hunting

Long-time science journalist Robin McKie has a long article in The Observer about the Neandertals this weekend: "Neanderthals: how needles and skins gave us the edge on our kissing cousins".

The article puts together several aspects of recent inquiry into Neandertal biology -- the genome sequencing, the dating questions over Châtelperronian artifacts from Grotte du Renne, and some of Steve Churchill's work on projectile versus thrusting weapons. There's a real interesting mix of stuff here, some that I agree with and basically find uncontroversial, and other stuff that I find to be outlandish or unsupported by any evidence.

For example, McKie talked to Brian Fagan, who has a new book out (Cro-Magnon) that tries to describe the human "edge" over Neandertals. A good topic, but this paragraph is completely misleading:

But which specific traits gave us such an advantage that we were propelled to global glory at the expense of the Neanderthals? In the suite of behaviours that we evolved in Africa 150,000 years ago, what were the characteristics that really made a difference and can therefore be considered as defining human attributes? There are many candidates – complex language and superior memory, for example. However, among many scientists there appears to be consensus that imagination and opportunism were critical attributes.

There is no "suite of behaviours that we evolved in Africa 150,000 years ago." There just aren't any. There's no good evidence of symbolic expression, no projectile points, no subsistence innovations, no evidence of long-distance raw material procurement or trade. That's the big problem we have substantiating a modern human advantage -- the "modern" humans didn't seem to get many behavioral innovations in Africa that the Neandertals didn't get, and the Neandertals got them almost as early.

It is an undeniable problem; there's no sense glossing over it. Churchill's (and John Shea's) ideas about projectile weapons are right now among the most reasonable suggestions, because there do seem to be relatively early (ca. 85,000-90,000 year old) projectile points in Africa.

It would be convenient if there were better evidence that projectiles were a singular innovation. But as John Shea [1] wrote in 2006, the idea of projectile weapons seems to have gotten around widely, possibly including Neandertals:

The evidence currently available instead favors an indigenous origin for projectile point technology in the Levant ca. 40–50 Ka. Similarly, the earliest European Upper Paleolithic stone artifacts that fit the TCSA criteria for projectile points, Chatelperronian points, Font Robert points (as well as Aurignacian split-based bone/antler points) do not have clear chronological antecedants in the Levant (though it is possible that other as-yet-unidentified projectile point types do). While it is possible that over-production of atmospheric radiocarbon between 30 and 50 Ka [39] obscures rapid geographical diffusion of projectile point technology the typological variability of the earliest likely stone and bone projectile points in Africa, the Levant, and Europe do not currently support a diffusion/migration hypothesis. It is vastly more likely that projectile point technology was developed convergently among African, Levantine and European hominin populations.

I probably wouldn't stretch so far as to say that the Châtelperronian Neandertals were using projectile weapons, even if the points are consistent with that hypothesis. But considering that a big element of McKie's story is the dispute over the Châtelperronian evidence of ornamentation (at Grotte du Renne), I think it's fair to remind people that those late Neandertals had a lot of things going on. All the skeletal associations with the industry are Neandertal, and there are multiple sites representing the interesting material culture elements.

I've actually been stunned lately by the number of people who have asked me about the Grotte du Renne paper and it's "demolishment" of the case for Neandertal ornamentation. I say stunned, because people seem completely unaware of the substantial Mousterian record of pigment processing and use.

My candidate for the most subtly controversial element of McKie's story: the opening passage about the Swanscombe skull:

Many treasures [at the Natural History Museum] compete for attention, but there is one sample, kept in a small plywood box, that deserves especial interest: the Swanscombe skull. Found near Gravesend last century, it is made up of three pieces of the brain case of a 400,000-year-old female and is one of only half-a-dozen bits of skeleton that can be traced to men and women who lived in Britain before the end of the last ice age. Human remains do not get more precious than this.

However, the Swanscombe find is important for another, crucial reason: the skull is that of a Neanderthal

I say that's controversial because it asserts that this 400,000-year-old skull is a Neandertal. The case for Swanscombe as a member of the Neandertal lineage has been mostly chronological, not because it has any pattern of derived Neandertal morphology. There were people in Europe before the Neandertals, they had a subset of Neandertal features, and so they were plausibly early members of a Neandertal lineage. But the genetic work this year, discussed later in the article, argued that humans and Neandertals shared a common ancestral population only 250,000-400,000 years ago. If that's true, the chronology is all wrong for Swanscombe to be a Neandertal itself. Indeed, this chronology would not permit Swanscombe to be a member of a population exclusively ancestral to Neandertals.

But what, then, is it?

I think the chronology is wrong, and I doubt whether the evidence will soon let us distinguish gene flow from isolation at this time depth. There's not much sense talking about the "human-Neandertal ancestral population" when some Neandertals were ancestors of some humans.

Still, the Middle Pleistocene European population focuses the problem. If Neandertals themselves had derived much of their gene pool from Africa in the Middle Pleistocene, as the genetic work has suggested, what does that mean for specimens like Swanscombe? And if we substantially lengthen the chronology of human diversification, what does that mean for Middle Pleistocene Africans?

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