Here is the third in our series of posts considering the paper by Hardy et al, that made so many headlines in the media.This time we are pleased to feature a post by PhD candidate Miki Ben-Dor from Tel Aviv University who brings his expertise of paleo-anthropology to the fore and questions Hardy’s ideas that cooked starches promoted rapid brain growth 800 thousand years ago.
“Big brains needed carbs” (???)
Big Brain Need Carb is the title of the press announcement that accompanied the publication of a paper by Hardy et al. that was slightly more mutely titled “The Importance of Dietary Carbohydrate in Human Evolution” (1).
I don’t think that big brains need carbs or that, until recently, carbs were important in human evolution so here is an initial rebuttal.
Several camps can be identified among those who think that it was indeed the diet that made us human:
- Stanford and Bunn’s “Hunting and meat” camp (2, 3),
- Wrangham’s “Cooking and plant food” camp” (4, 5),
- Anton, Aiello and Ungar’s “Dietary flexibility” camp (6, 7),
- Crawford’s “Omega 3″ camp (8)
- My lonely “Man the fat hunter” hut (9) at the corner of the “Hunting and meat” camp.
Hardy et al.’s hypothesis is a variation on Wrangham’s cooking hypothesis which claim that cooking of starch and meat 1.8 million years ago allowed an enlarging brain and thus the evolution of the Homo species. Hardy et al. push that date 1 million years to 800 thousand years ago (kya).
Hardy et al. argue that multiplication of the salivary amylase AMY1 gene coincided with the invention of cooking 800 thousand years ago (kya) in order to facilitate a large consumption of carbohydrates that a growing brain needed. They conclude their abstract so: “Although uncertainties remain regarding the antiquity of cooking and the origins of salivary amylase gene copy number variation, the hypothesis we present makes a testable prediction that these events are correlated“.
I will try to test the correlation in time between cooking and the multiplication of the AMY1 gene later but let’s start with a test of whether any of the two proposed events (AMY1 multiplication, cooking) happened 800 kya when, as Hardy et al. point out, the rate of increase in brain size accelerated.
Chimpanzees, with only two copies of the AMY1 gene, don’t eat much starch and therefore have no need for large amounts of salivary amylase. Some modern humans that eat plenty of starch, as Perry et al. found out (10), have on average a higher number of copies than those who eat little starch. Additionally, there is an inverse relationship between the number of copies people have and their chances of becoming diabetic or obese (11-13) so a high number of copies of the AMY1 gene seems to be a good indicator for genetic adaptability to the consumption of large quantities of starch.
So when did we start to accumulate more copies of the AMY1 gene? Hardy et al. state that although the exact date is unknown it is thought to be less than 1 million years ago. In a 2014 paper, which Hardy et al. do not cite, Perry et al. (14) concluded that the duplication event occurred after the split between H. sapiens and Neandertals 550-590 kya. Furthermore they raise the possibility that the multiplication of the genes occurred within the past 200 thousand years but prior to the origin of agriculture based on similarity between the AMY1 copies. So in summary it is quite likely, based on AMY1 studies, that humans began to consume large quantities of carbohydrates not 800 kya but after 550 kya and more probably between 200 kya and 12 kya after their brain already reached its maximum size.
But what about cooking? Was cooking prevalent 800 kya and was the beginning of cooking necessarily associated with increased starch consumption?
To archaeologists, the control of fire is indicated by the existence of hearths, preferably with burnt bones in them and stone tools around them. There is no good evidence for hearths prior to 780 kya anywhere in the world. At 780 kya there is a single claim for habitual use of fire at Gesher Benot Ya’aqov (GBY) in the Jordan valley, Israel (15). The problem with this site is that it is situated in an area that went through extensive volcanic activity and lava flows at that time. One such flow ran in the middle of the site and was actually used to date the site to 780 kya. One cannot rule out the possibility, therefore, that existing concentrations of organic material were burnt by the heat of the lava and formed a hearth like artifact. In any event, the evidence for a habitual use of fire at that period in hundreds of other sites in the world is null. In my opinion, the most important recent (2014) paper on the subject is Shimelmitz et al. (16). It is important because it examines a cave (Tabun in Israel) that was inhabited by three successive cultures from 800 kya to 100 kya, and you do not find many complete sequences like this. By examining bones and stone tools from all of the cave layers Shimelmitz et al. concluded that fire was not used in the cave prior to 350 kya but was used continuously ever since. In summary people at Tabun cave from the same culture (Acheulian), and the same period (800 – 350 kya) and only 80 km away from the people of GBY had not used fire at all, habitually or not.
This result ties perfectly with evidence from Qesem cave in Israel, which also provides a date of 350 kya for the extensive use of fire by people from the same subsequent culture (Acheulo-Yabrudian) to the one in GBY (17).
Ok’ so not 800 kya but did the AMY1/cooking presumed duo take place around 350 kya?
First of all, it must be said that by 800 kya the size of the human brain had already doubled, compared to Australopithecus, from 400 cc or so to approximately 800 cc. Following Hardy et al.’s hypothesis, humans didn’t really eat plenty of carbs until 800 kya. Since we are limited in our ability to process protein to energy, the only possible logical conclusion is that prior to 800 kya the brain received its sugar from protein by way of gluconeogenesis and that a large part of the rest of the energy was supplied by fat. The brain can live nicely on glucose from gluconeogenesis or from ketone bodies that the liver produces from fat. See the Maasai or the Inuit. In other words, if we accept Hardy et al.’s hypothesis, high protein high fat diet is the only possible diet during the first million plus years of human evolution.
I do not claim that Paleolithic humans ate only protein and fat. Hardy is an expert in finding plant residue in dental calculus and she indeed found them as early as 350 kya in Qesem Cave (18). However, as far as finding the proportion of plant food in the diet goes, there are other methods, mainly using isotopes.
I looked at Hardy et al. for references to isotope research and found this: “…stable isotope analyses indicate a mainly carnivorous diet for Neanderthals; a wider range of isotopic values have been observed in contemporary Middle Pleistocene H. sapiens (Richards and Trinkaus 2009), indicating that considerable differences in the levels of starch consumption existed between these two species”.
I was a little puzzled as I have cited this paper (19) in the past and could not recall any reference to starch in the diet of the examined population. The paper compared N isotope samples from 10 modern European humans from around 40-30 kya and 5 Neandertals. To cut a long story short here is a quote from the paper summary:
“There are now enough isotopic data to see patterns in the data, and they show that the Neanderthals and early modern humans had similar dietary adaptations, obtaining most of their dietary protein from animals…”. Hmm…1500 cc brains (of modern humans and Neandertal) obtaining most of the dietary protein, which comes with plenty of fat, from animals. Just to be sure – later, as we get close to agriculture and archaeological evidence indicate increased carbs consumption, isotope studies do pick it up very nicely (20).
So we are 900 cc brain humans that need to grow to 1500 cc and the extra 600 cc brain looks for energy. As an economist by training I see a substantial problem with carbs as economical solution to the problem. It transpires that collecting carbs by way of tubers and preparing them for consumption is around ten times less energy efficient when compared to hunting (21). With the men doing the hunting that provides glucose for the first 900 cc of brain volume, the collection and preparation of tubers must be done by the women, who can’t go hunting since they have to guard the young. Plant food is also seasonal and not necessarily found in the same patches where animal are found. In summary, it is difficult to see how carbs could be the solution. More fat, obtained more efficiently, seems to me to be a much more plausible solution. Presumably, a larger brain allows for smarter tracking of animals, which means reduction in locomotion needs per pound of flash and thus achieving the extra energy more efficiently.
There is plenty of other evidence that humans were basically carnivores until 30-20 kya, just prior to the agricultural revolution. Some of the evidence is shown in my AHS13 presentation which can be found here. If true, the evidence points to a 300 ky difference between the control of fire and the consumption of large quantities of starch, and hence the addition of multiple copies of AMY1, so synchronicity between the two also seems quite unlikely.
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21. Stiner MC, Kuhn SL; Paleolithic Diet and the Division of Labor in Mediterranean Eurasia. In:Hublin J-J, Richards MPs (eds). The Evolution of Hominid Diets: Integrating Approaches to the Study of Palaeolithic Subsistence: Springer, 2009, 155-168.