Homo sapiens are at the pinnacle of hierarchy of life on planet Earth. As a bipedal ape species, the phenotypic limitations of humans are numerous: humans are medium-sized, can neither run fast nor fly, are acutely intolerant to heat and cold, can hold their breath in water for no more than a few minutes and are clumsy swimmers. In addition, their vision is sensitive only to a very narrow range of the electromagnetic spectrum; their auditory range and acuity is poorer than that of many animals; and their sense of smell is vestigial at best.
And yet, humans have a uniquely supreme mental capacity: humans alone are capable of elaborate symbolic thought; of manipulating the environment to suit their needs; of creating sophisticated tools; of using logic and reason to study and understand the mysteries of the Universe; of developing many religions and the arts; of communicating ideas through a sophisticated medium called language; of having self-awareness; and of desiring to know the ultimate purpose of life. Humans have begun to understand that the human brain, arguably the most complex phenomenon in the known universe, makes all of these unique abilities possible and have begun investigating its inner workings. In other words, in the quest to find out who humans really are, the human brain is turning inwards and looking at itself.
Clearly, all of these incredible human abilities imply an unprecedented sophistication in cognitive abilities, which in turn implies a profound change in the structure and/or the function of the human brain. The important question regarding which cognitive trait makes humans unique still remains unanswered. Did various unique human cognitive and behavioural traits, such as thought, language, music, self-awareness and religion develop independently, or is there a more fundamental mental mechanism that facilitated the development of these diverse traits? Although controversial, there is considerable archaeological evidence that modern human behaviour appeared abruptly 50,000-40,000 years (50-40 ky) ago (Klein, 2000). Thus, the law of parsimony would argue for a single, fundamental mental trait causing fairly simultaneous sophistication of various cognitive traits, rather than independent development of diverse traits. In addition, it appears that anatomically modern humans had appeared by approximately 200 ky ago and that the human brain reached its present size by at least 100 ky ago (Klein, 2000). Thus, there seems to be a lag period of about 50-150 ky between the appearance of modern human anatomy and modern human behaviour. Further, it appears that the profound change in human cognition was achieved with only minor structural modifications of the human brain.
Therefore, any framework attempting to explain the uniqueness of human behaviour must preferably implicate a single mental trait, explain how the proposed trait facilitated sophistication of other cognitive traits, account for the abrupt appearance of modern human behaviour approximately 50-40 ky ago, and provide a neural framework to explain how such sophistication was achieved with only a minor structural modification of the human brain.
The aim of this article is to:
1. Hypothesise that sophisticated time-awareness (STA) is the cognitive trait that makes humans unique.
2. Explain how STA can explain various unique human qualities, such as tool-making ability, language, logical thought, notion of causation, episodic memory, mental time travel, self-awareness, religion and music.
3. Hypothesise a mechanism for the evolution of STA.
4. Explain that STA can account for the abrupt appearance of modern human behaviour.
3.1. Evolutionary Background
The theory surrounding the evolution of humans from hominids is controversial. Two major models exist: the Multiregional model (Wolpoff et al. 1988) and the Out of Africa model (Stringer, 2003). The Multiregional model contends that hominid ancestors migrated and settled in various parts of the Old World, and regional populations slowly evolved into modern humans. In contrast, the Out of Africa model proposes that modern humans evolved from hominid ancestors only in Africa and then migrated to the rest of the world, replacing all other existing hominid populations. Although inconclusive, the bulk of anatomical, archaeological and genetic evidence favours the Out of Africa model.
Approximately 2.5 million years (my) ago, the Homo genus branched out from the sub-tribe Hominina (Fig. 1).
There were likely many intermediate species of the Homo genus in the subsequent 1.8 my. Approximately 600 ky ago, Homo heidelbergensis appeared, which appears to be the common ancestor to both Homo neanderthalensis (the Neanderthals) and Homo sapiens (Tattersall, 2011). The Neanderthals migrated out of Africa and settled in various places, including Europe. The other species continued to evolve in Africa, and from this lineage, anatomically modern Homo sapiens appeared approximately 200 ky ago. It is notable that both the anatomically modern Homo sapiens of Africa and the Neanderthals of Eurasia, like their hominid predecessors, do not show any evidence of behavioural modernity such as sophisticated tool-making, specialised hunting techniques, structural “ruins”, rituals, cave art, or personal adornment. Such behavioural modernity appeared only in African Homo sapiens approximately 50-40 ky ago (Klein, 2000).
Considerable controversy exists as to whether modern human behaviour actually appeared 50-40 ky ago, whether it was abrupt or gradual, and whether the change was brought about by biological (genetic), demographic or socio-cultural factors. These and other controversies regarding the evolution of modern human behaviour are discussed by Klein (Klein, 2000). Even after conceding that archaeological records can be inherently noisy, there is fairly consistent evidence of a radical change in human behaviour 50-40 ky ago in Africa. I agree with Klein’s view that the change appears to be relatively abrupt. This sudden appearance of modern human behaviour implies a fairly simultaneous increase in the sophistication of various human cognitive abilities, such as symbolic thought, language, self-awareness and the arts. The question of how this simultaneous development in human cognitive abilities occurred remains unanswered. What is clear is that such abrupt sophistication in human cognition is the result of a profound change in the function of the human brain, without any significant increase in its size.
The size of the hominid brain increased for approximately 2.5 my, from ~650 cc in Homo habilis to ~1350 cc in Homo sapiens. The issue of whether the rate of growth was gradual or marked by periods of punctuated equilibria is controversial (Lee & Wolpoff, 2003). The possible reasons for the enlargement of the hominid brain have been discussed previously (Dunbar, 1998; Gibbons, 1998). Notably, the Neanderthal brain is as large as the human brain, if not larger. Additionally, it is significant that no differences are observed in the skulls between anatomically modern and behaviourally modern Homo sapiens (Johanson, 2001). Moreover, the brain size of early Homo sapiens reached that of modern humans by at least 100 ky ago (Klein, 2000). It follows that a large brain alone was not sufficient for the development of sophisticated cognitive traits in humans. One theory proposes that major neural reorganisation resulted in a significant enhancement of the manner in which the human brain processes information (Klein, 2000). Tattersall highlights that the profound change in human cognition that occurred approximately 50-40 ky ago was achieved with only minor structural modifications of or additions to the human brain (Tattersall, 2011). However, the question of what these changes were remains unanswered.
The concept of modularity of cognition proposes that cognition evolved as a collection of mental modules in response to environmental stimuli, with natural selection favouring the modules that were successful in aiding survival and reproduction (Barrett & Kurzban, 2006). The development and sophistication of such cognitive modules could have ultimately resulted in traits like tool-making, language, thought, music, and self-awareness. Notably, many of these traits are known to have existed in earlier hominids, especially in Neanderthals, but never reached the level of sophistication observed in modern humans. Some of the increased sophistication can be explained by adaptive evolution of existing modules, but this does not fully explain the relatively abrupt, simultaneous and wide-ranging changes in human behaviour that occurred 50-40 ky ago. These changes can be explained by the appearance and/or sophistication of a more fundamental, domain non-specific cognitive module, which could have simultaneously augmented the functions of other pre-existing modules, leading to an evolutionary phase transition. Thus, the unprecedented sophistication in human cognition could have occurred without a significant increase in brain size.
I hypothesise that the sophistication of such a mental module led to the sophisticated awareness of time in humans, and that this domain non-specific module acted as the catalyst for modern human behaviour. In essence, the human cognition is denominated by a sophisticated awareness of time, which is what makes humans unique.
3.2. Sophisticated Time-awareness (STA)
Time-awareness refers to the conscious perception that time exists. It is the awareness that there is a present, a past and a future. Pöppel lists the following ‘elementary time experiences’: (i) duration; (ii) non-simultaneity; (iii) order; (iv) past and present; and (v) change, including the passage of time (Pöppel, 1978). Time-awareness is the subject of intense philosophical and scientific inquiry, and various concepts pertaining to it, such as that of specious present, are discussed elsewhere (Brown, 1990; Le Poidevin, 2009).
Time is fundamental to life. The appearance and evolution of life on Earth is a function of time. All life processes are governed by time – for example, the life cycle of birth, development, reproduction, senescence and death; seasonal changes in the behaviour of flora and fauna; the circadian rhythm; the homeostatic mechanisms involved in maintaining the internal milieu; and the genetic code.
It is important to differentiate time-awareness from so-called ‘time-sense’. Since all biological processes are governed by time, the behavioural response of a living organism to an external stimulus is also a function of time. Thus, loosely applied, the term time-sense can be a property of all behavioural responses of living organisms, including those of unicellular organisms such as amoeba. Higher animals such as birds and mammals may exhibit even more complex behaviour with respect to time, which may give the impression that they have have an ‘awareness’ of time. An example is the presence of circadian rhythm; another is an aspect of animal behaviour called ‘chaining’. Clayton and her colleagues have found that scrub jays demonstrated an awareness of where and when a particular cache of food was stored, and an ability to alter their caching strategies to minimize stealing by other birds (Clayton et al. 2003). But it must be doubted whether non-human animals possess an awareness that time exists. For these reasons, it is better to avoid the term ‘time-sense’ and instead use the term time-awareness to denote cognitive awareness that time exists. Even among those few species which are purported to have some degree of time-awareness, it is doubtful whether they possess all of Pöppel’s elementary time experiences. The qualifier, ‘sophisticated’, was added to time-awareness to differentiate it from primitive time-awareness, which may be present in lower animals, and where one or more of Pöppel’s elementary time experiences are absent.
Closely related to time-sense is the concept of a priori knowledge of time. First expounded by Immanuel Kant, a priori knowledge of time involves the organisation of sensory inputs from various sense organs according to time, which helps an organism appropriately respond to sense data. A priori knowledge of time also exists in non-human animals. It has been shown experimentally that in lesser animals, the variable of time can be altered to elicit various responses. For example, rats can be conditioned to make a particular response, such as pressing a lever, after a specific time interval to avoid receiving an electrical shock (Mackintosh, 1983). Again, it must be doubted whether such animal behaviour ascribed to a priori knowledge of time implies sophisticated time-awareness.
Additionally, many lesser animals can sort sensory data with respect to three-dimensional space (a priori knowledge of space), sometimes with more acuity than human beings; however, it is well-established that the conscious ability to manipulate objects, images and symbols in a three dimensional mental space (space-awareness) is a unique human quality. Unlike other animals, humans thus have a mental world, a stage characterised by cognitive awareness of space and time, and are able to live for much of the time in the world as they recreate it in their heads (Tattersall, 2011). I hypothesise that this unique ability is the result of sophisticated awareness of time and space.
It is possible that time-awareness co-evolved with space-awareness because, as was shown by Einstein, the space-time continuum is a fundamental property of reality. It would therefore be appropriate to refer to an evolution of space-time-awareness, which could be the result of the evolutionary relationship of humans and their sensory organs to the relative space-time reference frame found on Earth, where objects move at a fraction of the speed of light, so that time and space appear as independent entities. In addition, Earth’s gravity is weak enough not to obviously distort the space-time fabric, so that space-time appears independent of gravity. The implication is that the mental concept of time may also be relative because it was shaped by the relative physical time in which humans evolved.
For the purpose of this article, the discussion will be restricted to STA, rather than space-time-awareness, because one could argue that STA is more fundamental, more intriguing, and needs further elaboration. However, I would like to emphasise that such an approach does not underestimate the possible contribution of sophisticated space-awareness in defining human uniqueness. The role of sophisticated space-awareness or, better, space-time-awareness, in human symbolic thinking could be the subject of another article.
3. 3. Evidence for STA
Section 3.3 will provide corroborative evidence showing how STA is the common underlying mental trait for most of the uniquely important human qualities. It is acknowledged that the evidence is empirical and observational, and not that obtained from formal experiments. However, it must be noted that formal experimental evidence is impossible for a hypothesis concerned with evolution of human uniqueness.
The ability to create and use tools is present in many animals. For example, it is well known that chimpanzees can fashion a tool from a twig or a branch to probe termite nests. But none of the animals can match the level of sophistication of humans. Modern airplanes, rockets, computers, nano-robots, and artificial satellites are all examples of the unprecedented ingenuity of humans as tool-makers.
The evolution of hominid tool-making has been discussed previously (Ambrose, 2001; Klein, 2000; Tattersall, 2011), with some salient points noted here. Hominid tool-making appeared at least 2.5 my ago in the form of simple stone flakes chipped off rock cores. The archetypal tool made by subsequent members of the Homo genus was the stone hand-axe, which appeared approximately 1.5 my ago. Over the subsequent hundreds of thousands of years, there was only a gradual, step-wise refinement in hominid tool-making. Even in the case of Neanderthals of Eurasia and the anatomically modern Homo sapiens of Africa, there was no significant innovation in tool-making. However, sometime during the Middle Stone Age (MSA)/Middle Paleolithic Age (Fig. 1), there was an abrupt increase in the sophistication and refinement of tool-making techniques resulting from the invention of a method of hafting stone bits on wooden handles or shafts by MSA/Middle Paleolithic hominids (Klein, 2000). This resulted in the archetypal throwing spear, which first appears in archaeological excavations dating to approximately 200 ky ago. The throwing spear confers the enormous advantage of being able to hunt from a distance and hunt animals that are much larger in size.
It is pertinent to review the cognitive advances associated with refinements in hominid tool-making. The stone flake tools made by the earliest hominids imply a cognitive leap that involved an appreciation of the qualities of materials and some degree of planning and foresight (Tattersall, 2011). The archetypal stone hand-axes made by later hominids were formed to a standard symmetrical teardrop shape, implying for the first time the existence of a ‘mental template’ in the minds of the tool-makers (Tattersall, 2011); this also implies the emergence, to some degree, of space-awareness. The predominance of one cerebral hemisphere in handedness also evolved with such tool-making. In addition, I propose that the appearance of the throwing spear represents the most radical of all cognitive advances associated with tool-making; this is because the throwing spear is a composite, multi-part tool, and its construction requires assembly.
The likely steps involved in the making of a throwing spear, constituting a mental rule, are as follows:
1. Make a stone blade with flaked edges (like the hand-axe, but smaller).
2. Fashion a shaft from the branch of a tree.
3. Make a slit at one end of the shaft.
4. Have some type of binding (natural fiber or animal sinew) ready.
5. Place the stone blade in the slit.
6. Tie it firmly with the binding (hafting).
Notably, each step is followed by an implicit ‘then’; i.e., the steps are sequential. Humans have an innate awareness that step 2 follows step 1 and so on. Such concepts as ‘sequential’ and ‘simultaneous’ are possible only because of STA, which is what enables humans to formulate mental rules with a sequence and to rearrange the steps of a mental rule; it also enables humans to nest multiple mental rules within one another to create a hierarchy. The lack of STA in non-human animals severely limits their ability to form sophisticated mental rules needed for multi-part tools.
Consider, for example, if the intuitive knowledge that the aforementioned steps should form a sequence were to be lacking. Then, the number of possible permutations of mental rule sequences for a given number of steps is provided by the formula n!/(n-r)! , where n is the total number of steps available and r is number of steps used in the permutation. Since, in this case, it is assumed that all the steps are used in the permutation, i.e., n=r, it follows that (n-r)!=1, and the formula is reduced to n!. Thus, when there are 2 steps involved, the number of possible mental rules is only 2, whereas for 3 steps it is 6. For 6 steps, as in the case of afore-mentioned spear-building mental rule, the number of possible rules is 720 (6!). Thus, a unit increase in the number of steps results in a combinatorial explosion of the number of possible rules.
It is possible that non-human animals are able to execute a two-step mental rule that can enable them to make primitive tools purely because of chance: because there are only two ways of performing a given task, the chances are high (50%) that they will be successful using a simple trial and error heuristic. Once accomplished, the correct sequence can be committed to procedural memory. It is appropriate to mention here that although the stone hand-axe made by early hominids implies some degree of cognitive sophistication, its creation can be accomplished by a simple mental rule (possibly involving no more than two or three steps) that is repeated. This repetition of steps is called iteration. Simple mental rules, such as trial and error heuristic or iteration, can also be nested to create a hierarchy, though this nesting is limited to two or three orders. As the number of steps or orders in a mental rule increases, because of combinatorial explosion, it becomes less probabilistic for non-human animals to execute the correct sequence without STA, especially when they are not equipped to remember that a particular permutation has already been tried before. The fact that humans possess a uniquely elaborate working memory may thus be directly related to the presence of STA. Note that humans can improve upon the sequence by moving, for example, Step 4 to Step 2; this ability to rearrange and refine mental rules of this nature is due to STA.
In essence, humans are able to perform a mental selection of the ‘fittest’ thought sequence out of many possible permutations based upon STA. Thus, STA effectively opposes the combinatorial explosion of possible thought sequences. It is interesting to note that combinatorial explosion is a type of randomness found in Nature. The similarity to natural selection, in which the fittest physical trait that confers survival advantage is selected out of many random mutations, is notable.
3.3.2. Logic and Reason
Logic, the foundation of reason, philosophy, mathematics, science and technology, could not have developed beyond a primitive level if it was not anchored by STA. The two important components of logic are inductive and deductive reasoning. Inductive reasoning, in which generalisations based on specific events are made, and deductive reasoning, in which specific conclusions based on generalisations are made, are modes of reasoning based on STA.
The abstract concept of numbers was one of the first mathematical concepts to evolve, and provides an interesting example of logical reasoning. The earliest known archaeological evidence of numbers is tally marks found on the Lebombo bone dating back to approximately 35 ky ago (Bogoshi et al. 1987), which is consistent with the hypothesis that modern human behaviour appeared approximately 50-40 ky ago. Although numbers were assigned sophisticated symbols (possibly due to sophisticated space-awareness) only much later, the concept of ordinality (for example, that 2 is greater than 1) is based upon the awareness that 2 ‘comes after’ 1. In other words, the concept of numbers may be a form of cross-modal abstraction of STA. Thus, the concept of numbers in sequence and the subsequent development of arithmetic and mathematics would not have been possible without STA. It is notable that geometry, another branch of mathematics to evolve early in human history, involves the study of objects and shapes, and is likely the result of sophisticated space-awareness. However, the thought process involved in solving geometric problems also involves logical mental rules that are underpinned by STA.
To further generalise this idea, mathematical theorems, proofs, formulae, equations, and algorithms are all embedded with a sophisticated concept of time. In essence, all human endeavours based on logic and reason, such as philosophy, mathematics, science and technology, are built upon STA.
Some form of primitive communication exists in many non-human animals, but language has reached a high level of sophistication only in humans. Considerable controversy exists on the subject of the evolution of human language (Corballis, 2009). This article does not attempt to propose a new theory regarding the origin of human language but rather shows how STA is fundamental to the development of human language.
Human language has three basic components: lexicon, semantics and syntax.
Syntax, in particular, is considered to be a uniquely human ability. Syntax is nothing but a mental rule specifying how words in a sentence should be sequenced to convey a specific message. It is notable that tool-making and language centres, which are among the earliest cognitive modules that developed in humans, are in close proximity in the Broca’s area in the human brain, which is the brain area almost entirely responsible for the syntax of language. It has been proposed that the syntactic rules for language are derived from the original rules that evolved for planning and assembling a composite tool (Ramachandran, 2010). Like the mental rules for multi-part tool making, such syntactic rules of language also must be anchored by STA.
Some early forms of language (proto-language) used by hominids, such as the Neanderthals, most likely involved making simple sounds and grunts. It is likely that their lexicon included only mono- or di-phonemic words, thus severely limiting the scope of the language. More complicated words can be formed only when many phonemes are combined. For this to happen, the phonemes must be in precise sequence to consistently denote the same object, event or action; this ordering requires STA. It follows that poly-phonemic words must have begun to be added to the lexicon, thus contributing to its richness, only when time-awareness became sophisticated in humans.
The remaining component of language, semantics, developed primarily because of the capacity to learn by imitation (see Section 3.7 on mirror neurons), and STA probably directly contributed little to its development. However, semantics, the meaning conveyed by language, would be incomplete if it does not provide an answer to the question of when. Thus, a mental concept of time is indispensable to semantics as well.
Human speech is uniquely characterised by vocalising sounds in a precise sequence. The fact that reversing phonemes in a phrase or a sentence, for example, “I am human,” is counter-intuitive and incomprehensible shows the extent to which time is ingrained in spoken language. The same applies to other forms of language. Writing or reading would be difficult if the symbols were not written and comprehended in precise sequence. STA is thus ingrained in every aspect of language. Of course, such language skills also involve the unique human ability to manipulate symbols in mental space.
Teleology, or the notion of causation, is a unique human cognitive trait that attempts to explain phenomena in terms of cause and effect. This is one of the earliest cognitive traits to appear during the course of human life; children as young as three years old can assign causes and effects to events (Keleman, 2004). It is not unreasonable to presume that teleology could have developed early in the evolution of modern human behaviour 50-40 ky ago.
Teleology gives humans an insight into cause and effect, and the ability to manipulate causes to achieve desired results. Moreover, the notion of causation is a central principle in the scientific method. It also gives rise to the philosophical doctrine that everything that exists must have a cause or a purpose. Misapplication of this doctrine may be what causes humans to believe in astrology, superstition, intelligent design, or a supernatural creator.
Cause precedes effect and effect follows cause. Thus the notion of causation would be impossible without STA. It is possible that some primitive notion of causation existed in those hominids that existed prior to 200 ky ago, as evidenced by their ability to create primitive tools, such as stone hand-axes, and by their probable mastery of fire. I would argue that the human notion of causation differs from such primitive notions in its ability to assign causes and events to more than two events. In other words, humans are able to sequence many events to form a chain of causation, e.g., “A is the cause of B, which in turn is the cause of C.”. The similarity of such a mental rule to those of multi-part tool-making, logical thought and language is striking. It is clear that such sophistication in the notion of causation is possible only because of STA.
3.3.5. Episodic Memory
Two main types of long-term human memories have been described. Procedural or non-declarative memory refers to unconscious memories, such as skills (e.g., learning to swim). Declarative memory refers to memories that can be consciously recalled, such as facts and events. Declarative memory contains two sub-types, semantic and episodic memories. Semantic memory stores general factual knowledge about the world that is independent of personal experience, such as the fact that bananas are yellow. Importantly, procedural memory and semantic memory are not unique to humans.
Episodic memory, on the other hand, is the memory of specific events and can be thought of as a catalogue of memories of events together with their spatio-temporal relationships, which are stored in a more or less correct sequence. This type of memory is unique to humans (Tulving, 2002). The distinguishing feature of episodic memory is the assignment of time and space to events before they are stored. The uniqueness of episodic memory in humans can thus be explained by the uniquely human trait of STA (and sophisticated space awareness).
3.3.6. Mental Time Travel
Because humans organise episodic memories in approximately correct sequence, they can engage in mental time travel, reliving past episodes in vivid detail. More interesting is the ability to engage in open-ended forward time travel to anticipate and plan for the future. Mental time travel has been proposed as a unique human quality (Suddendorf & Corballis, 2007) and also as a mechanism for evolution of language (Corballis, 2009). Although its uniqueness to humans has been challenged by others (Clayton et al. 2003), non-human animals have not been consistently found to have sophisticated mental time travel. Like episodic memory, mental time travel is only possible in humans because of STA.
The phenomena of foresight and free will are related to mental time travel. The ability to imagine the possible future effects of present actions (foresight) and to choose a particular course of action from among many (free will) would be impossible without STA.
3.3.7. Existence-Awareness and Self-Awareness
Another fundamental trait that characterises humans is the awareness of existence. Although it is often conflated with self-awareness, it is worth noting that the awareness of self-existence also implies the simultaneous awareness of existence of other people or things, so-called other-awareness (Ramachandran, 2010). Sense of self, as part of existence-awareness, gives humans a sense of identity and continuity, and a focal point for sensations, experiences, thoughts, emotions, desires, goals and actions. An important consequence of existence-awareness is the “theory of mind”, which refers to the unique awareness of thoughts and intentions in self and others.
Study the following axiomatic statement: We are traveling in time just by existing. It follows that awareness of time and awareness of existence are closely inter-linked; awareness of time can lead to awareness of existence and vice versa. This article proposes the primary appearance of STA; existence-awareness appeared or became more sophisticated as a consequence of STA. In other words, when humans developed STA, they became aware of the existence of self and others.
It is generally accepted that human consciousness was born when a sense of autobiographical self was introduced into the mind process. Since it is argued that self-awareness appeared as a consequence of STA, it follows that emergence of human consciousness is closely linked to that of STA.
Additionally, the idea that episodic memories may be intimately linked to a sense of self has been previously suggested (Schacter, 1996). Every episodic memory, by definition, entails a mental representation of the self as the agent or patient of some action or as the stimulus or experiencer of some state (Kihlstrom, 1997). This link between episodic memory and the sense of self in humans can be explained by the idea that both self-awareness and episodic memory are direct results of STA.
The appearance of STA and existence-awareness is a unique event in the history of evolution. Though they provide humans with unique cognitive gifts, which enable the manipulation and mastery of the environment for human benefit, there are also adverse effects. Humans are the first species to recognise some profound and disturbing realisations: that they exist (existence-awareness); that there is something called the future (STA); that all future events are uncertain except death. Such realisations, especially certainty of mortality, can be frightening. Most, if not all, ritualistic, religious, artistic, scientific and philosophical endeavours are likely the result of such realisations.
3.3.8. Religion/ Faith
As noted in Section 3.3.7, awareness of existence, uncertainty of future and certainty of mortality are frightening concepts for humans. Combined with the propensity to misapply the teleological module (note that all these are the result of STA), it is easy to appreciate the human tendency to draw conclusions such as the following: the Universe must be created by immanent god(s); there must be a purpose to life; part of self will live even after death; suffering endured in the present life must be due to sins of a past life; sinners will be punished in an after-life; disasters, disease and death are due to wrath of a god(s) and can be appeased by rituals; it is possible to predict life events by the position of planets and stars at birth; or, a failure in an endeavour is due to someone else casting an envious evil-eye. Thus, human religious beliefs, rituals, astrology and other superstitions can be explained by the appearance of STA.
The ability to create and appreciate music is not only a universal human quality but also a unique one. Various components of music, such as melody, harmony and polyphony, are differentiated by the factor of time. A musical piece takes shape and form because of the pattern of musical notes or chords with respect to time. Rhythm, another basic component of music, is also a manifestation of time.
Another idea pertains to the human ability to identify sounds of specific frequencies as musical notes. This ability is probably a direct result of the manner in which humans are hard-wired to appreciate time. Note that frequency itself is a function of time. Perhaps signals in the higher auditory neural network that represent frequencies of specific musical notes ‘resonate’ with the intrinsic activity of the neurons responsible for STA to produce the sensation of music. Thus, music cognition may be a by-product of the way human brains are wired for sophisticated time cognition.
The link between STA and the ability to create and appreciate works of art is difficult to deduce. Though creation and appreciation of art involves the unique ability to imagine symbols and objects (due to sophisticated space-awareness), art may have evolved as an evolutionary defence mechanism designed to distract humans from fear about the inexorable passage of time and ultimate death. Creation of a work of art may represent an attempt to freeze time or a means to defeat death and achieve immortality. Contemplation of a work of art is likely a method used to ignore, albeit temporarily, the sense of self and time. These ideas can be generalised further to argue that the human sense of aesthetics developed as a survival tool to counteract the adverse effects of STA.
The link between STA and humour is also, at present, tenuous. Humour invariably has a narrative that culminates in an unexpected and harmless twist. Narrative or story-telling, built-up anticipation, deflation of expectation and inconsequential new interpretation are due to STA. It is also well known that the most important determinant of good humour is timing. Humuor is thought to have evolved as a mechanism for managing the struggle against the ever-present fear of death (Ramachandran, 2010). As noted in Section 3.3.7, fear of death is a result of STA and sophisticated existence-awareness.
Thus, STA appears to be the cognitive trait that unites the three major areas of human endeavour, namely science, religion and the arts.
The ideas discussed thus far are schematically presented in Fig. 2.
3.4. Evolution of STA
The mechanism of evolution of STA in humans is unknown and can only be speculated at present. It is highly plausible that the overall evolution of time-awareness is non-linear.
As mentioned in Section 3.2, all biological processes are a function of time. It follows that the neural processing of sensory input and the resultant behavioural response is also a function of time. It is well-known that neural signals from sensory organs reaching the central nervous system are arranged in precise temporal sequence. Further processing of these signals in the central nervous system must preserve this temporal information. It follows that the information regarding time is fundamental to all neurological processes in lower animals with a nervous system as well as to the subconscious mental processes of the human brain.
As life evolved further, it is plausible that there was a meta-representation of time from the lower brain centers to the evolving neo-cortex, possibly leading to a proto-time-awareness. This proto-time-awareness may represent the information regarding time that is present in all biological processes of the body and in the external world events arriving as sensory data. After a few further million years of gradual evolution in bipedal apes, it is possible that the proto-time-awareness in hominids underwent a radical sophistication about 200 ky ago, leading to a full-blown sophistication of time-awareness, which includes all of Pöppel’s elementary time experiences. It is thus proposed that STA is an emergent property of neocortical neural circuits.
The underlying mechanism that led to the sophistication of proto-time-awareness to STA is unknown. Though it might have happened de novo due to genetic mutation, it is also possible that it could have happened secondary to sophistication of other pre-existing cognitive traits; there are a few such possible candidates.
One candidate is hominids’ attempts to create more sophisticated tools; such attempts could have resulted in mental rules for the creation of multi-part tools. It is not difficult to envisage the natural selection of such rules because of the enormous survival advantage they confer. Mental rules for creation of multi-part tools must involve the ability to sequence or rearrange the steps involved in such rules. Time-awareness could have been a by-product of such mental rules. Thus, STA could have been the result of a ‘positive, reinforcing feedback loop’ between multi-part tool-making and its mental rule. In other words, STA could have appeared as a second-order meta-rule of multi-part tool-making. It is well-known that, in complex domains, such positive, reinforcing feedback loops can lead to non-linear, dramatic results.
Another possible candidate that could have led to STA is communication. As hominid hunting techniques became more sophisticated involving larger and more dangerous animals, it was imperative for individuals to co-operate. Such co-operation likely entailed sophistication of communication. At first, such communication was probably done by simple hand gestures and/or simple sounds and grunts. Natural selection would have favoured sophistication of communication because of the survival advantage conferred by it. Sophisticated communication (language) must involve the ability to sequence gestures or sounds to convey complex information. As in the case of sophisticated tool-making, there could have been a positive, reinforcing feedback loop between sophistication of communication and its mental rule, and STA could have been a by-product of such a process.
It is notable that such feedback loops for tool-making and communication could have co-existed and co-evolved because the selection pressure posed by the environment was likely the same, namely the need to hunt larger and more dangerous animals. Thus, there likely must have been ‘cross-talking reinforcement’ of these feedback loops which could have accelerated the appearance of STA. Again, in complex systems with interdependence of many variables, due to ‘butterfly effect’, a small change in some of the variables can produce accelerated, non-linear outcomes.
This hypothesis about the evolution of STA correlates well with the finding that the skulls of anatomically modern humans about 200 ky ago, such as the Cro-Magnons, show a rapid expansion of the Broca’s area (Klix, 2001), the brain region predominantly involved in tool-making. The finding that tool-making and language centers are in close proximity in the Broca’s area of human brain also lends further credence to this hypothesis. Once established, STA could have, in turn, resulted in further sophistication of tool-making and language (Fig. 2).
Other possible mechanisms include hominid attempts to master fire, or geographical, social-demographic and climatic factors. Again, such mechanisms could have acted synergistically to produce STA.
3.5. STA and Human Behavioural Modernity
Whatever the mechanism of evolution of STA, I hypothesise that it is the cause for the appearance of modern human behaviour about 50-40 ky ago. Through millions of years of adaptive evolution, the human nervous system became aware of time, the fundamental but mysterious phenomenon of Reality that underpins all life, including all biological processes. Such a mental awareness of time also proved to be useful for other evolving cognitive processes. What likely followed is an example of the evolutionary mechanism of exaptation, in which awareness of time was applied to other pre-existing (and co-evolving) cognitive traits such as proto-self and memory resulting in an rapid increase in their sophistication, and an abrupt change in the behaviour of Homo sapiens in Africa. In other words, once sophisticated mental awareness of time arose by whatever mechanism, the entire neo-cortex with its cognitive modules was ‘synchronised’ to it – the human neo-cortex thus became a time-aware organ. This mechanism of exaptation could have occurred over a relatively short period of approximately 150 ky, from the appearance of anatomically modern humans ~200 ky ago to the appearance of modern human behaviour in Africa ~50-40 ky ago. Thus, STA is the innovation, the neural substrate permitting symbolic thought (Tattersall, 2011), and the biological (neural) change that led to human behavioural modernity (Klein, 2000).
Sections 3.6 and 3.7 compare the STA hypothesis with other extant theories about human uniqueness.
Recursion is a process or a function that calls itself as its sub-routine. Recursion is a well-known feature of human language. Recursive syntax enables us to embed phrases within phrases, e.g., “Jerry, who ate the cheese, was chased by Tom.” Corballis has argued that recursion is characteristic of many human traits in addition to language, such as “theory of mind”, mental time travel, manufacturing, the concept of self and perhaps religion, so much so that a recursive mind is the principal characteristic that distinguishes humans from all other creatures (Corballis, 2007).
A closer examination of recursion in language and other human traits reveals that although recursion may be unique to humans, it is unlikely that it is the principal distinguishing characteristic of the human mind. It must be noted that recursion itself is underpinned by a mental concept of time. Recursive syntax in human language involves embedding a new phrase within an existing phrase, which would be impossible to accomplish without STA. Recursion is but one type of mental rule based on STA. Other rules anchored by STA could include ordinality, iteration, and conditional logic. Therefore, STA is a more fundamental property of the human mind than recursion, and it makes human beings unique.
3.7. Mirror Neurons
Mirror neurons are an active area of research in the field of cognitive neuroscience (Rizzolatti, 2008). Mirror neurons are implicated in various human traits, such as self-awareness, theory of mind, ability to empathise, and to mimic and learn skills, such as language, from others. Mirror neurons are thought to be responsible for the unique ability of humans to transmit knowledge and skills to others, an essential component of culture, and have been implicated in the cognitive leap that resulted in the appearance of modern human behaviour approximately 50-40 ky ago. However, several questions remain unanswered. First, mirror neurons are not unique to humans. Mirror neurons are also present in the brains of lesser animals; for example, the brains of apes, such as orangutans and chimpanzees, are rich in mirror neurons. Orangutans are especially good at mimicking the actions of others (Ramachandran, 2010). It is not unreasonable to suppose that the brains of hominids were also rich in mirror neurons. Thus, it is still unclear why mirror neurons have attained such a degree of sophistication only in humans. Second, mirror neurons are not responsible for all unique human traits. Mirror-neurons are most likely not involved in the development of innate, creative traits, such as tool-making, language, logical thought and music, but they do play a major role in learning and transmitting the ideas and skills resulting from these traits.
When STA appeared approximately 200 ky ago, the mirror-neuron system was likely one of many neural circuits that incorporated that STA through the mechanism of exaptation. Thus, an awareness of time could have increased the sophistication of the mirror neuron functions. It is not difficult to imagine a synergistic effect between the sophistication in creative human cognitive abilities, such as tool-making, logical thought, or language, and the sophistication in the ability to transmit and learn the skills and knowledge that arose from those abilities. This synergy could have augmented the abrupt appearance of modern human behaviour that occurred 50-40 ky ago. STA can thus explain the sophistication observed in the human mirror-neuron system, which implies that STA is more fundamental than the cognitive traits enabled by mirror neurons.
3.8. Time Neurons
I propose that there is a sophisticated network of neurons in the high-level centres of the human brain that is responsible for conscious awareness of time; it is appropriate to call such a network of neurons “time neurons.” These time neurons likely assign a denominator of time to most, if not all, neural activities in the higher brain centres responsible for human cognition. This network of neurons must be distinct from the time-keeping neurons of low-level brain centers, also present in non-human animals, which are responsible for circadian rhythm.
The structural organisation of time neurons is open to speculation, but it is likely that they are not localised to one region of brain. Because this article hypothesises that STA probably evolved as a by-product of the mental rules for multi-part tool-making and language, time neurons are most likely found in Broca’s area. Because of their unprecedented utility, they likely made connections with other brain regions responsible for human cognition, such as Wernicke’s areas (language), the inferior parietal lobule including supramarginal and angular gyri (mental image of one’s own self and the world outside, logical thought, mental picture of intended actions, arithmetic, cross-modal abstraction, etc.) and the prefrontal cortex (goal-orientation, foresight, ambition, morality, self-worth, etc.). It is not difficult to imagine such interconnections developing without a significant increase in the overall size of the human brain. As an aside, the individual variation in human intelligence can be explained by a variation in the nature of the interaction between time neurons and neural networks responsible for other human cognitive traits. Additionally, variation in the distribution of time neurons between the right and left cerebral hemispheres can account for some of the differences in their functions.
Another possible role of time neurons is as a mediator of the meta-representation of temporal relationships between events from the low-level to the high-level brain centres. One study has analysed the sequence of brain activation in humans during time perception (Rao et al. 2001). Initial activation occurred in lower brain regions, such as the basal ganglia, followed by the activation of various cortical regions, such as the right inferior parietal cortex and bilateral premotor cortex. The meta-representation of time probably occurs through this or a similar pathway involving time neurons. It is open to speculation whether the lower brain regions involved in circadian rhythm and a priori knowledge of time contributed to the primary evolution of time-awareness.
Time neurons are also likely to be inter-connected to the neural network in the human brain-stem which is responsible for wakefulness. Because humans are unaware of time when sleeping, the state of wakefulness is essential for STA. Notably, brain-stem centres responsible for the state of wakefulness are also necessary for human consciousness. Because a sense of self is fundamental to human consciousness, and STA is fundamental to the sense of self (see Section 3.3.7), it follows that the state of wakefulness, time-awareness and human consciousness are intimately connected. The neurons responsible for the state of wakefulness represent the stage on which the time-neuron system is the conductor orchestrating the various neural networks responsible for human cognition to produce the grand symphony of human consciousness.
The mental perception of time is subjective and can be altered by the state of the brain. The curious phenomenon of déjà vu, in which the subject feels that a particular novel experience has happened before, is probably due to a misfiring of these time neurons. The fact that time appears to decelerate during periods of anticipation and accelerate during periods of enjoyment is evidence that these time neurons can be modulated by the activity of other areas of the brain. The fact that time appears to accelerate with age may be related to the possible variation in the activity of time neurons with age. It is also possible that alcohol or psychoactive drugs alter the sense of time by affecting the activity of these time neurons.
3.9. Critique and Areas for Further Inquiry
Because this article attempts to construct a framework based upon incomplete and controversial data from inexact sciences and using mostly speculative thought, the effort is vulnerable to potential pitfalls of reasoning such as narrative fallacy and the problem of induction. An attempt is made here to critically examine the resultant hypothesis. Although it is likely that other issues will be uncovered, the following sections will discuss some important concerns regarding the STA hypothesis.
The hypothesis has its origins in the insight that STA is the human cognitive trait that led to sophistication in tool-making, logical thought, mathematics, language and the notion of causation, and the common principle underlying this sophistication is a mental rule which enables humans to temporally sequence or rearrange the underlying steps or sub-units involved. Subsequent analysis revealed that many other unique human traits also involve STA as an underlying root. The fact that seemingly unconnected traits, such as elaborate working memory, episodic memory, mental time-travel, music, self-awareness, other-awareness, religious beliefs, rituals, and superstitions, can be explained by STA lends some substance to the hypothesis. A preliminary attempt has also been made to link STA with aesthetics and humour. Though speculative, it is possible that STA can also explain other human traits, such as emotions of love and compassion, sense of morality (ethics), and concepts such as money, debt and financial markets. It can also be hypothesised that STA is indispensable to the unique human ability to cook food.
Subsequent analysis was centered on the evolution of humans from hominids. Although controversial, this article assumes that modern human behaviour appeared abruptly because the bulk of evidence favours it. The hypothesis presented here can elegantly explain such a model by invoking the evolutionary mechanism of exaptation of STA to other evolving cognitive traits. Again, such an incidental and unexpected fit lends further credence to the hypothesis. However, some archaeological findings do not support the theory that modern human behaviour appeared abruptly. It must be noted that the question of whether human behavioural modernity appeared gradually or abruptly need not in any way undermine the hypothesis that STA is the cognitive trait that led to modern behavioural patterns; STA can also explain the hypothesis that modern human behaviour appeared more gradually.
Section 3.3.7 makes a claim that STA led to existence-awareness, which in turn led to a cascade of unique human behavioural traits. Such a claim is likely to be dismissed as a leap of faith rather than an act of logical deduction. The insight that STA and existence-awareness are intimately connected occurred upon analysis of the axiomatic statement that “we are traveling in time just by existing.” It is acknowledged that axioms are products of human cognition and may lack universal truth. However, short of attacking the very nature of axioms, the logical deduction that STA can lead to existence-awareness and vice versa is difficult to refute.
Although time-awareness is sophisticated enough in humans to make them unique, it is a matter of conjecture whether it exists in lesser animals. Elephants, for example, are thought to have some degree of self-awareness (Plotnik et al. 2006). If self-awareness is intimately related to STA, does it follow that elephants have some degree of time-awareness? Exploring such possibilities could provide another area of research.
Autistic children are known to have defective mirror neuron systems, which impairs their self-awareness and their ability to empathise. Perhaps there is a subset of autistic children who also have defects in STA.
It is fully acknowledged that the idea of time-neurons is purely speculative and that there is no evidence for its existence. However, absence of evidence does not necessarily mean evidence of absence. Nevertheless, it is acknowledged that the human brain may process time-awareness by an entirely different mechanism – it is possible that there is no dedicated neural network for STA, but that various regions of the human neo-cortex communicate the information of time-awareness by synchronised oscillations in neuronal activity. Brain imaging has indicated that remembering the past and imagining the future activate a common “core” network (Addis et al. 2007). Studies like these will clarify the question of if such a network is involved in STA. Such studies will, no doubt, be immensely helped by further sophistication in brain imaging techniques.
The speculative claim made in Section 3.3.9 that music cognition is probably a by-product of the way humans are wired for STA needs to be investigated.
Genetic studies exploring the differences between Neanderthal and Homo sapiens DNA could shed light on the question of if minute differences between their respective genotypes can explain the possible development of STA.
4. Summary and Conclusions
Sophisticated awareness of time is the cognitive trait that makes humans unique. STA probably appeared as a by-product of mental rules that evolved for multi-part tool-making and/or communication approximately 200 ky ago. Exaptation of this STA to other cognitive traits caused an evolutionary phase transition 50-40 ky ago that resulted in human behavioural modernity. Thus, STA led to the exponential development of thought, language, religion, philosophy, mathematics, music, the arts, science and technology. It is the trait that led to cultural explosion and ultimately to human civilisation. In short, humans became behaviourally modern by becoming aware of simple but profound concepts of time, such as ‘before’, ‘after’ and ‘simultaneous’, and that ‘the clock is ticking’. It is probably more appropriate to call behaviourally modern humans Homo horo sapiens.
Although there are important qualitative differences between the human mental concept of time and the relative physical time found on Earth, it is important to bear in mind that the former is a derivative of the latter and, therefore, may also be relative. The implication is that, at present, the human cognitive traits derived from STA, such as logical thought, language, notion of causation, self-awareness, and cultural heritage, such as theology, philosophy, mathematics, and science, may be relative. Nevertheless, the human cultural revolution resulting from STA, as part of evolution in general, is still ongoing.
It appears that, in humans, evolution has made a quantum leap from the biological level to the cognitive level. At the biological level, evolution is based upon natural selection; at the cognitive level, it is based upon mental selection. Both evolutionary mechanisms oppose the combinatorial explosion and other forms of randomness found in nature. Natural selection opposes random genetic mutations, and mental selection opposes random thought permutations. Both forms of evolution are rooted in a form of time; the root for biological evolution is the relative physical time (or space-time, to be more precise) found on Earth, and the root for cognitive evolution is a mental form of time (or space-time). Such a line of reasoning leads to some intriguing questions: was there a similar quantum leap in the evolution from the inorganic level to the biological level? Was inorganic evolution based upon another type of selection? Did that selection also oppose natural randomness? Was such inorganic evolution driven by another form of time (or space-time)? Just as consciousness is an inevitable result of the ‘transformation’ of the relative space-time found on Earth to a mental form of space-time, is the origin of life and its evolution on Earth simply an inevitable manifestation of the ‘transformation’ of a ‘primordial’ or ‘undifferentiated’ space-time to the relative space-time found on Earth? In other words, is life an inevitable result of the values taken by the fundamental variables of Reality that results in the particular space-time reference frame found on planet Earth? It is notable that such a line of reasoning opens the way to potentially reconcile the concepts of relativity of space-time and evolution by selection.
The hypothesis that STA makes humans unique leads to new avenues for inquiry and brings us closer to understanding the true nature of humanity and where humanity will go in the future.
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