Hubbe et al, makes it clear that there was no insitu neutral evolution of ancient Americans and that as a result there were two origins for the early Americans. These researchers detected morphological differtiation between the Late and Early American populations. The authors of this study maintain that there had to have been two crossing from Northeast Asia into the Americas beginning 17-15kya.
Although Hubbe et al suggest that the earliest population to cross the Berinigan were Melanesians the research suggest that these negroes came from Africa instead of East Asia because homo sapien sapiens were already settled in South America 40kya (1).
There was no way any population crossed the Berigan 40kya because of the last Ice Age. The last ice age in North America lasted between 110,000 and 17,000BP. The ice-free corridor on the eastern flank of the Rockies did not open before 13,000 years ago. Yet, Africans were in the Americas long before the end of the last Ice Age when the “Siberians” began to cross the Bearing Straits. By 12,500 BC Africans were already living in Chile.
Kitchen et al argue that Northeast Asians lived in Beringa between 43-36kya (2). Here they probably remained for 20,000 years (2). The glaciation in North America, led to the Beringan standstill due to the dryness and cold existing in North America at this time which would have prevented any population from migrating into North, Central and South America.
Although there was a Beringan standstill people were already settled in South America at this time (1). The first Americans did not cross the Bearing Straits to enter the Americas.The earliest sites for Negroes date between 20,000 and 40000 years ago Old Crow Basin Canada(38,000BC) Pedra Furada (45,000BC) Brazil (1, 3-7) . These people were pygmies and Khoisan types according to Dr. Marquez (3).
In very ancient times the American continent was inhabited by Asian and African blacks. The oldest skeletal remains found in the Americas are of blacks. Marquez observed that "it is [good] to report that long ago the youthful America was also a Negro continent"(3). Dr. Dixon noted that as early as 70,000 B.C., Austroloid and later negritos crossed the Bering Strait to reach the New World. And Lanning noted that "there was a possible movement of negritos from Ecuador into the Piura Valley, north of Chicama and Viru" in early times.
Today archaeologists have found sites from Canada to Chile that range between 20,000 and 40,000 years old (1). There are numerous sites in North and South America which are over 35,000 years old. These sites are the Old Crow Basin (c.38,000 B.C.) in Canada; Orogrande Cave (c.36,000 B.C.) in the United States; and Pedra Furada (c.45,000 B.C.) . Given the fact that the earliest dates for habitation of the American continent occur below Canada in South America is highly suggestive of the fact that the earliest settlers on the American continents came from Africa before the Ice melted at the Bering Strait and moved northward as the ice melted.
The appearance of pebble tools at Monte verde in Chile (c.32,000 B.P), and rock paintings at Pedra Furada in Brazil (c.22,000 B.P.) and mastodont hunting in Venezuela and Colombia (c.13,000 B.P.), have led some researchers to believe that the Americas was first settled from South America(1). C. Vance Haynes noted that: "If people have been in South America for over 30,00 years, or even 20,000 years, why are there so few sites?....One possible answer is that they were so few in number; another is that South America was somehow initially populated from directions other than north until Clovis appeared"(6).
P.S. Martin and R. G. Klein after discussing the evidence of mastodont hunting in Venezuela 13,000 years ago observed that "The thought that the fossil record of South America is much richer in evidence of early archaeological associations than many believed is indeed provocative....Have the earliest hunters been overlooked in North America? Or did the hunters somehow reach South America first"? “(7).
The early presence of ice-age sites in South America suggest that these people probably came from Africa. This would explain the affinities between African languages and the Amerind family of languages.
In the 1970’s in Brazil an interesting skull of a girl was found. This skull was reconstructed and dated back to 12,000 BC. Dr. Walter Neves professor of biological anthropology at the University of Sao Paolo, after reconstructing the “Luzia” skull found that this personage was either an African or Pacific island type Black . Other ancient skeletons have been found in Chile: Monteverde (12,500 years), Tierra del Fuego, Cueva de Fell, Tres Arroyos and some other places. There are older ones in the Argentinian Patagonia.
Hubbe et al claims that these skeletons are of Australian or Melanesian Blacks. This is highly unlikely given the fact that these skeletons have been found near the Atlantic Ocean . This location is suggestive of a migration from Africa to Mexico, like the migration of the Olmec 11,000 years later (8). This view is supported by the discovery of the so-called Eva Neharon skeleton (c.13,600 ) dating to around the same period found in the Caribbean. By 11,500 we see the appearence tall Negroes from Africa in Colombia, Venezuela and Brazil e.g.,Luiza. This suggest that Negroes settled America both from the Bearinga and Africa.
Archaeologist have found that the oldest boat in Nigeria at Dufuna was made over 10kya. The most interesting thing about the Dufuna boat is that it is associated with a culture that appeared in Africa around 12,000 BC. This is around the same time that Luiza and Eva Nahron and the skeletons found off the Yucatan settled Latin America.
The Dafuna boat suggest that Africans at this early date had the technology to sail to Latin America.

References:
1. Warwick Bray,"The Paleoindian debate". Nature 332, (10 March) 1988, p.107.
2. Kitchen A, Miyamoto MM, Mulligan CJ, 2008 A Three-Stage Colonization Model for the Peopling of the Americas. PLoS ONE 3(2): e1596. doi:10.1371/journal.pone.0001596
3. C. Marquez. 1956). Estudios arqueologicas y ethnograficas. Mexico.
4. "Man's New World arrival Pushed back", Chicago Tribune, (9 May 1991) Sec.1A, p.40.
5. A.L. Bryan, "Points of Order". Natural History , (June 1987) pp.7-11.
6. C.V. Haynes,Jr.,"Geofacts and Fanny". Natural History ,(February 1988)pp.4-12:12.
7. P.S. Martin and R.G.Klein (eds.),Quarternary Extinctions: A Prehistoric Revolution, (Tucson:University of Arizona Press,1989) p.111.
8. C.A. Winters. 2009. Olmec (Mande) Loan Words in the Mayan, Mixe-Zoque and Taino Languages, Current Research Journal of Social Science, Forthcoming 2011.