Thursday, April 10, 2008

Me loves that ceteris paribus!   posted by Razib @ 4/10/2008 01:02:00 AM

I've criticized economists for being a bit cavalier about nutritional basics before. A comment below points me to this working paper, Agricultural Specialization and Health in Ancient and Medieval Europe:
It has been argued that protein-rich milk and beef are major determinants of the biological standard of living for societies of the late 18th and 19th centuries: a high local supply of milk lead to better nutrition and taller stature (which is correlated with health and longevity), even if purchasing power is not necessarily high: The shadow price of milk was extremely low, because this food item could not be shipped, but was used for subsistence (and the butter was sold). In this paper we consider this proximity-to-protein production effect in ancient and medieval Europe. The decisive protein production can be traced quantitatively for the first time using a sample of 2,059,689 animal bones. The share of cattle bones is ceteris paribus an indicator of milk (and beef) supply, especially if controlling for population density. We compare information on cattle bone share with estimates of heights in three European regions (Mediterranean, Northeast, Centralwest) for the 1st to 17th century A.D. In an experiment, we suggest height estimates for today's Turkey, Greece, the Near East and Egypt during antiquity, based on the regression formulas we find.

Between 600 and 300 B.C. an astonishingly extreme decline in cattle share took place in Mediterranean Europe. During the time of the Roman empire cattle share stagnated on a very low level in the Mediterranean regions, whereas a high share of cattle bones was typical for North-Eastern, Central and Western Europe. In contrast, a high pig bone share became typical for the Mediterranean regions when the cities grew enormously during the Empire formation, because near those large cities like Rome or Pompeii cattle grazing became inefficient or impossible. Thus the poor and middle income groups substituted beef and milk with grain and vegetables, whereas the richer strata of society could afford pork. Pigs were easier to feed with the remains of human nutrition and imported fodder.

The central finding of this study is that the share of cattle bones, interacted with land per capita, was an important determinant of human stature. Thus, also for ancient times we can state that proximity to animal protein production was decisive for the level and development of nutritional status.

A paper with a lot of data which I found fascinating. But...I have to be somewhat skeptical of this note:

Lactose intolerance was probably not a decisive limiting factor in Europe. Crotty (2001) emphasized the importance of lactose intolerance in his bold attempt to explain the evolution of capitalism based on cattle farming patterns. Crotty argued that lactose-intolerant people could not make sufficient use of cattle. Lactose intolerance means that many people in the world have digestive problems, if they do drink large quantities of milk after age 5-7, because at that age genetically lactose intolerant people loose their ability to digest fresh milk without diarrhoea and similar problems. Especially East Asians (east of Tibet and Rajasthan), American Indians and some African people have problems with lactose intolerance. For Southern Europe, the results are mixed - one study on Spain categorized the country into the highest group of lactose tolerance (70 % and more lactose intolerance) and a Greek study found a middle position (30 - 70 % lactose intolerance); whereas in Italy and Turkey less than 30 % were classified as lactose tolerant (see Mace et al., 2003). But even lactose intolerant people can digest modified milk as Kefir, Lassi and similar products. Moreover, all people can drink about a cup of milk per day if they train their intestinal bacteria to live in a milk environment. Even many South Koreans today consume some milk, using this method of permanent training. We thank Barry Bogin, Anthropology Department U. Michigan/ Dearborn, and S. Pak, Seoul National U., for hints.

I've posted lactose tolerance rates across regions before, they vary a great deal. It is certainly true that milk is not cyanide for lactose intolerant individuals, but I don't think we should soft pedal the ramifications of the development of adult lactase persistence. Its evolution was due to one of the most powerful and recent selective events in our species' genetic history. Many of the Eurasian alleles seem to be descended from a recent common ancestor. Some of the African alleles are likely to be independent. The West Asian alleles also seem derived from an independent mutational form distinct from that of the more widespread Eurasian variant.

This isn't to deny the reality that milk is a great source of nurtition which might be an important variable which might explain variation in height across time & space. And I don't dismiss the R2 they can produce. But the geography of genes in this case strongly implies a lot of local ecological adaptation has been at work, and should be included in these models as opposed to brushed aside, e.g.:
...We provide two new lines of genetic evidence that this long, common haplotype arose rapidly due to recent selection: (1) by use of the traditional FST measure and a novel test based on pexcess, we demonstrate large frequency differences among populations for the persistence-associated markers and for flanking markers throughout the haplotype, and (2) we show that the haplotype is unusually long, given its high frequency-a hallmark of recent selection. We estimate that strong selection occurred within the past 5,000-10,000 years, consistent with an advantage to lactase persistence in the setting of dairy farming; the signals of selection we observe are among the strongest yet seen for any gene in the genome.

I don't know much economics or economic history, but I do know a little human population genetics, so I'm biased in hoping that everyone else gets hooked into this field. But I also believe that economic historians should be aware of the fact that the evolution of lactase persistence is one of the best case studies for recent gene-culture coevolution. One should be cautious of assuming that the maximal utilization cattle as milk producers is purely a function of economic or social conditions (though the long term impact of those economic and social conditions do count for a great deal). Here's a salient point from A Map of Recent Positive Selection in the Human Genome:
An important type of selective pressure that has confronted modern humans is the transition to novel food sources with the advent of agriculture and the colonization of new habitats...As noted above, we see a strong signal of selection in the alcohol dehydrogenase (ADH) cluster in East Asians, including the third longest haplotype around a high frequency allele in East Asians. A variety of genes involved in carbohydrate metabolism have evidence for recent selection, including genes involved in metabolizing mannose (MAN2A1 in Yoruba and East Asians), sucrose (SI in East Asians), and lactose (LCT in Europeans). Processing of dietary fatty acids is another system with signals of strong selection, including uptake (SLC27A4 and PPARD in Europeans), oxidation (SLC25A20 in East Asians) and regulation (NCOA1 in Yoruba and LEPR in East Asians). The latter gene (LEPR) is the leptin receptor and plays an important role in regulating adipose tissue mass.

Since then there's been the CNV & amylase work....