Are we meat eaters or vegetarians ?

[carnivore]

An interesting article about the Expensive-Tissue Hypothesis from Dr Eades :

http://www.proteinpower.com/drmike/low-carb-library/are-we-meat-eaters-or-vegetarians-part-ii/

[William]

Lots about meat, but IMHO the real difference is fat content of the human diet.

Brains! Marrow!

I think of the effective banning of fat from my diet by government, and wonder if I am wearing a paranoid conspiracy hat.

[goodsamaritan]

I could only last 2 months or raw vegan.  And I could only last 2 months or raw fruitarian.

I think many of our diet experimentations which eventually landed us here in raw paleo show that we run optimally on large quantities of raw animal food.

When class picnics happen it is so obvious that people desire meat.  Ho hum how they display the starches, the veggies, the fruits... but when someone opens an animal food basket.... zoooommmm everyone just gorges in and the meats are all consumed immediately.

A fellow parent saw my wife and I today and she was raving about how good both of us looked... getting sexy... she asked... you must be avid vegetarians!!! (I think she remembered our vegetarian days).  I loudly replied... Absolutely not! The secret is lots of raw fatty meat!

[SkinnyDevil]

Interesting piece. Thanx for posting it!

[TylerDurden]

I'm increasingly unconvinced re the notion that diet is responsible for the vast increase in hominid brain-size. If it had only been a question of eating meat that led to brains, then one would expect many other mammals to have attained human-like boosts in brain-size. And there are plenty of herbivores which are more intelligent than meat-eaters like tigers etc.(a good example is the gorilla which Eades mistakenly mentions).

[cherimoya_kid]

I'm increasingly unconvinced re the notion that diet is responsible for the vast increase in hominid brain-size. If it had only been a question of eating meat that led to brains, then one would expect many other mammals to have attained human-like boosts in brain-size. And there are plenty of herbivores which are more intelligent than meat-eaters like tigers etc.(a good example is the gorilla which Eades mistakenly mentions).

Exactly.  No one has proven the reasons for various evolutionary changes.  It's nothing but theory.  The fact of evolution is obvious.  The reasons for specific changes has not yet been proven, in my opinion.  Perhaps when computers get so fast that they can model extremely complex real-world systems, then questions like this one can be answered with mathematical certainty.  Until then, it's really beside the point.

[carnivore]

I'm increasingly unconvinced re the notion that diet is responsible for the vast increase in hominid brain-size. If it had only been a question of eating meat that led to brains, then one would expect many other mammals to have attained human-like boosts in brain-size. And there are plenty of herbivores which are more intelligent than meat-eaters like tigers etc.(a good example is the gorilla which Eades mistakenly mentions).

Yes, it's not only a question of eating meat. And the size of the brain is not proportional to the intelligence, otherwise woman would be less intelligent than man.
Not to mention that they are many kind of intelligences.
Just curious : How do you compare intelligence between herbivores and carnivores ?

[TylerDurden]

Yes, it's not only a question of eating meat. And the size of the brain is not proportional to the intelligence, otherwise woman would be less intelligent than man.
Not to mention that they are many kind of intelligences.
Just curious : How do you compare intelligence between herbivores and carnivores ?

That's the point, there are hyperintelligent herbivores such as elephants as well as carnivores which are rather stupid. So, it doesn't seem likely that meat is linked to IQ. I suppose the expensive tissue hypothesis came about because scientists assumed that one needed greater brain-power in order to prey on other animals than herbivores which don't have to use their brains so much to get food. The scientists forgot though that there is also evolutionary pressure on herbivores to develop bigger brains so as to be able to ward off predators more effectively.

[William]

So, it doesn't seem likely that meat is linked to IQ.

You are all doing it.

It is fat that is linked to IQ.

[PaleoPhil]

I'm increasingly unconvinced re the notion that diet is responsible for the vast increase in hominid brain-size. If it had only been a question of eating meat that led to brains, then one would expect many other mammals to have attained human-like boosts in brain-size. And there are plenty of herbivores which are more intelligent than meat-eaters like tigers etc.(a good example is the gorilla which Eades mistakenly mentions).

Here is what Eades said on gorillas:

Are we meat eaters or vegetarians? Part II
21. September 2009, 22:32
http://www.proteinpower.com/drmike/low-carb-library/are-we-meat-eaters-or-vegetarians-part-ii/

"Take the gorilla, for example, almost pure vegetarians that spend their entire ‘working’ day foraging and eating, which they have to do to get enough calories to maintain their enormous bulk.  They have large guts and pay for it by having small brains.  Even smaller than that of our most primitive ancestors, the australophthecines.

Gorilla has one of the lowest levels of encephalization of any haplorhine primate, and the much higher level of encephalization of all the australopithecines suggests a diet of significantly higher quality than that of this genus.

Which makes sense when you consider that carbon 13 isotope analysis has shown that Australopithecus africanus (the species that came right after Lucy) consumed meat.  As you go up the lineage from Australopithecus and through Homo, you find that more and more meat was consumed the higher up the tree you go."


Eades addressed the big cats vs. gorillas and other primates issue several times in the comments:

Kayaman, 22. September 2009, 6:27
Excellent post, as usual. Thank you.

Why doesn’t the ETH logic apply to other carnivores? Lions eat meat but don’t seem very smart. How do they maintain parity with Kleiber’s law?

Eades: Lions and other carnivores obviously didn’t have the selective pressures to develop larger brains that humans did.

Allen, 22. September 2009, 7:51
If it was meat consumption alone that lead to our increased brain size, then why didn’t other carnivores, especially cats, also develop large brains? Lierre Keith posits in “The Vegetarian Myth” that it is man’s unique ability to crush our prey’s skull and get access to the fatty brain (mostly saturated fat at that) which ultimately allowed our human ancestors to develop their own large brains. BTW, I love this book! Thanks for recommending it.

Eades: The big cats developed through a different line than we did. They may not have had the selection pressures we did – given our relatively small bodies and relative lack of strength and speed – to grow a large brain.

Jeff, 22. September 2009, 9:41
Long time reader, first time commenter. Great article and it makes a lot of sense.

I am apparently the third to pick out carnivorous cats as making this theory questionable. Even though you answered the question earlier regarding cats I am still a bit unsatisfied. Cats may not have had the same selective pressures, but they do have small(er) GI tracts from what I hear. If that is the case then either Kleiber’s law is violated and they have a lower metabolic rate OR there is some other metabolically expensive tissue that makes up for it (instead of brain matter). Is it their musculature that is larger in proportion to their bodies that grew as a result of their selection pressure? I would love to see the chart of a big cat’s actual and expected as in the organ weight chart above. Muscle is my guess but I am not sure since you mentioned it isn’t much of a contributor. I would be curious as to your answer.

Great stuff and keep up the good work,
Jeff(meat and paleoish low-carb eater)

Eades: I don’t have the data on the big cats. The Expensive-Tissue Hypothesis was written to address the rapid increase in human brain size. We are a totally different genus and species from lions and tigers, so what applies to us doesn’t necessarily apply to them. They are on the Kleiber line, but I don’t know what makes up the difference anatomically between their small GI tracts and brain size. Obviously something does – I just don’t know what because I’ve never studied it. Lions do have a considerably larger muscle mass on a per body unit basis than humans, so maybe some of it is there. Since they are in a different genus, maybe their hearts and/or kidneys are larger as well. Aiello and Wheeler compared us to primates, since that is the line from which we descended.

Rebekka, 22. September 2009, 10:10
Back in my anthropology days at school we were lectured about the importance of meat to human evolution, but I don’t recall that our professor talked so much about the GI tract as the quality of the diet, and the chewability. If I remember correctly (not guaranteed!), as the diet grew finer and contained less roughage and coarse plant fibers, there was a corresponding decrease in tooth and jaw size, but the attachment sites on the skull for the jaw musculature also decreased, which allowed the intracranial capacity to increase.

This could be a potential explanation for why other carnivores such as the large cats haven’t evolved human-like large brains – they need their massive jaw musculature for hunting and breaking apart carcasses.

Thoughts?

Eades: Interesting idea, but, as Aiello and Wheeler warned in the ECT, you’ve got to be careful deriving theories from specific anatomical points without looking at the total picture. But, could be the difference between us and the big cats.

[TylerDurden]

Eades' explanation for why the big cats didn't develop large brains despite a smaller gut is pretty weak, simply claiming that they were different species from humans etc. The claim re smaller jaws deriving from meat is not the only one. There's another claim suggesting simply a genetic mutation causing smaller jaws(and therefore a larger brain):-

http://www.bioedonline.org/news/news.cfm?art=887

Also, the gorilla example is a bad one. Didn't you yourself give an example of some small primate species eating mostly or wholly fauna? That species is undoubtedly far lower in IQ than a gorilla, despite its carnivorous diet.

The real problem Eades has is that some wild animal species have increased their brain-size on an evolutionary level without requiring higher amounts of meat, which invalidates the expensive tissue hypothesis. Even worse, Homo Habilis had a brain 50% larger than its Australopithecine ancestors(Homo Habilis started to appear c.2.5 million years ago just at the start when  humans began gradually to eat a little more meats in the diet- Australopithecines were mostly frugviorus/herbivorous ).So since Eades cannot explain the increase in brain-size of the Australopithecines or Homo Habilis, it seems far more likely that the cause of the increase in the brain-size of all hominids had nothing to do with diet but some other cause.

[William]

You all still do go on and on and on about meat.

The difference between us and our dimwitted fellow carnivores is FAT consumption.

The broken animal skulls and leg bones at all paleolithic campsites show this clearly. Brains are fatty, so is marrow.

Lions, wolves etc. can't get enough fat to support a big brain; we can.

From a post by delfuego who noticed it too:
"Read Weston Price. He makes a strong correlation between diet and human behavior.

Also look into the insanity of John Harvey Kellogg and his work with canines. By turning their diets from meat to grain he was "able" to create a "docile" more "easily manageable" beast. He followed the same reasoning for children and came up with Corn Flakes. His work led to the creation of a sanitarium where guests were treated to vegetarian diets and yogurt enemas. He made it his life's work to strip males of their sexual desire (his religious belief deemed sexuality as an evil urge) through the consumption of grains and a low fat diet."

When politicians/tyrants create a docile subject/slave population, they do so by convincing them to avoid fat meat. This is the recipe for how to make paleoman into neolithic man.

[TylerDurden]

There's little point in citing fat as opposed to meat. For one thing, many of the calories in brains cannot be used so that's misleading and brain and marrow fat is not a great deal more than the rest of the fat in the body, being only a fraction of the overall total of body-fat, so that extra fat as an explanation for brain-size increase does not compute either.

[William]

It is hardly opposed to meat, it is rather the component of a human diet that makes the difference between us and the other carnivores.

Not so much the amount of calories as the quality - see Taubes' GCBC.

[TylerDurden]

It is hardly opposed to meat, it is rather the component of a human diet that makes the difference between us and the other carnivores.

Not so much the amount of calories as the quality - see Taubes' GCBC.

Trouble is that wild animals ate the same sort of food as palaeo humans. Some animals  do indeed eat the marrow by cracking open the bones(eg:- wolves, lions and vultures)The vultures crack the bones by dropping them from great heights while lions and wolves use their teeth. I wouldn't be surprised if there are many animals which eat brains as well.

[PaleoPhil]

Eades' explanation for why the big cats didn't develop large brains despite a smaller gut is pretty weak, simply claiming that they were different species from humans etc. The claim re smaller jaws deriving from meat is not the only one. There's another claim suggesting simply a genetic mutation causing smaller jaws(and therefore a larger brain):-

http://www.bioedonline.org/news/news.cfm?art=887

Yes, but if that hypothesis is true, then what enabled that mutation to survive and replicate in those who had it? One of the study's scientists, Minugh-Purvis, argues in the article you cited that it may have been a change from "eating chewy leaves all day long to snacking on smaller portions of meat" that enabled this mutation to survive and spread.

Also, the gorilla example is a bad one. Didn't you yourself give an example of some small primate species eating mostly or wholly fauna? That species is undoubtedly far lower in IQ than a gorilla, despite its carnivorous diet.

I'm wasn't claiming that Eades had responded to all possible questions re: the hypothesis, just thought it was fair to point out that he had responded to the questions you raised about the big cats. It's still an area of much speculation and I don't know what all the answers are. I am neither convinced that meat/fat is the whole answer, as he seems to suggest, or that cooking is the whole answer. I think that the answer is probably more complex and multifactorial, as with most biological processes. It may involve both cooking and meat eating and other factors, such as sexual selection of neotenized traits. The tarsiers vs. gorillas question is an interesting one you could raise with Dr. Eades.

The real problem Eades has is that some wild animal species have increased their brain-size on an evolutionary level without requiring higher amounts of meat, which invalidates the expensive tissue hypothesis.

Remind me again please of an example of this and if you have a hypothesis re: the mechanism behind it, and are you talking about increases in brain/body ratio, or just plain increases in brain size?

Even worse, Homo Habilis had a brain 50% larger than its Australopithecine ancestors(Homo Habilis started to appear c.2.5 million years ago just at the start when  humans began gradually to eat a little more meats in the diet- Australopithecines were mostly frugviorus/herbivorous ).So since Eades cannot explain the increase in brain-size of the Australopithecines or Homo Habilis, it seems far more likely that the cause of the increase in the brain-size of all hominids had nothing to do with diet but some other cause.

Quite the contrary, Eades and others have argued that Australopithecines ate more meat than earlier primates, and Eades directly addressed this in his article:

<<Take the gorilla, for example, almost pure vegetarians that spend their entire ‘working’ day foraging and eating, which they have to do to get enough calories to maintain their enormous bulk.  They have large guts and pay for it by having small brains. Even smaller than that of our most primitive ancestors, the australophthecines.

|    Gorilla has one of the lowest levels of encephalization of any haplorhine primate, and the much higher level of encephalization of all
|  the australopithecines suggests a diet of significantly higher quality than that of this genus.

Which makes sense when you consider that carbon 13 isotope analysis has shown that Australopithecus africanus (the species that came right after Lucy) consumed meat.  As you go up the lineage from Australopithecus and through Homo, you find that more and more meat was consumed the higher up the tree you go.>>

Trouble is that wild animals ate the same sort of food as palaeo humans. Some animals  do indeed eat the marrow by cracking open the bones(eg:- wolves, lions and vultures)The vultures crack the bones by dropping them from great heights while lions and wolves use their teeth. I wouldn't be surprised if there are many animals which eat brains as well.

Didn't you write favorably of the scavenger hypothesis, which is based on the inability of any major predators beyond hominids other than giant cats and hyenas that went extinct around half a million years ago to access the brains and marrow of an intact skeleton? Have you abandoned the scavenger hypothesis?

[TylerDurden]

Yes, but if that hypothesis is true, then what enabled that mutation to survive and replicate in those who had it? One of the study's scientists, Minugh-Purvis, argues in the article you cited that it may have been a change from "eating chewy leaves all day long to snacking on smaller portions of meat" that enabled this mutation to survive and spread.

This is a bad claim as hominids had already moved away from a largely herbivorous(leaf-eating etc.) diet to a more frugivorous one. Raw fruit does not require massive jaws in order to be eaten.

I'm wasn't claiming that Eades had responded to all possible questions re: the hypothesis, just thought it was fair to point out that he had responded to the questions you raised about the big cats. It's still an area of much speculation and I don't know what all the answers are. I am neither convinced that meat/fat is the whole answer, as he seems to suggest, or that cooking is the whole answer. I think that the answer is probably more complex and multifactorial, as with most biological processes. It may involve both cooking and meat eating and other factors, such as sexual selection of neotenized traits. The tarsiers vs. gorillas question is an interesting one you could raise with Dr. Eades.

I would never dare to argue with a guru after one or two unfortunate experiences in the past. They are all of necessity extremely dogmatic, as they are defending their own turf etc. As for the issue of cooking and bigger brains, given that the advent of cooking(250,000 years ago) happened long after the really big increases in hominid brain-size, that makes the theory dead in the water. Perhaps cooking led to a further slight decrease in jaw-size at the tail-end of the Palaeolithic(with no effect on the brain), but that's about it.

Remind me again please of an example of this and if you have a hypothesis re: the mechanism behind it, and are you talking about increases in brain/body ratio, or just plain increases in brain size?

I was just talking about plain increases in brain-size of herbivores via evolution, such as elephants.Brain-size does have to increase with body-size as the larger body requires more brain-matter dedicated to breathing and other basic functions. The brain-size/body ratio is controversial with some claiming that brain-size/lean-mass ratio is more accurate, and others claiming that even that is wrong.

Quite the contrary, Eades and others have argued that Australopithecines ate more meat than earlier primates, and Eades directly addressed this in his article:

<<Take the gorilla, for example, almost pure vegetarians that spend their entire ‘working’ day foraging and eating, which they have to do to get enough calories to maintain their enormous bulk.  They have large guts and pay for it by having small brains. Even smaller than that of our most primitive ancestors, the australophthecines.

|    Gorilla has one of the lowest levels of encephalization of any haplorhine primate, and the much higher level of encephalization of all
|  the australopithecines suggests a diet of significantly higher quality than that of this genus.

Which makes sense when you consider that carbon 13 isotope analysis has shown that Australopithecus africanus (the species that came right after Lucy) consumed meat.  As you go up the lineage from Australopithecus and through Homo, you find that more and more meat was consumed the higher up the tree you go.>>

If Eades made this claim, he's being deliberately misleading. Any cursory check online shows that the recent data claiming an omnivorous diet for Australopithecines is controversial at best(much like that seriously flawed study some time back which claimed that Neanderthals only ate meats and was later disproven), and even those claims admit that the majority of the Australopithecine diet was still mainly consisting of fruits, tubers etc., with meat being only a lesser component(given that chimpanzees already eat c.7%(?) of their diet in the form of animal food, low intakes of meat can be largely discounted as an issue. And, of course, it doesn't explain either that meat-intakes were even lower the further one goes back before the Australopithecines, and yet brain-size increased as time went by despite those ancient hominids not eating meats at all.

Didn't you write favorably of the scavenger hypothesis, which is based on the inability of any major predators beyond hominids other than giant cats and hyenas that went extinct around half a million years ago to access the brains and marrow of an intact skeleton? Have you abandoned the scavenger hypothesis?

In that paragraph, I was only talking about wild animals scavenging, not humans. As for the hominid scavenger theory, I viewed it as a possibility in the past, but I now simply don't believe that just eating brains or marrow as a preference leads to bigger brains as if that were the case, wild animal-eaters of marrow and brain would be significantly more intelligent, and that's not the case. On the other hand, it's possible that scavenging forced humans to grow bigger brains as it required bigger brain-power  than standard consumption of fruits.

[William]

This whole discussion is moot, as it depends on evolution which is not even a theory, given the "Missing Link".

It is neither possible to define paleofood nor determine its function by observing the antics of a bunch of monkeys.

Off topic.

[PaleoPhil]

This is a bad claim as hominids had already moved away from a largely herbivorus(leaf-eating etc.) diet to a more frugivorous one.

It may be a "bad claim," but it is nonetheless a claim from the article you felt confident enough in to cite. Are you having second thoughts about that citation? What was the immediate predecessor of Australopithecines that you are saying was frugivorous?

I would never dare to argue with a guru after one or two unfortunate experiences in the past. They are all of necessity extremely dogmatic, as they are defending their own turf etc.

You could pose it in the form of a question. I would do so myself, but I would probably be less likely to think of follow-up questions on the subject than you.

As for the issue of cooking and bigger brains, given that the advent of cooking(250,000 years ago) happened long after the really big increases in hominid brain-size, that makes the theory dead in the water.

The majority yes, but not all of them. I think you've argued in the past that cooking had an effect on human morphology (such as contributing to neoteny) in the last 10,000 to 40,000 years or so, and you could be right.

Perhaps cooking led to a further slight decrease in jaw-size at the tail-end of the Palaeolithic(with no effect on the brain), but that's about it.

As we've discussed before, human total body size, along with brain size, declined during the last 35,000 to 70,000 years (depending on the source), IIRC.

I was just talking about plain increases in brain-size of herbivores via evolution, such as elpehants.

Yes, we those of us who have confidence in evolutionary biology know it's evolution (with certain exceptions, like William), but the question is, do you have any hypotheses re: the mechanism that triggered the evolutionary change of increasing brain size among elephants?

If Eades made this claim, he's being deliberately misleading. Any cursory check online shows that the recent data claiming an omnivorous diet for Australopithecines is controversial at best

Controversial is not quite the same as deliberately misleading. Perhaps he should have noted that there is disagreement on the evidence for increased meat eating among Australopithecines, but I have read about that myself and he may have discussed it in earlier blog posts.

even those claims admit that the majority of the Australopithecine diet was still mainly consisting of fruits, tubers etc., with meat being only a lesser component(given that chimpanzees already eat c.7%(?) of their diet in the form of animal food, low intakes of meat can be largely discounted as an issue.

Eades only said that meat intake increased with Australopithecines (and continued to increase among every following hominid up to Cro Magnon), for which there is evidence (I won't bother to cite it, as it seems you've already decided you don't accept it), not that it was a major part of the diet.

And, of course, it doesn't explain either that meat-intakes were even lower the further one goes back before the Australopithecines, and yet brain-size increased as time went by despite those ancient hominids not eating meats at all.

I would be interested in your references on that, as I don't think I've seen the research on this and would be interested to read it. As usual, I'm open-minded on this subject.

As for the hominid scavenger theory, I viewed it as a possibility in the past, but I now simply don't believe that just eating brains or marrow as a preference leads to bigger brains as if that were the case, wild animal-eaters of marrow and brain would be significantly more intelligent, and that's not the case. On the other hand, it's possible that scavenging forced humans to grow bigger brains as it required bigger brain-power  than standard consumption of fruits.

I would think that hunting, and the social cooperation and communication it produces, would require even more brain power than scavenging, and thus might have been an even bigger factor in brain development. Thus, increased hunting could explain the increase in brain size that accompanied increased meat eating without meat eating necessarily being itself a causative factor. The hunting hypothesis has been proposed by several highly respected scientists, as I recall. My own view, as I mentioned, is that there are probably multiple causes, which complicates matters.

[TylerDurden]

It may be a "bad claim," but it is nonetheless a claim from the article you felt confident enough in to cite. Are you having second thoughts about that citation

Not at all. One can quote scientific claims in an article without believing other (more tentative) claims.

What was the immediate predecessor of Australopithecines that you are saying was frugivorous?

There were plenty such as Ardipithecus etc.
 Ultimately, of course, we split from chimpanzees c. 5 million years ago, who are/were mainly frugivores.

The majority yes, but not all of them. I think you've argued in the past that cooking had an effect on human morphology (such as contributing to neoteny) in the last 10,000 to 40,000 years or so, and you could be right.

No, I argued that cooking might have led to smaller jaws from around 250,000 years ago. Some rawists, like Aajonus, like to claim that cooking only became a major thing c.10,000 years ago(which I doubt).

As we've discussed before, human total body size, along with brain size, declined during the last 35,000 to 70,000 years (depending on the source), IIRC.

Never heard of the 70,000 figure. Where did you get that from?

Yes, we those of us who have confidence in evolutionary biology know it's evolution (with certain exceptions, like William), but the question is, do you have any hypotheses re: the mechanism that triggered the evolutionary change of increasing brain size among elephants?

I don't know. I do know that elephants have been shown to grow smaller when living on islands isolated by oceans(this dwarfism effect is common to such stranded species). Perhaps, elephants grew larger as it became more evolutionarily advantageous to pick leaves etc. from the tops of trees - who knows?

Controversial is not quite the same as deliberately misleading. Perhaps he should have noted that there is disagreement on the evidence for increased meat eating among Australopithecines, but I have read about that myself and he may have discussed it in earlier blog posts.
Eades only said that meat intake increased with Australopithecines (and continued to increase among every following hominid up to Cro Magnon), for which there is evidence (I won't bother to cite it, as it seems you've already decided you don't accept it), not that it was a major part of the diet.

Here's a scientific paper which points out how the isotope-testing which claimed that meat-consumption was endemic among the Australopithecines was likely flawed and might have indicated plant-consumption instead:-

http://www.pnas.org/content/97/25/13506.full

"
Their thick-enameled, flattened molars would have had great difficulty propagating cracks through tough foods, suggesting that the australopithecines were not well suited for eating tough fruits, leaves, or meat. The dental microwear data agree with this conclusion, as the australopithecine patterns documented to date are most similar to those of modern-day seed predators and soft fruit eaters. "

I would be interested in your references on that, as I don't think I've seen the research on this and would be interested to read it. As usual, I'm open-minded on this subject.

See above report in full  and any online references re Ardipthecus and other types. I think we can agree that there was very likely  minimal consumption of meats(5-7%?), much like with chimpanzees(after all, hominids ultimately split from the ancestors of chimpanzees c. 5 million years ago or so). However, claims re higher-meat-intake are highly suspect so that alternatives to the meat-brain theory are more likely.

I would think that hunting, and the social cooperation and communication it produces, would require even more brain power than scavenging, and thus might have been an even bigger factor in brain development. Thus, increased hunting could explain the increase in brain size that accompanied increased meat eating without meat eating necessarily being itself a causative factor. The hunting hypothesis has been proposed by several highly respected scientists, as I recall. My own view, as I mentioned, is that there are probably multiple causes, which complicates matters.

I tend to favour the notion that tool-use enhanced intelligence or that some unusual genetic mutation developed due to extreme predation, leading to bigger brains.

[pfw]

I think you guys missed the point.

Eating meat didn't magically cause larger brains. Eating meat allowed for larger brains. Also, "larger" is the wrong thing to focus on. "More metabolically active" is correct in terms of Kleiber's law.

To reduce the argument to a strict syllogism:

1) Kleiber's Law establishes that animals of a given total size will have some maximum amount of metabolic activity.

2) If, over evolutionary time, one organ decreases its metabolic requirements, the others must pick up the deficit. If one organ increases its metabolic requirements, the others must decrease theirs.

3) Therefore, for humans to evolve more metabolically active brains, they must have decreased their metabolic expenditure elsewhere.

So, left with that conclusion, the next question that is "what did we give up?" Again, a syllogism:

1) The above syllogism establishes that for humans to increase the metabolic expenditure of their brains, other organs must have given up some metabolic activity.

2) The organs that appear to have given way are those related to digestion. This is evidenced by the fossil record and contemporary comparisons.

3) Therefore, for the human brain to increase its metabolic expenditure, a change in diet must have taken place.

Finally, we ask ourselves what change in diet:

1) The previous syllogism establishes that a change in diet must have taken place for the human brain to expand.

2) The most widely available food which was calorically and nutritionally dense enough to enable humans to live with smaller guts is meat.

3) Therefore, for the human brain to increase its metabolic expenditure, humans must have started eating meat in preference to other foods.

Notice the causal links here. Our brains were expanding, so we had to eat meat. Not, we ate meat so our brains magically grew.

The selective pressures that lead to humans evolving active brains are not considered by this theory. Those selective pressures demanded humans develop more active brains. As those pressures selected for those brains, humans that ate meat were able to support them, by giving up their large digestive tracts. The more meat you ate, the less you need to use your guts, the more you could use your brain, and the more likely you were to survive whatever selective pressure existed that was pushing you towards using your brain more. Thus selective pressure for meat-eating due to its ability to support active brains.

That pressure or pressures could have been anything. The claim is that eating meat enabled or was necessary for us to evolve large brains, not that it was the sole factor in our development of large brains.

EDIT:

Here's an important quote from the article:

Any or all of these hypotheses may be valid, but the problem isn’t really as much a matter of why as it is a matter of how.  Other primates deal with groups and have complex foraging strategies; and many deal with social problems within their groups, and some even hunt.  Yet they still have small brains.  (Granted, their brains are larger for their size than those of other mammals, but primates sport small brains as compared to humans.)  How did the human brain grow?

Note the emphasis on how as opposed to why.

[Raw Kyle]

Never heard that one before, everything reads reasonable except law 2: "If, over evolutionary time, one organ decreases its metabolic requirements, the others must pick up the deficit. If one organ increases its metabolic requirements, the others must decrease theirs."

Can't the organism increase it's overall metabolic expenditure, or decrease it?

Even without that part of the concept though it still makes sense as a possible link between meat eating and getting smarter.

[pfw]

Yes, but then it changes in size. Kleiber's law has held for many observed organisms (hence its status as a "law"). You can't be the size of a human with an elephant's metabolism. You'd need to be roughly the size of the elephant to have the same overall expenditure.

If you increased in size while also increasing the relative expenditure of your brain, your gut would be proportionally smaller when you settled at your larger size. The end result would be the same as if you had stayed the same size overall and just traded off between the two, with a smaller gut relative to overall body.

http://en.wikipedia.org/wiki/Kleiber's_law

No one has a good theoretical explanation as to why this is, but the observations are too consistent to dismiss the law.

And again, I must stress that the link between the two goes more like "brain growing -> meat eating" than the other way around. We ate meat to support larger brains - those brains didn't magically appear due to eating meat.

[TylerDurden]

The trouble with Kleiber's law seems to be that even the 3/4 model has exceptions to it. It's unsurprising, therefore, , some biologists discount the theory.

[pfw]

Perhaps it's wrong for those exceptions, but for humans it holds true.

It's actually not that important to the case being made here. It establishes the theoretical framework of why our brains didn't just start sucking energy while everything else remained the same: we only had a certain energy budget and something had to give. The observational framework, which is fairly well established, is that our guts shrank while our brains grew. From there it's easy to derive meat-eating as being an enabler along that path.