Pre Siberian Human Migration to the Americas: Possible validation by HTLV-1 mutation analysis
David H. Gremillion, MD, FACP (Fellow Infectious Diseases Society of America, Professor of Medicine, Nippon Medical School)
Our current understanding of human migration derives from advances in four more or less integrated disciplines: archeology, physical anthropology, DNA analysis and linguistics. In recent years progress has slowed as researchers enroll familiar tools to validate or reject what have become more or less entrenched theories. For archaeologists, it almost goes without saying that advances in seemingly unrelated disciplines have great potential for breaking “logjams” and producing new ideas. Such was the case in 2003 when population based data in microbiology demonstrated that an ancient human pathogen, Helicobacter pylori, carried a key to the timing and pathway of early human migration out of Africa.
A landmark article in Science by Falush and associates (2003) reported on strain variation in Helicobacter pylori, a chronic gastric pathogen. Using known mutation frequency as a “biological clock” they were able to match known patterns of human migration with the molecular clock approach. Spread of this chronic human pathogen accompanied the migration of their human host and not surprisingly, the oldest strains with the most mutations were in Africa (Linz et al 2007). Prior to this validation by pathogen genetic analysis, the “out of Africa” theory rested primarily on conventional archeological and anthropological evidence.
The “Ah Ha!” moment of insight occurred when rapid advances in microbial genetic analysis were used to validate existing theories. Our microbial “hitchhikers” carry with them the secrets of their past and by inference our past. Such leaps forward in science often occur when the insights from apparently disparate disciplines merge and provide validation for or discredit earlier theories.
Another rapidly developing body of knowledge is challenging rather than validating the orthodox understandings of human migration. Human T cell Lymphotrophic Virus 1 (HTLV-1) has been part of the human condition for thousands of years and has an epidemiology and migration suggesting that the populating of the North and South American continents began long before the trans-Siberian migrations.
Human migration into the Americas
Archaeologists are perhaps all too familiar with the long debate over the pathways and timing of human migration into the Americas. Based on oxygen isotope records from deep-sea cores, during the last major stage of the Pleistocene epoch (10,000-50,000 years ago), a land bridge formed across what now is the Bering Strait. With Siberia and the Americas connected, conventional accounts suggest nomadic peoples tracked the big game herds that migrated to modern-day Alaska. From here, this group (or groups) subsequently traveled south and east across America through an ice-free corridor. For better and worse these findings have been substantiated by the discovery of several ancient American archaeological sites, including those at Clovis, New Mexico in 1932. Dating these findings between 12,800 and 13,300 years BP the artifacts were deemed the remnants of the so-called “Clovis culture.” The land bridge theory has gained further credence with recent scholarship into the human genome. A 2007 study examining the genome of indigenous people from North and South America and two Siberian groups found one unique genetic variant across all populations, suggesting Native Americans descended from a common ancestor (Hellenthal et al 2008; also see “First Americans arrived from Siberia”). Furthermore, this study found increasing genetic variation radiating from the Bering Strait, correlating to a more recent migration.
While it is perhaps all but indisputable that land migration occurred over the Bering Strait, it is not conclusive that this path was the first or the only means of migration to the Americas. There are incongruities in the archaeological record when North and South America are compared. There are South American sites, including Monte Verde, which predate the North American Clovis remains by at least 1,000 years (Dillehay 1999 http://www.archaeology.org/online/features/clovis/). These findings lend support to what has been argued to be an earlier and faster maritime migration. Furthermore, excavations across the Americas, the most recent of which unearthed coprolites in the Paisley Caves of Oregon’s Cascade Range, suggest the first Americans were a maritime culture (Thomas et al 2008). Beyond the earlier absence of the ice-free corridor — an absence which makes an earlier land-based migration virtually impossible — one of the primary clues is diet.
The diet of these early Americans consisted of turtles, shellfish, and tubers, a stark contrast to the big game the group(s) associated with the Clovis culture tracked across the Bering Strait. Finally, while the genetic research presents interesting conclusions, it does not take into account the assimilation or destruction of people who may have pre-dated the genome of modern Native Americans. New observations in the field of retrovirology allow a different marker to weigh in on the possible existence of extinct proto-American cultures.
Human T-lymphotropic virus 1 (HTLV-1) was the first retrovirus (HIV-like) to be isolated and has infected human beings for thousands of years (Ishida et al 1995). Recent research has identified important elements of HTLV-1 molecular biology, epidemiology and pathogenesis. The Southern Japanese island of Kyushu has a surprisingly high prevalence as does certain areas of sub-Saharan Africa, the Caribbean, and South America. In some communities of Japan, clusters with prevalence in the 30% range have been reported. The Ainu of the Japanese island of Hokaido currently have a high prevalence and historically they were the ancient indigenous peoples of the Japanese archipelago. Most HTLV-1 carriers remain asymptomatic, but infection can be associated with severe diseases of immunodeficiency (Verdonck 2007). Recent characterization of mutations in the viral genome has allowed viral pedigrees to be traced back thousands of years using a mutational biological clock. The more highly mutated viruses are presumed to be the older because of a relative constancy of mutation over time (Switzer et al 2006).
Four major strains of HTLV exist worldwide and are currently endemic in Sub-Saharan Africa, Japan, and the Americas. All four strains, including their primate precursors, are present in Africa, suggesting HTLV originated there in its primate form, PTLV (Vandamme et al 1998; Van Dooren et al 2005). In southern Japan more than 10% of the general population is infected with HTLV-1a (Gessain et al 1992; Morofuji-Hirata et al 1993). This endemicity extends to antiquity, as HTLV-1a has been detected in the Japanese subpopulations of Ainu and Ryukyuans, the putative descendants of Japan’s original inhabitants. This strain is also present in the Caribbean and South America, providing hints of an ancient migration into the Americas.
Throughout the Americas, HTLV-1a is found predominantly among African descendants and indigenous populations. It is endemic in the Caribbean, Colombia, and northern Brazil and, to a lesser extent, found in Peru, Argentina but is rare in Central and North America. Some researchers have interpreted this data as consistent with the dissemination of HTLV-1a to the New World through the African slave trade. Contrasting this theory, HTLV-1 exists in aboriginal populations on both sides of the Bering Strait, suggesting an ancient introduction of the virus into the New World prior to the slave trade (Miura et al 1997; Picard et al 1995). Furthermore, in 2000, Sonoda and colleagues described an Andean mummy with ancient HTLV-1a, suggesting a prehistoric introduction of the virus similar to that prevalent in the Ainu population (Li et al 1999; Sonoda et al 2000). Their conclusions remain controversial and can be resolved only with additional fieldwork on new specimens (Vandamme et al 2000).
HTLV-1 is often transmitted from mother to child through breastfeeding. The infected infant caries the virus for their lifetime and passes the virion to their progeny through sexual intercourse (Roucoux et al 2005) or breastfeeding. Contemporary epidemiology now also includes transfusions of blood products, contaminated needles (including acupuncture) and IV drug abuse. The duration of breastfeeding (Li et al 2004) can be a major factor in transmission rates that range between 15% and 25%. Vertical transmission from mother to child is a prominent and efficient mode of transmission and likely occurred in ancient human populations even when there was little risk of blood borne transmission. The HTLV-1 is thus an ideal migration marker, serving as a kind of historical “GPS” that tags an individual and subsequent generations.
HTLV-1 high prevalence in the Caribbean and South America has been presumed to be due to the post-Columbian slave trade into the Americas from regions of high HTLV-1 prevalence in Africa. New technology however, allows identification of strains with greater precision. The strains found in South America and the Caribbean are more similar to the strains prevalent in ancient Japan. The Peruvian mummy (Sonoda et al 2000) further confirms that HTLV-1 was present in the Americas thousands of years prior to the slave trade. Similarly, a cluster of Japanese type HTLV-1 has been detected in a Coastal population of Amerindian natives in British Columbia (Picard et al 1995), although the significance of this may not have been appreciated in 1995 when first reported.
These new findings may help explain the relative paucity of HTLV-1 in the African American populations of Northern latitudes in the Americas and the concentration of ancient Japanese strains in the Caribbean and South America. Coastal migration of ancient Ainu into the Americas did not require the Bering land bridge and the “ice-free corridor.” Archeological evidence may be submerged off the coasts of Canada and Western America. If this migration occurred 20-40,000 years earlier than the Siberian migration, the human migrants would have found an impenetrable ice sheet that extended deep into the Americas to at least as far as Meadowcroft Rockshelter in Pennsylvania. Lower sea levels would have allowed easier movement to the Caribbean islands which may have been connected. South America would have been predominately free of glacial ice and the flow of migrants over thousands of years would have gravitated in that direction.
When the Bering land bridge opened and the ice free corridor formed (circa 12,500 BC), Siberian Asian populations flowed along with the mega fauna into the northern USA latitudes which by then were more habitable.
Certain findings from the more classic disciplines, which deal with human migration, begin to make more sense as we build a better picture of an earlier wave of coastal migrants to the Americas. These include the discovery of Kennewick man (potentially an ancient Ainu skeleton found on the banks of the Columbia River). Retrovirology adds to this big picture.
A rapidly accreting body of evidence suggests that human migration into the Americas occurred much earlier than previously thought. Two distinct waves of migration have been documented with the characteristics of each dictated by the timing of the last ice age. Coastal migration was favored at the peak of the ice age when sea levels were lower and abundant seafood was available. The ancient people of Japan were known to be excellent coastal seafarers but reluctant visitors to the open sea. Sea craft during that phase of human migration were more primitive and did not support open sea migration. Siberian migration became dominant after the receding of the ice sheet and these later migrants may have replaced or assimilated the earlier migrants.
Awareness of the broad spectrum of science advancement creates the possibility that new insights will occur when overlapping discoveries validate new theories. Overspecialization within a scientific discipline may be a handicap when it comes to the big questions of humanity. Whether as scientists, scholars, or professional field archaeologists, we are all well served by maintaining a broader view of advances in many fields. Such key advances which allow a great leap forward in our own fields of expertise might occur in an isolated and seemingly unrelated discipline, such as retrovirology.
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