The sapiens advantage

Why are we the only human species? This is one of the most intriguing unanswered questions in evolutionary biology. Many other Homo species once existed, and some at least (notably Neanderthals and Denisovans) are known to have interacted—and for some, based on recent genomic evidence, interbred—with modern humans. But this interbreeding resulted in no persistent hybrids—only a few alleles incorporated into the Homo sapiens genome. It is indeed remarkable that there is apparently — as Varki (2016) put it — no “...other example wherein a single (sub)species from one geographic origin completely replaced all extant crossfertile (sub)species in every planetary location, with limited introgression of functional genetic material from replaced taxa, and leaving no hybrid species. Typically, one instead finds multiple cross-fertile (sub)species, with hybrid zones in between.” We alone then are “the last ape standing” (Walter 2013). And so what is the sapiens advantage? This is explored as a conspicuous theme in a recent book: Denial: Self-Deception, False Beliefs, and the Origins of the Human Mind, by medical researcher Ajit Varki, and (the late) geneticist Danny Brower. The conventional view is that early humans, at some point, evolved a cerebral ‘toolkit’ that enabled a remarkable advance in social intelligence, variously called the ‘great leap forward’, the ‘human spark’, or the ‘mind’s big bang’. This is generally attributed to cognitive functions associated (especially) with awareness of time, theory of mind, capacity for symbolic thinking, and (eventually) complex spoken language and cooperative culture — and this advance became associated (at some point) with an awareness of personal mortality, and an anxiety evoked by this awareness. As Dobzhansky (1967) put it, “A being who knows that he will die arose from ancestors who did not know.” Connected with this is a long history of literature suggesting that immortality is one of the most universal of human obsessions (Schellhorn 2008, Gollner 2013). And several writers have interpreted human motivations for mortality-anxiety buffers involving self-deception of various kinds (Choron 1964, Becker 1973, Shneidman 1973, Cave 2012, Solomon et al. 2015), including some with interpretation in terms of explicit consequences for genetic fitness, i.e. involving Darwinian evolutionary roots (Aarssen 2007, 2010, 2015). Self-deception is, of course, characteristically human (Dobzhansky 1967, Becker 1971, Smith 2007; Trivers 2011). Poet T.S Eliot mused, “...humankind cannot bear very much reality” (Eliot 1943, No. 1 of Four Quartets), and as philosopher Albert Camus (1956) put it, “Man is the only creature who refuses to be what he is.” Homo sapiens, then, is apparently the only species that has (and possibly has ever had) motivational domains that function, adaptively, to buffer mortality anxiety. And from this Varki and Brower (2013) offer their main postulate (Figure 1a): other Homo species went extinct mostly because their reproductive success was compromised (relative to Homo sapiens) by, fortuitously, never having evolved an ability to deny reality and hence deflect the anxiety of being able to foresee their own death. This is an interesting and novel hypothesis for the sapiens advantage because it involves interpretation based explicitly on evolutionary (i.e. genetic fitness) consequences of mortality-anxiety buffers. However, it is based on some unvalidated assumptions regarding the relative timing for arrival of the four principal traits involved (‘A’–‘D’, Figure 1) within


Book Review
The sapiens advantage Lonnie W. Aarssen Lonnie W. Aarssen (aarssenl@queensu.ca),Department of Biology, Queen's University, Kingston ON,Canada K7L 3N6 Why are we the only human species?This is one of the most intriguing unanswered questions in evolutionary biology.Many other Homo species once existed, and some at least (notably Neanderthals and Denisovans) are known to have interacted-and for some, based on recent genomic evidence, interbred-with modern humans.But this interbreeding resulted in no persistent hybrids-only a few alleles incorporated into the Homo sapiens genome.It is indeed remarkable that there is apparentlyas Varki (2016) put it -no "…other example wherein a single (sub)species from one geographic origin completely replaced all extant crossfertile (sub)species in every planetary location, with limited introgression of functional genetic material from replaced taxa, and leaving no hybrid species.Typically, one instead finds multiple cross-fertile (sub)species, with hybrid zones in between." We alone then are "the last ape standing" (Walter 2013).And so what is the sapiens 1 advantage?This is explored as a conspicuous theme in a recent book: Denial: Self-Deception, False Beliefs, and the Origins of the Human Mind, by medical researcher Ajit Varki, and (the late) geneticist Danny Brower.The conventional view is that early humans, at some point, evolved a cerebral 'toolkit' that enabled a remarkable advance in social intelligence, variously called the 'great leap forward', the 'human spark', or the 'mind's big bang'.This is generally attributed to cognitive functions associated (especially) with awareness of time, theory of mind, capacity for symbolic thinking, and (eventually) complex spoken language and cooperative cultureand this advance became associated (at some point) with an awareness of personal mortality, and an anxiety evoked by this awareness.As Dobzhansky (1967) put it, "A being who knows that he will die arose from ancestors who did not know."Connected with this is a long history of literature suggesting that immortality is one of the most universal of human obsessions (Schellhorn 2008, Gollner 2013).And several writers have interpreted human motivations for mortality-anxiety buffers involving self-deception of various kinds (Choron 1964, Becker 1973, Shneidman 1973, Cave 2012, Solomon et al. 2015), including some with interpretation in terms of explicit consequences for genetic fitness, i.e. involving Darwinian evolutionary roots (Aarssen 2007(Aarssen , 2010(Aarssen , 2015)).
Self-deception is, of course, characteristically human (Dobzhansky 1967, Becker 1971, Smith 2007;Trivers 2011).Poet T.S Eliot mused, "…humankind cannot bear very much reality" (Eliot 1943, No. 1 of Four Quartets), and as philosopher Albert Camus (1956) put it, "Man is the only creature who refuses to be what he is."Homo sapiens, then, is apparently the only species that has (and possibly has ever had) motivational domains that function, adaptively, to buffer mortality anxiety.And from this Varki and Brower (2013) offer their main postulate (Figure 1a): other Homo species went extinct mostly because their reproductive success was compromised (relative to Homo sapiens) by, fortuitously, never having evolved an ability to deny reality and hence deflect the anxiety of being able to foresee their own death.This is an interesting and novel hypothesis for the sapiens advantage because it involves interpretation based explicitly on evolutionary (i.e.genetic fitness) consequences of mortality-anxiety buffers.However, it is based on some unvalidated assumptions regarding the relative timing for arrival of the four principal traits involved ('A'-'D', Figure 1) within 1 'sapiens' is used here as an abbreviated, non-italicized common name for modern humans (Homo sapiens) sensu Harari (2015).This work is licensed under a Creative Commons Attribution 3.0 License.different Homo lineages, and so there is uncertainty regarding whether this hypothesis provides 'inference to the best explanation' (sensu Lipton 2000).
In particular, some extinct Homo species may have known that they will eventually die, but possibly not as early as did Homo sapiens.And for those that may have known, there is no reason to be certain that they would have felt any foreboding as a consequence of knowing -and thus no reason to suspect that they would ever have been disadvantaged by failing to evolve capacity to deny reality.They would certainly have had the kind of primal fear that we also have (and share widely with other animals), associated with 'survival drive'-i.e. an instinctual anxiety response to a perceived imminent risk to survival (e.g.hunger, or attack by a rival or predator)-thus triggering impulsive action that promotes staying alive in the face of such risks.In contrast, knowledge that one will eventually die somehow at some unknown time in the future (even though safe and well fed) primarily involves only one's imagination, with only guesses about possible details of the experience.And so, not being an imminent risk to survival, it is unclear whether this would be expected to trigger the visceral panic response associated with the drive to protect corporeal survival.I suggest that it normally doesn't, and likely especially didn't in early humans, for which reproductive success would have depended less on speculative ruminations about the future, and more on being acutely cognizant of, and prepared for, the many clear and present dangers that threatened imminent survival.Even evidence of symbolic behaviour involving intentional burial of the dead in Neanderthals (e.g.Rendu et al. 2014) suggests only mortality salience, not anxiety.This could have been associated with just a sanitation/disinfection motivation, or just an expectation of (and preparation for) some kind of afterlife assumption-conjured not necessarily out of fear (personal mortality anxiety) but simply as a creative and intriguing myth for satisfying a curious mind about where people happen to go when they stop breathing.For all we know, these burials may have involved joyful ceremonies about being thankful that a beloved family or tribal member, now expired, no longer needed to endure the routinely brutal hardships of existence (just trying to get fed and stay alive)-conjured perhaps by a general inclination to conclude (from inevitable experience) that it would/might be "better never to have been" (Benatar 2006).
Early hominids who worried too much about eventualities in the uncertain distant future then probably did not become our ancestors-at least not, I suggest, until Homo sapiens, evolved a distinctly different kind of angst in response to knowledge of personal mortality: 'self-impermanence anxiety' (SIA) (Aarssen 2010).This is about worrying that one's life is absurd (sensu Camus 1942)-meaningless, pointless, without purpose.It is a fear that is rooted not in the literal experience of bodily death-i.e.failing to stay alive per se-but rather a fear of not possessing something of 'the self' that can transcend death (Aarssen and Altman 2006).Homo sapiens, then, got SIA not because (or not just because) of knowledge that time brings eventual death-but more specifically because of knowledge that time (in bringing eventual death), inevitably annihilates all that we do, and all that we are (Aarssen 2015).It results from a capacity to recognize a fundamental truth regarding virtually all people who have ever existed (or ever will exist): it is (or soon will be) as though they never did.
The success of our species therefore involved the evolution of SIA buffers.Elsewhere (Aarssen 2007(Aarssen , 2010(Aarssen , 2015)), I have proposed that these are of two distinct types: distractions of leisure (Leisure Drive; i.e. attraction to free-time indulgence in pleasurable activeities that deliver an 'escape from self'), and delusions of legacy (Legacy Drive; i.e. attraction to delusions of being able to project an 'extension of self'-a memetic legacy-into the indefinite future).The latter commonly involves belief or faith in 'after-life' promises of religion, or recognition/fame through accomplishment of some sort.But in its rudimental form, I suggest, Legacy Drive had (and commonly still has) a more direct effect on evolutionary fitness-by favouring motivations of legacy delusion through parenthood (Aarssen andAltman 2006, Aarssen 2007).Importantly, even if we assume that Neanderthals might have evolveed self-impermanence anxiety, they may have been less likely than sapiens to evolve effective motivation for legacy delusion through parenthood-for two reasons: (i) duration of the dependent juvenile/childhood development stage was probably longer for sapiens (Walter 2013); and (ii) average life span was also probably longer for sapiens, thus increasing the likelihood of experiencing grandparenthood (Caspari and Lee 2004).Both of these reasons, then, may have bolstered opportunity (for sapiens more than for Neanderthals) to evolve (as a response to SIA) palliative delusional perceptions of being able to leave something of one's identity/personhood (ideas, beliefs, values, character, esteem, conscience, ego, will) -a copy of the 'inner self', as a memetic legacytransmitted to the future through shaping the malleable minds (and hence the 'inner self') of both offspring and grand-offspring.
Mortality salience, and anxiety because of it, was managed in modern humans then, not by evolving just capacity to deny reality/death or to be distracted from it (e.g. through Leisure Drive), but also by evolving a Legacy Drive domain that promoted gene copying and transmission success directly.In other words, "[t]his cognitive response-fear and anguish from knowing there is no immortality, combined with delusional belief in amelioration through achievement of memetic legacy-actually propelled copies of our ancestors' genes into us" (Aarssen 2010).As Barash (2012) put it, "Maybe awareness of mortality isn't merely a tangential consequence of consciousness but its primary adaptive value, if it has the effect of inducing people to seek yet another way of rebelling against mortality: by reproducing."Interestingly, at the same time however, several writers have noted that some 'anxiety disorders', like depression (including because of awareness of eventual mortality) are commonly associated with creativity and problem-solving skills, and soancestrally -may not have been a 'disorder' at all.In other words, the anxiety here represents just a symptom of having a particularly acute grasp of realitya cognitive tool that (for some of our ancestors at least) would have been highly adaptive (and thus compromised by inclinations to deny reality) because survival and reproductive success in ancestral environments would have required quick and accurate interpretation of mortality risks and how to avert them using creative solutions (Andrews and Thomson 2010, Lehrer 2010, Ghaemi 2011, Ravilious 2011).
Here then I offer an alternative hypothesis for the sapiens advantage (Figure 1b), with, I suggest, more (or at least equally) plausible assumptions, involving the following components: (i) The four principal traits ('A'-'D') evolved at different times; i.e. the key component traits of the 'great leap forward' ('A')-awareness of time, self-consciousness, theory of mind-arose together with (or were shortly followed by) awareness of mortality ('B'), which preceded self-impermanence anxiety ('C'), followed shortly or imminently by SIA buffers ('D') (Aarssen 2007(Aarssen , 2010)); (ii) Homo sapiens got 'A' significantly earlier than other Homo species, and this 'head start' was the principal sapiens advantage, leading eventually to the extinction of other Homo speciespossibly exacerbated to some extent because some of the latter also (eventually) got 'C', but not with sufficient remaining time (before impending extinction) to also get an operative 'D', and thus without extended capacity to buffer SIA; (iii) Only Homo sapiens then evolved SIA ('C') of any consequence, in combination with effective SIA buffers ('D').In other words, "…selection that favored an intrinsic Legacy Drive followed directly and quickly on the heels of selection that favored an intrinsic anxiety from knowing that we have no capacity for immortality (Aarssen 2007)" (Aarssen 2010).
According to this hypothesis then, being the first species to develop the 'great leap forward' package of cognitive traits ('A' in Figure 1) was like being 'first out of the starting block', newly equipped with evolutionary fitness potential far superior to any extant competitor or foe, with a huge 'head start', therefore, in opportunity to refinethrough effects of natural selectionthe effectiveness of these traits.No other species (of Homo or otherwise) then had a chance to 'catch up' to this sapiens advantage, and so remained unequipped to escape the impact of domination by modern humans, or simply being out-reproduced by them.Also, according to this hypothesis (Figure 1b), "…it is conceivable (probable in fact), that our ancestors evolved an inconsequential awareness that immortality does not exist, long before they evolved intrinsic fear in response to that awareness, together with intrinsic capacity for amelioration of that fear through perceptions of meme transmission through offspring" (Aarssen 2010).And even if self-impermanence anxiety had started at some point to emerge within certain other Homo species, it was too late to make much or any difference; they were already severely marginalized or virtually annihilated because of the 'head start' of Homo sapiens (Fig. 1b) in the 'great leap forward'.
In conclusion, our two hypotheses here have one assumption in common: the capacity to buffer anxiety connected with mortality salience has only ever evolved in one species-Homo sapiens.It seems likely that this capacity is what allowed our ancestors to get the most 'bang for the buck'-in terms of elevated gene transmission success-after having evolved awareness of time, self-consciousness, and theory of mind.Strong inference in support of this hypothesis is provided by more than two decades of research in terror management theory (e.g.see Burke et al. 2010), although it remains uncertain whether we are truly smart enough to know whether other animals have mortality salience /anxiety, or to know anything for that matter about what other animals are thinking (de Waal 2016).One thing though is now certain: the sapiens advantage that has endured over the past many thousands of years-the hallmark traits of our contemporary human condition ('A'-'D', Figure 1), resulting from one temporal evolutionary configuration or another-have become our disadvantage in today's world.Our lives resemble the lives of chimpanzees more than any other animal.They have a rudimental theory of mind and capacity for culture through social learning.But as Sterelny (2012) put it, chimps "…live in a world as they find it"; while humans "…live in a world as they make it."And we have made it mostly a world of delusions for chasing legacy, and a world of distractions for chasing leisure (Aarssen, 2015)-a world then in which "…cultural evolution is just one damned delusion after another.It is no wonder, as history has shown, that all great civilizations inevitably collapse" (Aarssen 2010).Miller (2006) offers the somber speculation that the reason why we have not yet made contact with other advanced extra-terrestrial life is not because we are alone in the galaxy, but because the evolution of advanced intelligence (like ours) has some deep tendency to be self-limiting, even self-exterminating.We would do well, therefore, to work harder in promoting a deeper and more broadly public understanding of the evolutionary roots of what we are, and of what our future may hold, while there still may be some time left for it to make a difference.Varki and Brower (2013) add another compelling appeal for this-the fierce urgency of now.

Figure 1 .
Figure 1.Schematic illustration of two contrasting hypotheses for why sapiens is the only extant Homo species (and why all others went extinct), based on differential timing proposed for the evolution of four key cognitive traits ('A'-'D') (see text).