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Human Evolution: A Discussion of Antecedent Factors in Bipedal Movement and Brain Expansion

April 21st, 2012 by Robert DePaolo | Posted in Psychology | No Comments » | 242 views | Send article | Print this Article |


Abstract

This article discusses causation regarding two core aspects of human evolution: bipedal movement and brain expansion. Rather than focusing on their retroactive adaptive benefits, the discussion revolves around typical causes of mutation, particularly metabolic changes fomented by climate shifts over the course of time.

The causal origins of two distinctly human traits; bipedal movement and brain expansion are unclear. With respect to upright walking, a number of theories have been put forth over time. So- called “savanna theory” (generically) is based on the premise that nature selected bipedal locomotion because it facilitated food carrying (Hewes, 1961) and provided heat resistance in the African flatlands (Wheeler 1991) and facilitate a throwing capacity for hunting and self defense. (Calvin 1990).

Another is the“generalist/fisherman” theory (my term) (Niemitz 2010) which holds that an upright posture evolved because it is readily supported by water and that early hominids spent considerable time by water’s edge in search of a plentiful, convenient aquatic food supply. Still another, free-hands theory, suggests that an upright posture allowed the first hominids to make tools (Darwin 1861) and/or throw objects to hunt or defend themselves (Young, 2003) (Kirschmann, 1999).

Meanwhile, aquatic ape theory provides a scenario in which homo sapiens became adapted to water environments. It is based on the fact that our species has tear ducts and other features seen in aquatic creatures. (Morgan 1997)

Each of these theories has merit but each has been subject to criticism. With respect to Aquatic Ape theory, Preuschoft has argued humans are biomechanically suited to life in the water, that too many of us are afraid of water and/or poor swimmers for this to be a species indigenous trait (1991). It begs the question of how competitive would we be in an aquatic milieu; for example vis a vis crocodiles and boas? Could we submerge without air for long periods of time as they do? Could we employ stealth in fleeing from predators? Even the most adept Olympic swimmers splash too much to rely on stealth in the water.

Challenges to savanna theory are based on recent findings indicating that upright walking first emerged, not in the savanna but in woodland environments (WoldeGabriel, Selassie et al 2001) and that such a posture would have diminished the speed with which hominids could flee from predators – rendering it more of a handicap than a benefit. Others have argued that the visual perspective allowed by a taller posture provided more prescient powers of observation, which to an extent might have obviated the need for rapid flight behaviors.

The tool making hypothesis runs into difficulty in that Australopithecus afarensis (possibly the first upright walker) had little capacity to make tools, other than, like modern chimps in rudimentary fashion.

The throwing theory does seem to make sense, because apes (particularly modern chimps) habitually toss objects at predators and rivals when threatened. If improved upon by a bipedal posture such an act might inflict enough harm or fear in predators to buy an escapee time to get away. A little thing like that can go a long way toward preserving the line, especially if the thrower gained social/sexual status by virtue of his group defending talents.

While Calvin and this writer have supported such a view, it does present a problem. It presumes the coordination of the first lines of bipeds was good enough to facilitate accurate and forceful throwing. Obviously over time hominids and early humans became prolific projectile hunters. Yet the first bipeds did not necessarily have the neuro-functional synchrony (i.e realignment of neuro-motor, visual and cognitive systems to be that effective. Indeed assuming the upright posture wasn’t initially accompanied by a total neuro-physiological remake the first bipeds might have been a bit clumsy in that regard.

The food carrying hypothesis also seems somewhat problematic when one examines the behavioral dynamics of food carrying. It has been noted that chimps stand upright most often when carrying food, which would seem to offer a plausible explanation for the evolution of a bipedal posture. Yet to accept that premise one has to assume an upright posture made food carrying so much more efficient as to create an advantage. In that sense there are several things to consider.

First, is the biped able to carry more food than the quadruped? The answer would seem to lie in the purpose of the behavior. Bipedal movement would provide an advantage if food had to be carried over a relatively long distance but not otherwise. If that was its prime benefit than one might just as well say that long distance travel – whether in open savanna or not – was a selective factor. Secondly, what would be the purpose of carrying food in the first place? The prime reasons for this would be to escape from and/or secure food from predators and scavengers (which might be unnecessary for a fruit eating primate) and social. That is, the food carrying would be (as some have stated) part of a mating ritual whereby the male brings offerings to the female to persuade her that he is worthy of siring her offspring. The problem with that lies in primate mating patterns. They are primarily hierarchical, which means no matter how much food one brings to a female her preference will be to mate with the alpha male.

With respect to the fisherman theory, Niemitz suggested the upright posture facilitated fishing prowess in the waters of lakes and rivers of East Africa, citing observations of modern primates who tend to stand upright when in aquatic environments . Water does support the limbs and would certainly reduce the neuromotor stress incurred by a quadruped in picking fish out of water. On the other hand if modern primates can more easily stand upright in water, catch fish then revert back to the four legged posture why would they require a permanent bipedal posture?

Beyond that it is possible for an upright posture to be maladaptive as per the act of fishing. The fishing habits of bears and modern humans provides an illustration. First of all, fish are rather fast, elusive creatures that must be visually tracked before being caught. One doesn’t simply reach down as if picking a daisy. Second fish blend in with their aquatic milieu so that in order to see them beneath the water requires a stooped position. In other words the closer the eyes to the surface of the water the easier it is to find fish – as is the case with bears. To evolve an upright posture only to have to bend over into a stooped position to feed one’s self seems a bit phylogenically extravagant.

Also, some modern human tribes along the Amazon fish for a living. Even the most adept among them aren’t inclined to reach down from an upright position to catch fish. Instead they lower their heads quietly to avoid detection- assuming virtually an ape-like posture. When they locate aquatic prey they typically use spears which function as mechanical extensions of arm length, because fish are simply too fast and wave sensitive to grab by hand. There is little evidence to suggest early hominids had such tools (though not out of the range of possibility since chimps use sticks to catch termites).

Another problem with fishing theory is the underlying notion of a “practice-effect.” While Niemitz supports the notion that bipedal evolution was a general (amphibious) evolutionary change not selected for any single purpose he still argues for a congruence between the availability of aquatic food supplies and an upright posture.” The notion can be scrutinized on several counts. For example the fact that a primate often engages in an upright posture, whether fishing, tree climbing or carrying food, does not mean there will be a concomitant anatomical change. It would be like saying the repeated act by humans of playing basketball will ultimately lead to an increase in height and leaping ability among the isolated groups who play the sport. It may attract tall people but will not in itself make themselves and their children taller.

It seems a probabilistic leap of faith to assume that some trait emerges, followed by mutations that happen to select that trait and weed out those without that trait due to its adaptive value. Most current arguments on human evolution similarly suggest a congruent juxtaposition of mutation on functional purpose when in the strictest terms, all one can really say is that mutations occur as the result of several possible causes and that most have little or no impact on survival or propagation (Eyre-Walker, Wollfit et al (2006).

A Theory of Proactive Causation

The reason why any specific trait emerges is not completely knowable but certain things can be inferred. One of which is that the emergence of any new trait is likely to be for a time, disadvantageous, not just because it runs counter to previous behavioral habits but for socio-sexual reasons as well.

New traits first show up in a small number of individuals within a species. In order to proliferate within the population they must, despite being anomalous, appeal to members of the opposite sex. Since in most primate social groups preferred traits must coincide with perceptual recognition patterns and social traditions an unusual individual who walked upright might look rather strange to potential mates, that is, unless the trait offered immediate that were esthetically and functionally perceptible to potential mates.

That suggests that as far as potential mates are concerned a functionally and esthetically acceptable mutation would have to be a slightly different version of what had been previously recognized as a sexually attractive feature. Radical changes would not do, and from that perspective it makes sense to look at the evolution of the bipedal posture and brain expansion in a different context, bearing in mind that while upright walking seems to have evolved gradually human brain expansion seem to have proceeded at an unusually fast pace.

The Origin of Mutations

The causes of mutations are several but they ultimately converge at a single point. Whether due to increased radiation from ultraviolet light, diet or chance mutations typically correlate with metabolic changes. The reason creatures living in tropical environments reproduce and mutate faster is because increased sunlight and heat speed up their metabolism, which facilitates activity within the genes (Wright, Keeling et al 2006) and often leads to unusual pairings among the four base chemicals comprising DNA (thymine, guanine, adenine and cytosine) Therefore an organism will tend to mutate at a faster pace whenever it has a chronically high metabolic rate or when the climate changes from cold to more temperate.

Interestingly, it appears each turning point in hominid evolution occurred during a cold to warm climatic swing. This occurred in the transition from homo habilis to homo ergaster 1.7 million years ago and during the transition from ergaster to homo erectus, roughly one million years ago. An Ice Age occurred during the latter tenure of erectus followed by a rapid increase in the earth’s temperature half a million years ago, at which time another hominid appeared on the scene, Homo heidelbergensis. And, despite his reputation as the ultimate cold-adapted human, the initial appearance of Neanderthal occurred during a warming period 200,000 years ago. Homo sapiens sapiens who arrived on the scene during one of the most dramatic cold to warm climatic fluctuations in earth’s history.

As will be discussed below it is possible for organisms to mutate in cold climates. Furthermore the evolutionary sequence discussed above does not signify that one group was a genetic forerunner to another. Evolution is a generative rather than linear process. Nonetheless the time tables corresponding to the discoveries of those hominid species suggests a major shift from colder to warmer temperatures. That would lend support to the idea that metabolic changes created a certain amount of tumult within the genetic structure of hominid groups which can lead to mutations in brain and body.

The arrival of the first bipeds (Australopithecus afarensis and a possible predecessor, Ardipithecus Ramidus) occurred during a global warming period as well (Glickson & Pearman 2009) but theirs was not as profound a change as occurred later in hominid evolution. It appears the Australopithecine was almost identical to a chimp except for pelvic anatomical changes supporting an upright posture (Lewin, 1999). The relative insignificance of this in terms of human evolution can be further supported by the fact that it appears the bipedal posture was not perfected in this group. The first bipeds did not apparently walk or run on two legs as fluidly as a modern human. Their sense of balance, center of gravity and limb sequence coordinations might have been a bit awkward, perhaps even (in modern clinical parlance) somewhat dyspraxic. Indeed a recent biped discovery (not yet categorized as either Australopithecine or Ardipithecus) indicated that the first bipeds retained the long toes of an arboreal (Owen, 2012) which would have made pushing off in stride rather awkward, particularly if speed was required (Taylor and Rowntree (1973).

Still the transition from quadruped to biped set the stage for other important changes in structure and behavior so the reasons why this might have occurred with this particular species are important.

A number of questions arise from this point of discussion. First, if the first bipeds lived in the same environment as quadruped apes, and if an upright posture favored food finding, fishing and/or other adaptive behaviors why did only some branches of the tree evolve a posture suitable to upright walking? Chimps are phenomenal imitators, and if, as many have maintained, the forest environment was surrounded by lakes and rivers why would they not have wandered out to shore, observed bipeds fishing and copied that behavior? And if they did, why would nature not select for the most efficient (upright) fishermen among them as well? They also carry food and used makeshift tools, yet evolved no such structural modifications. To an extent evolutionary branching can explain this, i.e the notion that only a small number of individuals incurred mutations and only with time and reproductive success did they blossom into a new species. However the question remains as to why the branching occurred in the first place and perhaps more important, if upright walking was so successful as to be selected by nature, why were quadruped apes successful as well?

Part of the answer lies in the fact that engaging frequently an activity does not prompt evolutionary changes, even retroactively as per natural selection. The anatomical changes leading to upright walking had nothing to do with temporary standing activities employed by primates. The fact that two (or more) primate species virtually identical genetically to one another lived in the same environment, dealt with the same pressures, behaved in similar ways while only one evolved an upright posture suggests something peculiar occurred. Discovering what that was might tell us something about human origins.

A Blip on the Screen

One possibility is a relatively minor change in anatomy produced such an effect on metabolism as to provoke more elaborate mutations; specifically a restructuring of bones in the lower extremities which provided the first biped with a hybrid (bipedal and quadruped) posture and style of locomotion

Initially this change might have been somewhat maladaptive. This hybrid primate would have been less adept traveling on flat ground than later hominids and less adept in the trees than his quadruped ancestors.(Lovejoy 1981) In physiological terms neuro-muscular noise interference arising from this new anatomical feature, superimposed on extant quadruped anatomy and central nervous system could have produced systemic dyspraxia. The reason for this assertion lies in the homeostatic-integrative nature of biological systems.

All systems, biological, informational or chemical operate by a feedback mechanism which require three features. First, there has to be an enclosure, or membrane enabling the system to function at least to some extent within its own parameters. Second there must be a communications process whereby various structures within the system interact with one another to provide feedback and error correcting signals for purposes of keeping the system intact. Third there must be a pan-adaptive (cybernetic) potential whereby changes in one component are accompanied by changes in others to maintain systemic integrity. Since that applies to bodies as well as other information systems, one can assume that a change in one anatomical feature of the biped would ideally prompt changes in others. It also implies that since all brain systems are integrated, a change in one not accompanied by changes in another could result in dysphasia.

For example once the vertical column of the spine evolved into an upright support mehanism the biped’s brain had to process internal, postural feedback in terms of this new arrangement. To do that would have required either that the extant quadruped brain use old circuits for new functions or that new circuits evolve. The likelihood of brain expansion occurring simultaneous with anatomical changes would seem low. Indeed evidence suggests the hominid brain did not expand very much during at time upright walking first emerged. That means the existing circuits of the ape-hominid hybrids would have had to be converted to other uses. Arboreal primates have pliable central nervous systems which would have allowed for conversions but the initial conversion would not have been fluid.

By that line of reasoning, the first wave of developmentally awkward primate hybrids would have been versatile but less specifically skilled –a jack of two movement trades but master of neither. If bipedal locomotion conferred an advantage that might not have mattered but as Neimitz (2000) has also suggested, the limitations on speed inherent in bipedal locomotion makes that unlikely. Yet these hybrid new-comers still needed to fight, flee and survive. Therefore they needed a compensatory tool by which to keep up with comrades and competitors.

One way to compensate for confused neuro-motor functions is to increase global arousal levels in to enhance physio-cognitive focus, override noise interference and compensate for dyspraxic movement and other input disparities.(Jepma, Verdonschoft et al 20120). Since ongoing demands on alertness and movement regulation depend on higher metabolic rates (Oken, Salinsky et al (2006) one can assume that since the first bipeds required more effort, to fight, flee and climb and had to expend more energy to orchestrate movements they would have had higher resting arousal levels and perhaps a higher metabolism. Over time that, in concert with climate change, could have increased the likelihood of provoked more rapid mutations.

Metabolic changes would have also led to increased appetite to sustain energy for brain and body. One fortuitous byproduct of that could have been a penchant for on-going sensory vigilance, stimulus seeking and enhancement of the curiosity drive and an enhanced need for perceptual closure – all of which correlate brain arousal (Jepma, Verdonschot et al 2012.

The Pros and Cons of Self Regulation

A reasonable person might ask how an awkward, hybrid primate template could become the forerunner of sapient cognitive abilities. The answer is complex. In normal developmental circumstances a primate can take its motor actions for granted. A structure positioned above the hind known as the cerebellum operates much like a computer, storing, regulating and stabilizing movement patterns so they don’t have to be consciously monitored. As long as this circuit is working properly and in concert with other brain sites movement and cognitive byproducts of that can proceed without need of conscious support.

When there is discord in the form of dyspraxia the organism does not have the luxury of automaticity. More noise pervades its central nervous system, which creates the potential for chronically activated intra-cerebral activities geared to the act of noise reduction. Heightened arousal tends to stimulate collectively various brain circuits, resulting in increased levels of consciousness. Because movements have to be preplanned, monitored and over-regulated by internal means the creature must become more aware of himself within his environment. At that point he is able (and forced) to deliberate a bit longer. In other words his neuromotor dyspraxia requires a heightened contribution from various systems in brain and body and such pan-mobilization could have set the stage for lengthier periods of cognition.

Interestingly, for that to be a viable argument. one would have to demonstrate that chimp metabolism is lower than that of the descendants of these first “dyspraxic” apes and that a metabolic change would be impactful. With regard to the second point, the research of Clarke (2003) showed that one of the prime genetic differences between chimps and humans is the higher rate of amino acid protein metabolism in humans which enables us to digest meat more efficiently than our primate cousins. Thus while some have presumed that a shift to a meat diet set the stage for human brain expansion it now appears the earlier catalyst was metabolic change, which enabled our bipedal ancestors to eat more meat in the first place.

With regard to the first question it is well established that the metabolic rates for adult chimps and modern humans is roughly the same but that the metabolism of a human infant and preadolescent are much higher than that of a chimp. Therefore it is possible human evolution resulted in part from changes during early childhood and preadolescence rather than in utero.

It is a speculative point but not without inherent logic. Humans have extended periods of brain growth well into adolescence. Human females develop broadened pelvises and permanently enlarge breast upon reaching adolescence. This suggests many critical aspects of human development are post natal. While the adage about “ontogeny recapitulating phylogeny” is bit simplistic, some trends in utero do seem to mimic stages of evolution. In that context a speculative scenario comes to mind.

Emergence

A youngster is born and appears normal. Consequently it is accepted and nurtured in the usual way by mother and social group. Just prior to adolescence it develops an awkward neuromotor trait…which deviates from the norm. It rises up on two legs and can maintain that posture for longer periods of time. While its ilium, sacrum and spinal column have been altered the rest of its body and brain have not. It is in an intermediate postural stage, has difficulty walking on all fours and can’t flee or climb with the same explosive alacrity as the others. Yet it remains part of the group and is thus protected by them. Nonetheless its anatomical transformation has created movement challenges for which it must compensate to remain viable. Because its feet lack the exquisite symmetry of the quadruped it has to rely heavily on femoral and pelvic push off and upper body strength and to get by on sheer effort. To compensate it automatically relies on neuromotor and cognitive pan mobilization, which is a mixed blessing. On one hand it has to exert more constant effort, which means the youngster must maintain a more chronically high level of alertness. That creates stress on the brain-body system. On the other hand, since it is not terribly adept at fleeing from predators a higher, more chronic level of brain stem and cortical activation makes it more perceptually and emotionally vigilant and reactive. Interestingly this trait did not originate in a mutation – metabolic changes can and do result from pervasive changes in activity level absent genetic alterations. Yet it did produce a physiological trend that could be coupled with climate change to yield substantial mutative changes.

Thus far there is no guarantee that this creature will pass on its genes. To do that it must find a way to reproduce, more specifically persuade females that its unique postural status is not tantamount to a species anomaly. Here its higher metabolism and the nature of primate sexuality come into play.

Due in part to its higher metabolic rate this creature has created a trend toward early maturation and neoteny. Just as modern humans living in tropical climates tend toward higher metabolisms and early sexual maturation the hybrid youngster matures sexually faster than its fellows, thus has more time in which to reproduce. Moreover its metabolic rate fosters a strong sex drive, compelling it to mate often. That, combined with the urgent and fairly indiscriminate nature of primate sexuality (many species of which feature coital chains, whereby males line up behind females to take their turn) enables it to create offspring with its unique anatomical, metabolic-and developmental characteristics. As a result, over time it can separate itself from the pack. A new and viable species is created.

The story doesn’t end there. Its metabolic proneness to mutation and reproductive zeal do not place it on the road to human evolution but over time climatic swings occur and its mutative inertia is tweaked to that subsequent and varying branches of its biped line become susceptible to mutations like brain expansion, further juvenilization, loss of bodily hair, increased height, gracile physique, and sexual changes in the female.

The question of why this did not happen with other primates is open to conjecture. Perhaps climate change was not enough to produce postural and encephalic mutations in other primates. Or perhaps, as the fossil record suggests, other primates did mutate but in different ways. For example one possible ancestor to both early hominids and ape (Proconsul) had ape-like features, for example no tail and relatively large body mass. However its lumbar spine was longer than in modern apes and its ulnar lacked the flexibility in wrist movements now possessed by modern apes. Thus it is clear that chimps and other apes evolved over time, just not in a bipedal direction. Or it could be that the load factor, ie. The additive combination of metabolic acceleration and climate change was necessary to induce the transformation from quadruped to biped.

In examining the differences between human and chimp anatomy, one is struck as much by the differences as the similarities. Despite the fact that we share roughly 95 percent of our genes with them, the chimp anatomy is hardly human-like. The evolution of upright walking featured enlargement of knee joints, hip joints, femoral bones, flattening of the feet, lengthening of the legs, extension of the vertical spinal column to virtually fuse with the skull and of course eventually expansion of the brain.

The Roots of Encephalization

The mutation from smaller to larger, could, along with its affect on the skeletal system, produce an increase in brain size. Large bodies require larger brains, and in order to regulate increasingly larger organs, the brain must use up more energy. Prior to the development of modern human motor and cognitive abilities the first hominids might have been faced with a significant ergonomic problem. Their bodies and brains had a high metabolic rate. Yet neither body nor brain had become as energy efficient as they would become later. That meant the first bipeds needed and conceivably had an enormous appetite. The journey to satiation would have been a long one, such that food gathering and variety in the diet might have been emphatically important aspects of their daily life.

Appetite has a way of speaking to the organism. The body knows what it needs regardless of the environment in which it lives. When carbohydrate levels are low, the organism seeks sugary foods. When protein intake is low, meats and other sources appeal to the palate. All it takes for an organism to change its diet is to have some semblance of variety in its foods to begin with (familiarity with meats, fruits etc). Body wisdom will take care of the rest. The emergence of dyspraxic/high metabolizing upright walkers might have led to a ravenous pursuit of foodstuffs, and the gratification obtained by satisfying the needs of brain and body would have reinforced the food searching, hunting and perhaps fishing activities discussed by various theoreticians. It suggests the first bipeds might have been in the process of developing a distinct personality trait one discussed by Freud under the rubric of repetition-compulsion principle. A tendency to overdo experience, to eat beyond the point of satiation, to seek beyond the point of necessity, to mate beyond the parameters of reproductive time tables, to fret beyond the immediacy of experience. If so then the biped ,might have begun to evolve into a compulsive hominid whose further evolution would via its exorbitant needs and desires lead to the extinction of animals and resources in every environment it inhabited.

Change for the Sake of Change

One other factor will be discussed with respect to human evolution. It is introduced here via the absurd question of whether future human evolution is more likely to result in larger brains or gills; in other words, whether or not evolution entails any sort of directionality.

It is a question encompassed in Darwin’s notion of chance mutations. The fact that all creatures share common genes (for example of the 13,600 genes in a fly (drosophilia melanogaster) 50% have human equivalents) one would have to say mutations occur within a broad template. Organic life is biochemically thematic and that theme controls the range and nature of mutations. When it comes to species differentiations the template becomes even narrower; mutations more predictable.

The genes for specific human traits are obviously more dominant for us than for other creatures. so the probability of mutations is always skewed by mutative precedent. That implies that as the genetic descendants of large brained arboreal, humans will be more apt to mutate toward brain modifications, increased height, bipedal structural realignments, visual processing skills and vocal-linguistic capacities than say canine enlargement. In a sense one could say we are human due to the probabilities inherent in the primate genetic template. Thus while Darwin’s notion of chance mutations might refer to the rate of mutation in any given organism, it does not necessarily apply to the nature of mutations for any given organism. Like all other info-energetic systems, phylogenic development seems to have a template around which variances occur. That does not mean bipeds were somehow destined to become human, but given our arboreal origins, the odds on some biped evolving in that direction were probably good from the outset.

 

 

 

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