Little more than a decade ago, an evolution revolution turned the study of the mind on its head. It began with a small band of researchers challenging the traditional concept that culture and child rearing serve as prime shapers of people’s behavior. The renegade theorists instead touted a potent blend of evolutionary theory, genetics, and cognitive science for determining the underpinnings of conduct. Evolutionary psychologists, as they called themselves, regarded competition among genes as the driving force of evolution. Genes that had best promoted individuals’ survival in the past spread with a vengeance throughout humankind. This process yielded hundreds or perhaps thousands of genetically programmed brain networks during the Stone Age. In theory, each neural circuit arose to address a different problem that our ancestors repeatedly faced, such as selecting an appropriate mate or detecting cheaters when making trades.
Cultures boast lots of surface differences, but even the most exotic among them rest on a universal foundation of evolved mental concerns, according to this perspective. Consider language. Evolutionary psychologists argue that the thousands of tongues spoken throughout the world derive from a common core of grammatical rules that have been built into our brains.
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Their approach has sparked much debate, as evolutionary psychologists have expanded their numbers and influence. Many of the movement’s leading lights regard their theory as the only one able to integrate nature and nurture. In The Blank Slate (Viking, 2002), psychologist Steven Pinker of the Massachusetts Institute of Technology describes evolutionary psychology as the sole scientific defense against what he regards as undying efforts by other brain and behavioral researchers to deny the existence of human nature.
Yet for all of their revolutionary fervor, evolutionary psychologists now face a challenge from counterrevolutionaries. These upstarts regard individual biological development as central to evolution. Genes participate in development, but not as the preeminent players proposed by Pinker, they assert.
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For instance, alternative theorists often cite the work of so-called evolutionary-developmental biologists, who probe a small set of regulatory genes that coordinate embryonic growth and body shape in a variety of animals. Even minor adjustments in these genes can trigger brain-shaking evolutionary transitions from one species to another in a surprisingly short time.
“Aside from evolutionary psychology, other viable ways exist to take evolution seriously in studying human behavior,” says psychologist Steven J. Scher of Eastern Illinois University in Charleston. Examples of this upstart movement appear in Evolutionary Psychology: Alternative Approaches (Kluwer, 2002), edited by Scher and philosopher Frederick Rauscher of Michigan State University in East Lansing.
According to several new frameworks, discoveries in evolutionary developmental biology indicate that the interplay of genes, cells, organisms, and habitats orchestrates physical and mental growth in members of a species. Survival-enhancing adjustments in the interaction of these elements spread through a population under the impetus of Darwinian natural selection.
In this view, developmental forces assemble flexible brain networks that foster novel types of learning rather than play out ancient instincts. These latest theories posit that natural selection acts at the junctures of critical biological entities. For instance, the cellular machinery that coordinates gene action provides the environment for genes. Those genes’ products, mediated by the cell, that aid survival or reproduction then achieve dominance in a population.
Some alternative theorists take a more moderate stance. They argue that cultural innovations and human manipulations of the natural world have influenced genetic evolution, as well as the reverse (SN: 11/25/95, p. 366).
It’s tough to find ways to test such ideas in rigorous experiments. When the distinction between nature and nurture breaks down, as it often does in alternative evolutionary approaches, the empirical sledding suddenly gets much tougher than usual.
Evolutionary psychology works from the proposition that natural selection creates people’s depth perception, spiders’ web spinning, and other specialized traits out of random genetic mutations and recombinations within a fertilized egg. Yet it’s behavior, not genes, that jump-starts evolution, according to psychologist Gilbert Gottlieb of the University of North Carolina at Chapel Hill.
Changes in animals’ rearing environments in the womb and after birth unleash previously dormant forms of genetic expression and set the stage for behavior changes, Gottlieb contends. Successful new behaviors and the developmental arrangements out of which they grew then gain currency in a population.
Systematic tweaks in rearing styles instigate this process by disposing animals to behave in novel ways, Gottlieb proposed in the April Psychological Review. For instance, researchers have found that mice separated from their mothers for a few minutes each day during the first 3 weeks after birth exhibit more exploration of unfamiliar settings and superior learning of new tasks as adults than do mice with no early separations. These disparities in curiosity and intelligence seem to arise because mouse moms bestowed extra care and grooming on the pups after their brief disappearance. This fortified the pups’ budding physiological networks for handling stress, such as that evoked by strange surroundings, in Gottlieb’s view.
Also, much evidence indicates that people and other animals learn to prefer experiences–even initially troubling separations–that become familiar early in life, and they try to recreate those experiences as adults.
The apple maggot fly offers a glimpse of how such developmental effects translate into real-world evolution, Gottlieb adds. One variant of this creature in the United States mates and lays its eggs on apples, while a neighboring variant does the same on hawthorn fruits. It’s known that, over decades, the flies first preferred hawthorn fruits and some later switched to apples. The two sets of flies, presumably on their way to forming separate species, now follow distinct patterns of hatching, courting, and mating because apples mature earlier in the year than hawthorn fruits do (SN: 7/21/01, p. 42: Alarming Butterflies and Go-Getter Fish).
Flies often become strongly attracted to odors from their host fruits. However, experiments show that the apple variant routinely accepts both apples and hawthorn fruits as hosts. In contrast, only a small portion of the hawthorn variant tolerates apples. This suggests that in the ancestral hawthorn-reared flies, population pressures of some kind induced a small number of flies already able to endure apple odors to infest the new fruit, Gottlieb theorizes. Natural selection then spread changes in the population’s genome and in the timing of larval and pupal development that increased survival on apples.
Crucial behavioral shifts may also have sparked human evolution. Consider a surprisingly small-brained, 1.75-million-year-old Homo erectus skull from western Asia that’s described in the July 5 Science. This ancient individual belonged to one of the first groups of human ancestors to migrate out of Africa, say anthropologist David Lordkipanidze of the Georgian Academy of Sciences in Tblisi and his coworkers.
The skull, which housed a brain less than half the size of an adult’s brain today, challenges the longstanding view that our ancestors left Africa only after evolving large brains that enabled them to adapt to new environments.
This find makes sense to Gottlieb, however. One or more H. erectus groups may first have left their African homeland and learned to get by in the Asian wilds before experiencing the spread of genetic and developmental changes necessary for brain growth. Intriguingly, only a single change in a regulatory gene is needed to produce mice with big brains topped by wrinkled, humanlike outer layers, according to a report in the July 19 Science.
“We need to look for developmental processes that could have brought novel features of anatomy and behavior into existence among our fossil ancestors,” remarks anthropologist Ian Tattersall of the American Museum of Natural History in New York City. “Useful novelties would have taken on an evolutionary life of their own.”
On the levels
If evolution operates as a hierarchy of interacting biological levels, then DNA participates in a team effort rather than enjoying star status. From one generation to the next, genes work within a web of influences that collaborate to create individuals as well as cultural practices, argues psychologist Linnda R. Caporael of Rensselaer Polytechnic Institute in Troy, N.Y.
“Our physical and psychological characteristics emerge from interactions of recurring resources, including genes, their cellular machinery, social and cultural resources, regular features of the physical world, and the ongoing results of individual development,” Caporael says.
In other words, there’s far more to inheritance than DNA. Caporael’s argument dovetails with a theory, held by zoologist Kevin N. Laland of the University of Cambridge in England, that people and other animals affect their own evolution by reworking their physical and social environments in each generation. Cultural innovations, which Laland says occur in animals ranging from fish to primates, transform individuals’ behavior and thus have the potential to alter a population’s genetic makeup.
Caporael further argues that people have evolved to think not in isolation but within four “core” social settings: two-person relationships with intimate functions such as sex and infant care, five-person work or family groups, 30-person bands that provide the focus for social identities, and 300-member “macrobands” that periodically gather to exchange resources, members, and information.
The size of core groups may vary across generations and cultures, but people continually reassemble and experiment with these social units to meet environmental challenges, Caporael asserts.
The interrelated roles of people in such groups yield a social whole that is greater than its individual parts, proposes biologist David S. Wilson of the State University of New York in Binghamton. Even a society with a population of far more than 300 people can operate as a kind of organism, thanks to the unifying power of moral codes and religious beliefs, Wilson says. A cursory look at world history shows that spiritual belief systems sometimes degenerate or turn horribly destructive. However, the collective actions of religious groups have often enabled their members to survive and reproduce more effectively than they could have on their own, Wilson writes in Darwin’s Cathedral: Evolution, Religion, and the Nature of Society (University of Chicago Press, 2002). Religions evolve mainly through the transmittal through the generations of shared beliefs about the world and expectations about moral behavior toward other believers, in Wilson’s view.
Symbolic facets of culture, including religion, are outgrowths of a process of brain development that has evolved a tremendous thirst for adaptive discoveries, holds neuroscientist Steven R. Quartz of the California Institute of Technology in Pasadena. An extended period of human brain growth enables individuals to learn about all sorts of rewarding features in their surroundings, he theorizes. Human groups may have needed this open-ended neural capacity to survive the Stone Age’s jarring climate and habitat fluctuations (SN: 7/12/97, p. 26).
The expansion of individual development reshaped the entire human brain (SN: 3/16/02, p. 166: Heads Up: Problem solving pushed bright primates toward bigger brains), rather than sculpted separate brain networks for specific jobs, Quartz argues. Reward systems in the brain direct attention in general ways and enable brain cells to hook up to underlie learning.
For instance, research suggests that inner-brain structures instill a general preference in babies for looking at facelike configurations. With increasing exposure to faces, parts of the brain’s outer layer take over facial recognition and may contribute to other forms of visual expertise (SN: 7/7/01, p. 10: Faces of Perception).
“Human brain development is more protracted and more sensitive to environmental signals than evolutionary psychologists have supposed,” Quartz says.
Evolutionary psychology’s spotlight on genes leaves much of human sexual behavior shrouded in darkness, according to some upstart theorists.
The evolution of creatures throughout the animal kingdom depends not only on reproduction but also on survival, or “economic,” functions, says biologist Niles Eldredge of the American Museum of Natural History. That latter category consists of all the exchanges of matter and energy, such as eating food and excreting waste, that sustain life and make reproduction possible.
Moreover, in humans–and in one chimpanzee species, as well–sex no longer serves only as a way to make babies, Eldredge says. People use sex to make money, to exert power over others, and to escape into interludes of sensory pleasure, to name a few purposes.
Reproductive, economic, and sexual functions have interacted with cultural traditions throughout human history and prehistory, Eldredge points out. It’s thus dangerous to assume that only a reproductive imperative to spread one’s own genes and those of relatives orchestrated modern sexual behavior.
For example, some evolutionary psychologists have theorized that women everywhere prefer to mate monogamously with men who have lots of resources and status, as a way to ensure the survival of offspring. Anthropologist Sarah Blaffer Hrdy of the University of California, Davis argues that only when social rules require all marital property to be inherited through the husband’s family, as became common in the post–Stone Age world of agriculture and large cities, do women seek wealthy mates. In differently structured societies, women pursue more sexual variety, Hrdy says.
Evolutionary psychologists have tested and revised promising theories of short- and long-term mating strategies in both sexes, responds evolutionary psychologist Timothy Ketelaar of New Mexico State University in Las Cruces. The trick now is to devise experiments that pit predictions made by evolutionary psychologists against those proposed by upstart theorists, he says.
There’s a fitting Darwinian ring to this suggestion. Where better than the science of the mind to apply the concept of survival of the fittest?
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