Instructions:
There are arguments for the pro-side of hypothesis that ancient AM humans interbred with Neanderthals in Europe and Asia. Your assignment is to take a side: pro or con. There really is no middle ground either we have evidence of breeding or we do not. It cannot be that you think ancestral humans may be bred with Neanderthals but there is not sufficient evidence for it. YOU SHOULD TAKE A SIDE!
CON-SIDE: Disprove the arguments below with evidence. Paper you turn in would be each of the following paragraphs followed by a paragraph describing the evidence that disproves this argument.
OR
PRO-SIDE: Each of your three paragraphs (the evidence you have for the con-side) followed by a paragraph that refutes the evidence you presented.
6 paragraphs no more! Do not write an introduction or conclusion section. Just present the evidence that you believe discredits the evidence. You might quote the three paragraphs directly. The original writing is in each paragraph which follows the original paragraph. If you don’t understand please ask for clarification. We don’t have time to rewrite these.
PRO-Side arguments1
Similarities in morphology of mandible between Neanderthals and AMH suggest that features are output of interbreeding. In 1957, the mandible of RiparoMezzena, a Middle Paleolithic rockshelter in MontiLessini (NE Italy, Verona), was discovered (Condemi 2013). Mitochondrial DNA (mtDNA) analysis and direct dating confirm that mandible belongs to the late Neanderthal, the only genetically typed Neanderthal of the Italian peninsula (Condemi 2013). Condemi et al. used comparative morphology and shape analysis to show the morphology of the lower jaw appears to be in between the receding lower jaw (no chin) of classic Neanderthals and the projecting lower jaw (strongly developed chin) of AMHs. The incipient mental trigone of the late Neanderthal displays a much more modern morphology (Condemi 2013). Additionally, the results of linear Discriminant Function Analysis (DFA) depicts the position of the Mezzena mandible to be within the modern human shape space, while possessing strong shape similarities with some Neanderthal specimens (Condemi 2013). Change in morphology of the mandibular chin among the fossils of Mezzena and other late Neanderthals could have been the result of a small degree of interbreeding with AMHs.
Comparison of DNA sequences between Neanderthals and Anatomically Modern Humans (AMH) demonstrate that Neanderthals share more genetic variants with non-Africans than with Africans due to interbreeding. Recent analyses of 100 mg of bone powder removed from the bone of the Mezzena mandible for DNA extraction and PCR analysis revealed that Neanderthal genomes have closer genetic af?nities with modern non-Africans than with West Africans (Condemi 2013). Additionally, Green et al. identified single-nucleotide polymorphism (SNP) alleles by comparing one randomly selected sequence from an AMH against that of a Neanderthal, demonstrating Neanderthals are equally close to current day Europeans and East Asians. However, Neanderthals are significantly closer to non-Africans than to Africans (Green 2010).
Success rates of interbreeding are observed to be low and mean that there are efficient barriers to gene flow between Neanderthals and AMH. By
means of spatially explicit simulations, Currat and Excoffier calculated the expected amount of admixture among Neanderthals and AMH to be less than 2%. Observed low rates can be desribed by assortative mating and/or low fitness of hybrids, and are compatible with earlier observed lack of mtDNA introgression (Currat 2011). If both species interacted for 10,000 years during the range expansion of modern humans, to produce introgression levels of 1–3%, Neanderthals and AMH only have to mingle, on average, once every 23–50years, that demonstrates that they were extremely rare events (Currat 2011).
Argument No.2 - Spatially explicit model shows no evidence of admixture between Eurasians and Neanderthals.
The lack of spatial realism in the previously done studies tends to lead to overestimation of the level of hybridization of modern humans and Neanderthals. A spatially explicit framework was built by Eriksson and Manica to allow for investigation of the extent to which ancient population structure might drive spatial differences in genetic overlap between ancient humans and modern humans. The model was built to show that common ancestor was occupying the demes in Eurasia region and Africa at 500 kya, and then the split between the Neanderthals and Modern humans took place at 320 kya. The populations of African range eventually turned into modern humans and expanded out of Africa and subsequently colonized demes parallel to the ones occupied by Neanderthals in Eurasia, but no admixture took place². Eriksson and Manica’s spatial model results fit into the pattern of shared genetic polymorphisms between Neanderthals and modern humans. This model provided more realistically fit data than the nonspatial model that included hybridization. Although Eriksson and Manica do not deny the possibility of admixture, their spatial model data provides no supportive evidence of interbreeding, therefore their discussion part of the study indicated that admixture events are inconclusive.
Argument No.3 – Genetic and morphological evidence points to no interbreeding between the modern Eurasians and Neanderthals.
Several genetic evidence shows a separation of Neanderthals (HN) and modern humans (HS), including the absence of Neanderthal sequence motifs and the mitochondrial DNA. The mtDNA divergence and monophyly points to long separation of female lineages with a separation time of 300,000 – 750,000 years ago. Recent morphological studies also support a clear distinction between these two species ³. Even though European gene pool shows no evidence of Neanderthal mtDNA, it was previously argued that it might have been lost during the genetic drift. Some think that mitochondrial fitness differences might have had an influence for the natural selection of the mtDNA preference of modern humans. However, it is doubtful that newcomer modern humans had a better mitochondrial adaption than Neanderthals who had been living in this climate for several thousand years ³.
Argument No.4 – Admixture model points to very little (<0.1%) interbreeding between the modern Eurasians and Neanderthals.
A mixture model was built by Currat and Excoffier to show estimated rates of interbreeding. Their findings showed no to very little possibility (<0.1%) of interbreeding between the HN and HS species. Their estimated admixture rate is 400 times smaller than previously estimated by Nordborg, and Serre et al. Overall, Currat and Excoffier took into account a more realistic model of evolution, with population subdivision, constant population size, and the progressive nature of the range expansion of modern humans into Europe, with a single and instantaneous admixture event between the Neanderthals and modern humans, and came up with a conclusion that maximum initial input of Neanderthal genes into the European population can be estimated to lie between only 0.02% and 0.09%, if any interbreeding took place indeed. This admixture model might additionally imply that there was a bottleneck before the range expansion into Europe.
References
1. Condemi et al. “Possible Interbreeding in Late Italian Neanderthals? New Data from the Mezzena Jaw (MontiLessini, Verona, Italy).”PLoS ONE (2013) 8:e59781.
2. Currat M, Excoffier L. “Strong reproductive isolation between humans and Neanderthals inferred from observed patterns of introgression.”
3. Proceedings of the National Academy of Sciences.(2011) 108:15129–15134.
4. Green et al. "A Draft Sequence of the Neandertal Genome." Science. (2010) 328:710-722
5. Relethford, J.H., Genetic Evidence and the Modern Human Origins Debate. Heredity. 2008, 100,555-563.
6. Eriksson, A.; Manica, A. Effect of Ancient Population Structure on the Degree of Polymorphism shared between Modern Human Populations and Ancient Hominins. PNAS. 2012, 109, 35, 13956-13960.