In our earliest history, so far as we can tell, individuals held an allegiance toward their immediate tribal group, which may have numbered no more than ten or twenty individuals, all of whom were related by consanguinity. As time went on, the need for cooperative behavior – in the hunting of large animals or large herds, in agriculture, and in the development of cities – forced human beings into larger and larger groups. The group that was identified with, the tribal unit, enlarged at each stage of this evolution. Today, a particular instant in the 4.5- billion-year history of Earth and in the several-million-year history of mankind, most human beings owe their primary allegiance to the nation-state (although some of the most dangerous political problems still arise from tribal conflicts involving smaller population units).

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If we survive these perilous times, it is clear that even an identification with all of mankind is not the ultimate desirable identification. If we have a profound respect for other human beings as co-equal recipients of this precious patrimony of 4.5 billion years of evolution, why should the identification not apply also to all the other organisms on Earth, which are equally the product of 4.5 billion years of evolution? We care for a small fraction of the organisms on Earth – dogs, cats, and cows, for example – because they are useful or because they flatter us. But spiders and salamanders, salmon and sunflowers are equally our brothers and sisters.

I believe that the difficulty we all experience in extending our identification horizons in this way is itself genetic. Ants of one tribe will fight to the death intrusions by ants of another. Human history is filled with monstrous cases of small differences – in skin pigmentation, or abstruse theological speculation, or manner of dress and hair style – being the cause of harassment, enslavement, and murder.

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The time has come for a respect, a reverence, not just for all human beings, but for all life forms – as we would have respect for a masterpiece of sculpture or an exquisitely tooled machine. This, of course, does not mean that we should abandon the imperatives for our own survival. Respect for the tetanus bacillus does not extend to volunteering our body as a culture medium. But at the same time we can recall that here is an organism with a biochemistry that tracks back deep into our planet's past. The tetanus bacillus is poisoned by molecular oxygen, which we breathe so freely. The tetanus bacillus, but not we, would be at home in the hydrogen-rich, oxygen-free atmosphere of primitive Earth.

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There may be a time, as I describe in Part III of this book, when contact will be made with another intelligence on a planet of some far-distant star, beings with billions of years of quite independent evolution, beings with no prospect of looking very much like us – although they may think very much like us. It is important that we extend our identification horizons, not just down to the simplest and most humble forms of life on our own planet, but also up to the exotic and advanced forms of life that may inhabit, with us, our vast galaxy of stars.

Humans evolved to trust a small select group of individual, but as we live in a world community, a biologically diverse community, and eventually an outer space community, we must evolve culturally to see appreciate our differences.

The in-group-vs.-out-group double standard, which had and has such catastrophic consequences, could in theory be eliminated if everyone alive were considered to be in everyone else’s in-group. This utopian prospect is remote, but an expansion of the conceptual in-group would expand the range of friendly, supportive, and altruistic behavior. This effect may already be in evidence in the increase in charitable activities in support of foreign populations confronted by natural disasters. Donors identifying to a greater extent with recipients make this possible. The rise in international adoptions also indicates that the barriers set up by discriminatory and nationalistic prejudice are becoming porous.

The other potential benefit is genetic. The phenomenon of hybrid vigor in offspring, which is also called heterozygote advantage, derives from a cross between dissimilar parents. It is well established experimentally, and the benefits of mingling disparate gene pools are seen not only in improved physical but also improved mental development. Intermarriage therefore promises cognitive benefits. Indeed, it may already have contributed to the Flynn effect, the well-known worldwide rise in average measured intelligence by as much as three IQ points per decade over successive decades since the early twentieth century.

Marcel Kinsbourne explains how recognizing all human beings as part of our in-group can promote inter-marriage, which will diversify our genes and improve our overall health and well-being.