This essay will critically discuss the statement, that Neanderthals are an important link in the evolutionary line leading to modern humans. Specifically, this essay will aim provide evidence suggesting the genetic lineage of Neanderthals lived on, to some extent, through the interbreeding of Homo sapiens and Neanderthals, who shared parts of Europe for around 10,000 years co-existing and possibly interbreeding circa 50,000 years ago. Firstly, this essay will provide some context around Neanderthals and then the emergence of H. Sapiens. Then, in order to determine whether Neanderthals did in fact play any role in the evolutionary line leading to modern humans, we must determine which traits, cultural, genetic or morphological, which can be attributed to an interbreeding scenario. This essay aims to show that while Neanderthals are likely not part of the direct lineage of modern humans, that is, humans living today, some important traits of modern humans can be traced back to interbreeding of some H. sapiens and Neanderthals. The essay will lastly address some of the issues and debates concerning the possibility of whether Neanderthals and H. sapiens did indeed interbreed.
For up to 300,000 years, Neanderthal, one of the best known archaic Homo, Homo neanderthalensis (henceforth written as Neanderthal) thrived throughout Eurasia – Europe, parts of Western Asia, and parts of the Middle East (Mellars, 2011). Using stone tools, and in some cases, fire, as their ancestors from Africa had developed in aeons past, Neanderthals spent much of their time in caves, earning their early moniker as “cave-man”. Such cave dwelling would have, of course, been necessary considering the relative climate that Neanderthals evolved in, especially given that their reign coincided with a long period of glacial maximum (Hofreiter, 2009).
With a brain of between 1300 cc to 1600 cc, which is, in some cases, larger than that of modern humans, a squat torso, short extremities enabling very strong musculature, which also surpasses that of modern humans, Neanderthals adapted to their climate and surroundings perfectly. Their specialised physiology, altogether, may have also enabled them to survive at a lower temperature than that of modern H. sapiens providing a sincere advantage in such frigid climates (Balter, 2004).
The caves that Neanderthals lived in had the remarkable quality of preserving both their remains and the remains documenting their diet and changes within their morphology and culture. There is evidence that Neanderthals subsisted on a range of flora and fauna, with remnants of bones from gazelle, fish and seeds being found in some caves (Hardy, 2011).
Approaching 50,000 years ago, however, the specialised morphology of Neanderthals, began to work against then when the climate that it evolved in began to change and it faced a fierce new competition. Anatomically modern humans began penetrating Europe in numbers approaching ten times that of the population of Neanderthals. This influx of anatomically modern humans would have put significant pressure on the groups of Neanderthals, forcing them to compete for food, location and resources for the first time (Mellars, 2011). Whether or not the population of H. sapiens possessed more highly evolved mental capacities is not known, however the emergence of items of cultural significance, such as art and imported crafted ivory, and vastly more sophisticated tools, suggests that H. Sapiens was more culturally sophisticated than Neanderthals (Brose, 1971).
Rather interestingly, the diet of Neanderthals was not all that different to that of H. Sapiens and as the incursion into Neanderthal occupied territory continued, the cultural trait of H. sapiens travelling vast distances to hunt, ensured their survival (Roebroeks, 2001). Slowly, but surely, the vastly greater numbers of H. sapiens pushed Neandertals into harsher and less fertile environments, where they eventually went extinct around 30,000 years ago. Their slow decline is documented in the stratigraphic record, with the youngest specimens showing some evidence of malnutrition (Baskerville, 1989).
It is important to recognise that H. sapiens and Neanderthals were, in many ways, very similar. Culturally, both species recognised the significance of death, routinely and ritually acknowledging one’s passing, sometimes with tools and other artefacts apparently significant to the dead (Sommer, 1999). Most notably, while their phenotype provided a sincere morphological difference, the genetic distinction was not so great that they could not mate and produce viable, fertile offspring. This offspring, as has been found in recent comparative analysis of the nuclear DNA of Neandertals and modern humans, explains the origin of several traits (Green, 2010).
The first trait attributed to Neanderthals the gene microcephalin which regulates brain size during development, allowing for more thorough brain development, had been absent in the modern human genome for around 1.1 million years. Such an advantageous trait, which has been attributed to increased intelligence, arose in modern humans around 37,000 years ago and through positive selection, propagated throughout the human population (Evans, 2006). This gene is now present in close to 70% of the world’s population today and propagated with such speed that neutral genetic drift is discordant (Evans, 2005).
Perhaps the most compelling evidence for traits derived through interbreeding has been evidenced through research into the nuclear DNA of Neanderthals. The HLA (human leucocyte antigen) class 1 of genes play a vital role in developing resistance to viruses that attempt to proliferate the immune system. The specific set of antigens present only in modern humans with European and Asian lineage suggest that Neanderthals and other archaic Homos, who had developed their immunity in that environment for hundreds of thousands of years, had indeed interbred with anatomically modern humans (Gibbons, 2011).
Research into the genomic similarities between Neanderthals and modern humans has also confirmed that several populations around the world share a percentage of their genome with Neanderthals. Using both mitochondrial and nuclear DNA recovered from Neanderthals, researchers have been able to attribute between 2-6% of the human genome in modern humans that trace their lineage back to Eurasia. Researchers took samples from five groups of modern humans, French, Han-Chinese and Papuan from Eurasia and Yoruba and San groups in Africa and confirmed the presence of shared alleles between Neanderthals and the Eurasian groups and their absence in the African groups. This suggests genetic drift attributed to interbreeding of with Neanderthals (Green, 2006).
While the presence of Neanderthal genetic material in the modern human genome does indeed verify the theory of interbreeding, the low prevalence and percentage of shared genome, however, does raise the argument of such pairing being fairly uncommon. Recent simulated modelling into the spread of modern humans leaving Africa and their encounters with Neanderthals across Eurasia appears to validate the theory that sex between the two species may have been heavily discouraged or taboo. It may also have been quite the opposite and interbreeding may have been rampant, but the hybrid offspring may have either had some genetic disadvantage preventing it from reaching reproductive age, or offspring may have simply been infertile. The simulations concluded that it would have taken between only 197 and 430 successful ‘rendezvous’ to reach the current estimation of 2-3% of the human genome attributed to Neanderthals (Currat, 2011). Extrapolated, considering that humans and Neanderthals lived and competed for the same resources in the same areas for around 10,000 years, one human would only have to have mated with a Neanderthal, on average, every 23 to 50 years.
Is it so strange that these two morphologically different hominins interbred? The literature supporting or rejecting the case for H. sapiens and Neanderthals reproducing is mixed, though most do support some level of interbreeding. The reason for the lack of a firm position is for several reasons which we will discuss. Firstly, both species share a common ancestor, which, using cranial, mandibular and facial morphology, to determine common traits, might have been H. antecessor (Bermudez de Castro, 1997). Thus it is likely that the common ancestor of modern humans and Neanderthals diverged circa 706,000 to 650,000 years (Weaver, 2008).
As the two species of hominin shared a common ancestor, some of the traits common to both species could have a common origin. The obvious refutation to this, however, is the fact that alleles common to Neanderthals and people of Eurasian descent aren’t found in continuous African populations (Green, 2006). More specifically alleles related immunity, such as the aforementioned HLA series, come from and immune system subject to hundreds of thousands of years evolving to the conditions found in Eurasia during the Pleistocene (Potts, 1996).
Another theory that seems to hold some validity is the possibility that Neanderthal and H. sapiens populations did not interbreed, but were instead subjected to the same selective pressures. What this theory purports is that modern humans, upon arriving in Eurasia, were subject to the same ecological and pathological pressures that Neanderthal had lived and evolved in for some 300,000 years. In this scenario, modern humans superior numbers and ability to adapt to harsh environments through cultural means, rather than to lean on natural selection, may have been able to overcome a potentially high attrition rate expected from such a significant change in environment as that of leaving temperate Africa for frigid Eurasia (Piazza, 1981). As part of this, useful genetic mutations and traits would have been positively selected for, quickly spreading throughout the population (Kelley, 2008).
In conclusion, there appears to be enough evidence to suggest that modern humans and Neanderthals did indeed interbreed during their some 10,000 years spent cohabiting in Eurasia. With that said, the amount to which this interbreeding propagated any sort of important evolutionary traits appears to be either limited or indeterminate. The alleles linked to microcephalin and HLA’s certainly seem to provide some level of genetic differentiation between descendants of Eurasians who may have had contact with Neanderthals and those continuous cultures of Africa. However, again, the amount to which these alleles present of genetic advantage to those with those genetic traits appears to be either limited or indeterminate. As such, it doesn’t seem that Neanderthals were an important link in the evolutionary line leading to humans.
- Mellars, P & French, JC 2011, ‘Tenfold Population Increase in Western Europe at the Neandertal-to-Modern Human Transition’, Science, vol. 333, no. 6042, p. 623.
- Hofreiter, M & Stewart, J 2009, ‘Ecological change, range fluctuations and population dynamics during the Pleistocene’, Current Biology, vol. 19, no. 14, pp. R584-R94.
- Balter, M 2004, ‘Dressed for success: Neandertal culture wins respect’, Science, vol. 306, no. 5693, p. 40.
- Hardy, BL & Moncel, MH 2011, ‘Neanderthal Use of Fish, Mammals, Birds, Starchy Plants and Wood 125-250,000 Years Ago’, PloS one, vol. 6, no. 8, p. e23768.
- Brose, DS & Wolpoff, MH 1971, ‘Early upper Paleolithic man and late middle Paleolithic tools’, American Anthropologist, vol. 73, no. 5, pp. 1156-94.
- Roebroeks, W 2001, ‘Hominid behaviour and the earliest occupation of Europe: an exploration’, Journal of Human Evolution, vol. 41, no. 5, pp. 437-61.
- Baskerville, RF & Rosenberg, KR 1989, ‘On Neandertal pubic length’, Current Anthropology, pp. 482-8.
- Sommer, JD 1999, ‘The Shanidar IV ëFlower Burialí: A re-evaluation of Neanderthal burial ritual’, Cambridge Archaeological Journal, vol. 9, no. 01, pp. 127-9.
- Green, RE, Krause, J, Briggs, AW, et al. 2010, ‘A Draft Sequence of the Neandertal Genome’, Science, vol. 328, no. 5979, pp. 710-22.
- Evans, PD, Mekel-Bobrov, N, Vallender, EJ, Hudson, RR & Lahn, BT 2006, ‘Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage’, Proceedings of the National Academy of Sciences, vol. 103, no. 48, p. 18178.
- Evans, PD, Gilbert, SL, Mekel-Bobrov, N, Vallender, EJ, Anderson, JR, Vaez-Azizi, LM, Tishkoff, SA, Hudson, RR & Lahn, BT 2005, ‘Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans’, Science, vol. 309, no. 5741, pp. 1717-20.
- Gibbons, A 2011, ‘A Denisovan Legacy in the Immune System?’, Science, vol. 333, no. 6046, p. 1086.
- Green, RE, Krause, J, Ptak, SE, Briggs, AW, Ronan, MT, Simons, JF, Du, L, Egholm, M, Rothberg, JM & Paunovic, M 2006, ‘Analysis of one million base pairs of Neanderthal DNA’, Nature, vol. 444, no. 7117, pp. 330-6.
- Currat, M & Excoffier, L 2011, ‘Strong reproductive isolation between humans and Neanderthals inferred from observed patterns of introgression’, Proceedings of the National Academy of Sciences, vol. 108, no. 37, pp. 15129-34.
- Bermudez de Castro, JM, Arsuaga, JL, Carbonell, E, Rosas, A, Martinez, I & Mosquera, M 1997, ‘A Hominid from the Lower Pleistocene of Atapuerca, Spain: Possible Ancestor to Neandertals and Modern Humans’, Science, vol. 276, no. 5317, pp. 1392-5.
- Weaver, TD, Roseman, CC & Stringer, CB 2008, ‘Close correspondence between quantitative-and molecular-genetic divergence times for Neandertals and modern humans’, Proceedings of the National Academy of Sciences, vol. 105, no. 12, p. 4645.
- Potts, R 1996, ‘Evolution and climate variability’, Science, vol. 273, no. 5277, p. 922.
- Piazza, A, Menozzi, P & Cavalli-Sforza, L 1981, ‘Synthetic gene frequency maps of man and selective effects of climate’, Proceedings of the National Academy of Sciences, vol. 78, no. 4, p. 2638.
- Kelley, JL & Swanson, WJ 2008, ‘Positive selection in the human genome: from genome scans to biological significance’, Annu. Rev. Genomics Hum. Genet., vol. 9, pp. 143-60.