Neanderthal extinction

Skeleton and model of a Neanderthal from La Ferrassie

Neanderthal extinction refers to the sudden disappearance of Neanderthals during a time when modern humans began to emerge in Eurasia. Since the discovery of Neanderthal remains, both the Neanderthals' place in the human family tree and their relation to modern humans has been hotly debated. At different times, they have been classified as a separate species (Homo neanderthalensis) and as a subspecies (Homo sapiens neanderthalensis).

In August 2014, a team reported on a new analysis of 40 sites in Western Europe, concluding that Neanderthals died out about 40,000 years ago.[1] This date, much earlier than previous estimates, was established through improved radio carbon dating methods. Researchers want to expand their survey of sites to Eastern Europe and Siberia, as Neanderthals may have survived longer there.

Hypotheses on the fate of the Neanderthals include a failure or inability to adapt to climate change, competitive exclusion, or extinction by encroaching anatomically modern humans, who arrived in Europe long after Neanderthals had settled there.[2] Neanderthal hybridization with early modern human populations is also considered a viable hypothesis. Some interbreeding took place in western Asia about 50,000 to 60,000 years ago, as evidenced by 1 to 4 percent of the material of genomes carried by non-African people living today.[1]

Coexistence prior to extinction

The new evidence narrows the period in which both Neanderthals and modern humans coexisted in Europe to approximately 5,000 years.[1] The duration of this period is uncertain.[3] There is evidence that modern humans reached Europe sometime between 45,000 and 43,000 years ago.[4] The new evidence, based on conclusions from improved radiocarbon dating indicates that Neanderthals became extinct 40,000 years ago, which overturns older less accurate carbon dating which incorrectly indicated that Neanderthals may have lived as recently as 24,000 years ago in refugia on the south coast of the Iberian peninsula such as Gorham's Cave.[5][6] Inter-stratification of Neanderthal and modern human remains has been suggested,[7] but is disputed.[8]

Rapid extinction by violence

It is possible that Neanderthal extinction was either precipitated or hastened due to some form of conflict with Homo sapiens. Conflict and warfare are virtually ubiquitous features of hunter-gatherer societies, and this includes chiefly conflicts over resources, including the land itself (which contains prey animals and water sources), which are naturally limited.[9] As such, and given that there was the added feature of two different human species, it's very plausible that violence, including primitive warfare, would have transpired between the two groups. And in this conflict, Homo sapiens would have had one clear advantage, technological superiority (along with probable other advantages such as superior language abilities). The technological superiority of Homo sapiens, then, would not only have indirectly led to Neanderthal extinction through more effective hunting, but through direct conflict. It is also plausible that such violence and conflict was only one part of the reason for the extinction of Neanderthal, a contributing factor among many others; with other possible factors being: Homo sapiens out-hunting Neanderthal (both through superior technology and fitness); habitat change or destruction through the controlled use of fire (by Homo sapiens); Homo sapiens' domestication of dogs (which would have aided in hunting and warfare);[10] superior language skills; and the existence of trade networks, a greater level of cultural networks, and larger communities (made possible perhaps through improved language skills).[11][12] Conflict and competition between the two species may have also been combined with climate change, with the two forces leading to the extinction of the less advanced and less adaptable Neanderthals.[13]

Rapid extinction by parasites and pathogens

Another possibility, and one which could also account for rapid extinction, would be the spread among the Neanderthal population of some pathogen or parasite carried by Homo Sapiens, or a number of them.[14] Neanderthals would have evolved immunity to certain diseases, but not to those they hadn't been exposed to. A disease causing pathogen or parasite, carried by the Homo Sapiens who entered Europe, would have been alien to Neanderthals, with them not having evolved immunity to it. As such it may have been particularly lethal. A factor which may have contributed to the rapid spread of a pathogen in the first place, would have been the close presence of very similar species to one another. In the first place, it will be much easier for a pathogen to leap between two very similar species. Aside from that, two such populations being close to one another would allow a pathogen to rapidly infect many individuals, without defeating itself through killing too many of them and too quickly (and thereby not being able to spread to fresh organisms). The tolerance of the one of the two species for the pathogen would also aid this. In this scenario, the hypothetical pathogen would have been able to move between Neanderthals and Homo Sapiens, killing off the Neanderthals (and quickly), while remaining in the Homo Sapiens who had a tolerance for it. Examination of Human and Neanderthal genomes, and adaptations regarding pathogens or parasites, would shed light on this issue.

Competitive replacement

Even a slight competitive advantage on the part of modern humans could account for Neanderthals' replacement by anatomically modern humans on a timescale of 10,000–20,000 years.[15]

The hypothesis that early humans violently replaced Neanderthals was first proposed by French palaeontologist Marcellin Boule (the first person to publish an analysis of a Neanderthal) in 1912.[16]

Another supporter of competitive replacement is Jared Diamond who points out in his book The Third Chimpanzee that the genocidal replacement of Neanderthals by modern humans is similar to modern human patterns of behavior that occur whenever people with advanced technology invade the territory of less advanced people.[17]

Division of labor

In 2006, two anthropologists of the University of Arizona proposed a new explanation for the demise of the Neanderthals.[18] In an article titled "What's a Mother to Do? The Division of Labor among Neanderthals and Modern Humans in Eurasia",[19] they theorize that Neanderthals like Middle paleolithic Homo sapiens did not have a division of labor between the sexes. Under this hypothesis, both male and female Neanderthals participated in the single main occupation of hunting big game that flourished in Europe in the ice age, such as bison, deer, gazelles and wild horses. This hypothesis proposes that the Neanderthal's relative lack of labor division resulted in their poor extraction of resources from the environment — as compared to Homo sapiens — and contributed to their demise.

Anatomical differences and running ability

Researchers including Karen L. Steudel of the University of Wisconsin have proposed that because Neanderthals had limbs that were shorter and stockier than those of modern humans, and because of other anatomical differences in their limbs, the primary reason the Neanderthals were not able to survive is related to the fact that they could not run as fast as modern humans, and they would require 30% more energy than modern humans would for running or walking.[20]

Nevertheless, in the recent study, researchers Martin Hora and Vladimir Sladek of Charles University in Prague show that Neanderthal lower limb configuration, particularly the combination of robust knees, long heels and short lower limbs, increased the effective mechanical advantage of Neanderthal knee and ankle extensors, thus reducing the force needed and the energy spent for locomotion significantly. The walking cost of the Neanderthal male is now estimated to be 8–12% higher than that of anatomically modern humans, whereas the walking cost of the Neanderthal female is considered to be virtually equal to that of anatomically modern females.[21]

Other researchers, like Yoel Rak, from Tel-Aviv University in Israel, have noted that the fossil records show that Neanderthal pelvises in comparison to modern human pelvises would have made it much harder for Neanderthals to absorb shock and to bounce off from one step to the next, giving modern humans another advantage over Neanderthals in running and walking ability.[22]

Modern humans' advantage in hunting warm climate animals

Pat Shipman, from Pennsylvania State University in the United States, argues that the domestication of the dog gave modern humans an advantage when hunting.[23] The oldest remains of domesticated dogs were found in Belgium (31,700 BP) and in Siberia (33,000 BP).[24][25] A survey of early sites of modern humans and Neanderthals with faunal remains across Spain, Portugal and France provided an overview of what modern humans and Neanderthals ate.[26] Rabbit became more frequent, while large mammals - mainly eaten by the Neanderthals - became increasingly rare. In 2013, DNA testing on the "Altai dog", a paleolithic dog's remains from the Razboinichya Cave (Altai Mountains), has linked this 33,000-year-old dog with the present lineage of Canis lupus familiaris.[27]

Interbreeding

Further information: Neanderthal Genome Project

An alternative to extinction is that Neanderthals were absorbed into the Cro-Magnon population by interbreeding. This would be counter to strict versions of the Recent African Origin, since it would imply that at least part of the genome of Europeans would descend from Neanderthals, who left Africa at least 350,000 years ago.

The most vocal proponent of the hybridization hypothesis is Erik Trinkaus of Washington University.[28][29] Trinkaus claims various fossils as hybrid individuals, including the "child of Lagar Velho", a skeleton found at Lagar Velho in Portugal.[30] In a 2006 publication co-authored by Trinkaus, the fossils found in 1952 in the cave of Pestera Muierii, Romania, are likewise claimed as hybrids.[31]

Genetic studies indicate some form of hybridization between archaic humans and modern humans had taken place after modern humans emerged from Africa. An estimated 1 to 4 percent of the DNA in Europeans and Asians (e.g. French, Chinese and Papua probands) is non-modern, and shared with ancient Neanderthal DNA rather than with sub-Saharan Africans (e.g. Yoruba and San probands).[32]

Modern-human findings in Abrigo do Lagar Velho, Portugal, allegedly featuring Neanderthal admixtures, have been published.[33] However, the interpretation of the Portuguese specimen is disputed.[34]

Jordan, in his work Neanderthal, points out that without some interbreeding, certain features on some "modern" skulls of Eastern European Cro-Magnon heritage are hard to explain. In another study, researchers have recently found in Peştera Muierilor, Romania, remains of European humans from 30 thousand years ago who possessed mostly diagnostic "modern" anatomical features, but also had distinct Neanderthal features not present in ancestral modern humans in Africa, including a large bulge at the back of the skull, a more prominent projection around the elbow joint, and a narrow socket at the shoulder joint. Analysis of one skeleton's shoulder showed these humans, like Neanderthals, did not have the full capability for throwing spears.[35]

An analysis of a first draft of the Neanderthal genome by Richard E. Green et al. from Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany released in May 2010 indicates interbreeding may have occurred.[36][37] "Those of us who live outside Africa carry a little Neanderthal DNA in us," said Svante Pääbo, who led the study. "The proportion of Neanderthal-inherited genetic material is about 1 to 4 percent. It is a small but very real proportion of ancestry in non-Africans today," says Dr. David Reich of Harvard Medical School in Boston, who worked on the study. This research compared the genome of the Neanderthals to five modern humans from China, France, sub-Saharan Africa, and Papua New Guinea. The finding is that about 1 to 4 percent of the genes of the non-Africans came from Neanderthals, compared to the baseline defined by the two Africans. This indicates a gene flow from Neanderthals to modern humans, i.e., interbreeding between the two populations. Since the three non-African genomes show a similar proportion of Neanderthal sequences, the interbreeding must have occurred early in the migration of modern humans out of Africa, perhaps in the Middle East. No evidence for gene flow in the direction from modern humans to Neanderthals was found. The latter result would not be unexpected if contact occurred between a small colonizing population of modern humans and a much larger resident population of Neanderthals. A very limited amount of interbreeding could explain the findings, if it occurred early enough in the colonization process.[36]

Although some populations of modern humans share some nuclear DNA with the extinct Neanderthals, the two species do not share any mitochondrial DNA,[38] which in primates is always maternally transmitted. This observation has prompted the hypothesis that whereas female modern humans interbreeding with male Neanderthals were able to generate fertile offspring, the progeny of female Neanderthals who mated with male modern humans were either rare, absent or sterile (in line with Haldane's rule).[39]

While interbreeding is viewed as the most parsimonious interpretation of the genetic discoveries, the authors point out they cannot conclusively rule out an alternative scenario, in which the source population of non-African modern humans was already more closely related to Neanderthals than other Africans were, due to ancient genetic divisions within Africa.[36]

Among the genes shown to differ between present-day humans and Neanderthals were RPTN, SPAG17, CAN15, TTF1 and PCD16.[36]

Climate change

Neanderthals went through a demographic crisis in Western Europe that seems to coincide with a period of extreme cold in Western Europe. "The fact that Neanderthals in Western Europe were nearly extinct, but then recovered long before they came into contact with modern humans came as a complete surprise to us," said Love Dalén, associate professor at the Swedish Museum of Natural History in Stockholm. If so, this would indicate that Neanderthals may have been very sensitive to climate change.[40]

Although it is believed that Neanderthals had clothing,[41] it has been proposed that failure to adapt their hunting methods caused their extinction when Europe changed into a sparsely vegetated steppe and semi-desert during the last Ice Age.[42] Requiring more energy to move and hunt than their early modern human counterparts, Neanderthals would have been more susceptible to less food being available as a result of climate change. A string of major volcanic eruptions 40,000 years BP (including Campi Flegrei), affecting the region between Italy and the Caucasus Mountains, may have contributed to the Neanderthals' decline by reducing their food supply.[43] Studies of sediment layers at Mezmaiskaya Cave suggest a severe reduction of plant pollen.[43] The damage to plant life would have led to a corresponding decline in plant-eating mammals hunted by the Neanderthals.[43][44][45]

See also

References

  1. 1 2 3 Kenneth Chang, "Neanderthals in Europe Died Out Thousands of Years Sooner Than Some Thought, Study Says", New York Times, 20 August 2014, accessed 23 August 2014
  2. ↑ Hortolà P, Martínez-Navarro B (2013). "The Quaternary megafaunal extinction and the fate of Neanderthals: An integrative working hypothesis". Quaternary International 295: 69–72. Bibcode:2013QuInt.295...69H. doi:10.1016/j.quaint.2012.02.037.
  3. ↑ Mellars P (February 2006). "A new radiocarbon revolution and the dispersal of modern humans in Eurasia". Nature 439 (7079): 931–935. Bibcode:2006Natur.439..931M. doi:10.1038/nature04521. PMID 16495989.
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  17. ↑ Diamond, J 1992, The Third Chimpanzee: The Evolution and Future of the Human Animal, Harper Collins, New York, p 45.
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  40. ↑ http://phys.org/news/2014-02-neanderthals-extinction-modern-humans-emerged.html
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  42. ↑ Climate Change Killed Neandertals, Study Says, National Geographic News
  43. 1 2 3 Than, Ker (September 22, 2010). "Volcanoes Killed Off Neanderthals, Study Suggests". National Geographic. Retrieved 23 September 2010.
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