The Sacrifice of Admetus: How the Evolution of Altruism Reveals our Noblest Qualities

Heracles battles Death for generosity's sake / Frederic Lord Leighton (1869-71)

Whereas great scientific theories stand the test of time when they accurately predict the natural world through repeated empirical trials, great literature transcends the ages when it speaks to universal qualities of human experience. Such inspirational works can also, without the authors realizing at the time, reveal the sublime beauty and tragedy of our evolutionary drama. Few classical authors have tapped into this zeitgeist of biological experience as the Greek tragedian Euripides. The conflict between male and female reproductive strategy and the horrific choice of maternal infanticide is powerfully presented in the story of Medea (which waited some 2,400 years before being elucidated as an adaptive strategy in primates by the incomparable Sarah Hrdy). Electra chronicles the bitter feud between parent and child that would later be revealed as encompassing a biological reality by Robert Trivers in Parent-Offspring Conflict Theory. And Helen, the haunting tale of Helen of Troy's fateful decision, evokes the evolutionary importance of female mate choice revealed through Darwin's theory of sexual selection.

However, despite his focus on tragedy, Euripides could also reveal what we as a species have long prided ourselves as a uniquely transcendent gift: generosity even amidst the most terrible of circumstances. In his lesser-known work Alcestis, Euripides has the great hero Heracles (the Greek Hercules) arriving to the home of Admetus, the King of Pherae in Thessaly. Not realizing that his wife and true love, Queen Alcestis, has just been snatched by Death at a young age, Heracles asks his dear friend for harbor and a reprieve from his many adventures. Though wrought with grief, the tenderhearted Admetus cannot deny his friend the generosity of his home and so hides his mourning for the benefit of the visiting demigod. Ignorant of the great pain felt throughout the household, Heracles unwittingly offends his hosts with his Dionysian joviality only to be clued in by one of Admetus' less obedient servants. Overwhelmed by his breach of such generosity, Heracles descends to the Underworld to confront the "black and wingèd Lord of Corpses" and wrestle the dearly departed Alcestis from Death's icy grasp. Heracles understood the depth to which Admetus had sacrificed his own well-being for the sake of hospitality, and not even Death would prevent him from honoring his debt.

Such beneficence, in a decidedly less epic but nonetheless important way, has likewise been shown in the life of the great bard of biology himself. Having spent more than twenty years privately exploring the evidence for evolution, only mentioning his heretical research to his closest friends, Charles Darwin was faced with one of the great moral challenges in the history of science. In the summer of 1858 Darwin's collected work on the topic of natural selection exceeded a quarter of a million words (roughly five hundred pages), and was only half completed, when a parcel arrived from a young naturalist working in Borneo by the name of Alfred Russell Wallace. To Darwin's surprise he found that Wallace had independently developed a theory of natural selection (which he referred to as "progression") that outlined what Darwin had spent countless hours elucidating. Scientific culture places a premium on primacy of authorship and here Darwin was holding in his hand a document that could undermine the originality of his life's work.

Darwin knew what was at stake when he wrote to his friend and mentor Charles Lyell that Wallace "could not have made a better short abstract! Even his terms now stand as Heads of my Chapters. . . So all my originality, whatever it may amount to, will be smashed." But in an act that evokes Admetus' generosity, Darwin continued by stating that, "he does not say he wishes me to publish, but I shall, of course, at once write & offer to send to any Journal." And so Wallace's outline was included alongside an abstract of Darwin's theory and presented jointly before the Linnaen Society on July 1, 1858. On the Origin of Species was published just over a year later, the first edition selling out on the day of its release.

Ironic though it may be, the very act of generosity which gave origin to the Origin has posed tremendous difficulty to evolutionary biologists ever since. What Martin Luther King, Jr. described as a "walk in the light of creative altruism" has seemed, to many, contradictory to the "selfish gene" approach of natural selection. From a gene's-eye view of the world only those traits that are successful for an individual organism and allows the maximum level of reproductive success will live on in subsequent generations. Any trait that influenced one to benefit others at their own expense would be at a disadvantage compared to individuals who merely accepted the assistance and failed to reciprocate. The schoolyard dictum that "cheaters never prosper" wouldn't seem to have any place in such a system.

Much ink, and many hours in the field, have been spent working to resolve this seeming conflict. The latest papers to do so, hitting the presses back-to-back and reinforcing each other in a fitting metaphor of the mutual assistance they document, highlights how this perceived conflict is really no conflict at all. The first to be published (on June 26, 2007 in the public journal PLoS Biology) was by Felix Warneken, Brian Hare, Alicia P. Melis, Daniel Hanus and Michael Tomasello of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. In their study they compared the innate predisposition for generosity in wild-born adult chimpanzees (Pan troglodytes, named for another Greek demigod) and human infants (aged 18-months).

The researchers set up identical conditions by which their Pan and Homo subjects observed an unfamiliar person stretching to reach an object just beyond their grasp. In multiple individual trials the researchers recorded the frequency at which each group of 36 subjects would offer their assistance by retrieving the desired object and handing it to the stranger. Contradicting previous studies of chimpanzee altruism, the researchers found no significant difference between us and our evolutionary cousins. This result was upheld even when the subjects had to put in some effort, climbing over a series of obstacles, in order to deliver the object. In a variation on these initial trials the researchers also offered the subjects a reward to elicit their assistance (toy blocks for the infants and bananas, of course, for the chimpanzees). In both cases the only significant factor was whether the subjects observed the stranger attempting to reach the distant object; a factor that chimpanzees and infants both responded to selflessly. Offering a reward for their assistance had no effect on this display of generosity. Service, it seems, was its own reward.

However, perhaps the chimpanzees had previously learned to obey human researchers in their time spent under semi-wild conditions? Would chimpanzees go out of their way to help other chimpanzees? To test this possibility the researchers constructed a door that could be opened by pulling a chain in order to access food on the other side. The researchers fastened this chain to a peg that could only be removed by a second chimpanzee in an adjoining room. In order to access the food the first chimpanzee would have to rely on assistance from the second, who gains nothing in the bargain. As before, 8 out of 9 individuals consistently helped a stranger (this time of their own species) if they saw they needed help. This, the authors reason, suggests that "the roots of human altruism may go deeper than previously thought, reaching as far back as the last common ancestor of humans and chimpanzees."

But could the roots of altruism go back even further? Apparently yes, as suggested by an additional study appearing in PLoS Biology on July 3 by Claudia Rutte and Michael Taborsky at the University of Berne, Switzerland. In a study entitled "Generalized Reciprocity in Rats" the Swiss biologists constructed a similar cooperative task as used for the chimpanzees. A baited tray was attached to a stick that one rat could pull in order to bring food within range for a second, unrelated rat's benefit. Rutte and Taborsky then conducted subsequent trials to see how often rats who had benefited in the past would be willing to help other rats in the future, the Rattus norvegicus version of the movie Pay It Forward. On average rats were 21% more likely to help strangers if they had received such help themselves.

These findings seem to fly in the face of previous theory suggesting that individuals wouldn't perform an altruistic act unless they could expect such acts to be repaid. Known as reciprocal altruism, it has traditionally been held that an individual (human and non-human alike) would only be likely to help another if the recipient had previously shown they wouldn't take advantage of such generosity. This meant that only group residents whom the individuals had previous experience interacting with would warrant their aid. It was solely among kin members, depending on the frequency of shared genes, that individuals would behave altruistically without reciprocation. However, in both PLoS Biology papers, altruism was being displayed for the benefit of total strangers. And in the case of rats the decision to offer anonymous help was determined by how much anonymous help they'd already received. Rather than contradicting reciprocal altruism, what these studies instead suggest is an expansion of the evolutionary social contract. In an environment of cooperative strangers it pays to be cooperative yourself.

While much has been made of the Darwinian phrase "survival of the fittest" suggesting that natural selection operates purely through aggressive competition, credit for the term must go to the British sociologist Herbert Spencer who had a dubious political ax to grind ("Social" Darwinism remains as his misguided legacy). However, Darwin's Origin preferred the more neutral "struggle for existence," which evokes a race against the elements rather than between individuals. It is only through Spencer's understanding of natural selection that cooperation and altruism pose a problem. For Darwin, cooperation between individuals could be an adaptive strategy in many environments as individual reproductive success increases through the safety and support of the group. Such group dynamics have been examined in detail by Robert Sussman and Audrey Garber published in the edited volume The Origins and Nature of Sociality (which Sussman also co-edited). In their metanalysis including seventy-eight published studies that covered twenty-five genera and forty-nine species of non-human primates they determined that prosimians, monkeys and apes spend the vast majority of their social lives in cooperative interactions. The study also showed that the amount of social aggression was statistically insignificant, concluding that "affiliation is the major governing principle of primate sociality and that aggression and competition represent important but secondary features of daily primate social interaction."

It was in Darwin's second great treatise on natural selection, The Descent of Man, that he offered this very line of reasoning that would wait 130 years to return full circle. With the knowledge that human and non-human animals alike were often illuminated by such walks of creative altruism, Darwin suggested that "those communities which included the greatest number of the most sympathetic members would flourish best and rear the greatest number of offspring." It is this relatively unacknowledged aspect of Darwin's theory that offers us hope in troubled times. For as surely as aggression and greed are part of our human character, so also is our capacity for cooperation and generosity.

Such an evolutionary legacy is rightly explored and celebrated through our great works, scientific and literary alike. In this way a chimpanzee reaching across species lines to help a human stranger in need can be viewed with the same appreciation as a Samaritan woman reaching out to offer water to a traveling Jewish mystic in breach of the social customs of her time. And, just as a notoriously self-indulgent demigod will transcend the boundaries of the living to repay his host's hospitality, so will a beady-eyed vermin transcend its (clearly unjustified) reputation to help a stranger with food. Through this very idea of generosity we witness how evolution can reveal our noblest of attributes. As Euripides expressed in his play Temenidae (of which, like the fossil record, only fragments remain), "When good men die their goodness does not perish, but lives though they are gone." The sacrifice of Admetus persists today as our evolutionary inheritance thanks to an unbroken chain of cooperative ancestors, who even Death himself could not prevent from sharing their gift with us.


Felix Warneken, Brian Hare, Alicia P. Melis, Daniel Hanus, Michael Tomasello (2007). Spontaneous Altruism by Chimpanzees and Young Children PLoS Biology, 5 (7) DOI: 10.1371/journal.pbio.0050184

Claudia Rutte, Michael Taborsky (2007). Generalized Reciprocity in Rats PLoS Biology, 5 (7) DOI: 10.1371/journal.pbio.0050196

Sussman, RW, Chapman AR (2004) The nature and evolution of sociality: Introduction. In: The Origins and Nature of Sociality. Ed. by RW Sussman and AR Chapman. Aldine De Gruyter: New York, pp. 3-19.


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a reward to illicit their assistance

...should be "elicit"...

Silly nitpicking aside, nice essay!