aggression

GRRRRRR! Why do I have to keep reading about how spearchucky modern humans went around killing Neandertals?

It's all over the science news this week -- Shanidar 3, a 50,000-year-old Neandertal from Iraq, has a partially-healed deep cut to one of its ribs. The kind of cut that gets there when you're stabbed with a knife or spear, or shot with an arrow.

That's pretty interesting in itself, although not news -- the wound has been known and listed as an example of ancient interpersonal violence since the 1960's. It may be the oldest clear example of a wound to a living person from a stone implement, because all earlier cutmarked human remains (there are many) may be post-mortem. Still, not news.

The "news" part is that Churchill and colleagues (2009) have shown that the wound is most consistent with a small projectile, as opposed to a large thrusting spear. In their discussion, they suggest that modern humans may be the culprits:

The nature of the lesion to the left 9th rib of the Shanidar 3 Neandertal is most consistent with injury from a low kinetic energy, low momentum weapon. While this does not rule out accidental injury or attack by a conspecific wielding a hand-held weapon, the nature of the traumatic damage, combined with the wound track suggested by the placement and orientation of the rib lesion, is consistent with injury by a long-range projectile weapon traveling along a ballistic trajectory. Given the possible sympatry of this Neandertal with early modern humans, and given possible assymetries in weapon technology between the two species, the case of Shanidar 3 is a good candidate for an instance of Neandertal-modern human interspecific violence (14, emphasis in original).

So what do I think? I guess anything is possible. But any good murder case hinges on motive and opportunity. Churchill and colleagues don't really give us a way to place modern humans at the scene at the crime.

The authors point to the only radiocarbon dates available for Shanidar 3, which at 46,000 and 50,000 radiocarbon years are in a range that we probably should not trust for old, non-AMS dates (as they point out). There's no indication of a non-Mousterian industry in the immediate region before 35,000 radiocarbon years, although an earlier presence of non-Neandertals is possible.

As it stands, what are the odds that the Neandertal Shanidar 3 individual (setting aside the question of whether it or other West Asian specimens really are the same as European Neandertals) would ever have encountered a "modern" human (setting aside the question of whether the two populations were distinct)?

Well, we don't really know. Probably pretty low. Maybe impossible. But that probability is very important to this "forensic" question.

Do we know that Shanidar 3 wasn't hit by a Neandertal weapon? The study shows very well that seven blows from a Mousterian point-hafted spear are not enough to guarantee a wound like the one on the ribs of Shanidar 3. That wound has a clear cut to one rib but little involvement of the adjacent rib. The seven experimental blows from a "high-energy" weapon resulted in five wounds with significant slices in two adjacent ribs, and two that affected no ribs at all. In contrast, the "low-energy" trials generated a high fraction of wounds that involved slices to only one rib. Meanwhile, two of those "low-energy" trials were from a Neandertal-associated tool, stabs with a Levallois point.

But what does this tell us, really? If we just treat the probability of a Shanidar 3-type injury as a binomial with 7 trials in this study, we can't reject the hypothesis that it occurs at a true likelihood of 35 percent. I think the authors have done a cool study, and did a great job evaluating the wounds. The problem is that 7 trials just isn't enough to yield a high confidence. We don't really know that a weapon known to be associated with Neandertals couldn't have made the wound.

So it comes down to opportunity. Can we place modern humans at the scene of the crime?

We may not be able to estimate this probability, but we can use Bayes' theorem to evaluate what the likelihood would have to be to satisfy our jury. Let's say our jury is stacked with Neandertals, and is willing to convict a modern human if there is just a five percent chance he committed the crime.

1. We don't know the probability that Shanidar 3 encountered modern humans. We'll assume that Shanidar 3 would have been attacked with equal probability by whomever he encountered. Hence, the encounter rate with modern humans, the chief unknown, directly determines the chance of being wounded by a modern human.

2. We do know something about the probability that a random injury from a high-energy Neandertal-made weapon would have the characteristics of the Shanidar 3 wound -- it's something less than 35 percent.

3. We'll assume the probability of Levallois point stabs is zero, thereby stacking the deck in favor of a modern human killer.

4. We know that a random wound from a modern human projectile weapon would generate a wound having those characteristics with something like a 50 percent probability.

Plugging into Bayes' theorem, that means that an encounter rate of 1 modern human for every 28 Neandertals would yield a five percent chance that a modern human was the culprit. Did Shanidar 3 encounter one modern human for every 28 of his own? If not, we have to conclude its very unlikely -- less than five percent -- that his wounds were caused by a modern human.

At the moment, the best evidence from the site suggests that Shanidar 3 lived fifteen thousand years before any archaeological transition to the "low-energy" impactors. Which tilts the scales in favor of the alternative -- a slightly lower probability impact from a much higher probability weapon.

References:

Churchill SE, Franciscus RG, McKean-Peraza HA, Daniel JA, Warren BR. 2009. Shanidar 3 Neandertal rib puncture wound and paleolithic weaponry. J Hum Evol (early online) doi:10.1016/j.jhevol.2009.05.010

Last year, I pointed to an article that Jared Diamond had written in the New Yorker on revenge cycles in Highland New Guinea. Now Diamond has been sued by two New Guinea men, claiming the article is false.

Henep Isum Mandingo and Hup Daniel Wemp say in a single-page filing in Manhattan's state Supreme Court that Diamond's article published April 21, 2008, accused them "of serious criminal activity ... including murder."

The article was titled, "Vengeance Is Ours: What can tribal societies tell us about our need to get even?"

The New Yorker spokeswoman Alexa Cassanos said she had not seen the lawsuit and could not comment. She added: "We stand by our story; we stand by Jared Diamond."

There is a convoluted series of events behind the lawsuit. A media researcher named Rhonda Roland Shearer fact-checked the story, employing a team of New Guinea researchers to find the people mentioned in Diamond's article. The research resulted in a long report, which has been summarized on the StinkyJournalism.org website.

If you want to know more, I can recommend Savage Minds, where Alex ("Rex") Golub has written some detailed thoughts.

Shearer conducted punishingly scrupulous research on Diamond’s story, which included contacting Wemp and having researchers in Papua New Guinea investigate Diamond’s story. It looks like the New Yorker article is a hodge-podge of Diamond’s recollections of the stories Wemp told Diamond when Wemp drove him around the Southern Highlands. The actual history of fighting in the area Wemp describes is quite different—for instance, the man that Diamond says was paralyzed in a wheelchair is photographed standing and walking in Shearer’s piece. Diamond presents what appear to be verbatim quotations from Wemp which are probably Diamond’s reconstruction of the conversation, and so forth. So both the facts and their presentation are problematic.

I don't pretend to understand the legal issues, but it's certainly a cautionary story for anthropologists. Many like to hold out Diamond as a know-nothing dilettante, but the main thing that distinguishes this case is that it was published in the New Yorker, which gets read by people. Monographs of fieldwork in anthropology don't get read by people. They are peer-reviewed, but not usually fact-checked. The writers are often young scholars who need to publish a book for tenure.

Hopefully they have the sense to use fake names for their informants.

LiveScience reports on David Carrier's current paper in Evolution:

"The old argument was that [apes] retained short legs to help them climb trees that still were an important part of their habitat," said the study author David Carrier, a biologist at the University of Utah. "My argument is that they retained short legs because short legs helped them fight."

I won't be able to get the paper for awhile. The thesis is that short legs are adaptive to aggressive interactions because short, stout bodies are better for fighting. In addition to the primates mentioned in the article (of which there are only nine), Carrier has in previous papers (Pasi and Carrier 2003, Kemp et al. 2005) studied the relation of limb length and fighting ability among dog breeds.

It looks like Carrier is arguing that the biomechanical advantage of short legs in climbing doesn't really predict arboreality:

As indicators of aggression, Carrier looked at the weight difference between males and females and the male-female difference in length of canine teeth, which are used for biting during battle. Studies have shown greater aggression in primate species in which males tipped the scales relative to females.

Primates with the stoutest figures also ranked high on both aggression measurements. For instance, the gibbons boasted longer legs than other apes and also ranked low on the aggression scale. In contrast, male gorillas, which are more than double the size of females, were stout.

The lengthy legs didn't keep gibbons away from canopies either. "Gibbons are the best acrobats in the animal kingdom. There are no other animals that can move through the canopy the way a gibbon can," Carrier told LiveScience. "And they contrast male gorillas, which hardly ever climb. When they do climb they stay close to the trunk, they spend most of the time on the ground."

I don't know -- australopithecines don't have long canines, and their level of dimorphism has been subject to debate. So the comparison may not work. I'll have to see what the paper says.

References:

Pasi BM, Carrier DR. 2003. Functional trade-offs in the limb muscles of dogs selected for running vs. fighting. J Evol Biol 16:324-332.

Kemp TJ, Bachus KN, Nairn JA, Carrier DR. 2005. Functional trade-offs in the limb bones of dogs selected for running versus fighting. J Exp Biol 208:3475-3482.