Comment on "Evolutionary Implications for Human Brain development" by Prof.Loren Cordain et al.

Expensive Tissue and Dietary Quality

First is the expensive tissue point about body RMR, brain size and metabolic activity of tissues. The computation never seemed logical to me (I'm a computer professional) and in this version of your description the words "and metabolic activity" are the key point (and IMHO should need a little more emphasis. For example: Given the fact that brains take 25% of RMR in humans and 8% in other primates. And the brain has a high metabolic rate (h) compared to other tissues (l). So the rmr would be:
RMRh = 0.25*h+0.75*l for humans and
RMRp = 0.08*h+0.98*l for other primates
If we accept that RMRc and RMRp are the same (Klieber), then it's obvious that simply reducing the size of the "other tissues" with lower RMR, actually the RMR per weight would *increase*. Bbut as you told it's constant. If the gut is of the type "low RMR" and you decreased it's size, this would actually result in an *increase* of the summary RMR, because the brain would be relative bigger.

That's a contradiction. It only makes sense if the gut was also a tissue with elevated metabolic resting activity. Like reducing some of the highly active gut while increasing also highly active brain and leaving the rest at the same. "Expensive Tissue" only makes sense if the gut also was highly active, at rest. Maybe this is the case. I think it could be a little more active than other organs at rest because it has to move it's contents.

Personally I think a improved dietary quality alone can explain the gut size reduction. We should think of the purpose of each gut part. What is reduced in humans is the last part where cellulose fermentation is going on in other primates. They need it because they eat very much cellulose material (like leaves) and need to derivate their energy from it.

If a human get's more fruit (untrue) or more starch (true if you think of Wrangham/tubers) cellusole processing is less necessary. The fermenting part gut can be thrown overboard.

Increased dietary quality from animals can hardly explain how additional energy can come in. Game meat is a energetically a low density food, unless it's fat. As you often pointed out. A little added tuber starch can easily explain the gut reduction, because it

  1. delivers glucose energy (needed for the 25% of the brain) After all glucose is exactely the part of the energy demand which is increased
  2. It's easily digested with a small, non-fermenting gut

DHA and brain evolution

The assumption is, that brain size is limited because DHA synthesis in humans is limited. I see the following contradicting points to the points listed:
  1. Brain size and intelligence is the same between humans with high or low dietary intake of DHA/AA. Hunza or other inland dweller have the same size brain and intellegence as Inuit. The brain size and IQ of vegetarian humans (from birth) does not differ from that of fish eaters. Therefore the synthesis capacity for AA and DHA *must* be sufficient in humans to build up the big brain.

    N.B.: However in modern humans the low intake of short chain w-3 fats and/or the intake of synthesis hindering items like tFA, alcohol, excess w-6, excess saturated fats seems to result in some points of differece in IQ to fish oil eaters.

  2. The size of the human (female) breast. this is the Venus of Willendorf, a paleolithic piece of art, illustrating one point of my DHA arguments As I got to know from Prof.Cordain, the mammary glands are one of the tissues able to synthesize DHA. A new born baby has only 25% of it's later brain mass. The rest is built up slowly in childhood. A small weaned child therefore has not only it's own liver as a synthesizing organ, but in addition the mammary glands of it's weaning mother.

    Obviously in most women these organs are rather big, compared to the liver of a child. The amount of milk is not dependant on the size of breasts. But it may be that (I hypothetize this) the DHA synthesis capacity is dependant on the organ size (it creates more cell surfaces and more space for special enzymes to work).

    Anyway the size of human mammary glands is very much bigger than the mammary glands in any other mammal I could imagine. Also in primates. As a side effect, the attraction of human (men) to sizy breasts (mammary glands) can be explained with a real evolutionary advantage they provide.

    Both, the (relative) big brain and big breasts are unique in humans, because human kids need a longer brain growth time and more substrate.

  3. Not all carnivores (eating pure DHA) have a relative bigger brain. The brain size of lions, elephants and horses is almost the same in percent of bodyweight (~ 1:500 to 600). The brain size seems to be determined by other factors, like evolutionary advantages.
  4. The "inefficient pathways with a low product to substrate ratio" to make DHA. I think it doesn't matter how big the ratio is, as the supply of the EFA sources (LNA) in nature is high. It's easy to eat grams of it, because (e.g.plant) cell walls consist in parts of it. While the necessary DHA amount is very small E.g. for a baby with a 400g brain its 0.22g(per 100g) * 4 = 0.88g DHA to be built up in say 5 months of brain growth, makes only 0.88g/150 days= 0.006g DHA per day.

    My conclusion would be that DHA is not a limiting factor to brain growth, except in diseased states like intoxication with tFA, alcohol, excess w-6 acids, high insulin.

    If DHA is not a main limiting factor then the best food item in dietary quality are tubers. That supports the theory of Dr.Wrangham. Also the availability of tubers in a savannah seems to be very much better in the aspects of

    Amadeus Schmidt-Philipp. 1