BAF60c: Hero or villain?

UC Berkeley researchers have identified the gene that flashes dietary carbohydrate into body fat: BAF60c. When mice had this gene disabled, no matter how they gorged on carbohydrates, they stayed sleek and svelte, while those with enhanced BAF60c continued pumping out fat storage genes even when fasting. I’ll be asking Dr. Sul, some questions about this gene!


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Adam or Eve?

In my previous post, I said that a favourable mutation (in this case for increased fat deposition) in even one woman, in a very small population group such as those that occurred during the Middle Stone Age, would be fixed very rapidly – and suddenly had a thought that  I was taking it for granted that this mutation occurred in a woman when it might very well have occurred in a man! However, upon thinking it through, I am inclined to conclude that an African Eve, rather than an Adam, got the lucky roll of the genetic dice, for these reasons:

1. Women have two X chromosomes, one from each parent, while men have one X and one Y. Both sons and daughters get their X chromosome from their mother, who cannot contribute anything else. Daughters get an X and sons a Y from their father.

2. Women, across cultures, carry almost twice as much fat as men, even when matched for age, height, ethnicity, and weight.

3. This looks to me as if women have two copies of the fat-depositing gene, one on each X chromosome.

A woman would pass on the adiposity genes in an equal-opportunity fashion, to both sons and daughters, who would then disseminate it very rapidly. A man undergoing such a mutation affecting his X-chromosome would pass on the gene only to his daughters, and were the gene located on his Y-chromosome would pass it on only to his sons. While the daughters would pass it on to their sons, who would again pass it on to their daughters, and so on in perpetuity, my hunch is that there would then be little or no sexual dimorphism in fatness.

Over time, it’s possible that some individuals may have developed multiple copies of this adiposity gene, just as Asians have many more copies of the gene influencing amylase expression, making them better able to metabolize starches.

The paper linked above, Diet and the evolution of human amylase gene copy number variation, by Perry et al, 2007, notes that freeliving wild born chimpanzees have only 2 diploid copies of the AMY1 gene, which influences starch digestion, while even humans with a low number have 6. To me, this suggests that the initial mutation may have occurred during crossingover in meiosis, such that some lucky individual got an extra allowance of AMY1, allowing her to better digest starch and lay down fat. As daughter populations hived off and some underwent selective pressure through environmental determinism pushing their diet to starch as a staple, extra copies might then be generated.

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Minding the gap

I think I can narrow the timespan – if anatomically modern humans, in the Out of Africa II scenario, left their mother continent around 60,000 years ago, the ability to lay down fat would likely have been fully functional by then. This is getting exciting – it looks now to me as if we Scatterlings of Africa started getting plump between 75,000 and 60,000 years ago. With a small bottleneck population, of which an even smaller number would be nubile females, a favourable mutation in even one woman could be very rapidly fixed!

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Boom or Bust In The Middle Palaeolithic

Doubtless readers of my blog have noticed a serious hole in my process: where, you might well ask, is the evidence that anatomically modern human populations suddenly started to grow in the Middle Palaeolithic? All my other work has baked a nice big fluffy bun, but where’s the beef?

In an earlier post “Always Something New”, I cited Sarah Blaffer Hrdy to this effect, but have not, until now, brought any heavy guns to bear. Duck, cover, and hold, because the big artillery has arrived!

Here’s a pretty graph from the University of Michigan:

Note, there is a quantitative leap sometime before 10,000 years ago, shown by the insertion of a new line segment disconnected from the previous segment. The broad time scale, and inevitable drawing of the eye to the much more impressive population numbers and gains of our recent history make this little blip seem insignificant, yet it is the turning point for our species. The associated gains in cultural evolution have gotten the kudos for our subsequent success, but as Brown & Konner pointed out, we are unique among living creatures because our evolutionary engine has two pistons: culture and biology (29).

Heavy gun #1: Mary C. Stiner, Natalie D. Munro, Todd A. Surovell, Eitan Tchernov, & Ofer Bar-Yosef (2009), in their paper Paleolithic Population Growth Pulses Evidenced by Small Animal Exploitation, analyzed the proportions of small and large prey remains in dietary assemblages. Small prey proportions, they say, “are more sensitive indicators of the changes in human population density because small prey species vary much more than ungulate species with respect to life history and predator avoidance characteristics.” They conclude that early Middle Palaeolithic human populations were “exceptionally small and highly dispersed” and that population densities “increased abruptly during the late Middle Paleolithic and again during the Upper and EpiPaleolithic periods.”

While I am concentrating my attention on the first of these periods, it is compelling that the second part of the Paleolithic saw the appearance of the Venuses, AND a greater reliance on grains and pulses (carbohydrates) in the plant fraction of the diet (Maher 2012).

To me it looks like this: Once the thrifty gene/phenotype had been acquired, it was rapidly fixed and came without brakes: we raced from gaining significant fat, highly adaptive for survival and breeding success, to obesity as we incorporated more starches and sugars into our daily diets. While pulses are usually thought of as high-protein foods, they are also rich in starch and sugar.

Heavy Gun #2: Stanley Ambrose’s 1998 paper, Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans, explores reasons for the genetic bottleneck and release affecting anatomically modern humans around and after the time of the Mt. Toba eruption. He says, “Around 50ka, dramatic growth occurred within dispersed populations that were genetically isolated from each other. Population growth began earliest in Africa and later in Eurasia …”The reason for this, he says, is usually ascribed to better technology, “which developed in equatorial Africa” (623). Stanley then goes on to suggest that bottleneck/release mechanisms may be driven by factors of climate and geography.

Heavy gun #3: David E. Reich and David E. Goldstein (1998) delved into the genes of aboriginal populations worldwide. The results of running statistical tests on a number of genetic markers: a major population expansion occurred in Africa but nowhere else (paper abstract), sometime between 44,000 and 570,000 years ago (see correction to paper).

These three lines of evidence – archaeological, climatic/geographical, and genetic – support the hypothesis that there was a sudden surge in anatomically modern human populations, in Africa, during the Middle Paleolithic. Note: climate alone is unlikely to have driven this, as sapiens had previously enjoyed 50,000 years of warm wet weather without concomitant expansion in population size, and the cold dry interval appears to have killed us off in the Levant. After our return to the Levant, it seems that succeeding cold dry intervals had lost their biological power over us; secure in our fatty envelopes, we simply tweaked our technology and social coping skills.

Ambrose, Stanley. Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans. 1998. Journal of Human Evolution 34:623-651.

Brown, Peter J. and Melvin Konner. An anthropological perspective on obesity. 2006. Annals of the New York Academy of Sciences 499: 29-46

Hrdy, Sarah Blaffer. 1999. Mother Nature: a history of mothers, infants, and natural selection. Pantheon Books, Random House, Inc., New York, N.Y.

Reich, David E. and David B. Goldstein. Genetic evidence for a Paleolithic human population expansion in Africa. 1998. Proceedings of the National Academy of Sciences, USA. 95:8119-8123.

Stiner, Mary C., Natalie D. Munro, Todd A. Surovell, Eitan Tchernov, & Ofer Bar-Yosef. Paleolithic population growth pulses evidenced by small animal exploitation. 1999. Science 283:190-194.

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From African Eve to European Venus

Unfortunately there is little art from any period before the Upper Palaeolithic, so it isn’t possible to compare and contrast imagery to either refute or support my contention that the ability to fatten was gained late in our evolutionary history.

The Venus of Tan-Tan, from Morocco, if indeed it is a hominin-modified pebble and not a natural pebble, is too old for our species to have made it, as it is 500-300kya.

It may have been painted with red ochre, which would make it an artifact regardless of whether the shape is entirely natural. Image and information: If this is an artifact, it would have been made by H. erectus or H. heidelbergensis.  My take: is it intended to be female? It looks fairly phallic!

 Venus of Berekhet Ram: Found in the Golan Heights,  Israel (Levant), around 280,000-250,000 years old.

Alexander Marshack has intensively studied this object, and concludes on the basis of scanning electron micrographs that it was deliberately modified using flint tools to increase its resemblance to that of a robust female (Marshack 1997:). Again, there is a problem with the age: the figurine must have been made by H. neandertalensis  or a non-anatomically modern sapiens. She is definitely chunky, but it looks like heavy bone and solid muscle (small breasts, flat belly), and there’s no way to measure her body fat %.

Our oldest depictions of anatomically modern women all come from Europe – but we are all Africans anyway. From left to right: the Venus of Hohle Fels, Germany, 40-35kya, mammoth ivory; the Venus of Galgenburg, Austria, 30kya, serpentine; and the Venus of Dolni Vestonice, Czech Republic, 31-27kya, ceramic. All images from Wikipedia.

There has been a good deal of argument about whether these figurines represent real women or the yearnings of a population whose women are trapped in skinny bodies. On the side of the dreamgirl theorists is Eric Colman, M.D., whose 1998 paper, “Obesity in the Palaeolithic Era? The Venus of Willendorf” outright dismisses any possibility of obesity but is willing to consider Cushing’s syndrome. He argues that seeing hunter-gatherers were not sedentary and because their diet was lower in fat than that of today, nobody could have been other than lean and muscular. As this is his starting point, his conclusion is inevitable: “Obviously, we will never know exactly what inspired the creation of the Venus of Willendorf, nor will we know its true meaning. Nevertheless, this ancient work of art serves as a valuable reminder that obesity is a disease unique to the modern world and one in which environmental factors, such as diet and exercise, assume critical etiologic roles.”

Hmm. There’s a fair amount of circularity to this argument: Paleo people got a lot of exercise and ate a low fat diet, hence they could not be obese, hence the Venus is not drawn from life, hence obesity is a modern disease and looking at the Venus should drive us to high exercise, low fat diet living. Hmmm.

Another medical doctor, the practising gynaecologist Jean-Pierre Duhard, is equally certain that he knows what’s what. In the course of his work, Duhard has seen many naked women, and he says that modern women exhibit the same range of shapes and sizes as the Venuses. He backs up his contention with a plate of line drawings of modern women, below (554). Like Colman, he recognizes some medical conditions. However, Duhard’s conclusion is that Palaeo artists were simply working from life and producing portraits (1991:559).

Finally, Katheryn C. Twiss, in discussing the Neolithic of the southern Levant, includes this image (2007:27). Although stylized, the female figures are of physical types recognizable to any modern woman who looks around the locker room at the gym, and they have goodly fat deposits. The figures range from around 11,700 to 9,000 years old.

While I haven’t proved beyond reasonable doubt that humans, especially women, first acquired their modern ability to store large amounts of body fat during the Middle Palaeolithic, I think there is good reason to suspect this, given that normal fatness is strongly correlated with reproductive fitness, and that there was an uptick in our breeding rate when we recolonized the Levant. We exhibited a cultural blossoming on our return, and it seems that we acquired a full suite of anatomically modern behaviours during the critical period, 75-45kya, that we were gone from the Levant but present in Africa. It seems very likely to me that that our cultural evolution event was conjoined to a biological evolution event involving fattening.

I’m giving the last word to Johnny Clegg and Savuka: we are all “Scatterlings of Africa.”

Colman, Eric. 1998. Obesity in the Paleolithic Era? The Venus of Willendorf. Endocrine Practice. Vol. 4. No. 1. 58-9.

Duhard, Jean-Pierre. 1991. The shape of Pleistocene women. Antiquity 65:552-61

Twiss, K. C. 2007. The Neolithic of the southern Levant. Evolutionary Anthropology, 16, 24-35.

Wikipedia: The Venus of Tan-Tan.
                    The Venus of Berekhat Ram.
                    The Venus of Hohle Fels.
                    The Venus of Galgenburg.
                    The Venus of Dolni Vestonice.

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Always Something New

When Pliny the Elder commented, Ex Africa semper aliquid novi, (Out of Africa always something new), in 79AD, he was talking of African animals, and he was quoting Aristotle’s book on natural history, written 2,300 B.C. (Mayell, 2003)

Archaeologists could well make the same remark, in reference to just one animal: Homo. Africa, at each throw of the genetic dice, turned up hominin after hominin until it came to us – but the party didn’t stop with the first anatomically modern humans around 195 kya at Omo Kibish and Herto, Ethiopia. It was in Africa, as McBrearty and Brooks, showed in their paper The Revolution That Wasn’t, published 2000, that each single component of fully modern human behaviour evolved, whether harpoon, microlith and compound tool, or symbolic expression through hatching of ochre. I argue that during this same period, the Middle Palaeolithic, anatomically modern humans, H. sapiens, evolved a cunning new biological twist to accompany all this cunning new cultural stuff. I think that during the Middle Palaeolithic Africa dialed up our potential to lay down astounding amounts of body fat, which in turn led to increased survivability in the Levant and Eurasia after recovery from the H5 Heinrich Event.

We humans are fat, fat, fat, even when we’re skinny, compared to other primates and even to other mammals. In my earlier post, The Fats of Life, I concluded that our fattiness, and high sexual dimorphism in body fat percentage, must confer an adaptive advantage, and I’ve presented evidence that reproduction costs bear very heavily on women, who use this fat to fuel pregnancy and lactation.

There’s little evidence to show when this fattening ability came into our lineage; Leslie Aiello et al suggest that the tendency arose during the Homo erectus period, when there was a sudden increase in our body size (2002: 552). It was in this period that we probably started cooking, grew our brain, and shrank our gut. We may also have started talking at this time, and we may have lost our fur then. Caroline Pond presents evidence that other primates, notably the orangutan, have some ability to pack a paunch. Other plump animals lay down their fat along the back, which is why farmers pinch their cattle and sheep around the root of the tail to assess their condition. Only primates lay down their fat in front – Pond suggests that it may be a form of sexual signalling indicating good body condition. For primates, the belly is often on view so we can strut our stuff effortlessly (Pond 1998: 42-45).

My point: we were probably preadapted to fattening under the right conditions. I argue that these conditions arose in East Africa during the Middle Palaeolithic while Neandertals ruled the Levant and then disappeared into the dust of eternity: between 75 and 50 kya. I’m picking on East Africa because it’s almost certainly the birthplace of hominids, hominins, and H. sapiens.

This table is adapted from information provided in my Anthropology 128 class, Fall 2012, by Professor Lisa Maher.

Later sapiens could well have recolonized the Levant from East Africa, and the difference in behaviour and economy after only 30,000 years, a blink in our evolution, is striking. Something was going on during Marine Isotope Stage 4 that catapulted us into a new paradigm.

What was happening in East Africa during MIS 4?

While the degree of the effects of the Mount Toba supervolcano eruption are debated, there is consensus that a 6-10 year volcanic winter set in that affected the entire world, lowering average temperature, and probably reducing the amount of sunlight that reached Earth’s surface because of gigantic ash clouds and emission of aerosol gases. (Wikipedia: Toba)  East Africa, however, was probably buffered from the worst effects, becoming somewhat colder and drier, but not as markedly as the Levant (Maher 2012). Also, its position on the Equator would maximize its solar gain. However, less light and less water would have borne hardly on forests, so grasslands would have expanded as forests shrank.

Another point in its favour: East Africa does not have the geographic circumscription of habitable zones characteristic of the Levant. Humans would have been able to migrate, following the food, whether plant or animal, as well as exploit both forest and grassland. However, game herds may well have shrunk during this time, in lockstep with the diminishing productivity of the plants on which they feed, forcing humans to start exploiting their environment more intensively.

Small fast prey like rabbits and birds need different hunting weaponry, and new technology might well be needed to extract the maximum nourishment from plant foods. Tubers or seeds might need to be crushed or pounded, knives might be in demand for peeling and slicing, and perhaps new cooking methods might have to be devised. Of interest in this regard is the finding of Phillipson et al. that “the heavily worn grindstones” found in the Nile Valley had been used to process the tubers of Cyperus rotundus, commonly known as wild nutgrass , thus warning against the assumption that grindstones equal grain (147, 148). According to Wikipedia, these 1″ long tubers are somewhat bitter, but edible, and are very rich in minerals and vitamins. They also have medicinal properties (Wikipedia: Cyperus rotundus).

Necessity is the mother of invention: with the rapid onset of bad climate (within 3 years of the Mount Toba explosion, according to the Wikipedia article referenced above), humans, particularly women, would have had to turn to other foods. Leaves and fruits were scarcer. What’s a hungry human to eat? What about tubers?

Freeliving chimpanzees, who make digging tools to grub them out, eat tubers (Kiderra 2007).  The great Bob Brain claims that australopithecines were routinely extracting tubers at Swartkrans, South Africa, around 1 million years ago (Vincent 1985:131). For some nice pictures of this famous site: Probably humans have been eating tubers all along. Forests are not prolific tuber producers; in Tanzania, forest produces 0.5 tonnes-hectare of tubers, while savanna produces a bounty of edible bulbs – ranging from 5 – 2,000 tonnes/hectare, depending on species and local conditions, with only pigs and porcupines as competitors. These bulbs can be eaten raw as well as cooked, unlike grains. Anne Vincent, whose work I am citing here, says that one hectare of savanna produces enough tubers to feed three families for 148 days (1985: 139). Tubers are more reliable than grains, and can be left underground until needed, without deterioration in quality, while they regenerate faster than cereals.

Tubers are a rich source of carbohydrate, with varying proportions of starch and sugar. Gary Taubes, in his intensively researched book Good Calories, Bad Calories, argues persuasively that the prime driver of fat deposition is carbohydrate (see summary p454). This is certainly in line with the work of Leslie Aiello and Jonathan C.K. Wells, who show that humans can metabolize only enough lean meat to meet half of their daily requirements (2002: 326), and with that of John D. Speth and Katherine E. Spielmann, who discuss the   phenomenon of rabbit starvation, where tribes forced to eat lean meat as a staple, although having more than enough calories can essentially starve to death (1982: 3,4), and find that carbohydrate is non-replaceable by fat when it comes to protein sparing and muscle maintenance (6,7). Without sufficient body fat, calcium is not properly metabolised and essential vitamins are not stored (17,18).

Returning to Vincent’s work, women gathering tubers have full control of the resource, and they often eat a considerable amount of what they gather in situ. The same is true of berries and fruit . The high-status foods, meat and honey, are obtained and controlled by men, who share them out according to the societal norms (1985:135). It seems probable that natural selection would favour women able to rapidly fatten on a high-carbohydrate diet, as they would reach menarche earlier (Frisch 1990; Guo 2011), be more likely to retain a pregnancy to full term, and successfully lactate for 3 – 4 years, thus weaning a child more likely to survive till puberty.

One last tempting titbit: Sarah Blaffer Hrdy says that at the beginning of the Palaeolithic our species doubled its population size only every 15,000 years. Then, starting with a single small band of hunter-gatherers sometime around 100,000 years ago, we became “newly fecund“ and the brakes came off the breeding train (1999:184). Could the reason be a thrifty gene or thrifty phenotype that allowed carbohydrate-eating women to get fat and get fertile?

Aiello, Leslie C. and Jonathan C.K. Wells. 2002. Energetics and the Evolution of the Genus Homo. Annual Review Anthropology. 31:323-38.

Frisch, Rose E. 1987. Body fat, menarche, fitness and fertility. Human Reproduction vol 2. No 6. 521-533.

Guo, Xiao Yan & J.I. Chenge Ye. 2011. Earlier menarche can be an indicator of more body fat: study of sexual development and waist circumference in Chinese girls. Biomedical and Environmental Science, 24(5). 451-458.

Hrdy, Sarah Blaffer. 1999. Mother Nature: a history of mothers, infants, and natural selection. Pantheon Books, Random House, Inc., New York, N.Y.

Kiderra, Inga. “Chimps Dig Up Clues to Human Past?” 12 November 2007. UC San Diego News Center. Accessed 7 November 2012.

Maher, Lisa. 2012. Anthropology 128, Fall semester, University of California, Berkeley, USA. Class lecture notes.

Mayell, Hillary. “Out of Africa” Phrase in Use Since Ancient Greece.”  19 February 2003. National Geographic News. 7 November 2012.

McBrearty, S. & Brooks, A. S. 2000. The revolution that wasn’t: a new interpretation of the origin of modern human behavior. Journal of Human Evolution, 39, 453-563.

Phillipson, D. 2009. African Archaeology, Cambridge, Cambridge University Press. Chapters 4-5.

Pond, Caroline. 1998. The Fats of Life. Cambridge University Press, Cambridge, United Kingdom.

Speth, John D. & Katherine A. Spielmann. 1983. Energy source, protein metabolism, and hunter-gatherer subsistence strategies. Journal of Anthropological Archaeology, 2:1-31.

Taubes, Gary. 2007. Good Calories, Bad Calories. Alfred A. Knopf, New York, N.Y.

Vincent, Anne S. 1985. A preliminary report of tubers eaten by the Hadza of northern Tanzania. World Archaeology, 17(2):131-148.

Wikipedia. “Cyperus rotundus.” Last updated 28 October 2012. Wiki entry. Accessed 7 November 2012.

Wikipedia. Toba catastrophe theory. Last updated 23 October 2012. Wiki entry. Accessed 7 November 2012.

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Say It Again

Given the potential importance of Pontzer et al’s paper on hunter-gatherer energetics, I checked my table, corrected some arithmetical errors, and added a line for kg of body fat.

Then I made a graph.

This makes things clearer.

The data points are in the same order as Table 1: Hadza women and men, Western women and men, Bolivian farmer women and men. The data now reveal, in terms of energy output per kg of body mass per day, apparently marked differences between hunter-gatherers and farmers on the one hand, and Westerners on the other.

While the differences even out somewhat in terms of lean body mass for hunter-gatherers and Westerners and farming men, farming women are still way out there.

However, I still don’t see the point of working with lean body mass, given that fat is by no means inert and given that even lean people have a large amount of fat.

Further points to consider:

1. Energy output was measured at rest. Subjects sat quietly while breathing into apparatus that measured resting metabolic rate, from which basal metabolic rate was then estimated. For pregnant and lactating women, this may not be a good way to estimate total energy expenditure, as it does not take into account the increased difficulty of movement as pregnancy progresses – each task takes longer and is more awkward,and the throwing out of whack of normal balance by the increasing size of the belly in itself requires more energy to keep the woman upright. This increased energy usage is unlikely to show up while she is sitting peacefully.

2. While a lactating woman may not show increased energy output either while sitting or while walking, this may be because the cost of producing milk is being cancelled out by the simultaneous release of energy from her fat stores, which would probably not show up through the doubly-labelled water measurements as the fat was laid down long before the event. Hence, all that’s visible is her standard cost of metabolism and transport.

Women use fat from their body stores to support lactation, and they preferentially use fat from specific depots – the thighs, and the region called hips by Americans and buttocks by many others – which store considerably more DHA than any other female fat depot. Mark’s Daily Apple has a post stuffed with fascinating factoids about fat, his and hers, over here:

In the pipeline: my reasons for arguing that the cultural evolution taking place in Africa during the Middle Pleistocene were accompanied by a biological evolution event affecting the ability of anatomically modern humans (us) to accrete large amounts of body fat.

Pontzer, Herman, David A. Raichlen, Brian M. Wood, Audax Z. P. Mabulla, Susan B. Racette, Frank W. Marlowe. 2012. Hunter-Gatherer Energetics and Human Obesity.

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