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The Magic Stick of Human Evolution: RUNX2?

May 25, 2010 Leave a comment

Evolution is a slow process, not a magic stick that turns one species into another one. Could our species be the exception? This is worth a thought, now new genetic results reveal each and everyone of us descends from Neanderthal as well as other hominins.There is nothing gradual about the change of Neandertal into “us” and moreover, there is a growing awareness that other hominins besides early Anatomically Modern Humans (AMH) contributed their genetic share to the current human genepool. Maybe this traditional concept of evolution, that evolved from Darwins observations of the animal world, explains why Green et al. in their 2010 article “A Draft Sequence of the Neandertal Genome” rather picture the genome of modern humans conform the traditional view, ie. as the end point of a single human lineage “out of Africa”, that now according to new results amalgamated along the way at random with some stray genes from other hominins. However, what those allegedly exogenetic contributions to the modern genepool have in common are their sheer incompatibility with this hypothetised single human lineage, and the emergence of rapid genetic changes.

Rapid genetic change affected the global population once the genes were available. Next to evolution came the Magic Stick.

Green et al. investigated the differences between Africans and non-Africans in relation with the (draft) Neanderthal genome and revealed 1-4% of the modern human genepool could be accounted for by Neanderthal only. Green et al., 2010:

“Under the assumption that there was no gene flow from Neandertals to the ancestors of modern Africans, the proportion of Neandertal ancestry of non-Africans, f, can be estimated by…”
“Assuming that gene flow from Neandertals occurred between 50,000 and 80,000 years ago, this method estimates f to be between 1 and 4%
“We note that a previous study found a pattern of genetic variation in present-day humans that was hypothesized to be due to gene flow from Neandertals or other archaic hominins into modern humans (81). The authors of this study estimated the fraction of non-African genomes affected by “archaic” gene flow to be 14%, almost an order of magnitude greater than our estimates, suggesting that their observations may not be entirely explained by gene flow from Neandertals.”
“We expect that further analyses of the Neandertal genome as well as the genomes of other archaic hominins will generate additional hypotheses and provide further insights into the origins and early history of present-day humans.”

But let us define “gene flow”: the study explicitly excluded shared ancestral genes and gene flow from Neanderthals to the ancestors of modern Africans. This 1-4% only reflects the measurable genetic differences between African and non-African haplotypes explained by Neanderthal, and thus is just a minimum amount. What we miss is an assessment of the maximum amount, that would instead reveal the percentage of early African genes that exclude any Neanderthal input in the genome of non-African modern humans. This could prove to be a much more cumbersome excercise, not in the least because we don’t know yet how to tell apart all truly African from other non-Neanderthal hominin genes.

Only the genetic deviations in modern humans with respect to the Neanderthal genome were investigated. None were found or mentioned in genes that nowadays harbour truly cross-hominin haplotypes. Previously the “European” H2 haplotype of the gene MAPT and the “Eurasian” haplotype D of the Microcephalin-1 gene were recognized as definitely different from the haplotypes associated to Homo Sapiens, and consequently dubbed (prematurely) “Neanderthal” by some. The sheer differences between the various haplotypes of such genes already suggested cross-breeding of hominins that share a common origin with modern humans that coalescence to dates that vastly exceed the AMH-Neanderthal split under investigation. Indeed, neither haplotype was found on Neanderthal and by default the Neanderthal version of MAPT and Microcephalin-1 must be considered compatible with the haplotypes commonly associated with modern humans. Green et al., 2010:

This analysis shows that some old haplotypes most likely owe their presence in present-day non-Africans to gene flow from Neandertals. However, not all old haplotypes in non-Africans may have such an origin. For example, it has been suggested that the H2 haplotype on chromosome 17 and the D haplotype of the microcephalin gene were contributed by Neandertals to present-day non-Africans (12,79, 80). This is not supported by the current data because the Neandertals analyzed do not carry these haplotypes.

Likewise, the study doesn’t resolve the precise origin of the other newly identified non-Neanderthal genes. Was the AMH gene pool already amalgamated with a wide variety of exogenetic hominin contributions at the moment African AMH and Neanderthal met? Such contacts should have increased the genetic differences between early African AMH and Neanderthal about the time of ingression. On the other hand, as far they could verify with modern genes, Green et al. were pretty determined that their Neanderthal samples did not feature ingression:

Thus, all or almost all of the gene flow detected was from Neandertals into modern humans.

This emerging picture of the AMH genome being like a sponge to exogenetic influences against isolated Neanderthal communities that preserved their genetic purity until extinction is amazing. Admixtures being confined to quite recent developments confirm the effects of culture in human evolution, that thus must have included increased contact and communicaton between hominin groups. To a certain degree admixtures may have involved even African AMH’s own African neighbours. Green et al.:

[…]old population substructure in Africa has been suggested based on genetic as well as paleontological data.

However, in the article this substructure suggested by Green et al. only concerned a possible African’s AMH or sub-saharan special relationship to Neanderthal up north:

If after the divergence of Neandertals there was incomplete genetic homogenization between what were to become the ancestors of non-Africans and Africans, present-day non-Africans would be more closely related to Neandertals than are Africans.

For sure, recent cross-hominin gene flow of the H2/MAPT or D/Microcephalin-1 type would require an African substructure much deeper than the one implied by this study. Older cross-hominin events may include “African” V and M haplotypes of gene ASAH1, where a coalescent-time depth of 2.4 million years ago tend to marginalize the differences with Neanderthal. The Green paper did not give a clue about the Neanderthal haplotype and thus does not resolve the question of potential post-split date hybridization in Africa. Based on dates and global currency, haplotype M could have been of heidelbergensis and haplotype V a successful African cross-hominin addition. Kim et al. 2007:

it should be noted that the pattern of genetic diversity of ASAH1 and other loci is compatible with the proposal that the human population was once geographically structured and genetically differentiated in Africa

The predominance of the V lineage is observed in both Africans and non-Africans
[…] Furthermore,the number of haplotypes in the V lineage is only 6, yet it is 11 in the M lineage. These hold true in both African and non-African samples

The most parsimonious explanation for the sharing of this pattern across all four [worldwide reference] populations is that the sweep occurred prior to the radiation of modern humans out of Africa.

the TMRCA of the V lineage is estimated as 200 +/-50KY from Genetree analysis […] and 340 +/- 80 KY on the basis of the average nucleotide diversity […] On the other hand, the TMRCA of the M lineage is 320 +/- 70KY from the Genetree analysis and 680 +/- 180 KY from the nucleotide diversity. Compared with the M lineage, the relatively recent origin of the predominant V lineage implies that it has been rapidly increasing in frequency.

Haplotype M/ASAH1 would not be surprising for Neanderthal, but haplotype V/ASAH1 would raise questions about the extend of early human hybridization events, and where it all started. Such and other potentially recent African cross-hominin evidence should put the impact of the Neanderthal lineage on modern humans in a completely different perspective. Unfortunately the Green paper didn’t intend to resolve the interbreeding questions already raised in previous publications on the subject. Moreover, single genes are hardly significant to the degree of admixture and the significance of African non-AHM contributions are still a matter of debate. Wall et al. (2009) insist on at least a considerable West African non-AMH hominin component:

We find evidence for this ancient admixture in European, East Asian, and West African samples, suggesting that admixture between diverged hominin groups may be a general feature of recent human evolution.

Instead, the announced results of a New Mexico investigation “didn’t find evidence of interbreeding in the genomes of the modern African people included in the study” (Nature News,20 April 2010). Conventional wisdow still has it that hominin admixture was a recent AMH feature, primarily linked to Out of Africa expansions and post-dating the AMH-Neanderthal split, but at least an African origin of haplotypes H2/MAPT and D/Microcephalin-1 would apply for special pleading. A specifically eastern Asian RRM2P4 haplotype even computes a coalescent-time of 2.3 million years ago and a quite short LD sequence virtually rules out a recent adquisition of this gene – that moreover must have been confined to the people of eastern Asia!

Let us return to the 1-4% measurable admixture. Like John Hawks already put it, this estimate “is so high that this is not just a few genes introgressing in from Neandertals — it is a big fraction of the neutral, non-coding part of the genome.” This portion was deduced from the signal of gene flow retrieved from the segments with the lowest divergence to Neanderthal: [Non-African] “segments, with few differences from the Neandertals, tend to have many differences from other present-day humans, whereas African segments do not“. However, I don’t agree it is just this “visible” part of the Neanderthal genome that survived. This must be just the low-mutation-rate part of Neanderthal that pops up from the graphs, where strongly homogenizing selective forces in Africa about 200 kya must have reduced variability among AMH. Subsequent strong selective forces during AMH expansion kept variablity lower at non-African high mutation rate segments.

Only non-African high variability segments with an unusually low divergence to Neanderthal (box) tend to have an unusually high divergence to other modern humans (here Craig Venter) because the segments are not shared by all, being specifically derived from Neanderthal. One additional reason may be some of these segments are typified by low mutation rates and bottlenecked in Africa, where the registered variability of these segments is low.

The premise of this certainty of dealing with Neandertal admixtures is that other causes of a low divergence to Neanderthal, such as low mutation rates, “would produce monotonic behaviors” on the above diagram. Here it shows that African and non-African segments that most closely resemble the Neanderthal genome may diverge considerably from the reference genome of mister Craig Venter, thus confirming high variability, and 94% of these diverging, high variability genes are of confirmed European ancestry. Moreover, the results of “identified regions in which there is considerably more diversity outside Africa” (Green et al.) show 80% tag SNPs (133 out of 166) and 83% of the regions (10 out of 12) pointing to a specific Neanderthal origin. Note the potential contribution of other hominins to this unexpected variability seems to be rather low, and we can only assume the remaining non-Neanderthal tag SNPs and regions could be African – if this weren’t so contradictory to Out of African type bottleneck scenarios. All these results seem to be in sheer contrast to concurrent genetic and phenotype results that rather indicated a “smooth loss of genetic diversity with increasing distance from Africa” (Manica et al., 2007). The contradictory clines involved inhibit a simple extrapolation of the diagram to the Neanderthal survival of the remaining 96% of their genome, ie. somewhere hidden on the part where diagram behavior is “monotonic” (ie. a straight line). Basically, in line with the comments made by Relethford, 2008, the current data doesn’t exclude this possibility at all:

[…] the observation of higher African diversity supports the other genetic (and fossil) evidence for an African origin for modern humans, but does not distinguish between an African origin with replacement and an African origin with admixture outside of Africa except to say that if there was any admixture it was not of sufficient magnitude to erase the genetic signature of an African origin.

High non-African variability on certain regions is remarkable, but so is the implied low African variability at these same locations. Let us reverse the assessment and consider the potential backmigration or diffusion from a hybrid source in Eurasia into Africa, conform the one already implied by the purported cross-hominin D haplotype of the microcephalin-1 gene found in only 30% of subs-saharan Africans. The Green paper is not conclusive about the origin of African counterparts of the mentioned non-African high variability genetic regions, but for sure the ingression back into Africa of a reduced set of “bottlenecked” non-African haplotypes would have a similar result on the diagram if regions of higher Neanderthal divergence were caused by higher mutation rates of these regions, and thus bottlenecked were to remain within the bandwith of African variability. However, the low world-wide frequency of this off-modal variability would also imply the reduction of high mutation speed variability among non-African populations, possibly due to some kind of post-Neanderthal event. This scenario would be in agreement with a more recent ingression of new genes and the heavy selective pressures implied during this process.

It should be investigated if much of the higher African variability indeed involve segments having a higher mutation rate, since strong natural selection at an early stage, contrary to the Neanderthal situation, could have obliterated much of the deeper African substructure. If this would be the case, there wouldn’t be any argument left against extrapolating the survival of virtually the whole genome of Neanderthal among non-Africans – i.e. except for the few genes and sweep areas associated to AMH related evolutionary changes elsewhere. This result would be irrespective of the African substructure proposed by Green et al. or sheer Neanderthal relatedness of early AMH, and in agreement with the announced New Mexican results, that Africa remained largely exempt of reminiscent cross-hominin interbreeding variability. African hybridization, if any, thus would have been concurrent with strong selective processes.

Long time, bidirectional genetic diffusion should be taken in consideration, and we still don’t know to what extend the advance of AMH into Neanderthal territory also involved hybridization with other hominins. However, having this assumption of 1-4% of the modern human genome just being the detectable part of the surviving Neanderthal genome in mind, it will be much easier to estimate Neanderthal survival. Green et al. (2010) estimate the frequency of candidate Neanderthal regions among non-Africans averaging “13%, and all less than 30%“. Now even the Mesolithic survival of Europeans as a result of the Neolithic wave of advance is still a matter of scientific dispute, this possible 13% Paleolithic result of potential Neanderthal survival may shed more light on this issue.

The real Neanderthal differences were found in regions where “the Neanderthal carries fewer derived alleles than expected relative to the human allelic states. A unique feature of this method is that it has more power to detect older selective sweeps”, ie. neighbouring DNA of a mutation in a region that defines a haplotype where reduced variation is an indication of few chromosome cross-overs, and thus of recent and strong positive natural selection. Such haplotypes are often referred to as a region of LD (linkage desequilibrium) for being characterized by DNA patterns that are overrepresented in the population, thus being not as random as it should be after a certain count of cross-overs. The Green et al. investigation yielded a limited set of such AMH related genetic regions that are low or depleted of Neanderthal derived alleles, and that thus could prove useful in the search for the genome or genomes of African hominins.

We identified a total of 212 regions containing putative selective sweeps […] We ranked the 212 regions with respect to their genetic width […] because the size of a region affected by a selective sweep will be larger the fewer generations it took for the sweep to reach fixation […] Thus, the more intense the selection that drove a putative sweep, the larger the affected region is expected to be. […] The widest region is located on chromosome 2 and contains the gene THADA,where a region of 336 kb is depleted of derived alleles in Neandertals. […] Changes in THADA may thus have affected aspects of energy metabolism in early modern humans. (Green et al., 2010)

Mutations in several genes on the 20 widest regions have been associated with diseases affecting cognitive capacities. One single AMH gene at such a location (RUNX2) was suggested to be related to the most striking morphological changes associated with the Neanderthal “extinction”.

The Green paper is not very clear about the reasons why these sweep areas are lower in derived Neanderthal DNA compared to other regions, except for saying that the method to identify regions where the Neandertal carries fewer derived alleles than expected relative to the human allelic states […] has more power to detect older selective sweeps. This by itself has the potency to even put the truly ‘Sapiens’ origin of this cherrypicked sweep areas in doubt, if the difference with Neanderthal would turn out to be too much. Note it is useless to play down the Neanderthal component based on unrelated selective sweep areas, while even larger sweep areas from a different hominin origin exist that are not even as closely related to AMH as Neanderthal. E.g. Hardy et al. (2005) about the (non-Neanderthal) H2/MAPT haplotype:

Interestingly, a recent assessment of LD (linkage disequilibrium) across the genome in different populations suggested that the MAPT locus was the longest region of LD in Europeans.

A close relationship between AMH and Neanderthal only solves part of the problem of knowing what DNA haplotypes originated where. Indeed, it complicates matters considerably knowing that especially the larger sweep areas of the AMH genome segments found exceptionally low in derived Neanderthal DNA, qualify most for being admixtures from hominins unrelated to AMH. Successful genes are most likely to reach saturation and to become fixed into the whole human population, and also are most likely to reduce overall variability. A deep African substructure as a source for new genes that are low in derived Neanderthal DNA would only make sense if evolution accelerated also among groups whose genes were less related to AMH’s proposed Neanderthal-like kin in the neighbourhood. However, the African origin of AMH is currently sought in the north of sub-saharan Africa, not south. Those AMH thought to have ultimately passed the selective sweep out of Africa thus should have received at least some genetic improvements from further south, what is remarkable considering the attested focus of AMH development around Ethiopia. If the alternative could be sought rather in local hybridization then one might wonder about the fate of southern African hominins. Where did the evidence of interbreeding go in the genomes of the modern African people? Indeed, the team of the University of New Mexico that conducted this analysis, due for publication in the near future, claims no such evidence exists. The gradual increase of variability towards the south is no argument in favour of interbreeding, nor acceptable as evidence of a southern African origin of AMH. It is evidence of continuity.

At the annual meeting of the American Association of Physical Anthropologists in Albuquerque, New Mexico, on 17 April, this team asserted that extinct species interbred with the ancestors of modern humans twice, allegedly first at about 60,000 years ago in the eastern Mediterranean. Nature News: “The researchers suggest that the population from the first interbreeding went on to migrate to Europe, Asia and North America. Then the second interbreeding with an archaic population in eastern Asia further altered the genetic makeup of people in Oceania.” The precise details are still unpublished, but especially the genetic interbreeding model of the second event “at about 45,000 years ago in eastern Asia” raises questions “about the range of species, like H. heidelbergensis”, since “Human skeletons found at Lake Mungo in New South Wales, Australia, have robust features, which may represent the result of interbreeding“.

Homo heidelbergensis is often considered the last direct ancestor of both Neanderthal and modern humans, though others rather consider Homo antecessor in this role. The remains of this hominins center in Spain and are dated between 1.2 million and 800,000 years ago. About 600 kya the first heidelbergenses in Europe already appear to be on the the evolutionary line to Neanderthal. Ever more robust forms of purported heidelbergensis derivation arrived on the scene, but not only in future Neanderthal territory. Robust phenotypes appeared almost everywhere, including southern Africa where apparently closely related forms (Rhodesia Man) became prominent between 600 – 250 kya. However, not even the mtDNA split date between Neanderthal and modern humans exactly indicate Homo antecessor as the most recent common ancestor (MRCA):

mean = 465,700 years ago; 321,200–618,000 years ago, 95% HPD (Krause et al., 2010)

The specimen of Rhodesian Man found at Broken Hill, Zambia, also referred to as Kabwe skull, put the mark of a general development towards maximum robustness before this African lineage gave away to a geographically rather scattered development towards gracile AMH. Unfortunately there is a dating problem here. Rhodesian Man shared many morphological features with rapidly modernizing precursors of AMH in northeast Africa that could have been contemporaneous. Like European Neanderthal, it doesn’t look like Rhodesian Man was much in a hurry to evolve into the “right” direction. Human modernization probably reached southern Africa from the north. It is striking that geographic isolation probably wasn’t a precondition to the development of African AMH, since an Ethiopian origin is rather somewhere in the middle between Neanderthal and Broken Hill. Indeed, rather quite close to the eastern Mediterranean, where according to the New Mexico team, in a Recent Out of Africa (ROA) scenario about 60,000 years ago, the first Neanderthal hybridization event should have taken place.
The current substructure and global distribution of mtDNA is unlikely to resolve the origin, for being too young. The mitochondrial “Eve”, the female MRCA that was the ancestral mother of all modern humans, was born only 200,000 years ago, making it impossible to even define an older African origin. In the vein of recent insights this discrepancy could be interpreted as a strong signal of natural selection involving mitochondria, even within Africa. In his blog article on the new paper of Green et al. John Hawks reproaches everyone that still don’t know, and that take their misconception as an argument in favour of extinction scenarios:

I’ve been saying it for years. I’ve published it. Will you learn to listen to me, already?
The mtDNA of Neandertals is gone because it conferred some disadvantage. There are many reasons to suspect this — the Neandertal variation is itself apparently recently derived; the human variation is clearly in disequilibrium, especially outside Africa; the mtDNA genes affect functions that differ greatly in Neandertal and recent populations, including energetics, longevity, and brain; there are clear signs of mtDNA selection in many recent human populations.

But where did the pre-sapiens mtDNA in Africa go in view of a heidelbergenis takeover, and did current mtDNA really originate in Africa? Neanderthal mtDNA isn’t so very different to take this for granted anymore. The overwhelming success of apparent heidelbergensis mtDNA and the absence of a significant signal of African hybridization may imply the death blow to older African lineages before heidelbergensis. Current African variability can’t even beat the overall post-heidelbergensis variability deduced from a draft of the Neanderthal genome. Extinction scenarios like those that traditionally accompany Out of Africa hypotheses may not be utterly useless after all.
Mitochondrial extinction was not just an issue of Neanderthal: the age of mtDNA recovered from LM6, the early modern human of Lake Mungo, Australia, even exceeds the age of modern mtDNA. Similar DNA at chromosome 11 of especially Eurasian modern humans indicate an inclusion event of a type of mtDNA that may have been common outside Africa once, and that may point to an eastern expansion of heidelbergenses dating back up to even 300,000 years ago. This mtDNA coalescent-time depth suggests that eastern Asia and Oceania may have been overrun by the heidelbergenses that were geographically closer to East Africa. In between, in the Middle East, the type of mtDNA might have changed with the fortunes of Neanderthal, or otherwise early Asiatic Neanderthal may have been the carriers of this type of mtDNA, in which case there should have been a reversed link to eastern Africa. The findings of Xinzhi Wu are in agreement to the latter:

There is a morphological mosaic between H. s. erectus and H. s. sapiens in China. The existence of common features and the morphological mosaic suggest continuity of human evolution in China. That there are a few features which are more commonly seen in the Neanderthal lineage, occurring in a few Chinese fossil skulls, probably suggests gene flow between China and the West. (Xinzhi WU, 2004)

Based on the evidence of continuity and gene flow, a new hypothesis, Continuity with Hybridization, was proposed in 1998 for characterizing human evolution in China. (Xinzhi WU, 2004)

At least across the eastern fringes of generally accepted Heidelbergensis/Neanderthal territory, hybridization was noticed in the fossile record, suggesting this process started indeed long before the purported exodus of AMH out of Africa.

“It is also suggested the Homo heidelbergensis is represented in Asia by the Dali skull and the Jinnishuan skeleton, both from China, and dated at between 200,000 and 300,000 years old. Precise dating of these fossils is important, because they might be contemporaneous with the last Homo erectus fossils in China” (de Arsuaga & Martinez, 1998)

According to the New Mexico announcement, hybridization in East Asia has been confirmed on the genetic level. A possible origin in a hypothetized African substructure in a Recent Out of Africa scenario is no issue here, though the investigators are careful to link a possible pre-Sapiens reminiscence in the East Asiatic (and Oceanic) genes to more or less related heidelbergensis rather than local hominins in the region that derive directly from much older forms. Nature News:

Theodore Schurr, a molecular anthropologist at the University of Pennsylvania in Philadelphia, said the genetic model showing interbreeding raises questions about the range of species, like H. heidelbergensis.

The exotic genetic results of the Denisova hominin, for that matter, proves that we can’t think too light about the close genetic distance of eastern Asian people to the rest of the world. The genetic differences of hominins that developed in isolation for such a long time should have left a mark impossible to miss. The few haplotypes of genes that indeed attest an extremely high age, just don’t add up to appreciable levels of admixture that, like now attested with Neanderthal, represent a certain percentage. Thus all along the bulk of exogenetic influences appears drowned amidst very much related heidelbergensis genes, whose differences to modern genes can be assumed to be more moderate. The coalescent-time depth of heidelbergensis is much less than a million years while the genus Homo might be three times that age.

The early arrival of heidelbergensis as a new hominin quickly replacing older hominins, also created an opportunity to differentiate into geographical phenotypes. Naturally, the degree of differentiation would increase closer to the place of the heidelbergensis origin. The proximity of Europe and West Africa to Iberia, home to the above mentioned ancestral Homo antecessor, would locate potential hotspots of heidelbergensis variability in those same places, thus making heidelbergensis hybridization feasible. Contact zones could be assumed at the fringe of heidelbergensis dispersal, but also close to more archaic forms that may now be assumed in Neanderthal Europe and West Africa. Prospected contact zones for hybridization can thus be hypothetized to include geographic regions where diverged branches of a wider heidelbergensis family (also Neanderthal, early sapiens etc) probably met:

  • Middle East, the most likely contact zone between African sapiens and Neanderthal
  • West Africa, probably not so far from the oldest heidelbergenses and deepest heidelbergensis substructure in case of an important role of nearby Homo antecessor as an inmediate precursor
  • China, the likely scene of prolonged contact that most probably involved much earlier local hominins as well

Without exception, these prospected zones of genetic interaction and the earliest hotspots of modern features in the fossile record turn out to be close together. Indeed, the earliest attestations of AMH also include Morocco at the western side of Africa:

… an early Homo sapiens juvenile from Morocco dated at 160,000 years before present displays an equivalent degree of tooth development to modern European children at the same age. (Smith et al., 2007)

These include China, where “existence of common features and the morphological mosaic suggest continuity of human evolution” (Xinzhi WU, 2004) from about 200 kya, when hominins like Dali start to appear.

Attestations, of course, also include NE Africa, at the perifery of the Middle East, where the first emergence of AMH (Omo I and II) appears to be especially typified by variety, rather than being evidence of a single, isolated lineage. McDougall et al., 2004:

Here we confirm that the Omo I and Omo II hominid fossils are from similar stratigraphic levels in Member I of the Kibish Formation, despite the view that Omo I is more modern in appearance than Omo II

Our preferred estimate of the age of the Kibish hominids is 195 5 kyr, making them the earliest well-dated anatomically modern humans yet described.

The first unequivocal modern human features emerge in Ethiopia. It is difficult to define the differences as extreme variability or progressive hybridization.

Hominin variety has been noticed and studied before, but rarely to this degree at a single site. The evolutionary trend towards the modern features of AMH is not unlikely to have been preluded by inter-hominin contact, cq. starting about 200kya, rather than that inter-hominin contact was the unequivocal result of quite recent AMH expansions “out of Africa”. Like Garrigan et al. already put it in 2005:

Alternatively, the AMH phenotype may be the by-product of such admixture events.

What new clue would this observation give us about the evolution of AMH?

Cosmopolitan behavior and universal physial acceptance might have been of prime importance to those early humans that were located at the inter-hominin contact zones, and whose survival depended on their ability to cope with human differences, both physical and behavioural. The underlying assumption is that communication and language can be properly understood by taking into account their relation with other important behavioural, social, and cognitive processes – and the corresponding genetic modifications to ascertain a selective advantage. About 200 kya the level of human development must have reached a critical point, when the most economic response to first contact with other groups had changed. The nature of selective forces changed as well, thus accelerating cognitive improvements that boosted the evolution of e.g. that select set of genes mentioned by Green et al., 2010. Indeed, this must have happened right in the middle of contact zones.
Note accelerated evolution in contact zones doesn’t strictly imply hybridization, since it basically involves an adaptive response to increased environmental stress due to the demands of frequent “cosmopolitic” contacts.
Physical acceptability might have been another factor. Wearing clothes could have been one strategy to conceal the differences and this custom seems to originate from about the same time. Science News, 8th of May 2010:

Using DNA to trace the evolutionary split between head and body lice, researchers conclude that body lice first came on the scene approximately 190,000 years ago. And that shift, the scientists propose, followed soon after people first began wearing clothing.

Cloths as a social invention would overcome the practical arguments against a correlation with decreasing body hair in hot climates.
The human response to the physical appearance of others might also have accelerated the development of typical “modern” features that accompanied the rise of AMH. A positive response that commonly derives from typical “child-like” features associated with AMH might have invoked another strategy involving physical change. The investigation of Green et al., 2010, teaches us that much of these modernizing changes are possibly regulated (suppressed?) by a single new gene, considered exempt from a Neanderthal origin:

One gene of interest may be RUNX2(CBFA1). It is the only gene in the genome known to cause cleidocranial dysplasia, which is characterized by delayed closure of cranial sutures, hypoplastic or aplastic clavicles, a bell-shaped rib cage, and dental abnormalities (70). Some of these features affect morphological traits for which modern humans differ from Neandertals as well as other earlier hominins. For example, the cranial malformations seen in cleidocranial dysplasia include frontal bossing, i.e., a protruding frontal bone. A more prominent frontal bone is a feature that differs between modern humans and Neandertals as well as other archaic hominins. The clavicle, which is affected in cleidocranial dysplasia, differs in morphology between modern humans and Neandertals (71) and is associated with a different architecture of the shoulder joint. Finally, a bell-shaped rib cage is typical of Neandertals and other archaic hominins. A reasonable hypothesis is thus that an evolutionary change in RUNX2 was of importance in the origin of modern humans and that this change affected aspects of the morphology of the upper body and cranium.

A natural selection-driven advance of a small set of modernizing genes would do the rest of the trick. Neanderthal did not evolve slowly to AMH, but neither did Neanderthal disappear. Neanderthal survived, because they tricked human evolution by swaying the magic stick. The Neanderthal phenotype disappeared as rapidly as it took for a small set of AMH genes to gain prevalence. Cultural changes were the precondition for the success of these new genes, including RUNX2. The physical change could have been a matter of a couple of generations.

Or does all of this mean that Neanderthal disappeared anyway, because Neanderthal hybridization already happened long before AMH entered Europe and the rest of the world? Very unlikely, since on their natural selection-driven way through European Neanderthal territory, the new AMH genes were brought by those that also carried the exclusive haplogroup H2/MAPT gene. The Neanderthal genes may have been the same east and west, so instead we have to focus on the way how AMH genes entered. Now understanding hybridization better as the trigger for AMH related change, we should recognize this cross-hominin genetic MAPT admixture as the genetic marker to be associated with the “modernization” of European Neanderthal. This is the ultimate indication that AMH related hybridization didn’t stop in the Middle East. The magic stick even touched some unknown, unrelated hominin that exclusively roamed European Neanderthal borderland on the eve of modern ingression, but none could override the Neanderthal genes already there.


Referenced:

  • Green et al. – A Draft Sequence of the Neandertal Genome, 2010, link
  • Xinzhi Wu – On the origin of modern humans in China, 2004, link
  • Xinzhi Wu – Fossil Humankind and Other Anthropoid Primates of China, 2004, link
  • Garrigan et al. – Evidence for Archaic Asian Ancestry on the Human X Chromosome, 2005, link
  • Burbano et al. – Targeted Investigation of the Neandertal Genome by Array-Based Sequence Capture, 2010, link
  • John Hawks Weblog – Neanderthals Live! 2010, link
  • John Hawks Weblog – Multiregional evolution lives! 2010, link
  • Hardy et al. – Evidence suggesting that Homo neanderthalensis contributed the H2 MAPT haplotype to Homo Sapiens, 2005, link
  • Neanderthals may have interbred with humans – Nature News 20 April 2010, link
  • Wall et al. – Detecting Ancient Admixture and Estimating Demographic Parameters in Multiple Human Populations, 2009, link
  • Manica et al. – The effect of ancient population bottlenecks on human phenotypic variation, 2007, link
  • The Human Lineage – Matt Cartmill,Fred H. Smith,Kaye B. Brown, 2009, link
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