Foreword:
While I have characterized this view of H. s. sapiens speciation as "speculation", almost all of it is well supported by published and peer reviewed research. My suggestion that 'nakedness' forced archaic Eurasian sapiens into elaborated culture is the only unique and speculative element of this hypothesis. Accordingly, I feel it is useful to provide a brief introduction to the genetics of hominid hairiness, or, in the case of humans, near nakedness. The following excerpt is from Klein and Takahata's, WHERE DO WE COME FROM? The Molecular Evidence for Human Descent, Springer-Verlag, 2002, beginning on page 203. Discussing the differences between the chimpanzee and human genomes, they say:
"Several laboratories are currently hunting the genes associated with sapientine characters, but no successes have as yet been reported. At present, three genes are known that differentiate humans from chimpanzees and other primates functionally, but their involvement in the development of any sapientine morphological or behavioral character is not obvious."
The first gene they discuss is of no particular interest, but the second is most apposite!
"The second gene is a member of a large family of loci which specify keratins, the proteins of intracellular filaments in cell layers covering body surfaces, such as the epidermis of the skin, and in hair and nails, which are derived from the epidermis. The "soft" keratins of the epidermis and other epithelia are encoded in a different set of genes (KRT) than the "hard" keratins of hair (KRTH). Both the soft and the hard keratins are of two types, acidic (A) and basic (B) and are organized in A-B pairs in the filaments. In humans, there are nine genes encoding acidic keratins (KRTHA) and six genes specifying basic keratins (KRTHB). One of the KRTHA genes has a stop codon in the middle of its sequence: it is a nonfunctional pseudogene (KRTHAP1). This defect has been found in all humans tested, but not in the chimpanzee or gorilla, in which the corresponding gene appears to be functional. The inactivating mutation is estimated to have occurred 240,000 years ago."
Note that date for inactivation of the gene for hairiness. I feel that it tends to corroborate the theory advanced below, in two respects. First, the era of inactivation so closely precedes other evidence of incipient modernity (the FoxP2 gene, domestication of the dog, Hss fossils) that it is plausible to speculate regarding its possibly causative correlation. Second, the late date of inactivation lends support to the common-sense view that early, low-culture Eurasian hominids had effective pelts. We may be assured of that because there are occasional cases of modern humans who grow a complete coat of hair. Klein and Takahata point out that such 'atavisms' reveal that the underlying genetic capacity still exists, and can be re-activated by a single, minor mutation.
"In the same way as a character can be lost as a result of a single mutation, it can be regained if the function of the mutated gene is restored by another mutation. The return-to-function mutation can either take the form of a reversal of the original change or that of a different mutation that compensates for the effect of the first one.
The reappearance of a character believed to have been present in a remote ancestor is known as 'atavism'(from Latin atavus, ancestor). The resurrection of the character indicates that the complex pathway leading up to it is still intact and that the gene responsible for the character is still present in a form that can be reactivated."
Then, writing about the specific case of interest, they say:
"consider, as an example, human nakedness. One of the characteristic features accompanying the emergence of mammals from theriodont reptiles was the appearance of hair on the outer body surface.
There can be little doubt that the ancestors of humankind had bodies which were covered by a coat of hair similar to that found in other primates. The strongest evidence of this is the observation that rare atavistic mutations lead to the recurrence of hair growth in humans to produce a coat of hair resembling that of certain other primates. Since the Middle Ages, some 50 cases of a condition clinically classified as "generalized congenital hypertrichosis"have been recorded. The surface of their bodies was covered by dense hair as observed in our ape ancestors. In one of the best studied cases involving 19 individuals of a Mexican family, the origin of the condition could be traced back to a gene on the X chromosome. Presumably a mutation in this gene restored its function and thus initiated the expression of the pathway leading to full hair cover. This pathway must therefore still be intact in every one of us and it takes only a small step to its manifestation.
The difference between possessing a coat of hair or appearing naked is actually far less dramatic than most people think. We are not really 'hairless 'on most parts of our bodies; in fact we have almost the same density and distribution of hair follicles in our skin as chimpanzees or gorillas. Most of our body hair is generally much shorter and finer, however, so that it is almost invisible. Regardless of the mechanism responsible for the transformation from clearly visible to almost invisible hair, the cases of hypertrichosis demonstrate that this mechanism can be flipped on or off rather easily at the molecular level."
Speculations on Speciation
Homo ergaster and a taxon with affinities to H. habilis (tentatively georgicus) were in Eurasia before 1.8 million years ago. Future discoveries may establish yet more ancient antecedents of humanity in Eurasia. Certainly, our ancestors were adapting to temperate and higher latitudes, even before H. erectus evolved.
There is evidence (Mishmar, et al. 2003) of mtDNA adaptations to cold that persist in modern times. One may assume that directly selected nuclear genetic adaptations occurred early, and (at that low culture level) one of the first was for hirsute specimens. It may be inferred, from knowledge of genetic potentials that, within a few thousand generations, Eurasian hominids, living in temperate and higher latitudes, had effective pelts.
After more than seventy-five thousand generations of selection, H. heidelbergensis lived in northern Europe. They were large, robust hominids with capacious brains. The need to provide for winter sustenance was a powerful selective force, and their mere survival in northern Europe indicates that they possessed initiative and providence, 'common sense', and forethought: this is, at least, rudimentary intelligence.
There are spears, designed to be thrown, in northern Europe dating from more than 300 thousand years ago, with archaic sapiens remains from about the same era, and there are tools from Siberia and Finland, dated to that era. Taxonomists have recently affirmed Heidi's classification as Homo, while propinquity, dental anatomy, and cranial capacity make him at least a potential human ancestor. Whether archaic sapiens differentiated from Heidi or evolved independently, they were adapted to and living at high latitudes in Eurasia, several hundred thousand years ago. It is reasonable to assume that high-latitude populations of archaic sapiens also possessed pelts, having inherited that characteristic from their adapted, low culture progenitors.
Infant mortality was high, longevity low, and a sparse population experienced continuous, heavy selection, with interludes of extreme pressure and regional depopulation followed by recolonization. Only females who made heavy investments in parental care had much chance of attaining replacement fertility; this selected them for behavioral propensities that may be termed 'sustaining virtue'. Unsupportive males were selected against, resulting in behavioral patterns that may be thought of as incipient 'family values'. Females who were fortunate enough to acquire such mates gained relative fitness. To the extent that females had and made choices leading to sustaining mates, and/or reinforced pair bonding they experienced relative fitness. This sort of behavior is only an elaboration of that which is observed in chimps.
This elaboration and intensification of mutual sustenance and pair-bonding created a nascent, nuclear family and facilitated reproduction. A feedback-cycle became established, in which the most committed to parental care and mutual assistance achieved such superior reproductive success that those behavioral patterns became normal. While selection would have remained so severe that no seriously debilitated individual was likely to achieve replacement fertility, such families would have been able to preserve marginally fit infants. If they matured into fit specimens, they too could achieve reproductive success. Under reasonably favorable circumstances, infants could be preserved who would have been too 'helpless' to survive without elaborated caregiving. Consequently, neotenic young could survive and mature into fit individuals.
Only those phenotypes whose expressions of neoteny were favorable, neutral, or only slightly detrimental, survived, even with parental care. Expressions of neoteny that could not be compensated for by elaborated parental care continued to be eliminated. Given the association of neoteny with paedeomorphic traits such as nakedness, some females would have been born, who didn't grow an outer pelt until puberty. Only the most committed caregivers would have been able to raise such female young, who possessed only a downy undercoat for the first decade of their lives, except in favorable eras and areas. Neotenic males should have been viable, and inbreeding would have been common in sparse populations, so some lineages would have acquired a propensity for producing neotenic young, and those clans would have developed the most elaborated caregiving behavior.
We may infer that there was an established, if somewhat casual, use of animal skins for rudimentary protection from the elements. It would have been natural for parents, with an established propensity for caregiving, to use such skins to protect children without a full pelt. When they did survive until puberty, and gained a normal coat, they had certain advantages. We are entitled to assume that a longer period of neurological development produced a more highly developed form of cognitive function in these ancestral hominids. If they were able to survive to puberty, they would have benefited from higher levels of 'g' and experienced relative fitness. They would have been more gracile, but, while not as strong as the ancestral species, they were still robust by comparison to modern humans.
Higher intelligence would have facilitated cultural developments such as processing skins to make, perhaps at first, capes, sleeping covers, and shelter enhancements. Later, rudimentary clothing, at least for the young, and regular use of fire, would have enhanced fitness. As neoteny was associated with 'g', and intelligence facilitated social interaction, as well as cultural elaborations, those lineages which were most neotenic would have become the most fit and successful, particularly in variable conditions and new environments. Neotenic individuals might also have enjoyed certain physical advantages, such as needing less food to survive. In hot weather it would have been easier for them to transpire excess body-heat; they weighed less, so they could have run faster and farther than the ancestral type. It is widely believed that the ability to run fast, for long distances, has been selected for, and it is easy to imagine its survival value.
Neotenic lineages would have been more successful at recolonizing depopulated areas, or seizing opportunities in general, and successful lineages would have become clans. Their 'social skills' would have facilitated conflict resolution and enabled larger groups to pursue mutually beneficial endeavors, such as competing with and displacing solitaries or smaller family groups. A proto culture emerged, in which neotenic young were normal, and routinely sustained, to mature into intelligent, fit specimens who cooperated in extended kinship groups. Social interaction became very important, and selection for the capacity to interact successfully simultaneously selected for yet more 'g'.
A mutation of the FoxP2 gene, crucial for the interpretation of rapid and complex speech, may be as much as two hundred thousand years old, though some research indicates it is recent enough to be associated with the upper paleolithic cultural explosion (Enard et al., 2002). Communication skills fostered increasingly sophisticated verbalization and recognition of social cues and signals. The feedback cycle of selection for intelligence was intensified, as the basis for real language was established and elaborated. That permitted new levels of social interaction, which led to even more success in exploiting resources and competition with other hominids.
In East Asia, by a hundred thousand years ago, casual scavenging of predator kills had developed into systematic exploitation of the wolves' capacity to pursue fleet game. That lead to domestication of dogs, who were, themselves, differentiated from wolves by neoteny. They matured into an amenable creature that could pattern on humans as pack-alphas, and behave with 'puppyish' submission even when mature. A rationalized relationship, exploiting the canids' physical capacities, elevated the clans who possessed them to even higher levels of resource exploitation, permitting greater population density. That created another cycle of selection for social skills and intelligence. This population may have been isolated from central Asia until the ice sheets retreated, around 60 thousand years ago. On the other hand, elements of this population may have entered central, and S W Asia prior to Oxygen Isotope Stage 4 and the eruption of Mt. Toba.
Inhabitants of central Eurasia were savagely selected by the ''nuclear winter'' following the eruption of Mt. Toba, sometime after 75 thousand years ago. Authorities are in disagreement as to the severity of the eruption's effects. Some claim that Toba caused ''temperatures around the world to plummet''; created ''a broad human extinction zone in India'', ''a six year nuclear winter'', and was ''a global catastrophe for all living things''. Others say the effects were less severe, but genetic research indicates that, at one time, the proto-human population was reduced to only a few thousand breeding pairs, and it may have been in this era. The Eurasian Eves' mtDNA lineages are at least 65 thousand years old, and (considering effects of recombination and repetitive mutation) probably date to approximately the era of Toba's eruption. See: Plural Lineages in the Human mtDNA Genome.
One imagines that some fled south to escape the devastated environment. The Herto skulls, and other fossils, reveal that radiations into the tropics, by Eurasian sapiens, had been going on since ancient times. This gene flow had 'modernized' the indigenous erectus, and created an advanced, hybrid population. By 100 thousand years ago there were proto-anatomicly-modern sapiens in S W Asia. The effects of Toba probably precipitated a new radiation of advanced Eurasian sapiens into S W Asia.
By 55 thousand years ago, the East Asian population that had domesticated dogs was established in Central Asia, at the beginning of the interglacial era. True-human artifacts have been discovered there, dated from 50,000 years ago. From that time on, the Aurignacian culture spread through Europe and S W Asia; the Cro-Magnon type developed and expanded into Europe; the original, East Asian type radiated throughout Asia. The tropics were relatively densely populated by adapted and disease resistant erectus-hybrid populations created by previous radiations of archaic sapiens. Those tropical populations were further hybridized by the initial and subsequent radiation of modern humans. Such populations have persisted into the present era, as revealed by the genetic research discussed in Age and Origin of the Human Species.