I can imagine slaver ships picking the worm up in Brazil and conveying it to Africa and Southern Asia. That is a lot easier than trying to figure out how it managed to cross the frozen Siberian steppes, Beringia and Alaska to reach America during the peak of the Ice Ages.
But since Out Of Africa prevails and man is supposed to have reached America via Beringia, scholars must explain its entry through that route.
Hookworms and cold, they don't get along
Despite spending their adult lives inside our guts, they must live for a while in the soil to fulfill their biological cycle: An infected human host spreads the eggs of mature hookworms by means of his feces. Once in the soil, the eggs turn into larvae capable of infecting other hosts. These eggs can hatch in less than 1 day (under special conditions of humidity and temperature) and molt twice into larvae after a period of 5 to 14 days.
These larvae then move upwards through the soil and wait for their new host, which they enter by piercing its skin (A. duodenale can also infect if ingested). They can survive between 2 and 10 months if the temperature is higher than 14°C (if lower, they die).
The special soil conditions for eggs to hatch are quite warm: between 17 and 35°C. And this raises an important question:
Now, how can a band of humans trekking across the Arctic wastelands during the height of an Ice Age come across soil which has a temperature above 17° for the eggs to hatch and above 14° for the larvae to survive?
The paper considered different alternatives:
- These hookworms belonged to a "cold resistant" strain that could develop at lower temperatures
- The migrants stopped at "warm" places to rest and during this period the hookworm went through its cycle there and reinfested them
- They moved across the cold area quickly, so that the adult hookworm survived inside its human hosts until they reached warmer lands (it's maximum life span is 8 years).
Option 1 was rejected because cold-tolerant hookworms have not been discovered yet in America or Asia. Option 2 was also discarded because it is improbable that the migrating humans would have found caves with temperatures above 14 - 17°C. Currently caves are cooler than that. Even if they warmed the caves (i.e. using fire), it is improbable that they would have defecated close to the warm sectors of the cave -that would have been quite unpolite.
That leaves only leaves us with Option 3: a quick march from warm spot in Asia to warm spot in America, crossing the frigid zones of Western Siberia, Beringia and Northern North America during the lifespan of an adult hookworm, comfortably lodged in the warm guts of a migrating human. All this in the context of an Ice Age -otherwise there would be no land bridge to cross and reach America: sea levels dropped during the Ice Ages since most of the water was packed into the continental ice sheets.
Nevertheless, even assuming hypobiosis (a suspended animation state for the larvae inside the hosts during the cold spell), the study found that "contagion [...] was impossible because temperatures were too low even during the warmest periods of the year." [1]
For Option 3 to be possible, the band of migrants would have had to encounter "extraordinary circumstances and even then would have required very rapid displacement rates, rates that appear to have no parallel in the archaeology of the continent." [1], something that the authors consider very improbable.
Therefore they propose other routes (i.e. a trans Pacific or Trans Atlantic route across the sea) for the parasite to enter the continent, pointing out that a viaable route of entry would require "regional temperatures significantly higher than at 13,000 years" [1].
So how did hoowkorm reach America?
The trans Pacific route either at low latitudes (close to the Equator) is feasible and even probable: there is evidence of Chinese / Japanese contact with Peruvian - Ecuadorian Amerindians, but it is more recent than the 7,230 years required to explain the Brazilian coprolites.
The coastal Pacific route (canoes paddling along the coast of Asia, Beringia, Alaska and Western Canada - U.S.), would still be crossing the cold Northern regions. But may have moved faster than a group of walking humans. This could shorten the time needed to get from warm place to warm place and allow the hookworm to survive the journey.
Another option is that the hookworm originated in America and spread elsewhere from there by boat during modern times - post 1492 discovery by Europeans. To validate this, mutation rates would have to be checked (to find the age of the most recent ancestors of the different regional worm lineages).
However N. americanus' great affinity with humans and its notable virulence make me wonder if it has been plaguing us for tens of millennia. Could it be a parasite of Neanderthals or (considering its range in Eurasia), even H. erectus?
But this still leaves us with the problem of getting it into America by a means accessible to our older ancestors. Setting aside the transoceanic routes (too complicated for a band of Neanderthals or H. erectus), the only option is a land route across Beringia. Which, as explained above is not feasible.
Were older glaciations different from recent ones? That is, did they allow a low sea level that made crossing the land bridge possible but at the same time ensured local temperatures above the 14°C minimum threshold?. If this is the case, then our ancient ancestors could have brought the hookworm with them to America. I will research this for a future post.
New April 2, 2014: I did some more research and posted it as a "Second part" to this post.
Sources
[1] Alvaro Montenegro, Adauto Araujo, Michael Eby, Luiz Fernando Ferreira, Rene´e Hetherington, and Andrew J. Weaver, (2006). Parasites, Paleoclimate, and the Peopling of
the Americas Using the Hookworm to Time the Clovis Migration. Current Anthropology Vol 47, No. 1, Feb. 2006 pp +193
[2] Yat T Tang et al., (2014). Genome of the human hookworm Necator americanus. Nature Genetics 46,261–269(2014)doi:10.1038/ng.2875
[3] Romstad A, Gasser RB, Nansen P, Polderman AM, Chilton NB., (1998). Necator americanus (Nematoda: Ancylostomatidae) from Africa and Malaysia have different ITS-2 rDNA sequences. Int J Parasitol. 1998 Apr;28(4):611-5
[4] Hu M, Chilton NB, Abs El-Osta YG, Gasser RB., (2003). Comparative analysis of mitochondrial genome data for Necator americanus from two endemic regions reveals substantial genetic variation. Int J Parasitol. 2003 Aug;33(9):955-63
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