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