Does a Small Brain Make You Dumb?

When you look back at the path evolution took to come up with something there is only one. It is probably not the only one possible, see convergent evolution, it is just the one that was taken. Since there is only one example of human level intelligence that pretty well says there is only one path and says nothing about any other possible paths that were not taken.

That would be a modern human brain with Version 2 of the architecture.

I don’t think anyone could argue that such a path is impossible. But we know it wasn’t taken. So what is your issue? Evolution doesn’t always take what appears to be the easy path when viewed in hindsight. I am reminded of the Robert Frost poem.


Either I misunderstood Jay’s argument, but that seems to be exactly what he is arguing…

What he said just before the quote was:

Which I take to mean human intelligence resulted from the combination of larger brain sizes and the changes in parenting and cooperation. That is just the path that was taken. I don’t think he meant to imply any other path was impossible, but I might be wrong.

But you didn’t answer my question. Why do you want to focus on the path that wasn’t taken?

As he was specifically responding to my claim that “it was not necessary for a human-chimp-ancestor to evolve a larger brain to achieve human intelligence,” I can only read him as, yes, in fact claiming that there was no alternate pathway. Hence the wording of “could not,” rather than “did not.” But I’ll let him clarify if I have in fact misunderstood him.

My basic “thesis,” if I might call it that, is that while humans do have larger brains than primates, this is incidental, not necessary. This is patently obvious to me given the reality of humans that have smaller brains than primates but who have normal human intelligence. This tells me unambiguously that internal structure and arrangement of neurons, and not “raw size,” is the causal factor of intelligence.

I completely grant what I think you were saying, that increased brain size may have been the “canvass,” or working material, that facilitated new neural arrangement. This well may have been the pathway used, as your earlier metaphor suggested. But I would object to the idea that this pathway was the only possible path.

So as to my focus on the “path not taken”… If human intelligence actually, absolutely required increasing brain size from earlier primates, then there would not be any alternative evolutionary pathways.

But this I find untenable. Jay’s very specific argument aside (that increased caloric intake resulting from a larger brain causing scarcity of food resources was the only possible means by which humans would be induced to behave cooperatively as a prerequisite for human intelligence), I think it clearly within the realm of possibility, given evolutionary assumptions, that human intelligence could well have been achieved while the brain decreased in size.

If that is in fact so, if human intelligence could have been achieved while the brain grew smaller, then it remains the case (Jay and Stephen’s protestations notwithstanding) that intelligence is entirely the result of the structure and arrangement of neurons within the brain, and not a factor of “raw size.” And that, while increase in size may have been part of the overall process, or the specific pathway utilized, it was not a necessary pathway to achieve said endstate.

Thus, rephrased: human intelligence did not require increase in brain size from primates.

Would certainly appreciate your further thoughts or any corrections.

While it could have been achieved it isn’t what happened. So why is the could so important? Are you arguing for an outside influence that resulted in human intelligence? In fact it is my belief that this is what happened, but it was done in a way that leaves no evidence it was done. All we have to go by is what the Bible says, man was created in the image of God.

The “could” is not so important to me… I am rather trying to fathom the importance to others here of why they adamantly claim it “could not” have happened any other way.

If someone is claiming that it could not have happened any other way, this seems to my admittedly inferior understanding to suggest a misunderstanding of evolutionary theory, of the possibility of convergent evolution, of multiple pathways to achieve certain outcomes, etc.

Perhaps better to ask Jay why the “could not” is so important?

I said it was impossible, but I was just messing with Daniel because the whole thing is getting silly. What is the point? To disagree for the sake of disagreeing?

I understand why @gbob is focused on the small brain. He understands human evolution and argues that Adam and Eve were literal H. erectus several million years ago. That long ago, of course A&E would have smaller brains than modern humans. The question then is what level of intelligence they might have possessed. Glenn wants to argue that erectus had intelligence comparable to ours. Okay. We can have that discussion. But let’s base the argument on the evidence that erectus left behind. Comparing erectus to modern sapiens who have smaller-than-normal brains is not apples-to-apples. Not even close. It tells us exactly nothing about the intelligence or capabilities of erectus. All it says is that modern humans with exceedingly small brains still think and behave like modern humans. Yawn. What else ya got?

You can’t be serious.

We weren’t discussing the causal factor. (That would be God in my book.) We were discussing the correlation between absolute size and intelligence, which has been empirically verified for nearly 20 years. (See Reader & Laland 2002 above.) You’re only half right, but I suppose that’s better than none.

The conclusion doesn’t follow from the premises. I can dream up a thousand alternative pathways to arrive at human intelligence. So what? Not a single one of them was actualized. The fact that I can theorize alternatives doesn’t negate the premise. Only one path was taken, and it involved increasing brain size from the first primate to the last. The evidence at our disposal points to increasing brain size as essential to the evolution of human intelligence. The only refuge is to discount the evidence, which you have done (and are free to continue doing!).

I wouldn’t say the only possible means. The story that I told just scratches the surface. It’s from an older theory called the Expensive Tissue Hypothesis (“Brains and guts in human evolution,” 1997). Metabolically, the four most “expensive” organs in mammals are the brain, heart, liver, and gastrointestinal tract. Heart and liver are tied directly to body size. To balance the metabolic demands of a growing brain, the gastrointestinal tract has to shrink, and the only way to accomplish that is with a high-quality diet. That’s exactly what we see among the great apes. Those that eat primarily leaves have a large gut and smaller brain; those that eat fruit, nuts, and occasional animal protein have smaller guts and larger brains.

In fact, I could tell a simplified tale of human evolution in terms of just brain growth and food resources. Millions of years ago, our ancestors in Africa had an ample supply of fruit and nuts, which is when we lost the ability to produce our own vitamin C. Their brains grew larger, and their guts shrank. Then, the climate changed. The rainforest began to disappear, and our ancestors had to come down from the trees and look for food to survive. Australopithicus started walking on two legs and scavenging meat. He invented tools to crack bones and dig tubers. Erectus arrived with a bigger brain. He invented better tools and started hunting larger game. A million years ago, erectus learned to control fire. Cooked meat is more easily digestible, so more calories for the brain, less need for a large gut. Brains grew larger, we continued improving our diet, and the next thing you know … BAM! Here we are. Human intelligence.

It wasn’t. I made the statement forcefully just to jerk your chain. All that we have is the one pathway that evolution took. Could it have taken a different path? I don’t know. To me, it’s the same as people who complain, “God wouldn’t have done it that way!” Well, apparently he did, so I’m more interested in understanding what actually happened than speculating whether he could have done it some other way.

My point still stands, though. The metabolic requirements of bigger brains first brought us down from the trees to walk on two legs. The metabolic requirements of bigger brains caused us to invent tools and learn to hunt. Infants with larger brains require longer gestation and take longer to mature. Mothers could not do the job by themselves anymore. Big-brained babies necessitated a change in rearing strategies and social structure, which was the likely beginning of cooperation and sharing. Both language, as a shared symbolic system, and morality, as a shared system of values, rest upon that foundation of cooperation.

Without the metabolic requirements of larger brains, there would have been no pressure to leave the trees behind. Without larger brains, there would have been no pressure to eat meat and hunt animals. Without larger brains, infants would mature more quickly and require less food, so we would never have needed to change the same old child-rearing strategy that worked for millions of years of primate existence, and we never would’ve needed to learn to cooperate with one another instead of compete.

So, along the particular pathway that actually was taken, none of the things that made us who we are would have taken place if we had smaller brains. Whether some other selective pressure could have achieved the same ends, we’ll never know.


Great attitude. Thanks. I agree that’s the way we learn.


This topic has proven to be of great intellectual interest to me, but, honestly, for BioLogos purposes, shouldn’t more attention be devoted to the question of how an evolving animal brain somehow knows that it is a creature, and then develops the ability/desire to communicate with its Creator? For example, I consider the evidence is compelling that supports an average decrease in brain size of almost 10% between Neanderthal and Homo (disregarding questions of direct ancestry). I also give credence to the evidence that, after some 100,000 yrs of successful competition with Neanderthals on the basis of tool making and hunting techniques, Homo sapiens took a Great Leap Forward (Jared Diamond, Ian Tattersall, etc) as witnessed by splendid cave art, musical instruments, and a much more caring treatment of their dead (‘expensive’ grave goods). I think it reasonable to associate this GLF with a “spiritual awakening” that led to a desire to covenant with their Creator beginning some 50,000 YBP.

Of course there are many gaps to fill in to solidify this world view. The mechanism for such a GLF would have to be epigenetic rather than some mutation(s) passed on through sexual reproduction. Current evidence would support its origin as somewhere in Europe or the Mideast, but just how such an epigenetic advantage would spread through migration of H.s. thru central and east Asia and thence to Australia is not at all clear, but it almost certainly had to be thru shared ideas (thru pidgin language?).

So…However it came about that the Homo sapiens brain was enabled to operate as Mind and thereby dominate most of life on this planet, it behooves us to learn as much about how to control the process while preserving human freedom–IF that is possible. I’m sorry now that in high school and college I did not give the Social Sciences the respect they deserve. I wish I could be more confident that religion(s) could fill the bill, but currently they seem to make the tribalism problem even worse.
Al Leo

I have to say, I do like your choice of words here…

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Well, I’ll wrap the discussion up here, then. Sure sounds like many will remain in disagreement, but my initial impressions I’m afraid have been confirmed.

It remains an empirical and indisputable fact that brain size simply does not play a role in the cognitive capability of modern humans, at least within a certain scale. People across the world with brains a fraction of “normal” human size are no less intelligent than the rest of us. A human can evidently be just as intelligent whether their brain weighs 3 pounds or 3 ounces.

Thus I would think it beyond obvious that how our brains are programmed, the specific arrangement and networking of our neurons, is what provides our mental cognition and extraordinary multifaceted human intelligence; raw size of the entire brain, or raw size of any component thereof, is unquestionably irrelevant in our species.

This seems rather relevant information to the overall theory. But this obviously relevant fact, when not outright ignored, is waved aside as an outlier, statistically irrelevant, or as an irrelevant comparison. Wave this very significant fact aside if you desire, but it will simply reinforce my suspicion that what we have is adherence to a narrative that can only be maintained by ignoring obvious, indisputable, and relevant data.

You observation is quite true when you are discussing behaviorally modern H sapiens. The question remains how you think we got to this point given the fossil record records increasing brain size. It gets harder to judge cognitive capability when you only have the tools and art left behind.

Bill, thanks, this is a good question. In the absence of archaeology or other such evidence, I simply suggest humble ignorance, rather than making assumptions about intelligence based solely on brain capacity. I would go either way, in fact. Short of the archaeological evidence you mention, I would neither conclude that a small brain indicated less intelligence, nor conclude a larger brain automatically indicated larger intelligence.

Whether or not I agree with the larger evolutionary theory (I personally am very skeptical), I don’t particularly take significant issue with the proposal in itself that brains increased in size over the course of hominid evolution, as would be inferred from the fossil record; nor with the idea in itself that intelligence increased over that same general period as inferred from archaeology (tools, burial, etc.). Thus if someone wants to argue they are “correlated,” I certainly have no objection. But correlation in itself simply does not indicate causation, otherwise as mentioned above, I could argue a causal and necessary relationship between microwave oven use and low infant mortality. There must be some evidence that they are causally related beyond the mere fact of their correlation.

Thus I simply object to what seems an unwarranted assumption that those two things (raw brain size and intelligence) have a causal relationship, especially given the empiric reality right in front of us that tells us that, in the “final product,” the two factors are clearly not causally related.

I do plan to wrap this up, but I’m happy to respond if you have further thoughts.

The technical term for your “data” is “anecdotal evidence.” In any case, you keep coming back to the raw size of one modern human brain compared to another (“in our species”), but we are trying to compare the brain and intelligence of an extinct species (erectus) to modern humans. Just for comparison, here’s what we’re talking about …

I said before that you were half right. Stephen laid down a clue a long time ago …

From the abstract of the latter paper:

The best fit between brain traits and degrees of intelligence among mammals is reached by a combination of the number of cortical neurons, neuron packing density, interneuronal distance and axonal conduction velocity—factors that determine general information processing capacity (IPC), as reflected by general intelligence. The highest IPC is found in humans, followed by the great apes, Old World and New World monkeys. The IPC of cetaceans and elephants is much lower because of a thin cortex, low neuron packing density and low axonal conduction velocity.

You were right about the arrangement and networking of neurons being a key to human cognition. You missed the boat on size also making a difference, especially in the cortex. Notice the importance of the number and density of cortical neurons. More cortical neurons tightly packed together adds up to more brainpower. That, my friend, is a numbers game. Size does matter.

Unfortunately, as @sfmatheson alluded, soft tissue doesn’t fossilize, so we can’t make these kind of measurements on erectus. We can make endocasts (see picture above) and infer some things by the relative sizes (there’s that word again!) of particular regions of the brain, but that’s a limited tool. Maybe Stephen could elaborate on this.

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Tough question. By your definition here, I’m not sure that some folks walking around right now would qualify as “human.” haha.

By the time period that you’re talking about, humans had modern language, so that’s no longer a barrier.

I really want to wrap this up, but this is worth inquiring… and please correct me if I have completely misunderstood you… but what you said here baffles me… I don’t follow the logic…

I have no issue whatsoever with the idea of greater numbers or density of neurons being significant. (This could well be consistent with empiric evidence in the brains we can study, both small and large.) But this specifically doesn’t translate to bigger size by what appears to be your own reasoning.

By very definition, the more tightly packed together something is, I.e., the more dense it is, the less space it takes up. Unless you’re using the term “density” or “tightly packed” here completely differently than I am familiar.

But if something has greater density, then by definition it specifically means you can have more “numbers” within equal or smaller size. So by your own logic, if I follow it, size wouldn’t necessarily matter.

What am I misunderstanding?

Right. So our cortical neurons are more tightly packed than other species. All things being equal, we should have less cortical matter by volume. But all things aren’t equal. We also have more neurons. So even though our neurons are packed more densely, theoretically yielding a smaller volume, we still have a large cortex because we have (way) more neurons than other species.


I haven’t followed all the technical details, but this reminds me of how long chromosomes emerge. Early species don’t have a way to “spool” their chromosomes, so they are physically limited to what can fit in the cell nucleus without getting tangled. But in species that do have the ability to wind and unwind chromosomes, the DNA strands can grow much longer. So, once the ability to spool chromosomes exists, there is less pressure to keep them short, resulting in less pressure to remove unused or redundant bits.

The growth of the brain seems similar. Without the developments in folding, etc. that allowed for greater density, a greater size (relative to the size of the creature) was necessary to have a more capable brain. But once those developments came about, it relieved some pressure on size and allowed the same capacity to be maintained in a smaller brain (again, relative to total body size).

I know I’m hugely simplifying and that probably contains some errors. I usually avoid science discussions because I don’t know much about this. But is that general comparison relevant?

Offhand, I don’t know. Maybe @sfmatheson can answer that one when he has a chance.

Some of the most devastating causes of intellectual disability involve specific effects on the cortex. There is no point (and no hope of success) in reiterating the plain fact that both size (of the brain/cortex) and the network elements themselves are major influences on cognitive capacity.

If we’re talking about brains in long-gone ancestors, our two best tools are braincases (from fossils) and comparative genomics. Another tool is experimental: we have ever-improving animal models of cortical development in which we can study the genetics of brain development, especially the central questions about cortical development. Here’s a great example from last year: