The literary critic Harold Bloom once wrote that Shakespeare "invented the human." But that's not true, of course. Evolution did. Homo sapiens, like all other living things, is a biological critter, a product of evolution by natural selection. This means that whatever else we may be -- artful manipulators of language and symbols, composers of symphonies, splitters of atoms no less than of logs -- we are also concatenations of genes that have evolved to do their best at copying themselves and then to kick those copies into the future.
In theory, there could be worlds of the imagination -- novels, movies, plays, stories and songs -- in which human biology played no role, in which people didn't eat, sleep, communicate with each other or reproduce. Or in which there was no sexual identity, no predisposition to care preferentially for one's own children or relatives, no predictable patterns of love, anger, competition or cooperation. The result would be a kind of science fiction or wild fantasy.
But such imaginary excursions of extreme "inhuman-ness" are rarely undertaken, almost certainly because wholesale departures from the recognizably human are not only very difficult but also incomprehensible and thus unlikely to be interesting.
Even the bizarre creatures conjured up in Harry Potter books, "Lord of the Rings" or the various "Star Wars" movies (especially the wonderful bar scene in the first film), for all their imaginative anatomical variety, retain demonstrably human motivations and relationships, just as -- centuries after their conception -- Hamlet, Don Quixote and Achilles retain their vitality and their relevance because they retain their humanity.
That's how we like our literary figures: real, believable, true to human nature. Like us, they must be gooey, breathing, eating, sleeping, defecating, reproducing, evolving and evolved Homo sapiens, shaped by genetics and evolution, and then twisted and gnarled by life itself.
This is what lies behind Beowulf's foolhardy courage, Heathcliff's obsessive passion, Jane Eyre's spunkiness, Huck Finn's mixture of naivete and wisdom, Augie March's antic yearning for self-realization.
There is something instantly recognizable about such basic, obviously natural traits as Romeo and Juliet's hormonally overheated teenage love, Hamlet's intellectualized indecisiveness, Lady Macbeth's ambition as well as her remorse, Falstaff's drunken cavorting, Viola's resourcefulness, Lear's rage.
Take Othello. Evolutionary scientists know that males are especially vulnerable to sexual jealousy simply because of their biology. Whereas women can rest serene in the confidence that they are genetically related to any offspring that emerges from their bodies, men have to take their mate's word for it. Othello, as a perfectly good male mammal, is therefore susceptible to suspicions of marital infidelity by his wife, Desdemona. Add the fact that sperm-makers are selected (naturally) to compete (often violently) with other sperm-makers for access to egg-makers, and Shakespeare's tragedy makes biological sense.
The flip side of male-male competition is "female choice." While the sperm-makers are busily jousting with each other, egg-makers are positioned to choose among them, and the poet laureate of such choice is Jane Austen. The bottom line in each of her novels is getting each of her main characters appropriately paired, and in each case, Austen serves up textbook examples of what females -- including but not limited to Homo sapiens -- are inclined to choose: good genes, good resources and good behavior. Nor are these examples unique. The key concepts in evolutionary psychology are regularly reflected in the great works of literature; we have, for instance, kin selection in "Little Women," reciprocal altruism in Steinbeck and parent-offspring conflict in "The Brothers Karamazov."
Biology is not the only science present in literature. Physics, for example, is also relevant; there are certainly calculations of force, mass and momentum that could yield insights into Anna Karenina's encounter with a train. No one, however, would suggest basing literary analysis on quantum mechanics, string theory or general relativity.
But biology is different. Human nature pulses inside every writer, and, when artfully communicated, is understood by every reader because it is so deeply shared. It is the breath and beat of living organisms embodied in an organic world of sex, blood, food, fear, anger, love, hopes, trees, animals, air, water, sky, rocks and dirt. Now that biologists have begun clarifying their perspective on what it means to be human, it is time to look for it for ourselves where it has always been -- in our greatest, most resonant stories.