hunter-gatherers

Kim Hill and colleagues described in last week's Science a study of kinship within bands of hunter-gatherers known from ethnographic research [1]. They couched their study to dispute the idea that most of the members of hunter-gatherer bands are kin.

Why? Because the idea that hunter-gatherers live in bands composed mainly of close kin has been a very common answer to the question, "Why do humans cooperate?"

Traditionally, anthropologists have suggested that hunter-gatherer co-residence is almost entirely based on kinship [e.g., (15, 16)], and evolutionary psychologists have embraced this idea in order to develop “mismatch hypotheses” about cooperation among non-kin in modern societies (17). Evolutionary researchers have also argued both that female philopatry and maternal grandmother provisioning is ancestral (5) and that male philopatry, typical of other African hominoids (18, 19) and leading to adult male provisioning (8), is the ancestral human pattern. If either of these is correct, and if foraging bands are mainly collections of close kin, inclusive fitness gains might be the primary motivator of ancestral human cooperation.

True, many evolutionary psychologists have adopted this view with gusto: Hunter-gatherers cooperate because they live in small bands with their kin. But as many have realized, actual bands of hunter-gatherers pretty quickly show that this isn't true. Either men or women generally move when they marry, so bands have close relatives in them, but they also have unrelated individuals. In practice people often move with a sibling, or their adult parents may come to live with them as well (both practices described by Hill and colleagues).

The net effect of residence changes is to reduce the levels of inbreeding within bands, and also reduce the genetic variation among bands. Some evolutionary psychologists occupy themselves with group selection precisely because the relevant level of cooperation in hunter-gatherers is the band, and bands include many people who are distantly if at all related. But (as Hill and colleagues also note) the high rates of intermarriage among bands greatly reduces the strength of group selection possible among them.

I think the paper sets up a straw man when it claims that the "traditional view" in anthropology is that bands are made up of kin. That's certainly not the view I learned. Their "traditional" citations are Elmer Service and June Helm, writing in the 1960's, and I don't doubt that they and others have argued for kin-structured co-residence. My understanding of "kin-structured" was never that bands were made up of close kin, but instead that residence was determined by kinship pattern. Exogamy is driven by the incest taboo, for example. Whether men or women typically change residence to marry depends on other kinship rules. Humans recognize kin, and their movements are not incidental to this, resulting in a kin-structured society. Some rules force people to live in different groups from their close kin, not the same groups.

At any rate, Hill and colleagues confirm that the groups in their dataset do not have bands composed mainly of close kin. They suggest that we need a new theory of human social evolution to explain the emergence of cooperative behaviors:

The hunter-gatherer social structure we describe has important implications for theories about the evolution of cooperation and cultural capacity. First, bands are mainly composed of individuals either distantly related by kinship and/or marriage or unrelated altogether. In our sample of 32 societies, primary kin generally make up less than 10% of a residential band. For example, in the Ache we estimate the mean genetic coefficient of relatedness (Hamilton’s r) between adults in 58 precontact bands to be only 0.054 (n = 19,634 dyads, SE = 0.0001). This agrees with Ache informants who reported that during the precontact period they often lived with people described as “friends, not relatives.” The Ju/’hoansi results in Fig. 2 suggest that mean relatedness in other groups is not too different from the Ache. Thus, we cannot necessarily assume that cognitive features such as inequality aversion and enhanced prosocial emotions evolved in ancestral environments composed mainly of close kin. Given the constant flow of individuals between groups, genetic group selection at the level of the band also seems improbable. Instead, cultural group selection (27) may lead to the spread of cooperative institutions within ethnic groups, which might then create a context favoring the genetic evolution of prosocial cognitive mechanisms through individual-level selection.

I have two reactions. The paper is missing a null model. It is true that the hunter-gatherers are much less related to each other within a band than would be predicted if they were choosing to live near close kin. But are they more related to each other than would be the case if they moved randomly? What if individuals move around without consideration of kinship at all, according to a simple distance function? If individuals behave as they would without any knowledge of kinship, it's clear that our explanation for cooperation needs to work at the individual level. Cultural group selection may help explain the persistence of institutions and rules, but I think it's insufficient to explain the evolution of people who can form institutions and rules.

My second reaction is that humans are self-interested. In my experience, children learn to share when they can grasp the concept of reciprocity. Let me suggest that, like numbers, cooperation may be a cognitive technology in humans. We may have many biological changes that facilitate cooperation -- for example, I would look for human-specific changes to oxytocin regulation. But those changes may be mostly tuning a system to facilitate prosocial behavior (and reduce aggression). They don't explain the occurrence of specific prosocial behaviors, because those behaviors themselves did not evolve. They were invented.

References

I want to run through some examples of how we can apply game theory to consider hunting decisions in human groups. First, I describe a simple Snowdrift model applied to hunting. This is part 2 of a series, part 1 introduces the topic of the Snowdrift game.

A reader sent along a story after reading the first post:

In reading your snowdrift blog post, I was reminded of an experiment that does not require game theory to understand. You may have heard of it. Two pigs are in a pen. One is dominant. To get food one of them presses a bar, but the food is dispensed at the other side of the pen. If the subdominant pig presses the bar, it gets no reward, as the dominant pig hogs the food, eating it all. The result is that the dominant pig presses the bar while the subdominant pig waits at the food trough. Then the dominant pig rushes over to the trough to push the subdominant pig aside. Both pigs get fed, but the dominant pig does all the work

It's a great example of asymmetrical rewards. I'll get to those in the next few posts on this topic, because the asymmetries are very important to understanding dynamics in hunter-gatherers. But first, we have to describe the simple symmetrical case, including the algebra defining the evolutionarily stable equilibrium between the two simple strategies.

Suppose we have two hunters, who will share whatever game either of them kills. A man may choose on a given day to hunt. By hunting, he suffers a cost c and brings back a large fixed benefit b for each man. The two men may both choose to hunt on the same day, resulting in the same benefit b but a lowered cost c∕2 for each man. The two men decide whether to hunt simultaneously and without conferring — that is, there is no information transfer between them that would affect their decisions.

Here is the payoff matrix of the game for player 1 (choices on left) given the strategy selected by player 2 (on top):

Given the existence of the two strategies, “hunt” and “no hunt,” the ESS is the ratio at which the two strategies have equal expected returns. If individuals select a strategy and do not vary, the ESS represents the frequency of these variants in the population. If in contrast, individuals can choose to adopt either strategy, then the ESS also will be the optimal proportion of the two strategies in one individual’s repertoire. The two strategies will yield equal payoffs when the ESS satisfies the following equation, where p represents the proportion of “hunt” and 1 - p the proportion of “no hunt”:

(1)

…which simplifies to p = 2(b - c)∕(2b - c). That expression is positive where b > c, and approaches unity where c is very small relative to b. If in contrast b ≤ c then the scenario is the Prisoner’s Dilemma, where the only ESS is a pure “no hunt” strategy.

Let’s also look at a slightly different case. As above, each man’s return from hunting will be b regardless of whether one man or both choose to hunt. But in the payoff matrix below, the cost of hunting is also the same whether one man or both choose to hunt. So there is no reduction in the cost of hunting if both men do it.

Now, in this case, the ESS satisfies the equation:

(2)

Again, p is the frequency of the “hunt” strategy. This simplifies to p = (b-c)∕b, which again yields the Prisoner’s Dilemma when b < c.

OK, that’s the simple Snowdrift game model, described in the language of hunting instead of winter car accidents. It is quite simplistic in many ways. We might expect real hunters to have successes and costs that vary as stochastic functions of the environment. A real hunter must decide whether to hunt based not only on the odds his companion will hunt, but also upon some appraisal of the companion’s likelihood of success. Men in hunting societies are not paired up by the buddy system, but instead make their decisions about hunting in the context of a larger group’s activities.

Maybe most confusing, there are two possible kinds of currency in which benefits and costs may be expressed. A benefit from hunting may be most naturally measured in calories. If we average hunting returns across many episodes, then our result would be mean calories per day, or per hour of effort. Likewise, it might seem natural to discuss costs in terms of calories, as we might consider the cost of locomotion or cost of transport associated with foraging.

But the only currency that matters to evolution is fitness. We cannot assume that maximizing caloric returns will maximize fitness. Transport and locomotor costs may be minor compared to the mortality risk from predation when foraging far from camp. The caloric benefits from hunting matter more to a starving child than to a satiated adult.

So the measures of costs and benefits that define the ESS should be expressed in terms of fitness. That’s a problem, because fitness outcomes are a lot harder to measure than caloric returns. To figure out caloric expenditure and returns, you can measure oxygen consumption, work out distances, and weigh meat. To measure fitness, you have to record lifetime reproduction. To assess the relationship between caloric returns and fitness, you need a lifetime of caloric returns.

So far, hunter-gatherer demographic data and hunting returns are both known from a small number of transverse studies. Longitudinal data on hunter-gatherer demography are limited, and mostly known by retrospective methods — that is, informants share their knowledge about the history of their groups. The fitness effects of a single individual’s hunting effort over time are not known.

If fitness outcomes are hard for the scientist to measure, they are equally hard for a social actor to predict. Even intelligent actors like humans know little about the effects of their actions upon their future reproduction. Men sometimes do poorly with information directly relevant to fitness, like “Is the child mine?” That’s not to say that men may not follow highly sophisticated strategies to allocate hunting effort. But we should develop explanations that do not assume that a man knows the fitness benefits and costs of his choices.

Next: Life history and asymmetrical strategies

I have had a New York Review of Books essay by Richard Lewontin, titled, "Why Darwin?" on my desktop for a week without getting to the last section of it.

Like many essays in the NY Review of Books, Lewontin's shoehorns small points from the books into an argument of his own. As you might guess from the title, Lewontin's theme is that Darwin has been overrated -- a result of biologists overemphasizing a "great man" story of the history of their science, and an unjustified belief in the ubiquity and power of natural selection. Lewontin mobilizes his argument against Jerry Coyne's Why Evolution Is True.

I don't really find the "pluralist versus adaptationist" debate very interesting. Despite the vocal complaints of some, I can't ever seem to locate the mythical "adaptationists" who deny that non-adaptive evolution ever happens. So the "debate" always comes down to whether particular adaptive hypotheses are true. Since no scientific hypothesis is true a priori, and since "those adaptationists are always saying stupid things" is not a scientific argument, I don't see the point.

Still, I meant to get to the last section of Lewontin's essay, and this morning I finally read it. To close his case for the weakness of natural selection, Lewontin turns to another new book by Greg Gibson, titled, It Takes a Genome: How a Clash Between Our Genes and Modern Life Is Making Us Sick. The book is an extended account of "diseases of civilization", a topic that I discussed here last week ("Arrested adaptation and the 'diseases of civilization'"). Here's a passage from the book's promotional material (on the Amazon page):

In It Takes a Genome, Greg Gibson posits a revolutionary new hypothesis: Our genome is out of equilibrium, both with itself and its environment. Simply put, our genes aren’t coping well with modern culture. Our bodies were never designed to subsist on fat and sugary foods; our immune systems weren’t designed for today’s clean, bland environments; our minds weren’t designed to process hard-edged, artificial electronic inputs from dawn ‘til midnight. And that’s why so many of us suffer from chronic diseases that barely touched our ancestors.

Set aside for a moment how "revolutionary" this hypothesis is -- I'll revisit the idea in another post. The question is whether this mismatch between our environments and our genetic variation means that human evolution "stopped" or that we are still "adapted to the Pleistocene". As I pointed out in my earlier post, both propositions are true: human populations are mismatched with their current environments, and human populations have been recently adapting very rapidly to new environments. Here's what I wrote last week:

[M]any of today's chronic diseases reflect the reaction of human biology to novel environments for which our genes are not well adapted. But we don't need to exaggerate the slowness of human evolution to arrive at that conclusion. Recent rapid evolution of humans does not mean that humans are perfectly adapted to the present. Far from it -- if human populations have undergone rapid genetic changes into the past thousand years, it is a strong sign that fitness has not yet maximized in the post-agricultural environment.

I can contrast my point of view with Richard Lewontin's, who perfectly reiterates the "human evolution stopped in the Pleistocene" version of events.

An important property of adaptive evolution is that it is usually a slow process. Certainly there are cases where a single genetic change can mean the difference between life and death in a hostile environment. The classic cases are the mutations that give pathogenic microorganisms the ability to resist antibiotics or mutations that allow crops to resist pathogens, for example insects or herbicides. But these are not representative models for how species adapt, by accumulation of mutations of small effect, to changes in food availability, temperature modifications, and the thousand shocks that flesh is heir to. The usual small differences in fitness among genotypes are therefore manifest as detectable evolutionary change only after thousands of generations.

This deliberate tempo has presented the human species with a problem of adaptation. With a human generation of about twenty-five years, there have been roughly only one hundred generations since the founding of the Roman Republic. Yet the changes in the human environment caused by changes in human activity have been enormous. Changes in diet, habitation, working conditions, the pollution of air and water, and especially the considerable increase of lifespan that result in major alterations and breakdowns in the bodily machinery have all been too rapid for genetic adaptation.

Notice the false premises: Adaptive evolution is "usually a slow process." Species adapt by "accumulation of mutations of small effect." It's as if he were transported back in time to 1908 where no one had heard of the breeder's equation.

There's nothing impossible about long series of small changes. But they are not the only mode of adaptation, or even the most likely one. Populations with additive genetic variation that correlates with fitness will change rapidly under selection. The structure of the additive variation may lead to strong selection on one gene of large effect, or selection in parallel across many genes of varying effects. Series of small changes may be required for some adaptations, but a rapid environmental change (as Lewontin observes for humans) may cause bursts of rapid changes in allele frequencies.

To maintain the slowness of human evolution, Lewontin must do three things:

1. Assume humans are genetically uniform.

2. Where humans obviously are not uniform, argue that variations are uncorrelated with fitness.

3. Ignore any historical or genetic evidence that might contradict 1 and 2.

Keeping in mind the short length of this section of the essay, Lewontin does manage all three of these conditions.

I think it's downright sneaky the way Lewontin reinforces the assumption of human genetic uniformity. He refers to "the human genotype" as if there were only one! By emphasizing that "parts of the human genome are out of correspondence with modern life", he precludes the possibility that some human genomes may be more in correspondence than others. Sure, if humans share a single genome, they can't possibly differ in any adaptive way.

But diversity is the reality. Examples of recent human evolution are fixtures in biology textbooks, from sickle-cell to lactase persistence. These are traits that have rapidly changed in frequency during the last 2500 years, due to changes in recent human environments -- disease for the former, diet for the latter. These rapid transformations in precisely those that Lewontin says are impossible -- environmental changes being "too rapid for genetic adaptation." A number of morphological changes are also evident when comparing archaeological and recent skeletal samples in many parts of the world. Somehow the relevance of these recent changes goes unmentioned in the essay.

One of the best-characterized examples of evolution in recent populations is the rapid Holocene evolution of pigmentation phenotypes. It's a textbook example of human variation, and several adaptive hypotheses may explain it. So pigmentation would seem an unlikely example of how human evolution has been too slow to cope with the environment. But Lewontin finds a way:

[H]igh doses of solar radiation that is experienced by surfers on the California beaches might induce an eventually fatal skin cancer, but the cancer death almost always occurs well after reproductive age, so there is no opportunity for selection to act.

I agree that current patterns of cancer mortality of light-skinned surfers may have little impact on their fitness. In other words, this chronic disease is a sign of an environmental "mismatch" that future genetic evolution is unlikely to erase.

But why turn to false arguments about the speed of evolution to make this point? Surely Lewontin knows that "reproductive age" in humans is not synchronous with reproductive effort? Skin cancer is one of the earliest-killing cancers, with a good fraction of victims dying at ages when they might otherwise be helping raise their kids or grandchidlren. Lewontin must also know that human populations vary greatly in their skin cancer susceptibility, and that some surfers (the dark pigmented ones) have lower skin cancer rates after the same sun exposure. Skin cancer may or may not be the best explanation for dark pigmentation in low-latitude human populations (there are others, none mutually exclusive), but this example works strongly against Lewontin's claims that natural selection is "slow" and that human environmental changes have been "too rapid for genetic adaptation." We aren't perfectly adapted today, and the rate of our evolution in the recent past was very fast.

References:

Lewontin RC. 2009. Why Darwin? New York Review of Books 56(9) May 28, 2009. Online