Why the indigenous still dominate the Andean region

400-500 years ago in the midst of the Great Dying somewhere the indigenous inhabitants of the New World suffered mortality rates on the order of 90-95%. This was almost certainly due to the facts of evolutionary history; the indigenous peoples had little defense against Eurasian pathogens. A result has been the reality that most of the New World is inhabited by European, African or mixed populations. But there are exceptions. In Mesoamerica there is still an indigenous dominated region from southern Mexico into the highlands of Guatemala. More substantially the highlands of the Andes, and therefore Peru and Bolivia, remain strongholds of indigenous groups.

Why? We know that prior to the arrival of the Spaniards the Inca Empire was being hit by plagues, no doubt of Eurasian provenance. So there was almost certainly demographic contraction. But there are also records which suggest that Spanish women had a difficult time with carrying children to term in the highlands, so whatever population crash there was, unlike other regions of the New World world Europeans and part-Europeans did not expand into the demographic vacuum. The fact that the indigenous peoples of the Andes have adaptations to highland living is not news. But there is a new paper which reviews data which suggest that Andean women give birth to healthier infants at high altitudes, Augmented uterine artery blood flow and oxygen delivery protect Andeans from altitude-associated reductions in fetal growth:

The effect of high altitude on reducing birth weight is markedly less in populations of high- (e.g., Andeans) relative to low-altitude origin (e.g., Europeans). Uterine artery (UA) blood flow is greater during pregnancy in Andeans than Europeans at high altitude; however, it is not clear whether such blood flow differences play a causal role in ancestry-associated variations in fetal growth. We tested the hypothesis that greater UA blood flow contributes to the protection of fetal growth afforded by Andean ancestry by comparing UA blood flow and fetal growth throughout pregnancy in 137 Andean or European residents of low (400 m; European n = 28, Andean n = 23) or high (3,100-4,100 m; European n = 51, Andean n = 35) altitude in Bolivia. Blood flow and fetal biometry were assessed by Doppler ultrasound, and maternal ancestry was confirmed, using a panel of 100 ancestry-informative genetic markers (AIMs). At low altitude, there were no ancestry-related differences in the pregnancy-associated rise in UA blood flow, fetal biometry, or birth weight. At high altitude, Andean infants weighed 253 g more than European infants after controlling for gestational age and other known influences. UA blood flow and O2 delivery were twofold greater at 20 wk in Andean than European women at high altitude, and were paralleled by greater fetal size. Moreover, variation in the proportion of Indigenous American ancestry among individual women was positively associated with UA diameter, blood flow, O2 delivery, and fetal head circumference. We concluded that greater UA blood flow protects against hypoxia-associated reductions in fetal growth, consistent with the hypothesis that genetic factors enabled Andeans to achieve a greater pregnancy-associated rise in UA blood flow and O2 delivery than European women at high altitude.

Figure 1 shows the difference as a function of altitude when across the two groups of women:

Figure 3 shows the differences among the women of Andean origin as a function of their indigenous ancestry:

This being Latin America, there was a non-trivial proportion of European and African ancestry among the Andean women, and a non-trivial proportion of indigenous and African ancestry among the European women. A major issue though is that the indigenous ancestry of many of the European women is likely to be of lowland origin, e.g., Guarani. The markers selected could not distinguish between these ethnic groups, and those of the Andes. But in terms of this physiological characteristic one would assume that the lowland indigenous groups would would cluster with Europeans, and not with the Andean ethnicities. In any case, it seems plausible that the R-squared above is an underestimate of the effect of Andean ancestry as there is likely some indigenous lowland ancestry which arrived along with the "European" ancestors within the lineages of the highlanders.

Not only does this research illuminate current demographic distributions across this region of Latin America, it does emphasize the interrelationship of some of these genetic techniques and medical applications. The final paper of the paper:

Given the widespread importance of fetal growth not only for neonatal well-being but also for a growing range of disorders later in life the identification of parental, fetal, and environmental attributes that protect fetal growth and birth weight is increasingly appreciated. Studies at high altitude provide a unique natural laboratory to identify specific genes involved in the protection of fetal growth and the molecular signaling pathways through which they exert their effects. We consider likely candidates to be factors involved in maternal vascular response to pregnancy, including those influencing maternal vascular reactivity, growth, and remodeling, such as oxidative stress, angiogenic and/or immunological factors.

Until the genetic architecture of these physiological processes are elucidated sufficiently it seems that any risk assessment by medical professionals should consider the ancestry of pregnant women at high altitudes as determined by scientific techniques.