Marc Hauser's Horizons article 'The possibility of impossible cultures' (Nature 460, 190–196; 2009) carries an implicit assumption that cardinal aspects of human uniqueness arose by positive natural selection because they were beneficial to ancestral hominins. But this may not be the whole story.

Among key features of human uniqueness are full self-awareness and 'theory of mind', which enables inter-subjectivity — an understanding of the intentionality of others (see, for example, N. J. Emory and N. S. Clayton Annu. Rev. Psychol. 60, 87-113 2009). These attributes may have been positively selected because of their benefits to interpersonal communication, cooperative breeding, language and other critical human activities.

However, the late Danny Brower, a geneticist from the University of Arizona, suggested to me that the real question is why they should have emerged in only one species, despite millions of years of opportunity. Here, I attempt to communicate Brower's concept.

He explained that with full self-awareness and inter-subjectivity would also come awareness of death and mortality. Thus, far from being useful, the resulting overwhelming fear would be a dead-end evolutionary barrier, curbing activities and cognitive functions necessary for survival and reproductive fitness. Brower suggested that, although many species manifest features of self-awareness (including orangutans, chimpanzees, orcas, dolphins, elephants and perhaps magpies), the transition to a fully human-like phenotype was blocked for tens of millions of years of mammalian (and perhaps avian) evolution.

In his view, the only way these properties could become positively selected was if they emerged simultaneously with neural mechanisms for denying mortality. Although aspects such as denial of death and awareness of mortality have been discussed as contributing to human culture and behaviour (E. Becker The Denial of Death; Free Press, 1973), to my knowledge Brower's concept of a long-standing evolutionary barrier had not previously been entertained.

Brower's contrarian view could help modify and reinvigorate ongoing debates about the origins of human uniqueness and inter-subjectivity. It could also steer discussions of other uniquely human 'universals', such as the ability to hold false beliefs, existential angst, theories of after-life, religiosity, severity of grieving, importance of death rituals, risk-taking behaviour, panic attacks, suicide and martyrdom.

If this logic is correct, many warm-blooded species may have previously achieved complete self-awareness and inter-subjectivity, but then failed to survive because of the extremely negative immediate consequences. Perhaps we should be looking for the mechanisms (or loss of mechanisms) that allow us to delude ourselves and others about reality, even while realizing that both we and others are capable of such delusions and false beliefs.