A Clovis projectile point created using bifacial percussion flaking (that is, each face is flaked on both edges alternately with a percussor)
The Clovis culture is a prehistoricPaleo-Indian culture, named after distinct stone tools found at sites near Clovis, New Mexico, in the 1920s and 1930s. The Clovis culture appears around 11,050 RCYBP (radiocarbon yearsbefore present), at the end of the last glacial period, characterized by the manufacture of "Clovis points" and distinctive bone and ivory tools. Archaeologists' most precise determinations at present suggest that this radiocarbon age is equal to roughly 13,200 to 12,900 calendar years ago. Clovis people are considered to be the ancestors of most of the indigenous cultures of the Americas.
The Clovis culture was replaced by several more localized regional cultures from the time of the Younger Dryas cold climate period onward, about 9,000 years ago. Post-Clovis cultures include the Folsom tradition, Gainey, Suwannee-Simpson, Plainview-Goshen, Cumberland, and Redstone. Each of these is commonly thought to derive directly from Clovis, in some cases apparently differing only in the length of the fluting on their projectile points. Although this is generally held to be the result of normal cultural change through time, numerous other reasons have been suggested to be the driving force for the observed changes in the archaeological record, such as Younger Dryas impact event or post-glacial climate change with numerous faunal extinctions.
After the discovery of several Clovis sites in western North America in the 1930s, the Clovis people came to be regarded as the first human inhabitants of the New World. However, this theory has been challenged, in the opinion of many archaeologists, by several archaeological discoveries, including sites like Cactus Hill in Virginia, Paisley Caves in the Summer Lake Basin of Oregon, the Topper site in Allendale County, South Carolina, Meadowcroft Rockshelter in Pennsylvania, the Friedkin site in Texas, Cueva Fell in Chile and, especially, Monte Verde, also in Chile. The claim to the oldest human archaeological site known in the Americas belongs to the Pedra Furada human remains and hearths, a site in Brazil that precedes the Clovis culture and the other sites already mentioned by 19,000 to 30,000 years, but this discovery has become an issue of contention between North American archaeologists and their South American and European counterparts. Available genetic data shows that the Clovis people are the direct ancestors of roughly 80% of all living Native American populations, with the remainder having entered afterwards.
Clovis points from the Rummells-Maske Cache Site, Iowa.
A hallmark of the toolkit associated with the Clovis culture is the distinctively shaped, fluted stone spear point, known as the Clovis point. The Clovis point is bifacial and typically fluted on both sides. Archaeologists do not agree on whether the widespread presence of these artifacts indicates the proliferation of a single people, or the adoption of a superior technology by diverse population groups.
The culture was originally named for a small number of artifacts found between 1936 and 1938 at Blackwater Locality No. 1, an archaeological site near the town of Clovis, New Mexico. People began collecting artifacts at this site in the late 1920s but artifacts and animal remains that had not moved since the Pleistocene were not recovered until 1936. The in situ finds of 1936 and 1937 included most of four stone Clovis points, two long bone points with impact damage, stone blades, a portion of a Clovis blade core, and several cutting tools made on stone flakes.
Clovis sites have since been identified throughout much, but not all, of the contiguous United States, as well as Mexico and Central America, and even into northern South America.
It is generally accepted that Clovis people hunted mammoths as Clovis points have repeatedly been found in sites containing mammoth remains. Mammoth was only a small part of the Clovis diet; extinct bison, mastodon, sloths, tapir, palaeolama, horse and a host of smaller animals have also been found in Clovis sites where they were killed and eaten. In total, more than 125 species of plants and animals are known to have been used by Clovis people in the portion of the Western Hemisphere they inhabited.
The most commonly held perspective on the end of the Clovis culture is that a decline in the availability of megafauna, combined with an overall increase in a less mobile population, led to local differentiation of lithic and cultural traditions across the Americas. After this time, Clovis-style fluted points were replaced by other fluted-point traditions (such as the Folsom culture) with an essentially uninterrupted sequence across North and Central America. An effectively continuous cultural adaptation proceeds from the Clovis period through the ensuing Middle and Late Paleoindian periods.
Whether the Clovis culture drove the mammoth, and other species, to extinction via overhunting – the so-called Pleistocene overkill hypothesis – is still an open, and controversial, question. It has also been hypothesized that the Clovis culture saw its decline in the wake of the Younger Dryas cold phase. This 'cold shock', lasting roughly 1500 years, affected many parts of the world, including North America. This appears to have been triggered by a vast amount of meltwater – possibly from Lake Agassiz – emptying into the North Atlantic, disrupting the thermohaline circulation.
The Younger Dryas impact hypothesis or Clovis comet hypothesis originally proposed that a large air burst or earth impact of a comet or comets from outer space initiated the Younger Dryas cold period about 12,900 BPcalibrated (10,900 14C uncalibrated) years ago. The hypothesis has been largely contradicted by research showing that most of the conclusions cannot be repeated by other scientists, and criticized because of misinterpretation of data and the lack of confirmatory evidence.
A cowboy and former slave, George McJunkin, found an Ancient Bison (Bison antiquus, an extinct relative of the American bison) skeleton in 1908 after a flash flood. The site was first excavated in 1926, near Folsom, New Mexico, under the direction of Harold Cook and Jesse Figgins. On August 29, 1927, they found the first in situFolsom point with the extinct B. antiquus bones. This confirmation of a human presence in the Americas during the Pleistocene inspired many people to start looking for evidence of early man.
In 1929, 19-year-old Ridgely Whiteman, who had been closely following the excavations in nearby Folsom in the newspaper, discovered the Clovis Man Site in the Blackwater Draw in eastern New Mexico. Despite earlier legitimate Paleoindian discoveries, the best documented evidence of the Clovis tool complex was excavated between 1932 and 1937 near Clovis, New Mexico, by a crew under the direction of Edgar Billings Howard from the Academy of Natural Sciences/University of Pennsylvania. Howard's crew left their excavation in Burnet Cave, New Mexico (the first truly professionally excavated Clovis site) in August, 1932, and visited Whiteman and his Blackwater Draw site. By November, Howard was back at Blackwater Draw to investigate additional finds from a construction project.
The American Journal of Archaeology (January–March, 1932 V36 #1) in its "Archaeological Notes" mentions E. B. Howard's work in Burnet Cave, including the discovery of extinct fauna and a "Folsom type" point four feet below a Basketmaker burial. This brief mention of the Clovis point found in place predates any work at the Dent Site in Colorado. Reference is made to a slightly earlier article on Burnet Cave in The University Museum Bulletin of November, 1931.
The first report of professional work at the Blackwater Draw Clovis site is in the November 25, 1932, issue of Science News. The publications on Burnet Cave and Blackwater Draw directly contradict statements by several authors (for example see Haynes 2002:56 The Early Settlement of North America) that Dent, Colorado was the first excavated Clovis site. The Dent Site, in Weld County, Colorado, was simply a fossil mammoth excavation in 1932. The first Dent Clovis point was found November 5, 1932 and the in situ point was found July 7, 1933. The in situ Clovis point from Burnet Cave was excavated in late August, 1931 (and reported early in 1932). E. B. Howard brought the Burnet Cave point to the 3rd Pecos Conference, September 1931, and showed it around to several archaeologists interested in Early Man (see Woodbury 1983).
Known as "Clovis First," the predominant hypothesis among archaeologists in the latter half of the 20th century had been that the people associated with the Clovis culture were the first inhabitants of the Americas. The primary support for this was that no solid evidence of pre-Clovis human inhabitation had been found. According to the standard accepted theory, the Clovis people crossed the Beringia land bridge over the Bering Strait from Siberia to Alaska during the period of lowered sea levels during the ice age, then made their way southward through an ice-free corridor east of the Rocky Mountains in present-day Western Canada as the glaciers retreated.
This hypothesis came to be challenged by studies suggesting a pre-Clovis human occupation of the Americas. In 2011, following the excavation of an occupation site at Buttermilk Creek, Texas, a prominent group of scientists claimed to have definitely established the existence "of an occupation older than Clovis."
According to researchers Michael Waters and Thomas Stafford of Texas A&M University, new radiocarbon dates place Clovis remains from the continental United States in a shorter time window beginning 450 years later than the previously accepted threshold (13,200 to 12,900 BP).
Alternatives to Clovis-first
Evidence of human habitation before Clovis
Archaeological sites that predate Clovis that are well documented include the following:
Predecessors of the Clovis people may have migrated south along the North American coastlines although there are arguments for many migrations along several different routes. Radiocarbon dating of the Monte Verde site in Chile place Clovis-like culture there as early as 13,500 years ago and remains found at the Channel Islands of California place coastal Paleoindians there 12,500 years ago. This suggests that the Paleoindian migration could have spread more quickly along the Pacific coastline, proceeding south, and that populations that settled along that route could have then begun migrations eastward into the continent.
The Pedra Furada sites in Brazil include a collection of rock shelters, used for thousands of years by diverse human populations. The first excavations yielded artifacts with C14 dates of 48,000 to 32,000 years BP. Repeated analysis has confirmed this dating, carrying the range of dates up to 60,000 BP. The best analyzed archaeological levels are dated between 32,160 ± 1000 years BP and 17,000 ± 400 BP.
In 2004, worked stone tools were found at Topper in South Carolina that have been dated by radiocarbon techniques possibly to 50,000 years ago, although there is significant dispute regarding these dates. Evidence of humans at the Topper Site definitely date back to 22,900 cal yr BP.
A more substantiated claim is that of Paisley Caves, Oregon, where rigorous carbon-14 and genetic testing appears to indicate that humans related to modern Native Americans were present in the caves over 1000 14C years before the earliest evidence of Clovis. Traces and tools made by another people, the "Western Stemmed" tradition, were documented.
A study published in Science presents strong evidence that humans occupied sites in Monte Verde, at the tip of South America, as early as 13,000 years ago. If this is true, then humans must have entered North America long before the Clovis Culture – perhaps 16,000 years ago.
The Tlapacoya site is located along the base of a volcanic (remnant) hill on the shore of the former Lake Chalco. Seventeen excavations along the base of Tlapacoya Hill between 1956 and 1973 uncovered piles of disarticulated bones of bear and deer that appeared to have been butchered, plus 2,500 flakes and blades presumably from the butchering activities, plus one non-fluted spear point. All were found in the same stratum containing three circular hearths filled with charcoal and ash. Bones of many other animal species were also present, including horses and migratory waterfowl. Two uncalibrated radiocarbon dates on carbon from the hearths came in at approximately 24,000 and 22,000 years ago. At another location a prismatic micro-blade of obsidian was found in association with a tree trunk radiocarbon dated (uncalibrated) at approximately 24,000 years ago. This obsidian blade has recently been hydration dated by Joaquín García-Bárcena to 22,000 years ago and the hydration results published in a seminal article that deals with the evidence for pre-Clovis habitation of Mexico.
Coastal migration route
Recent studies of the mitochondrial DNA of First Nations/Native Americans suggest that the people of the New World may have diverged genetically from Siberians as early as 20,000 years ago, far earlier than the standard theory suggests. According to one alternative theory, the Pacific coast of North America may have been free of ice, allowing the first peoples in North America to come down this route prior to the formation of the ice-free corridor in the continental interior. No evidence has yet been found to support this hypothesis except that genetic analysis of coastal marine life indicates diverse fauna persisting in refugia throughout the Pleistocene ice ages along the coasts of Alaska and British Columbia; these refugia include common food sources of coastal aboriginal peoples, suggesting that a migration along the coastline was feasible at the time.
The controversial Solutrean hypothesis proposed in 1999 by Smithsonian archaeologist Dennis Stanford and colleague Bruce Bradley (Stanford and Bradley 2002), suggests that the Clovis people could have inherited technology from the Solutrean people who lived in southern Europe 21,000–15,000 years ago, and who created the first Stone Age artwork in present-day southern France. The link is suggested by the similarity in technology between the projectile points of the Solutreans and those of the Clovis people. The model envisions these people making the crossing in small watercraft via the edge of the pack ice in the North Atlantic Ocean that then extended to the Atlantic coast of France, using skills similar to those of the modern Inuit people.
In a 2008 study of the relevant paleoceanographic data, Kieran Westley and Justin Dix concluded that "it is clear from the paleoceanographic and paleo-environmental data that the Last Glacial Maximum (LGM) North Atlantic does not fit the descriptions provided by the proponents of the Solutrean Atlantic Hypothesis. Although ice use and sea mammal hunting may have been important in other contexts, in this instance, the conditions militate against an ice-edge-following, maritime-adapted European population reaching the Americas."
University of New Mexico anthropologist Lawrence G. Straus, a primary critic of the Solutrean hypothesis, points to the theoretical difficulty of the ocean crossing, a lack of Solutrean-specific features in pre-Clovis artifacts, as well as the lack of art (such as that found at Lascaux in France) among the Clovis people, as major deficiencies in the Solutrean hypothesis. The 3,000 to 5,000 radiocarbon year gap between the Solutrean period of France and Spain and the Clovis of the New World also makes such a connection problematic. In response, Bradley and Stanford contend that it was "a very specific subset of the Solutrean who formed the parent group that adapted to a maritime environment and eventually made it across the north Atlantic ice-front to colonize the east coast of the Americas" and that this group may not have shared all Solutrean cultural traits.
Mitochondrial DNA analysis has found that members of some native North American tribes have a maternal ancestry (called haplogroup X) linked to the maternal ancestors of some present day individuals in western Asia and Europe, albeit distantly, and has also provided some support for pre-Clovis models. More specifically, a variant of mitochondrial DNA called X2a found in many Native Americans has been traced to western Eurasia, while not being found in eastern Eurasia.
Spearheads and DNA found at the Paisley Caves site in Oregon suggest that North America was colonized by more than one culture, and that the Clovis culture was not the first. There is evidence to suggest an east/west dichotomy, with the Clovis culture located to the east.
In 2014, the autosomal DNA of a 12,500+-year-old infant from Montana was sequenced. The DNA was taken from a skeleton referred to as Anzick-1, found in close association with several Clovis artifacts. Comparisons showed strong affinities with DNA from Siberian sites, and virtually ruled out any close affinity with European sources (the "Solutrean hypothesis"). The DNA also showed strong affinities with all existing Native American populations, which indicated that all of them derive from an ancient population that lived in or near Siberia, the Upper Palaeolithic Mal'ta population. Mal'ta belonged to y-dna haplogroup R and mitochrondrial macrohaplogroup U. The data indicate that Anzick-1 is from a population directly ancestral to present South American and Central American Native American populations. This rules out hypotheses which posit that invasions subsequent to the Clovis culture overwhelmed or assimilated previous migrants into the Americas. Anzick-1 is less closely related to present North American Native American populations (including a Yaqui genetic sample), suggesting that the North American populations are basal to Anzick-1 and Central and South American populations. The apparent early divergence between North American and Central plus South American populations may or may not be associated with post-divergence gene flow from a more basal population into North America; however, analysis of published DNA sequences for 19 Siberian populations did not favor the latter scenario. The boy Anzick-1, which is 12,6 thousand years old on the territory of Montana, belonged to Y-haplogroup Q-L54(xM3). Q-L54 is by far the largest haplogroup among native Americans.
In approximate reverse chronological order:
Pedra Furada, Serra da Capivara National Park, in Piauí, Brazil. Site with evidence of non-Clovis human remains, a rock painting rupestre art drawings from at least 12,000–6,000 BP. Hearth samples C-14 dates of 48–32,000 BP were reported in a Nature article (Guidon and Delibrias 1986). New hearth samples with ABOX dates of 54,000 BP were reported in the Quaternary Science Reviews. Paleoindian components found here, have been challenged by American researchers as Meltzer, Adovasio, and Dillehay. Niède Guidon is the head archaeologist at the Serra da Capivara National Park.
The Monte Verde site in Chile, was occupied from 14,800 years BP, with bones and other finds dating on average 12,500 yrs BP. The earliest finds at the site were from between 32,840 and 33,900 years BP, but are controversial.
Lagoa Santa, Minas Gerais, Brazil, is erroneously asserted to be Clovis age or even possibly Pre-Clovis in age. The recent discussion of this site (specifically Lapa Vermelha IV) and the Luzia skull, reportedly 11,500 years old by Neves and Hubb, makes it clear that this date is a chronological date in years Before Present and not a raw radiocarbon date in eastern Brazil. Clovis sites mostly date between 11,500 and 11,000 radiocarbon years which means 13,000 years before present at a minimum. "Luzia" is at least 1,000 years younger than Clovis and Lapa Vermelha IV should not be considered a Pre-Clovis site.
Cueva del Milodón, in Patagonian Chile dates at least as early as 10,500 BP. This is a site found particularly early in the New World hunt for Early Man, circa 1896, and needs additional basic research, but 10,500 B. P. would be 1,500 years younger than Clovis, or if the dating is 10,500 RCYBP, it would still be roughly 500–700 years younger than Clovis. In either case this should not be considered a Pre-Clovis site.
Cueva Fell and Pali Aike Crater sites in Patagonia, with hearths, stone tools and other elements of human habitation dating to at least as early as 11,000 BP.
The Big Eddy Site in southwestern Missouri contains several claimed pre-Clovis artifacts or geofacts. In situ artifacts have been found in this well-stratified site in association with charcoal. Five different samples have been AMS dated to between 11,300 to 12,675 BP (Before Present).
Taima Taima, Venezuela has cultural material very similar to Monte Verde II, dating to 12,000 years BP. Recovered artifacts of the El Jobo complex in direct association with the butchered remains of a juvenile mastodon. Radiocarbon dates on associated wood twigs indicate a minimum age of 13,000 years before the present for the mastodon kill, a dating significantly older than that of the Clovis complex in North America.
The Schaefer and Hebior mammoth sites in Kenosha County, Wisconsin indicate exploitation of this animal by humans. The Schaefer Mammoth site has over 13 highly purified collagen AMS dates and 17 dates on associated wood, dating it to 12,300–12,500 radiocarbon years before the present. Hebior has two AMS dates in the same range. Both animals show conclusive butchering marks and associated non-diagnostic tools.
A site in Walker, Minnesota with stone tools, alleged to be from 13,000 to 15,000 years old based on surrounding geology, was discovered in 2006. However, further examination suggests that the site does not represent a human occupation.
In a 2011 article in Science, Waters et al. 2011 describe an assemblage of 15,528 lithic artifacts from the Debra L. Friedkin site west of Salado, Texas. These artifacts (including 56 tools, 2,268 macrodebitage and 13,204 microdebitage) define the Buttermilk Creek Complex formation, which stratigraphically underlies a Clovis assemblage. While carbon dating could not be used to directly date the artifacts, 49 samples from the 20 cm Buttermilk floodplain sedimentary clay layer in which the artifacts were embedded were dated using optically stimulated luminescence (OSL). Eighteen OSL ages, ranging from 14,000 to 17,500 ka were obtained from this layer. The authors report "the most conservative estimate" of the age of the Buttermilk clays range from 13,200 to 15,500 ka, based on the minimum age represented by each of the 18 OSL ages.
Human coprolites have been found in Paisley Caves in Oregon, carbon dated at 14,300 years ago. Genetic analysis revealed that the coprolites contained mtDNA haplogroups A2 and B2, two of the five major Native American mtDNA haplogroups.
The Mud Lake site, in Kenosha County, Wisconsin consists of the foreleg of a juvenile mammoth recovered in the 1930s. Over 100 stone tool butchering marks are found on the bones. Several purified collagen AMS dates show the animal to be 13,450 rcybp with a range of plus or minus 1,500 rcybp variance.
Meadowcroft Rockshelter in southwestern Pennsylvania, excavated 1973–78, with evidence of occupancy dating back from 16,000 to 19,000 years ago.
Cactus Hill in southern Virginia, with artifacts such as unfluted bifacial stone tools with dates ranging from c. 15,000 to 17,000 years ago.
Sixty-eight stone and bone tools discovered in an orchard in East Wenatchee, Washington in 1987, excavated in 1988 and 1990. Five of the Clovis points are on display at the Wenatchee Valley Museum & Cultural Center.
^Lepper, Bradley T. (1999). "Pleistocene Peoples of Midcontinental North America". In Bonnichsen, Robson; Turnmire, Karen. Ice Age People of North America. Corvallis: Oregon State University Press. pp. 362–394.
^Firestone, Richard; West, Allen; Warwick-Smith, Simon (4 June 2006). The Cycle of Cosmic Catastrophes: How a Stone-Age Comet Changed the Course of World Culture. Bear & Company. p. 392. ASIN1591430615. ISBN1591430615.
^Pigati JS, Latorre C, Rech JA, Betancourt JL, Martínez KE, Budahn JR; Latorre; Rech; Betancourt; Martinez; Budahn (April 2012). "Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis". Proc Natl Acad Sci U S A109 (19): 7208–12. Bibcode:2012PNAS..109.7208P. doi:10.1073/pnas.1200296109. PMID22529347.
^Boslough, M.; K. Nicoll, V. Holliday, T. L. Daulton, D. Meltzer, N. Pinter, A. C. Scott, T. Surovell, P. Claeys, J. Gill, F. Paquay, J. Marlon, P. Bartlein, C. Whitlock, D. Grayson, and A. J. T. Jull (2012). "Arguments and Evidence Against a Younger Dryas Impact Event". Geophysical Monograph Series198: 13–26.Cite uses deprecated parameters (help)
^ abcdMann, Charles C. (2005). 1491: new revelations of the Americas before Columbus.
^ abFagundes, Nelson J.R. et al. (2008) "Mitochondrial Population Genomics Supports a Single Pre-Clovis Origin with a Coastal Route for the Peopling of the Americas" The American Journal of Human Genetics 82(March): pp. 1–10, doi:10.1016/j.ajhg.2007.11.013
^Dunbar, James S. (2006). "Paleoindian Archaeology". In S. David Webb. First Floridians and last mastodons: the Page-Ladson site in the Aucilla River. Dordrecht, The Netherlands: Springer. pp. 403–435 . ISBN978-1-4020-4325-3.
^Guidon, Niède. 1986 "Las Unidades Culturales de Sao Raimundo Nonato – Sudeste del Estado de Piaui-Brasil"; New Evidence for the Pleistocene Peopling of the Americas: 157–171. Edited by Alan Bryan. Center for the Study of Early Man. University of Maine. Orono.
^Science, May 9, 2008, Ancient Algae Suggest Sea Route for First Americans
^José Luis Lorenzo and Lorena Mirambell (coordinadores). "Tlapacoya: 35,000 años de historia del Lago de Chalco" Colección Científica núm 155, serie prehistória, Instituto Nacional de Antropología, México, 1986
^Joaquín García-Bárcena. "Primeros pobladores: La Etapa Lítica en México." Arqueología Mexicana, Vol. IX, núm. 52, pp. 28–29, 2001.
^Meltzer, David J. First Peoples in a New World Berkeley: University of California Press, 2009, p. 129
^Jacobs et al. 2004. GENES, DIVERSITY, AND GEOLOGIC PROCESS ON THE PACIFIC COAST. Annual Review of Earth and Planetary Sciences 32: 601–652
^ abcdRasmussen, M.; Anzick, S. L.; Waters, M. R.; Skoglund, P.; Degiorgio, M.; Stafford, T. W.; Rasmussen, S.; Moltke, I.; Albrechtsen, A.; Doyle, S. M.; Poznik, G. D.; Gudmundsdottir, V.; Yadav, R.; Malaspinas, A. S.; White, S. S.; Allentoft, M. E.; Cornejo, O. E.; Tambets, K.; Eriksson, A.; Heintzman, P. D.; Karmin, M.; Korneliussen, T. S.; Meltzer, D. J.; Pierre, T. L.; Stenderup, J.; Saag, L.; Warmuth, V. M.; Lopes, M. C.; Malhi, R. S.; Brunak, S. R.; Sicheritz-Ponten, T.; Barnes, I.; Collins, M.; Orlando, L.; Balloux, F.; Manica, A.; Gupta, R.; Metspalu, M.; Bustamante, C. D.; Jakobsson, M.; Nielsen, R.; Willerslev, E. (2014-02-13). "The genome of a Late Pleistocene human from a Clovis burial site in western Montana". Nature506 (7487): 225–229. doi:10.1038/nature13025.edit
^"Monte Verde Archaeological Site". Tentative List of Properties of Outstanding Universal Value. World Heritage - United Nations Educational, Scientific, and Cultural Organization. Retrieved 1 November 2010.
Kennett, D. J.; Kennett, J. P.; West, A.; Mercer, C.; Que Hee, S. S.; Bement, L.; Bunch, T. E.; Sellers, M.; Wolbach, W. S. (2009). "Nanodiamonds in the Younger Dryas Boundary Sediment Layer". Science323 (5910): 94. doi:10.1126/science.1162819. PMID19119227.
Madsen, David B. (2004). Entering America: northeast Asia and Beringia before the last glacial maximum. Salt Lake City: University of Utah Press. ISBN0-87480-786-7.
Stanford, Dennis; and Bruce Bradley (2002). "Ocean Trails and Prairie Paths? Thoughts About Clovis Origins.". In Nina G. Jablonski (ed.). The First Americans: The Pleistocene Colonization of the New World (Memoirs of the California Academy of Sciences, No. 27.). edited proceedings of The Fourth Wattis Symposium, 'The First Americans: The Pleistocene Colonization of the New World,' October 2, 1999. San Francisco: California Academy of Sciences. pp. 255–271. ISBN0-940-22849-1.Cite uses deprecated parameters (help)
Stanford DJ, Bradley BA (2012). Across Atlantic Ice: The Origin of America's Clovis Culture. University of California Press. ISBN978-0-520-22783-5.