Explaining the evolution of cooperation — one of life’s most common, complex, and paradoxical phenomena
It’s easy to take cooperation for granted. Children team up to complete a project on time. Neighbors help each other mend fences. Colleagues share ideas and resources. The very fabric of human society depends upon working together. Cooperation is also ubiquitous in the natural world: lions collaborate on hunts, flowers share nectar with bees, and even bacteria produce essential resources that benefit their neighbors. But cooperation goes beyond mere quid pro quo — mutual aid for mutual benefit — and also takes the form of extreme self-denial. Worker ants give up reproduction to help their colony, and humans’ very bodies – like the bodies of all multicellular organisms – are the product of cooperative self-sacrifice. In a sense, all forty trillion cells in the human body sacrifice themselves in order for the few sperm or egg cells to pass on. This sort of cooperation presents a profound puzzle that dates back to Darwin: if traits persist or disappear according to their contributions to reproductive fitness, why would a trait — like resource-sharing or self-sacrificial behavior — that could reduce or eliminate an organism’s reproductive ability ever manage to stick around?
Key parts of that puzzle have been answered, but some big questions — and many fascinating avenues for research — still remain in the study of cooperation. Sir John Templeton was a firm believer that cooperation was one of the most important human virtues, and that insights from biology and psychology could help people learn to cultivate and optimize its benefits. In that spirit, the John Templeton Foundation recently commissioned a white paper providing a deep dive into the current scientific understanding of cooperation, at scales ranging from single cells to complex societies. It’s a story that shows how much the scientific understanding of cooperation has advanced, and highlights exciting new areas for experimentation as researchers develop ways to watch cooperation evolve in the laboratory.
In its initial form, Darwinian natural selection had trouble explaining most kinds of self-sacrificial cooperation. An organism willing to defend its colony to the death might feel noble, but the behavior didn’t make evolutionary sense: if having the most offspring was all that mattered, then individual self-preservation should be the rule. The key insight that began to unravel this paradox came from the realization that natural selection operates not just on organisms but, more fundamentally, on their genes. An organism will adopt a cooperative behavior, the theory states, if the benefits of cooperation will flow toward gene-sharing relatives. The bee, sacrificing itself to sting an intruder, gives up one whole set of its genes in exchange for hundreds of half-sets of its genes – a fair trade in the eyes of selection.
This idea was formalized by English evolutionary biologist W.D. Hamilton in 1964, and is understood most straightforwardly through the simple dictum called Hamilton’s rule: an altruistic trait will spread if its benefits to the altruist’s relatives are greater than the cost to the altruist itself. Hamilton’s insight, which has been termed kin selection, and the more general concept of inclusive fitness (the measure of fitness that takes into account social effects) has guided recent empirical work across the tree of life, from microbes to mammals.
EVOLUTION OF COOPERATION: THE SACRIFICES OF SLIME MOLDS
One of the most intriguing model species for the study of cooperation is the simple slime mold Dictyostelium discoideum, one of the rare types of organism that can be either single-celled or multi-celled. For most of its life cycle Dictyostelium exists as a single-celled amoeba, crawling around the soil eating bacteria. When the amoebas begin to run out of food, however, tens of thousands of them can aggregate to form a multicellular “slug” a few millimeters long. This slug crawls along the soil to a good anchoring location, at which point a remarkable transformation begins to take place. Some of its cells, roughly 20%, form a vertical stalk jutting from the ground. To do so, they have to sacrifice themselves, dying to form rigid stalk material. The remaining 80% of the cells climb up the stalk where they become reproductive spores. From there, they can be dispersed by the wind to new locations, where they start the life cycle over as single-celled organisms.
In a pioneering series of experiments, Joan Strassmann and David Queller at Washington University in St. Louis have shown how cooperative traits function and evolve in populations of Dictyostelium. They were able to separate lines of Dictyostelium cells that were essentially cheaters unwilling to engage in self-sacrificial behavior. Predictably, those cells had more offspring initially (since they climbed over their more-sacrificial neighbors to become spores). Once the Dictyostelium colony produced a generation with too many freeloaders, however, stalks couldn’t develop, making reproduction impossible. Notably, the cheater cells were more likely to develop in colonies where the individual amoebas were not closely related — underscoring the truism of kin selection, that self-sacrifice makes evolutionary sense if it enables an organism’s close genetic relatives to reproduce. Observations of Dictyostelium in the wild have underscored this finding and offered added insights due to another cooperative behavior, this one between species: Dictyostelium engages in a form of farming, ingesting certain bacteria without killing them, carrying them through their dispersal stage, allowing them to colonize the new area, and then harvesting them prudently. The bacteria benefit from being farmed, and sometimes even infect non-farming groups of Dictyostelium to induce them to take up the practice.
Humans may not practice the most extreme forms of cooperation — that award might go to the ant or the bee — but we do seem to be unique in how our cooperative behaviors are shaped not only by the rules of genetic evolution but also by the ways that cultures change and evolve. Cultural evolution is similar to genetic evolution in that it is shaped by a process of selection, in which the cultural replicators, or “memes,” are spread through the population. Yet there are also important differences, including the ways in which genes and memes are inherited, and the much faster pace of cultural evolution. While genetic evolutionary theory makes clear predictions about what traits will evolve (those that maximize fitness), the same can’t be said for cultural evolution, in part because it has rapidly pushed humans out of the environment to which they had adapted.
The concepts of kin selection and inclusive fitness can still explain a lot of human cooperative behavior. The most obvious example is parental care, in which benefits flow to offspring, who share half of a parent’s genes. There is a wealth of evidence that humans prefer to help relatives, that they help closer relatives more than distant ones, and that humans evolved in small groups where relatedness was high, making cooperation a successful adaptation from a genetic perspective.
While it might take several generations to calculate whether an individual act of human self-sacrifice was genetically effective, the costs and benefits of mutually beneficial forms of cooperation can be measured within a single generation, or even a single interaction.
Growing empirical evidence gathered through fieldwork and through lab models suggests that humans have evolved psychological mechanisms (including ethical systems and reputational status) to regulate cooperation. In addition, cooperative behaviors vary across individuals and cultures, and institutions, such as religions, can impact individual cooperation. One study of Christian participants found that priming them with passages from the Bible was uniquely effective in increasing their willingness to cooperate in a game.
One of the most fascinating long-term studies of real-life cooperation was carried out by Lee Cronk and his colleagues, who examined the implicit rules governing the osotua system of mutual self-help practiced by the Maasai in East Africa. In osotua, the requests for help — typically for cattle — are only made according to need, without reference to the kind account-keeping used in other forms of debts. Gifts are given as long as the benefactors can do so without harming their own herd’s survival. In osotua return gifts need not match the original gift — in fact, how much the giver provides to the partner is negatively correlated to how much they expect to get back. Nevertheless, studies have shown that osotua significantly increases cattle herd survival outcomes as a result of pooling and minimizing risk.
Other studies have examined the mechanisms of human cooperation on the individual cognitive and biochemical levels. Some studies have shown that the cooperative impulse is more associated with reactive “fast thinking,” while slower deliberative thinking pushes people towards self-interested choices. Researchers have also begun to identify brain mechanisms related to cooperation, revealing oxytocin as a crucial chemical in regulating social cooperation. As a whole, this body of work is painting a picture of both the ultimate (adaptive) and the proximate (mechanistic) drivers of cooperative behavior in humans.
Does this mean the puzzle of the evolution of cooperation is completely solved? Not quite. While the gene-centric inclusive fitness theory provides a general explanation for adaptive cooperation, in recent years, critics such as Harvard sociobiologist E.O. Wilson have argued that models of inclusive fitness require significant assumptions to work, and that when you modify inclusive fitness to make fewer assumptions, it becomes far more difficult to draw meaningful conclusions from the models. (Wilson and fellow detractors have yet to provide a viable alternative that can be used by biologists doing empirical studies, however.) Eventually, new theoretical work and refined definitions may provide a version of the theory that rests on fewer assumptions; or it may be the case that natural selection simply pushes organisms towards the target of inclusive fitness, but that inclusive fitness will never be able to predict behavior across all scenarios.
Advances in the experimental realm may help sort out some of the controversies remaining with theoretical models of the evolution of cooperation. Microbes are continuing to expand the frontiers of cooperation research, in part because they violate many of the norms of higher organisms that are built into 20th-century evolutionary theory. They have high mutation rates, exchange genetic information between individuals as well as through reproduction, and often lack mechanisms to reduce internal genetic conflicts. In addition, they are easy to keep in the lab and can undergo evolution over human timescales. Many of the successful empirical tests of inclusive fitness theory have been carried out on microbes in the last two decades. What will they tell us next about the murky fringes of cooperation theory, which includes concepts like multiplayer games, strong selection, and non-additive fitness effects — areas where the concept of inclusive fitness are claimed to be less applicable?
Humans, meanwhile, may be the hardest cooperators to study. We reproduce slowly, many adaptive experiments are unethical in humans, and opportunities for comparative tests are few and far between. But we also possess traits which make our cooperation particularly interesting: we have memory, consciousness, and emotions, and are strongly shaped by cultural evolution. The use of new tools for studying human cooperation, such as economic games, has spread rapidly. Advances in game theory models, experimental design, and technology for studying humans over their lifetimes and across populations suggest a bright future for the field.
EVOLUTION OF COOPERATION: PARADOX FOUND
It is a pet peeve among some social biologists that many articles about cooperation start with some version of the phrase “cooperation is a puzzle.” To them, cooperation was a puzzle, but theories of inclusive fitness and reciprocity resolved it. And yet, it is often these same social biologists who still dedicate their lives to the study of cooperation. Though some of the fundamentals of cooperation have been resolved, the meaty details – why this ground tit gave up its meal in this specific instance, why this tribe was more pro-social than the other – continue to pose questions that challenge, intrigue and inspire scientists to further exploration.
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