Thoughts on human evolution: what if modern behaviour and cognition come first?

The emergence of Homo sapiens, has been as contentious an issue as is the emergence of the particular  “Anatomically Modern” variant since the late Pleistocene, with more fragile jaws, chins, reduced or absent brow ridges, and elevated cranial vault.  Much has been made of brain size increases throughout human evolution, as indicative of conceptual augmentation – to explain traces of both technological and organizational behavior we recognize as “us”.

“Behavioral modernity” is a subtle proxy for intellectual or cognitive abilities – and it seems that there are some who question whether this can be automatically assumed to coincide with the appearance of “anatomically modern” human.

I have a suggestion: there was no time lag.

Obviously, quality of the mind is inferential, not empirical. Students of human evolution do the best they can to interpret the traces that remain: but is there any plausible ultimate causality, some independent variable, besides the almost default setting of “intergroup competition”, that can account for changes in gene frequencies specifically affecting neurological tissue? Can any such changes be reliably dated to support the idea that there was a time lag between the emergence of an anatomically modern form of Homo sapiens and modern cognitive capacity?

Here is what we know from the study of the impacts of modern human subsistence economies. The genus Homo was behaviourally and physically shaped by selection within an ecological niche.  In every known environment members of this genus appear to have represented a keystone species. Where they entered an ecosystem, it became more diverse and possibly more stable, due to four results of their presence:

 As a provisioning primate, each individual adult gatherer redistributed millions of fruit seeds and nuts, as well as roots, bulbs, and corms, across the landscape. (Among modern hunter-gatherers this has been observed during gathering trips, as small quantities of gathered material spill from containers over several hours of walking, collecting, and digging. This augments the redistributive impact of seeds through scat that we see in many other species).
 
 They also hunted, initially with sharpened wooden spears. Their presence altered the feeding behaviour of prey species: contributing to the effects of other predators in preventing  over-grazing, especially near water features. Initially a mesopredator in a whole suite of larger established predators, early hominin populations appear to have been subject to control by apex predators like leopards and lions.

 The temporary camping sites they used became hotspots of extra nutrients, and even, of concentrated new plantations of the kinds of wild plant foods humans favoured.  Thus the development of the “safe” camping site where provisions, gathered and hunted, would be shared, created middens of organic debris as well as a downwind zone where urine and defecation would be concentrated.

 Evidence of contained use of fire at camping sites has been dated back at least two million years. Actual “control” of fire has been debated; like behavioural modernity, it has often been thought to lag behind occasional indications of its use.  It would be extraordinary to find, after this length of time, contained “camp” fires not associated with cave sites.  For one thing, cave sites were often reused for thousands of years; in contrast to ephemeral open camping sites. Nevertheless, contained camp fires offer protection from night-prowling predators, permitted heat modification of material technology, and the possibility of cooking food.  Millions of abandoned campsites used for weeks or months would  however have a significant ecological consequence: concentrated ash lens left behind contributed to the soil fertility.

The degree of cognitive intelligence required to be this kind of keystone species, however, is not sufficient to explain the relentless evolutionary pressures that appear to have increased human brain capacity.  Clearly, the human cognition and behaviour was subject to another driver as well. In human evolution, many innate behavioural tendencies appear to have been scraped in favour of plasticity. Much behaviour was learned and shared.

Human evolution was due to more than an ecological niche – it was also an adaptation to a cognitive niche.

While comprehension of immediate “cause and effect” may be a function of individual trial and error learning (dark clouds and rising winds means there might be a rain storm) it is another thing to translate cause and effect into a communicable explanatory paradigm. (People see the regularity of days and nights; of the moon waxing and waning, and develop stories to “explain it” to children).  It is the sharing of these explanatory stories, these exemplars that illustrate the meaning of an event sequence, that goes beyond individual observations. Eventually longer term event-sequence models, and comprehension of all the possible consequences, of whole cascading feedbacks involving multiple variables, became normalized, at least in most cognitively mature adults.

Thus it is collective understanding, shared in symbolic form – as words, illustrations, and causal explanations – that permits each generation to by-pass individual trial and error. This was more than a cognitive niche – it was a collective cognitive niche that could organize cultural ecologies adaptively to  variety of geographies and ecosystems.  In a sense it was self-organizing: no single individual needed to comprehend the whole omnibus of cumulative and shared information and ideas about that information represented by any one culture; they just needed to know the consensus and learn the practices consistent with the received wisdom.

Perhaps the kind of processional learning, involved in the intergenerational communication to transmit skills in the manufacture of tools, and especially for making compound technologies, is critically important?  Perhaps it was, by analogy,  in the genesis of processional thinking applied to other things?  homo-erectus-african-savannah

When processional learning results in the ability to comprehend and then to effectively communicate causality, that a kind of collective cognitive niche becomes a powerful environment in its own right.   Every culture we know of has stored information so vast it exceeds any single human mind; specialists and experts each represent  repositories of knowledge not available to most other people in their culture.  Moreover the expert in microbiology, plant poisons, making leather boots, or elephant behaviour may have no clue about how to straighten a spear, bake bread, play a piano, or tie proper knots.

The greater the variability of the information base, the more adaptable the culture. Cognitive capacity, for memory of a multitude of symbolic representations of real and imagined people, animals, places, things, past and present, plus the capability to operationalize  paradigms to achieve desired results, was clearly one of the profound co-adaptations between biology and culture.

How can I be so sure of this? Because it is exactly the kind of analytic thinking seen among modern people in all economies, even if they apply it to controlling wildfires, city planning, architecture, compound interest, fine cuisine, plotting novels, designing fashions, creating nuclear plants, annual sales increments, retirement financing, or putting a man on the moon.

Culture – the “collective cognitive niche” –  produced cognitive “modernity”.  And it very likely produced it long before anatomically modern Homo sapiens appeared and began deliberately experimenting with more intensive ecosystem management. Thus, I would suggest,  it  was a cultural change, not a biological change or “great leap forward” that eventually made “us” anatomically human.

Culture – the “collective cognitive niche” –  produced cognitive “modernity”

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Recent evidence has suggested there might have been considerable genetic diversity among the populations of early Homo species and subspecies, not only in Africa, but also in Eurasia during the middle Pleistocene. If these early humans were all occupying “keystone” roles in a variety of local ecologies, could this explain the diversity?  It might be possible that they competed with each other but given the low population densities it seems unlikely: but how to account for the fact that, within a mere 200,000 years, only one (sub)species survived of the whole genus Homo?

According to many estimates from the genetic data, the Neanderthals were reduced to a genetically fragile state – inbred and, according to some, almost irretrievably headed for extinction – by the time the penultimate ice age advance was completed.They may not have been the only ones: humans at various times and locations apparently survived a horrific series of climatic changes: glacial advances in Eurasia tied up vast quantities of water and were at least partially responsible for “mega-droughts” in Africa. I’m not kidding about the “Mega” – these droughts lasted up to twenty thousand years and reduced Lake Malawi to puddles of brackish sludge at the bottom of an African version of Death Valley at times.  If surviving in refugia produced too narrow a bottleneck, some communities might have declined to extinction.  Not a single bottleneck, perhaps, but for some, a series. These very probably affected all varieties of humans, simultaneously with many other species on the planet as well.

Within Africa we have good evidence of Homo naledi, a small species in southern Africa, that might have been contemporaneous with early “archaic” Homo – a possible pan-African species of early Homo sapiens. Although often referred to as a possible Homo heidelbergensis, it is apparent that this Florisbad specimen with brain volume 1,400 cm3 has remarkable similarity with a slightly earlier skull from Moroco. The Florisbad skull was  found with Middle Stone Age tools. Tooth enamel samples dated by electron spin resonance dating to 294,000 to 224,000 years old. Now compare this skull with the jebel Irhoud specimen from Morocco which was declared to be Homo sapiens in a recent publication.

Top: Florisbad skull reconstructed; archaic Homo sapiens?  Bottom: Jebel Irhoud skull of archaic Homo sapiens.

These various species or subspecies of archaic Homo plausibly all made similar cultural adaptations to the recurrent shrinkage of habitat and numbers of the whole integrated Pleistocene ecology. By the time the latest cycle of mega droughts ended in Africa, I propose that the humans who survived, and thrived, had developed new twist to their collective cognitive niche. This was so good it permitted them to start growing their numbers, expand their range, and tackle adaptation in every African ecosystem by the period between 74,000 and 70,000 years ago.  Numbers of “archaic” Homo sapiens had already begun to extend their ranges into parts of southern Arabia and the Mediterranean by 150,000, perhaps even parts of southern Asia.

However, it appears that all of these more archaic humans were absorbed into a wave of cultural shape-shifters – the Anatomically Modern humans, who in short order, tackled every ecosystem out of Africa as well as in it..  It is noteworthy that they did a lot of this IN THE MIDDLE OF AN ICE AGE. The evidence of traces of mysterious, as well as “known”, archaic contributions in literally ALL modern Homo sapiens, in Africa and in Eurasia, is now clear. There is also the evidence that archaic Homo sapiens, and even earlier species, had been highly successful as keystone species, that even was able to spread to islands.

What was it, that made the cultural ecology of this latest model so much more successful? Why did this cultural change alter selection in a direction favourable to leaner people with lighter skeletons?

What was this twist?

It was probably a small change at first.

That major change was the following:  they appear to have intensified “eco-engineering” and conceptually more nuanced “keystone” practices.  These were practices that rose out of information on ecosystem consequences of fires and foraging strategies that already had been part of the culture of archaic Homo sapiens like the ones in Eurasia and Africa. I think it likely that this information and the preferred practices that resulted from experiments designed to keep ecosystems viable. I think they were cultural practices that were were deliberated upon and discussed over many generations and across many groups. The practices that were adopted were those that were considered most promising in terms of preserving the plants and animals within that community’s range. Even Neanderthals appear to have used fire to modify landscapes.

Ecological engineering became a more fully articulated and operationalized cultural system earliest in southern and possibly eastern Africa because these regions would have witnessed massively destructive wildfires during each severe Megadrought period.  They seem to have worked out ways to counteract the scale of these.

No previous human seems to have deliberately replanted important food plants. They could do even more; they could begin to monitor critical prey species more closely, and forego hunting those that were down to critical levels.  Wild animals that are relatively unafraid of humans are much easier to identify and keep track of. Prey switching, and adopting hunting practices that reduce predator fear, facilitate such monitoring.  Especially important is  “distance hunting” – the silent arrow that obscures the source of an attack, made possible by the development of the bows, augmented by use of poisons. Additionally, long range spear throwers, as well as snares, blowguns, and boomerangs, all appear to prevent an obvious link to be made between the appearance of a human as the source of the harm.

The archaeological evidence shows that such distance hunting technologies all appear sporadically in Africa during the late Pleistocene, especially at sites near oceans or rivers.  These technologies eventually integrated as “middle” stone age industries. Something about this cultural shift appears to have preceded the appearance of  anatomically modern humans.

Our earliest evidence of  technological innovations that signalled hunting methods so modified as to reduce predator fear appears in coastal cave sites in southern Africa.

Archaic humans, like Neanderthals, seem to have used hand thrown spears, cooperatively hurled by hunting parties, to wound and kill. It is most unlikely that the animals did not see who attacked them. In that case, any individual animal that survived would have remembered what happened.  This fear,  of the human pack hunter, would then be communicated as increased flight distance to any future companions and offspring.

The range of the spear would become useless unless the hunters resorted to a silent stalking approach. This would not work well in open country, but would be possible in woodland in windy wether.  This may explain why, for example, there is some evidence that Neanderthals only butchered a few of the herd of animals they had killed.  Ensuring no survivors by killing an entire herd was the only way people could keep fight distances short enough. Dealing with migratory game this was less critical, perhaps, but territorial species that became more common in woodland during interglacials might have become locally too difficult to get near, so driving them into pits, snares, and ambushes would be more common.

The same problem would of course plague spear hunters everywhere. It became a crisis, however, whenever glaciation, or mega drought, reduced whole ecosystems to survival in refugia.  Prey species, especially megafauna such as elephants, giraffe, eland, or rhinoceros, could have been driven to extinction in small fragmented ranges. Already  endangered by small numbers and slow reproduction, their reproductive rates could have been further damaged by the extra predator fear due to human presence. There was little humans could do to reduce predation by lions and other large predators, but they could alter their own hunting behaviour.

We cannot, of course, know the precise antiquity of the deliberate use of fire to set back ecological succession, the strategic management of prey species, the careful use of “distance hunting” technologies employing compound tools like bows and arrows, of use of poisons, of innovations like snares and nets, and of the replanting of roots and wild plant seeds, that I observed among the Kua. However, the frequency of such practices among foragers worldwide suggests considerable antiquity preceding the spread of anatomically modern people. These behaviours signify a subtle alteration in the way these people conceptualized ecosystems. They saw themselves as care-takers: and clearly “took care” of their food sources in ways that paid off in better long term survival during leaner years.

The occasional use of fire to attract game or clear camping sites of ticks was probably much older (see link), and the shift from unconscious dribbling of seeds by gathering women probably long predated the deliberate replanting of desired wild species. So too, did many hunting practices. However, tinkering with a plan to more effectively stalk an antelope, or to drive a herd into an ambush – or over a cliff – is taken to a whole new level in the late Palaeolithic.

When you begin to devise and execute plans that will reward you with higher numbers of buffalo in twenty years, and to devise and executive plans for increasing secondary growth meadowland and younger forest, to achieve resource goals unlikely to be realized in a single lifetime, indicates a different order of conceptual models.  These kinds of cultural paradigms cannot become normalized, nor even appear, overnight. images-5

To develop such a sophisticated understanding, even if it originates among a minority of discussants, involves the integration of many empirical observations, verified and accumulated, over many lifetimes. The wider this effort is shared, the more people are involved in discussions of all relevant data and concepts, and the larger the geographic area involved, the more comparative material can be assessed, and the more continuities – as well as exceptions and special cases – can be discovered. Provided the dedicated observers and discussants can explain and demonstrate that certain new ideas and practices achieve better results than the previous customs and technologies, there can be a shift within the whole society, a shift that may accelerate the “borrowing” of any successful innovation over much larger culture areas. It takes generations of observation and discussion to reach the level regularly found today among hunter-gatherers all over the world. The empiricism that is evident in their conceptualization of relationships between plants and animals, as parts of integrated communities, and the operationalization of this understanding into deliberate and practical interventions, goes beyond mere “planning ahead”.

It goes further: into the realm of imaginative moral equivalences to the natural world, with the expectation that actions showing respect will forestall future retaliatory danger. The practices resulting from empirical observations are thus tagged with mystical reciprocity reminiscent of the generalized reciprocity among humans. This makes it all fall into place within a socially-centred worldview: a kind of sacred naturalism infuses both the concepts and their practical application.  If you plant seeds, and replant dying roots, then you show respect for the spiritual connection between the plants, which feed everyone, and those who are fed. Thus a kind of balance is honoured: we know we do harm by eating the “children of trees” and the flesh of our fellow creatures, but we repay this with gratitude and care. It is a kind of bulwark against the risky future, so that grandchildren’s grandchildren will find such plants and animals in sufficient abundance. For the seventh generation to be fed,  we must also ensure the survival of the whole ecosystem.

I find it telling that, in documenting indigenous environmental knowledge in every sustainable economy on the planet, we can catch a glimpse, even today, of the kind of conceptually challenging synthesis of empirical observations – a synthesis that must have taken many generations, across many resource landscapes, shared among many cultural groups, to achieve.

The quality of minds, capable of understanding on this level, clearly existed before humans spread all over the planet. I will take that further: the level of cognition required, to visualize how a whole ecosystem could be manipulated – “taken care of”- over generations, was clearly in place before people could even begin to do it. Or, to state this another way: the level of thought and reasoning required, to understand an entire ecosystem well enough to effectively manipulate it, had to already exist before this kind of cultural adaptation could happen, just as the level of thought and reasoning required to compute and design a way to launch humans into space, to land on the moon, and then get safely back to the earth, had to exist before Kennedy made his famous speech.

Not everyone had to be a genius of course. But there had to be a certain number of reflective, questing intelligences in each deme, and a receptive majority fully capable of grasping the concepts well enough for practical application. No wonder our human genome has accumulated so many polymorphisms of small effect. You need quirky tinkerers as well as obsessive geniuses, but this is leavening: for culture to work, you also need retentive rationality. The successful application of concepts reinforces their utility. The culture would change: it does not depend on every individual being equally insightful. Just a few geniuses per generation are enough. Even if only a minority of people pondered particular issues and came up with practical innovations or conceptual breakthroughs, these resulting practices, and the reasoning behind them, could be demonstrated to others. I would even go further: I think there would have been at least some people born, among the hunter-gatherers of the late Pleistocene, who already possessed the cognitive capability required to get a team of astronauts to the moon.

Thus, critical cultural shifts appear to have permitted early Homo sapiens to survive despite a punishing series of droughts.  After this, at least some human demes in Africa were more than just another keystone species within a predator suit. Conscious analysis of the causal processes within those systems, and the deliberate application of such analyses, seems to have created a pattern of culturally mediated ecosystem engineering, set in motion, not on the basis of instinctual or evoked behavior as is dam building in beavers, but rather by applying conceptualizations to fully rationalized practices: these people had cultural mindscapes about landscapes.

This cultural refinement can be glimpsed in the traces it left in the archaeological record: the appearance, here and there, of technologically “sophisticated” compound spears, as well as bows and arrows and other hunting technology.  The earliest indication we have of whole pattern coming together is from a series of cave shelters excavated along the coast of South Africa.  Also, the increasing frequency of such technological and cultural shifts across much of middle stone age Africa speaks volumes about the increasing importance of information exchange through far-reaching social networks. It was a function of signalling both local and common identity that we see the spread of ochres used in body decoration, jewelry, and the creation of rock art.

Network size appears to be relatively large in humans, and larger personal networks have been linked to cognitive and emotional intelligence.  As observed among the Kua, the “hubs” – the people with the largest networks – are “popular” – their presence enhances group size and harmony – which in turn stabilizes food supply and enhances the safety of camp sites, since in a larger group there is always somebody awake to keep fires burning and watch out for approaching predators.

The network “hubs” I observed among the Kua were men and women who pulled together the largest camping parties. In the case of men, it was not the hunting ability that did this. They were men who personified valued qualities: generosity, competence, courage, compassion, and humour. Such men had the biggest camps because more households gravitated to join the band that contained such a man. Perhaps, even 100,000 years ago, they already had a certain sex appeal? Sexual selection may even have taken off in the times of greatest hardship, honing female preference for popular men; but at the same time, the personal integrity that was in evidence in popular men, also was exactly the quality that attracted male companions. Can we imagine such a thing as social selection? These men were lifelong friends, not necessarily relatives, and clearly trusted each other. They did not appear to imperil each other’s marriages; and were observed to behave responsibly towards each other’s daughters and sisters.  Attraction to people who embody ideals is not a minor drive, it situates the fate of one’s genetic legacy within an intensely social life, where the choice of living companions has as much to say about chances of one’s children’s survival as the choice of a mate.

Charisma became a thing, yeah, but so did ethics.

Of course ethics and charisma are not just male attributes. Some of the most influential people I met among the Kua were older women, who often served as a counterpoint to male authority within kin-groups. They were vocal advocates, who reinforced fair sharing, justice, and insisted on self-control, sometimes in raucous and hilarious language and gesture. Grandmothers were far more than babysitters and helpful with provisions, they had a powerful a moral authority in these societies, as did grandfathers. And since the role of morality seems to be about living in conformity with a group it can be a source of bigotry and hatred of “outsiders” or minorities, as well as a force for tolerance and diplomacy. We can wonder if this is why it was so significant that people invented “party time” manners.

As Ernst Mayr once noted, speaking specifically of human adaptation to an intensely social life and the role of altruistic behaviour: “Reciprocal helpfulness worked as successfully with outsiders as with group members. Yet far more important was the diversity within human populations. Every population contains individuals with a particularly friendly disposition, and they help in the making of bridges between groups and populations.” (What Evolution Is, 2001; page 287)

Evolutionary implications?

The last 300,000 – 100,000 years, then, might just have been the time in human evolutionary history when it was the mental and emotional qualities of individuals made the greatest difference to group survival. Periods of isolation, during glacial advances in Eurasia, and mega-droughts in Africa, would have made inbreeding depression a real danger; it is plausible that even the persistence of “weak ties” would have been critical to avoid this.  Popular people, with the farthest mutually cordial links, might offer us a glimpse of how genetic and cultural “cul-de-sacs” were avoided. In addition to the utility – to genetic exchange – of having a certain number of such people who acted as “hubs” of communication between demes, useful innovations and concepts could easily be ghosted (as memes and curios) through half a dozen degrees of separation.

How to account for a pattern of “friendly” connections among scattered and somewhat dissimilar populations, especially if they infrequently encountered one another over the course of millennia of hardship? A mechanism for the passage of information through many degrees of separation actually does suggest itself: the plethora of non-material treasures I witnessed being shared among the different language groups during seasonal aggregations in the Kalahari.

People in every culture looked forward to lively gossip, music, and storytelling; so did the Kalahari hunter-gatherers. They chatted in eager groups, showing each other ostrich egg water containers where children had etched symbols for constellations and from popular mythology. I think the careful absence of outright trade and balanced exchange of material objects is very significant on such occasions. They clustered in the evenings to share in the singing and dancing performances, the shamanistic rituals evoking healing magic and divination: in short, they revelled in all the modest symbolic richness that can be displayed without hinting at any crass seeking after selfish material benefit from these encounters.

One of the most unexpected conflicts I ever observed occurred when a person casually passed a gift to a youngster from another language group. This was swiftly punished – the father took the red t-shirt (originally from a Red Cross shipment I had helped distribute) and flung it angrily at the head of the woman who had given it away. How dared she presume that kind of intimacy when she and the boy’s parents were merely acquaintances? It spoke volumes to me about how sensitive people were to the possibility of “altruistic” behaviour being feigned, or perhaps only mistaken, as potentially manipulative, unless trust had been established; otherwise, to deliberately create a debt – implying future reciprocity – was considered outrageous presumption.

This explained why people took great care to avoid any hint that they wanted any material advantage from these encounters. A gift imposes indebtedness: and is thus a demeaning or even potentially hostile act. Offers of trade and barter could be construed as intentionally rude behaviour.

This delicacy was especially striking in contrast to the casual mockery, bawdiness, sarcastic wise-cracks, and eagle-eyed, jealous attention to sharing of meat – as well as other desirable material goods – that went on within Kua camps in everyday life.

Gifts require a certain level of intimacy; teasing and sarcastic wit require even deeper mutual trust.

So I was surprised to see ANY gifts. I was also surprised some of these objects, mostly beadwork and baubles, passed along with an offhand request that they might eventually reach some distant third party. I assumed, at the time, that these were tokens of affectionate longing and good wishes. I can’t be sure; not from the accompanying language. Clearly, with body decoration like red ochre pigment, and objects like bracelets, necklaces, and headbands, the best way to transport them is to wear them. A person so decorated is displaying social network connections, not material “wealth”.  Personal objects like jewelry or lumps of red pigment, might even be decorated with individual markers, objects can be given away and become conduits of sharing. This is very similar to the Hxaro sharing networks observed by Polly Wiessner hundreds of miles away in another language group.

There are other similarities. Each hunter marks the arrows he makes with distinctive grooves that are often darkened with ochre or ash. But then men frequently exchange arrows, the arrow itself is spoken of as being responsible for a successful hunt: so too is the maker of that arrow. If game is taken during an assembly where G/wi and Kua are camped together, a G/wi arrow, deployed by a Kua hunter, can mean that it is a G/wi who takes charge of distributing the meat to the mixed camping party.

Far more significant, however, were the immaterial joys that could be shared. Enhanced imaginative expression – especially in the new comedies, jokes, “magic”, and games, are among those ephemeral and intangible things that create a core of shared identity among people, showing each other we are human, even if we are strangers at first. How else can we explain the fact that every human being appears to have an evolved capacity to learn more than one language, and to adapt themselves to multicultural contexts? Consider how useful this is, precisely in these inter-group contexts. We are none of us strangers to these delights, no matter what corner of the human family we live in today. Of such flimsy stuff channels are opened, whereby information and personnel can cross ethnic boundaries.

Thus information and innovation flow out – and into – every local cultural grouping. How old is this pattern? Many subtle traces of these cultural elements (rituals, jewellery, use of ochre, and so on), begin to appear sporadically in the middle Pleistocene, even among European Neanderthals, and later, these symbolically significant indicators turn up all over Africa; beginning at least 90,000 years ago. If they were used as icebreakers and attractors to create positive exchanges, along with a whole complex of other elements that can bring different communities together in an atmosphere of festivity, then here is a mechanism whereby ideas and skill-sets could have trickled between groups for a quarter of a million years – or more.

Even if not put to immediate use, the regular exchanges of “memes” and innovations enlarged the diversity of information available to everyone. From this, new ingenious solutions could be cobbled together as problems came up. The “ratchet effect”, then, is not just a product of local cumulative culture, but gains much more traction from cross- fertilization. 

I am suggesting, therefore, that this was a cultural, not a biological, breakthrough. But it ultimately could not help but change selection pressures on regulatory, and other genes, effecting skeletal and muscular tissue systems. A shift to “distance hunting” technologies and practices not only reduced predator-fear and improved human wildlife management, it reduced the risks involved in more carnivore-like behaviour. When men cut down the probability that they will be direct grappling with desperate animals, they also reduce risks of broken bones, and of getting gored and trampling. Successful hunting no longer favoured robust bone and heavy muscle where lighter and leaner options returned more calories at lower risk.
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What could have begun, with a cultural change incorporating more ecosystem management, might eventually have contributed to altered frequencies of polymorphisms affecting genes for tissue systems regulating cell division rates and targeting organ sizes.

You cannot ask for a more profound example of bio-cultural feedback. Hunting with bows and poisoned arrows and other “stealth” technology, using quiet methods of distance hunting in preference to outright aggressive predatory attacks, are practices that may well have changed the selection pressures operating on gross anatomical tissue systems. It was not necessary, anymore, to be robust and large to be a successful hunter – not when using bows and arrows, spear-throwers, blowguns, snares, nets, or boomerangs. In fact, a lighter and leaner hunter would be energetically more efficient. He would expend fewer calories during the hunt, leaving more to be shared. Also, of course, a smaller woman could expend fewer calories on her activities and metabolism, and devote more to her unborn child. She could support the costs of lactation more efficiently too.

Under severe climatic conditions during the African mega-droughts, higher energy efficiency, very plausibly, saved lives. Even the more robust individual could continue to eat when many in his group required more modest inputs. Most importantly, the less food was needed for adults, the more could go to the growing children. Over at least a hundred thousand years of intense environmental turmoil, shifts in hunting technologies and practices, by lifting selection pressures that had previously favouring heavy bone and muscle, added up. Any genetic polymorphisms the contributed to conserving energy, or created adults with less massive skeletons and muscles, would be advantageous.

Thus, I would suggest that incremental shifts in the overall energy budget of a human deme, resulting from a cultural change, could have lowered the frequencies of polymorphisms linked to robust skeletal growth and large body size. Set in motion by the mega-droughts, each population bottleneck may well have shifted these selection pressures enough to change their bodies, not just through ontogeny, but through a shift in regulatory genes for bone growth and muscularity.

They became anatomically modern humans.

Anatomical modernity arrived, according to this model, with a slow drumbeat of feedbacks, as droughts blasted sub-Saharan Africa again and again for a over a hundred thousand years. The transformation that aligned cultural practices with a dual ecological niche may have accelerated the bio-cultural tango to a fast crescendo through that final series of climatic shifts. This was what was gained: the essential collective shape-shifting cultural competence of those hunter-gatherers who had walked through the shadow of the valley of death, not once but at least four times, in late Pleistocene Africa.

Wherever these people went after this, they were carrying a rich inventory of successful applications and conceptual models. When the traditions that worked in a lowland forest by the sea coasts did not work well on a steppe environment, or in denser tropical forests, trial and error experimentation – even if only undertaken by a few, could be rapidly communicated, and demonstrated, and over the course of a dozen generations, the cultural ecology of the whole group adjusted to be a better and better fit with the new environment.

“…The ancient settlements of the tropics are also a reminder that Homo sapiens is an incredibly adaptive, flexible species, said Roberts. That’s why we can “occupy every environment on the planet, through periods of dramatic climate change, and became the last remaining hominin.” (see reference 4 below) 

When the Holocene began, this keystone species, now clever enough to engineer whole ecosystems, became sedentary in some parts of the world and started tinkering with the animals and plants, to not only keep them “tame”, but to remake them as part of a more energy-efficient domestic economy.

A high frequency of inter-group fighting would hamper this process. A longer reach and greater strength is advantageous in hand-to-hand combat, and would stall positive selection for higher energy efficiency. Under conditions of intractable threat from another group, in fact, sudden ambush and total genocidal attack would be far more effective than what we call “warfare”.

Further reading and references

  1. Polly Wiessner “Hunting, healing, and hxaro exchange: A long-term perspective on !Kung (Ju/’hoansi) large-game hunting” Evolution and Human Behavior 23 (2002) 407–436
    and see also Polly Wiessner Style and Social Information in Kalahari San Projectile Points  American Antiquity, Vol. 48, No. 2. (Apr., 1983), pp. 253-276. Stable URL:  http://links.jstor.org/sici?sici=00027316%28198304%2948%3A2%3C253%3ASASIIK%3E2.0.CO%3B2-6

2. http://www.sciencedirect.com/science/article/pii/S0031018209001667

3.  https://phys.org/news/2017-07-early-human-ability-survive-prolonged.html

4.  Iain Davidson and William Noble (I989) “The Archaeology of Perception: Traces of Depiction and Language” CURRENT ANTHROPOLOGY Volume 30, Number 2, April I989.
See also Joseph Henrich, James Broesch “On the nature of cultural transmission networks: evidence from Fijian villages for adaptive learning biases” 28 February 2011. 12 April 2011
 Volume 366, issue 1567 from Discussion Meeting issue ‘Culture evolves’ organized and edited by Andrew Whiten, Robert A. Hinde, Christopher B. Stringer and Kevin N. Laland ”  Philosophical Transactions of the Royal Society of London B: Biological Sciences 366.1567 (2011):1139-1148. Web. 01 Sept 2017. DOI: 10.1098/rstb.2010.0323  and see also:
Laland, K.N., et al., Niche construction, innovation and complexity. Environ. Innovation Soc. Transitions (2013), http://dx.doi.org/10.1016/j.eist.2013.08.003

5. Early human’s ability to survive through prolonged arid areas in southern Africa 

6.  https://arstechnica.com/science/2017/08/evidence-that-humans-had-farms-30000-years-earlier-than-previously-thought/

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