Neanderthal DNA and modern human origins
Introduction
The debate on the origin of anatomically modern human (Homo sapiens sapiens) has lasted for decades. The well known “out-of-Africa” replacement model postulates that modern human ancestors evolved in Africa, spread throughout the globe and subsequently replaced all other hominids (Stringer and Andrews, 1988). However, the multiregional origin model suggests that the gradual transition from Homo erectus to modern human took place on different continents in the Old World (Wolpoff et al., 1984). Other, intermediate models acknowledge an African origin of modern humans and theorize that archaic hominids outside of Africa also made genetic contributions (Smith et al., 2005).
Now, the debate focuses on the evolutionary relationship between archaic hominids and anatomically modern humans. It also seeks to address whether gene flow could have occurred between archaic hominids and modern humans, thereby leaving a footprint on the modern humans' current gene pool.
Neanderthals (Homo neanderthalensis) are archaic hominids, supposed to be most similar to modern humans. These hominids, extinct members of the Homo genus, populated Europe and parts of western and central Asia before their disappearance 25,000 years ago (Tattersall, 1995). Fossil evidence suggests that Neanderthals probably coexisted with anatomically modern humans (i.e. Cro-Magnon) for 20,000 years (Finlayson et al., 2006). Although there is no evidence of contemporaneous cohabitation at any single archeological site, their long period of coexistence (including cohabitation in the Middle East and Europe) raises the possibility of genetic admixture between Neanderthals and modern humans. The shared morphological features between Neanderthals and early modern Europeans also have been interpreted as evidence for the gene flow between Neanderthals and modern Europeans (Wolpoff et al., 2001; Voisin, 2006). Since new developments in fossil-preserved ancient DNA analysis arose in the late 1980s, genetic evidence has shed more light on the relationship between Neanderthals and modern humans.
Section snippets
Neanderthal mitochondrial DNA
In 1997, a segment of hypervariable control region I (HVRI) of maternally inherited mitochondrial DNA (mtDNA) was determined from a Neanderthal-type specimen found in 1856 in Neander Valley, near Düsseldorf, Germany (Krings et al., 1997). Since then, mtDNA sequences have been determined from multiple Neanderthal samples (Krings et al., 1999, 2000; Ovchinnikov et al., 2000; Gutierrez et al., 2002; Schmitz et al., 2002; Serre et al., 2004; Beauval et al., 2005; Caramelli et al., 2006; Lalueza-fox
Neanderthal nuclear DNA
The development of high-throughput DNA sequencing technology and improved computational and statistical methodology have allowed for the large-scale sequencing of nuclear DNA from Neanderthal fossils. Noonan et al. (2006) used a cell-based approach to create meta-genomic libraries of fossil DNA extracts by performing traditional Sanger sequencing and 454 pyrosequencing. They obtained 65,250 bp of Neanderthal genomic sequences. Green et al. (2006) used an emulsionbead-based approach and obtained
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