I have seen this new article about how metabolism affected brain evolution emerge in several places this week (1). I think it is, for the most part, a great article, and would recommend it to any one with interest in this area. Anything with math usually gets my attention, and this article uses math to support its hypothesis in an easy to understand way. However, I am not sure that I can fully support the conclusions the authors, and many others, have come to about how their data relates to cooking.
If we take a closer look at the article we can see that the main point being made is that there is simply not enough time in a day to take in enough energy to support both a large body and a large brain. They make the point that the human brain accounts for only 2% of body weight, but consumes 20% of our energy. Another interesting fact they have is that for every billion neurons we have we need to consume about 6kcal to fuel them. Using this data they hypothesize that if we were to eat the same diet as our great ape relatives, we would have to spend over 9 hours every day eating to get enough energy to support our brains and body. Do any of you spend 9 hours eating? I didn't think so.
This eating constraint is the real result of the paper, and not the ways we overcame it. It is in the discussion they discuss reasons why we, as early humans, were able to overcome the constraint and develop our large brains. To quote the authors:
"such a metabolic limitation was overcome in the human lineage by the advent of cooking food, which greatly increases the caloric yield of the diet, as a result of the greater ease of chewing, digestion, and absorption of foods"
However, I think a lot of other adaptations were taking place in early humans that allowed us to develop our brains. I have written before about how important fat is a brain fuel, and when we switched from the herbivorous diet of our great ape relatives into one that included more animal products we also started consuming more fat. Let's review our basic nutrition facts quick: Carbohydrate=4 kcal/g Fat=9 kcal/g. For every one hour we spend eating fat we gain over twice the amount of energy.
I think our reliance on fat as a primary source of energy was probably more important than our adoption of cooking. This doesn't mean we can throw out cooking, I do believe that cooking played an important role in making us distinctly human. I just don't think that it was the sole, or even largest, factor in our transformation like these authors seem too. Again, I think I think this is a great article, I just wanted to provide some food for thought for those of you who have read this article or have seen it popping up around the web!
You are bringing up some interesting ideas. Are you suggesting a reliance on fat facilitated the development of the human brain?
ReplyDeleteAs long as we're brainstorming, I'll suggest the key lies more so within glucose metabolism. I think the reason humans are so vulnerable to diabetes is due to the large demand in neural tissue for glucose. If you want to explore the phylogeny, I'd look at the projections for where cetaceans differentiated. As you may know they far more closely represent human proportion of central nervous tissue as compared with non-humanoid great apes.
If you notice parallel evolutionary trajectories after the point of projected differentiation it suggests the metabolic pathways that are most important. Of note, dolphins don't cook! They eat a high protein diet (yes certainly fat too and particularly PUFA), but protein is a better substrate for gluconeogenesis.
Another key to dolphins specifically is that while they also have stunningly high rates of diabetes relative to non-humanoid great apes, they also seem to self regulate their insulin resistance in ways that may hold promise for human disease.
http://rspb.royalsocietypublishing.org/content/early/2012/06/26/rspb.2012.0869.full
http://www.sciencenews.org/view/generic/id/56441/title/Dolphins_may_offer_clues_to_treating_diabetes
I think a reliance on fat, as well as several other factors, were very important in the development of our brains. The authors of this particular study cite cooking as one explanation, but I think that eating fat is an equally plausible explanation, as it is another way to increase energy intake on a per gram basis.
ReplyDeleteI think that glucose metabolism is definitely a contributing factor, and found this study that does show that our brains have a higher expression of a glucose transporter than those of chimpanzees (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3237107/#B39). However, the authors also found that chimps have a higher expression of a glucose transporter from the same family in their muscle. This to me looks like it could an adaptation to a higher fat diet in humans. Something would have to supply our muscles energy and it could very well be fat, and this would also work as a mechanism to spare glucose for our brain, by decreasing the amount of glucose that can even get into our muscles. However, I may be letting my personal biases factor too much into these conclusions!
I think your example of dolphins is extremely interesting, and has had me stumped all day. I do think that dolphins are under uniquely different selection pressures, due to the marine environment. The authors of the study you linked even mention that dolphins have a higher metabolic rate than similarly sized land mammals, They also say that their is a similar acceleration of the gene for a specific part of the electron transport chain inside mitochondria of both primates and cetaceans.
Perhaps what is important is not what types of substrates and energy densities a mammal is consuming, but how efficiently the energy is used. One of the common threads between us and the dolphins were mitochondrial proteins. Maybe we have both developed very efficient systems for dealing with our energy and especially finding ways to supply our brains with the extra energy they need. This can be seen during times of low energy intake in both species, as we both have similar mechanisms to deal with energy shortage. The similar problems we see with diabetes and glucose dysregulation might stem from being too efficient with our energy, having too much of it, and having it too often.