The unlikely interbreeding between Homo neanderthalensis and Homo sapiens

 

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Schematic of possible Neanderthal and Human interbreeding.  Source: public domain.

1.  Introduction

The Neanderthals are extinct hominids who inhabited Europe and western Asia circa 300,000 to 30,000 years ago. During this time they co-existed with modern humans. The issue of whether Neanderthal populations, circa 35,000 BP, interbred with and/or were acculturated by anatomically modern humans, or became extinct in isolation, is still a matter of controversy and debate. The transition from the Middle to Upper Palaeolithic in Europe occurred some 40 to 30,000 years ago, and saw the extinction of the Neanderthals and appearance of anatomically modern humans (AMH).

Three basic hypotheses have been postulated to explain the fate of the Neanderthals: (1) Neanderthals were transformed in to modern Europeans, and thus direct ancestors of modern Europeans; (2) anatomically modern humans from the Near East completely displaced the Neanderthals who contributed nothing to the AMH gene pool; (3) partial admixture took place in the expansion of AMH from Asia and thus contributed some genes to AMH. (Trinkaus, 1993; Stringer, 1993). General support, based on molecular genetic variation analysis, is usually given to the second view – the Neanderthals constituting a separate species that went extinct without contributing genes to modern humans. (Cann, 1987; Vigilant, 1991; Hammer, 1995).

2.  The Matter of Interbreeding

Mitochondrial DNA, or mtDNA, is maternally derived non-nuclear DNA and underpins the concepts of an African ‘Eve’, with mtDNA the mitochondrial ‘Eve’. Gene flow studies are set by observations of human mtDNA. Analysis of morphological differences between skeletal remains classify Neanderthals and modern humans as separate species (Zollikofer, 1995; Hublin, 1996). Again, skeletal evidence associated with Aurignacian technology shows it was produced by AMH (Stringer, 1990), and the archaeological evidence “…seems to indicate that Chatelperronian Neanderthals and early Aurignacian modern humans were biologically different groups with important similarities in the cultural development.” (d’Errico, 1998).

However, the mtDNA results “…do not rule out the possibility that Neanderthals contributed other genes to modern humans.” (Krings, 1997). The first AMH in western Europe were the Cro-Magnon people, who were typically modern, even though they were “…continuously exposed  to potential admixture with Neanderthal. Cro-Magnon seems to emerge essentially unmixed.” (Cavalli-Sforza, 1997). It is known that archaic Neanderthals lived alongside for 30,000 years at Qafzeh in the Near East. AMH who had been there 92,000 years BP (Fagan, 1996), and further thermo-luminescence dates gave 36,000 years ago for Neanderthal artefacts in France. Despite lack of signs of admixture in most western European AMH it has been argued that, before they disappeared, Neanderthals may have established relations with AMH and hence “…may have contributed to the gene pool of modern Europeans.” (Cavalli-Sforza, 1997).

The ‘Out of Africa’ scenario, considering the Neanderthal mtDNA sequence shows that modern humans arose in Africa as a distinct species and “…replaced Neanderthals with little or no interbreeding.” (Krings, 1997). An alternative view postulates that expanding AMH populations or groups possibly admixed with earlier types but social and cultural barriers to inter-fertility may have been more important than biological factors (Cavalli-Sforza, 1997). The debate continues concerning the nature and timing of any interactions “…between ancient modern humans and Neanderthals with…emphasis on the degree to which Neanderthals contributed genetically to the earliest modern human populations in Europe.” (Weaver, 2005). Neanderthals and AMH lived in greater proximity in the Middle East where intermingling could have been a possibility – except the fact there is ”…no evidence Neanderthals and AMH lived at the same time: in fact in the Middle East, they are widely separated in time.” (Cavalli-Sforza, 1997). With regard to ancient DNA, the late survival of Neanderthals, and modern human-Neanderthal genetic admixture, most anthropological geneticists, human palaeontologists, and palaeological archaeologists “…now favour a predominantly extra-European origin for the earliest modern human populations in Europe.” (Weaver, 2005; see also Stringer, 2003; Trinkaus, 2003; Underhill, 2001). The analysis of diversity and global patterns points to a recent African origin of modern humans (Armour, 1996;

Bone analysis indicates that Neanderthal DNA is separated from modern humans by a great distance in evolutionary terms, with mtDNA separation some 500,000 years ago. Therefore the analysis of mtDNA from multiple fossil Neanderthal remains confirms ”…abundant skeletal evidence that Neanderthals were biologically distinct…from Holocene (recent) humans and at least some more ancient modern humans (Weaver, 2005; see also Knight, 2003; Serre, 2004). It seems that Neanderthal mtDNA is actually distinctive which means they differed substantially from AMH. (Krings, 2000; Krings, 1997; Ovchinnikov, 2000). This points to no interbreeding, possibly due to incompatibility, and the reclassification of Homo neanderthalensis sapiens to Homo neanderthalensis. Placing Neanderthal mtDNA variation outside modern human ranges is used to study outcrops in phylogenetic analyses in order to “…assess the geographic origin of the human mtDNA ancestor…however new methods of phylogenetic analysis have continued to support an African origin of human DNA variation.” (Penny, 1995).

3.  The Matter of Displacement and Isolation

The minimum date for the divergence of Neanderthals and modern humans has been given at 250,000 to 300,000 years ago. The mtDNA divergence date suggests 550,000 to 600,000 years ago, whereas the archaeological record indicates some 300,000 years ago. Therefore, if the evolutionary trajectories of Neanderthals and AMH separated over 500,000 years ago, then “…the possibility of such genetically based divergences in brain structure, neurology, and cognitive capacities can in no way be ruled out.” (Mellars, 1998). Some 28,000 years ago modern humans had replaced Neanderthals in Europe after their expansion into the east and west (Stringer, 1996).

neanderthals

Questions concern possible replacements, admixtures, and complex evolutionary phenomena, involving interaction between Neanderthals and expanding AMH (Cavalli-Sforza, 1997). Before 45,000 BP Neanderthals had the landscape alone, by 35,000 they disappeared from France, with the last Iberian outpost lasting until 27,000 BP. Some argued that localised Neanderthals evolved into AMH (Wolpoff, 1989), others that replacement occurred by migrating AMH because, after 15,000 years of interaction Neanderthalers “…were driven to extinction by the technologically and presumably socially more advanced Homo sapiens sapiens.” (Conard, 1998). Though the two groups shared a large territory, low demographic intensity, and discontinuous settlements, few biological interactions occurred with a few behavioural imitations (Hublin, 1998). If the Chatelperronian is unique then independent parallel development – ending 35,000 BP – occurred which excludes acculturation implying cultural divergence (Svoboda, 1998), indicating Neanderthals “…were the victims of us modern humans, who indisputably presided over their demise even if only as observers.” (Vega-Toscano, 1998).

migration

Interactive actuality was not violent, there being no archaeological record of fighting (Cavalli-Sforza, 1997), the incoming AMH seeking different landscapes with different hunting strategies, different social structures to Neanderthals (Gamble, 1999). The replacement hypothesis answers questions not answered by local evolution theory (Lewis-Williams, 2002), because widespread rapid migration is faster than evolutionary change by scattered, isolated, and different Mousterian populations. Replacement was not inexorable but a series of stepwise, discrete advances, and as the Neanderthal populations contracted and retreated, coexistence was not universal (Bocquet-Appel, 2000), with isolated pockets of Neanderthals living on until extinction.

The fate of the Neanderthals during the transition has to be considered in terms of speed of change, geographical extent, and type of change – cultural versus biological. There is a need to define incoming modern behaviour, social, cognitive, symbolic and ritual in comparison to that of Neanderthals. There are also contrasting hypotheses concerning the origins of AMH themselves. No hybridisation occurred but more evidence is required to resolve the issue of acculturation and displacement.

4.  The Matter of Molecules and Genetics

Neanderthal mtDNA was sequenced at the Max Planck Institute (Hundt, 1994), where differences between European humans for the derivative haplogroup H (CRS) were studied, and which revealed the Neanderthal genome branched from humans more than 300,000 years before Haplogroup H reached Europe. With regard to genetic differences the first draft of 63% of the Neanderthal genome comprised 3.7 billion base pairs (Saey, 2009). The entire Neanderthal genome was elucidated in 2010. It transpired that some of the Neanderthal genome has more in common with chimpanzees than humans.

Following indications that Neanderthals were ‘distinct’ from us it was broadcast that scientists “…studying the DNA of Neanderthals say they can find no evidence that this ancient species ever interbred with modern humans.” (Morgan, 2011). However, humans and Neanderthals do share the FOXP2 gene variant associated with brain development and speech. In addition the study results some inbreeding between humans and Neanderthals because the genomes of non-African humans have from 1 to 4% of Neanderthal DNA.

The DNA of the Neanderthals was gleaned from fossil specimens from Croatia. Neanderthals in Europe and parts of Asia became extinct around 30,000 years ago. Modern humans left Africa circa 100,000 years ago. Neanderthals lived side by side with AMH for many thousands of years, but without evidence of interbreeding. One gene focus studied was that of microcephalin-1. The Croatian Neanderthals did harbour an ancestral form of the microcephalin-1 gene. This implies that the Neanderthals at most, contributed a very limited fraction of the total variation found in contemporary human populations. Despite a potential interaction over a long period of time, it has to be recognised that the populations concerned “…had been separate for hundreds of thousands of years and I think there would have been significant physical and behavioural differences between them.” (Stringer, 2011).

Evidence from Neanderthal mtDNA shows that they survived late in Europe and their “…per generation contribution to early modern human populations must have been fairly small (less than 0.2%) or we would find Neanderthal mtDNA lineages in living humans.” (Weaver, 2005). Sequence comparisons of human mtDNA sequences “…show that the Neanderthal sequence falls outside the variation of modern humans.” (Krings, 1997). Modern archaeology supports the theory of rapid population growth (Klein, 2004), as does living human DNA (Ingman, 2000). Neanderthal mtDNA has demonstrated a distinctiveness when compared to modern human which means “…either modern human and Neanderthal populations diverged deep in the past or human mtDNA diversity was much greater and more subdivided in the past than in the present. (Weaver, 2005). This brings us back to the evidence that the mtDNA sequence of Neanderthals “…supports the scenario in which modern humans were recently in Africa as a distinct species and replaced Neanderthals with little or no interbreeding.” (Krings,1997). The likely explanation is that the Neanderthals became extinct without donating any of their DNA to modern human populations. Finally, recent studies have clearly shown that the age of the common ancestor of Neanderthal and modern human DNA “…is estimated to be four times greater than that of the common ancestor of human DNA.” (Kings, 1997), and therefore a single “…randomly mating population of modern humans and Neanderthals is not consistent with the mtDNA evidence.” (Weaver, 2005).

5.  The Matter of Recent Analysis 

The controversy concerning possible interbreeding between Homo neanderthalsensis and Anatomically Modern Humans (AMH) which unfortunately has contained inaccurate journalistic interpretation and emotive reportage of learned articles. An example recently has been the view that it has “…been around 30,000 years since the ancestors of modern-day humans are thought to have wiped out the ancient Neanderthals.” (Pleasance, 2014). The implication is obvious – the Neanderthals were violently eliminated by incoming Cro-Magnon in Europe. There is not a shred of evidence to support this assertion. With regard to inbreeding between Neanderthals and Modern Humans many of the myths “…that plague archaeologists come from outside the profession, the product of overtly imaginative minds untrained in the scientific study of the past and unfamiliar with the archaeological evidence.” (Chase, 1987).

New DNA evidence indicates that anatomically archaic Neanderthals did not contribute greatly to the gene pool of modern populations, and therefore did not previously interbreed (Stringer, 1993; Krings, 1997; Krings, 1999). The age of the common ancestor, derived from mtDNA analysis, of Neanderthals has been estimated as four times greater than the common ancestor of human mtDNA, which suggest “…Neanderthals went extinct without contributing mtDNA to modern humans.” (Krings, 1997). Human Y chromosome studies support the view that Neanderthals made none or little contribution to the modern human gene pool (Hammer, 1995; Trishkoff, 1996). Unfortunately assertions that there are differences between non-African and African populations due to European interbreeding with Neanderthal populations can be construed as implicitly racist. In the late 1990’s research on remains recognised that the DNA of Neanderthals showed “…at least 500,000 years of evolutionary divergence between our own species and the c. 40,000 year old Neanderthal in question, diminishing the likelihood that the two species intermixed.” (Pettitt, 1999).

The expansion of pre-modern humans into east and west Europe occurred, as we know, some 40,000 years ago and this eventually “…led to the replacement of the Neanderthals by modern humans – 28,000 years ago (Golovanova, 1999). At this time a second radio-carbon analysis of Neanderthal remains them to be 29,000 years old and one of the latest specimens found (Smith, 1999), but still distinct from modern humans (Ovchinnikov, 2000). The specimen from the northern Caucasus (Ovchinnikov, 1999) was separated from the German Feldhofer cave in he Neander Valley by some 2500 kilometres (Krings, 1997; Krings, 1999). The Neanderthal skeleton from the Caucasus showed a 3.48% divergence from the Feldhofer specimen. Despite the distance both sets of remains displayed closely related mtDNA, which was distinct in phylogenetic terms from modern humans,  which provided “…further support for the hypothesis of a very low gene flow between the Neanderthals and modern humans…the data reduce the likelihood that Neanderthals contained enough mtDNA  sequence diversity to encompass modern human diversity.” (Ovchinnikov, 2000). With regard to western and eastern Neanderthals, the estimated age of the  mtDNA of the most common recent ancestor (MCRA) is between 151,000 and 352,000 years (Gamble, 1998). In terms of the palaeontological record the divergence “…of modern human and Neanderthal DNA was estimated to be between 365,000 and 853,000 years.” (Ovchinikov, 2000).

Humans replaced Neanderthals in Europe between 42,000 and 35,000 years ago but however “…no Neanderthal mitochondrial DNA (mtDNA) lineage is found to date among several thousands of Europeans…interbreeding rates as high as 25% could not be excluded between the two subspecies.” (Currat, 2004). This simulation model with its interbreeding rate of 0.1% means an admixture over some 12,000 years thus “…results complement recent genetic and morphological evidence indicating that early human and Neanderthal interbreeding was unlikely.” (PLOS, 2004). Later analysis and more current thinking “…suggests that the modern human and Neanderthal lineages diverged before the emergence of contemporary humans.” (Noonan, 2010). The evidence from fossil mtDNA strongly indicates that interbreeding did not occur between humans and Neanderthals (PLOS, 2004). The low rate of 0.1%  “…strongly suggests an almost complete sterility between Neanderthal females and modern human males, implying that the two populations were probably distinct biological species.” (Currat, 2004). A report on another study (Rincon, 2010) claiming that Neanderthal genes ‘survive in us’ claimed between 1% and 4% of Eurasian genomic structure was Neanderthal in origin. The genomes of populations of non-Africans, such as those from Europe, China, and New Guinea, were closer to Neanderthal sequences than those of African origin (Rincon, 2010), suggesting limited gene flow took place between Neanderthals and the ancestors of modern Eurasians.

The debate continued with the proposition that Europeans never actually had Neanderthals as neighbours (Pinhasi, 2011), and carbon 14 dated Neanderthal remains from the Caucasus “…suggest that the archaic species had died out before modern humans arrived.” (Callaway, 2011) the remains 100,000 years older than expected. In 2010 the genome of the Neanderthals was decoded (Green, 2010) with the implication that humans Neanderthals never met in Europe and instead “…humans departing Africa encountered resident Neanderthals in the Middle East.” (Callaway, 2011). It was claimed earlier that there was a Neanderthal settlement in Gibraltar some 24,000 years ago (Finlayson, 2006). Another source states that the last Neanderthals of southern Spain did not co-exist with modern humans (Heritage Daily, 2014), the academic opinion being that it was “…improbable that the last Neanderthals of central and soutjen Iberia would have persisted until such a late date, approximately 30,000 years ago.” (Jorda, 2010). The opinion thus was expressed that the results of DNA research therefore show “…that there was admixture at some stage in our human ancestry, but it more than likely happened quite a long time before humans arrived in Europe.” (Pinhasi, 2010).

Finally it was reported that a fifth of Neanderthal genetic code survived in modern humans but furthermore the “…last of the Neanderthals may have died out thousands of years ago, but large stretches of their genetic code live on in people today.” (Sample, 2014). This journalistic exercise was based on research concerning the resurrection of surviving Neanderthal lineages from modern human genomes. Essentially the claim was that AMH overlapped and interbred with Neanderthals to the extent “…that non-African humans inherit 1 to 3% of their genomes from Neanderthal ancestors.” (Vernot, 2014). The journalistic and somewhat exaggerated interpretation continued by stating that the traces of Neanderthals in our make up “…are the lasting legacy of sexual encounters between our direct ancestors and the Neanderthals they met when they walked out of Africa and into Eurasia about 60,000 years ago.” (Sample, 2014). Is this the whole issue. Are there other non-genetic factors that need to be considered? In other words attention has to be paid to non-biological factors that may preclude the possibility of interbreeding even if Neanderthals and modern humans had met  or co-existed. More research is not only needed into acculturation and the archaeological background but also into social and cultural factors. Interbreeding has to be considered in the light of clan exogamy, systems of taboo, ritual practices and totemic relationships between separate groups – especially if they are indeed separate sub-species. To resort to descriptions of ‘sexual encounters’ or ‘wiping out’ Neanderthals only clouds the issue. Anyway – humans share 98.8% of their DNA with the chimpanzee but there is no claim the two species ever interbred. The situation elsewhere is a commonality of 50% with banana DNA. Have we ever interbred with bananas?.

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