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The Tall and the Short of It: Can Milk Drinking Explain Increases in Stature Among Prehistoric Agriculturalists?

    stature

    Written by: Lauren Milligan Newmark, Ph.D. | Issue # 114 | 2023

    • The transition from hunting and gathering to agriculture had profound changes on the human diet that are believed to have negatively impacted human health, including growth rates.
    • A new study on over 3,500 skeletal remains across Europe, Asia, and Northern Africa spanning the last 34,000 years suggests that declines in stature and body size preceded the adoption of agriculture in many regions, and in some places stature and body size were stable over time.
    • Increases in body size and stature in Central and Northern Europe that began approximately 8,000 years ago were argued to be the result of milk drinking, facilitated by selection on lactase persistence genes among these populations.
    • The ability to drink milk without digestive issues may have buffered the nutritional stresses associated with a farming mode of subsistence.

    You may have heard the claim that if you double your child’s height at age two, you can determine how tall they will be as an adult. Before you go and check the pencil marks on the doorframe at your childhood home to confirm, take note: this method for estimating height fails to consider the fact that adult stature is a complex trait that results from the interaction of genes and environment [1]. Not every three-foot-tall two-year old grows up to be a six-foot-tall adult—a taller than average toddler could have stunted growth due to chronic infections or inadequate nutrition during childhood and adolescence. 

    Because of the influence of the environment on an individual’s growth, the World Health Organization uses height throughout childhood and adolescence as a measure of health in at risk populations. For the same reasons, archaeologists also relate height to health in prehistoric populations. A long-standing hypothesis in anthropology argues agricultural populations will be shorter than their hunter-gatherer ancestors because of nutritional deficiencies that accompanied the shift from foraging to farming (e.g., decrease in dietary breadth, increase in calories coming from carbohydrates compared with protein). A new study [2] challenges this assumption and instead proposes that agricultural populations that consumed animal milk and had the genetic variant allowing for lactase digestion (aka lactase persistence, or LP) would be taller than their non-dairying, non-LP contemporaries. Called the Lactase Growth Hypothesis, it predicts that the shift to milk consumption, facilitated by selection for LP, would have “turbo-charged” growth rates by making up for any nutritional or immunological challenges that accompanied an agricultural mode of subsistence [2]. The tallest modern-day Europeans are those that get more protein from milk and less protein from wheat [3]. Could the same be true among Neolithic farmers?

    To test the Lactase Growth Hypothesis, researchers examined changes in stature and body mass from over 3,500 skeletal remains representing 366 archaeological sites across Europe, Western Asia, Northern Africa (Nile Valley), South Asia, and China. The dataset ranged from 34,000 years ago to present, including Upper Paleolithic (Late Pleistocene) foragers, populations in transition from foraging to farming (early Neolithic), and populations that relied solely on agriculture (including herding) for subsistence. Estimates for stature and body mass of each skeleton were calculated using established regression equations that derive overall height from the length of the femur or tibia and overall body size from the diameter of the femoral head or tibial plateau [2]. 

    The study authors identified a long-term decrease in average height and body mass that preceded the transition to agriculture, starting in the Upper Paleolithic (20,000–15,000 years ago) in most of the regions examined [2]. Moreover, not every region demonstrated the same response to the transition to agriculture in body size. In areas known to be associated with independent adoption of agriculture (rather than cultural transmission), stature and body size were relatively stable. On the other hand, regions where non-native crops were introduced, like Northern Europe, experienced declines in stature. Together, these findings suggest that the pace (gradual vs. rapid) and trajectory (introduced vs. independent development) of the transition to agriculture must be considered when discussing the effect of agriculture on human health. The classic dichotomy of tall foragers and short farmers was not supported by this dataset. 

    Importantly, the study also reported that following declines in stature, some farming populations demonstrated increases in body size over time [2]. Northern Europe (from 7,000–4,000 years ago) and Central Europe (from 8,000–5,000 years ago) experienced the most significant increases in height and body mass compared with other regions during these same time periods. The timing and geography of these stature increases correspond with archeological evidence for dairy farming and genetic evidence for selection on LP, supporting the predictions of the Lactase Growth Hypothesis [2]. 

    The study authors believe that environmental stressors that could impact growth, including crop failure or lack of diversity of crops (both of which could have been common in more northern latitudes), were buffered by milk drinking in these European LP populations [2]. Individuals that lack LP genes can consume milk, but the side effects (particularly diarrhea) would be especially problematic during times of nutritional stress. Dairy foods with reduced lactose content, like yogurt and cheese, would allow consumers to avoid digestive issues but may not have the same impact on skeletal growth because fermentation destroys a key ingredient: insulin-like growth factor I (IGF-I). IGF-I is a growth hormone, present in mammal milk to support infant growth [4]. Cow milk consumption by humans is associated with increases in circulating IGF-I and increased IGF-I is known to promote skeletal growth [2, 4]. As a result, animal milk consumption after weaning (and as a weaning food) would be associated with continued exposure to IGF-I, which is predicted to mediate skeletal growth throughout childhood and adolescence [2, 4]. 

    Milk drinking and LP selection coincide with the observed increases in body size, but are they a coincidence or the cause? The study design doesn’t allow the authors to do more than speculate; they did not collect ancient DNA (aDNA) from the skeletons they measured so they cannot say with certainty that the changes over time in Central and Northern Europe were exclusively in individuals with LP gene variants. Additionally, the authors concede that there are other, non-dietary explanations for the increases in stature observed among the Northern and Central Europeans, including gene flow with other populations (new gene variants introduced by mating with other populations), genetic drift (some genes randomly increase in frequency because of small population size), or genetic changes in energetic allocation to growth, immune function, and reproduction [2]. These factors could be operating alongside selection for LP in explaining stature increases or instead of selection for LP. 

    An important next step to untangle these potential cofounding factors would be to test the Lactase Growth Hypothesis in other populations that depended heavily on milk, rather than cheese or yogurt, for nutrition over the last 10,000 years and underwent rapid selection for LP gene variants. The study authors are particularly interested in understanding if LP and milk drinking explains variation in body size among modern-day East African populations, including the Maasai herders of East Africa. Unfortunately, the data required to search for trends in stature over the last 30,000 years are currently lacking for this population. Testing the Lactase Growth Hypothesis among non-European populations is critical to understanding how milk drinking and the evolution of LP shaped body size variation in modern humans.

    References

    1. Visscher PM. Sizing up human height variation. Nature Genetics. 2008;40: 489-490.
    2. Stock JT, Pomeroy E, Ruff CB, Brown M, Gasperetti MA, Li FJ, Maher L, Malone C, Mushrif-Tripathy V, Parkinson E, Rivera M, Siew YY, Stefanovic S, Stoddart S, Zarina G, Wells JCK. Long-term trends in human body size track regional variation in subsistence transitions and growth acceleration linked to dairying. Proceedings of the National Academy of Sciences. 2023 Jan 24;120(4): e2209482119.
    3. Grasgruber P, Cacek J, Kalina T, Sebera M. The role of nutrition and genetics as key determinants of the positive height trend. Economics and Human Biology. 2014;15: 81-100.
    4. Wiley AS. The evolution of lactase persistence: milk consumption, insulin-like growth factor I, and human life-history parameters. The Quarterly Review of Biology. 2018 Dec 1; 93(4): 319-345.

    The views and opinions expressed in this publication are those of the contributing authors and editors and do not necessarily represent the views of their employers or IMGC sponsors.”