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List of Bookmarks One of the most persistent debates about the process of evolution is whether it exhibits directionality or inevitability. This is not limited to a biological context; Marxist thinkers long promoted a model of long-term social determinism whereby human groups progressed through a sequence of modes of production. Such an assumption is not limited to Marxists. William H. McNeill observes the trend toward greater complexity and robusticity of civilization in The Human Web, while Ray Huang documents the same on a smaller scale in China: A Macrohistory. A superficial familiarity with the dynastic cycles which recurred over the history of Imperial China immediately yields the observation that the interregnums between distinct Mandates of Heaven became progressively less chaotic and lengthy. But set against this larger trend are the small cycles of rise and fall and rise. Consider the complexity and economies of scale of the late Roman Empire, whose crash in material terms is copiously documented in The Fall of Rome: And the End of Civilization. It is arguable that it took nearly eight centuries for European civilization to match the vigor and sophistication of the Roman Empire after its collapse as a unitary entity in the 5th century (though some claim that Europeans did not match Roman civilization until the early modern period, after the Renaissance).

It is natural and unsurprising that the same sort of disputes which have plagued the scholarship of human history are also endemic to a historical science like evolutionary biology. Stephen Jay Gould famously asserted that evolutionary outcomes are highly contingent. Richard Dawkins disagrees. Here is a passage from The Ancestor’s Tale:

…I have long wondered whether the hectoring orthodoxy of contingency might have gone too far. My review of Gould’s Full House (reprinted in A Devil’s Chaplain) defended the popular notion of progress in evolution: not progress towards humanity – Darwin forend! – but progress in directions that are at least predictable enough to justify the word. As I shall argue in a moment, the cumulative build-up of compelx adaptations like eyes strongly suggest a version of progress – especially when coupled in imagination with of the wonderful products of convergent evolution.

Credit: Luke Jostins
Credit: Luke Jostins

One of those wonderful products is the large and complex brains of animals. Large brains are found in a disparate range of taxa. Among the vertebrates both mammals and birds have relatively large brains. Among the invertebrates the octopus, squid and cuttlefish are rather brainy. The figure to the right is from Luke Jostins, and illustrates the loess curve of best fit with a scatter plot of brain size by time for a large number of fossils. The data set is constrained to hominins, humans and their ancestors. As you can see there is a general trend toward increase cranial capacities across all the human populations. Neandertals famously were large-brained, but they exhibited the same secular increase in cranial capacity as African Homo. On the scale of Pleistocene Homo and their brains the idea of the supreme importance of contingency seems ludicrous. Some common factor was driving the encephalization of humans and their near relations over the past two million years. This strikes me as very strange, as the brain is metabolically expensive, and there are plenty of species with barely a brain which are highly successful. H. floresiensis may be a human instance of this truism.

But what about the larger macroevolutionary pattern? Is there a trend toward larger brain sizes in general, of which primates, and humans in particular, are just the most extreme manifestation? Some natural historians have argued that there is such a trend. But, there is a question as to whether increased brain size is simply a function of allometry, the pattern where different body parts and organs tend to correlate together in size, but also shift in ratio with scale. The nature of physics means that very large organisms have to be more robust because their mass increases far faster than their surface area. By taking the aggregate relationship between body size and brain size, and examining the species which deviate above or below the trend line, one can generate an encephalization quotient. Humans, for example, have a brain which is inordinately large for our body size.

And yet there are immediate problems looking at relationships between body and brain size, and inferring expectations. Different species and taxa are not interchangeable in very fundamental ways, and so a summary statistic or trend may obscure many fine-grained details. A new paper in PNAS focuses specifically on various mammalian taxa, corrects for phylogenetics, and also relates encephalization quotient by taxa to the proportion of social animals within each taxon. Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality:

Evolutionary encephalization, or increasing brain size relative to body size, is assumed to be a general phenomenon in mammals. However, despite extensive evidence for variation in both absolute and relative brain size in extant species, there have been no explicit tests of patterns of brain size change over evolutionary time. Instead, allometric relationships between brain size and body size have been used as a proxy for evolutionary change, despite the validity of this approach being widely questioned. Here we relate brain size to appearance time for 511 fossil and extant mammalian species to test for temporal changes in relative brain size over time. We show that there is wide variation across groups in encephalization slopes across groups and that encephalization is not universal in mammals. We also find that temporal changes in brain size are not associated with allometric relationships between brain and body size. Furthermore, encephalization trends are associated with sociality in extant species. These findings test a major underlying assumption about the pattern and process of mammalian brain evolution and highlight the role sociality may play in driving the evolution of large brains.

A key point is that the authors introduce time as an independent variable, so they are assessing encephalization over the history of the taxon. This is clearly relevant for humans, but may be so for other mammalian lineages. The table and figures below show the encephalization slope generated by using time and body size as the predictors and brain size as the dependent variable. A positive slope means that brain size is increasing over time.

[nggallery id=21]

Two major points:

– Note that the slope is sensitive to the level of taxon one is examining. A closer focus tends to show more variance between taxa. So, for example, humans distort the value for primates in general. Bracketing out anthropoids paints a more extreme picture of encephalization, a higher slope. In contrast, the lemurs and their relatives exhibit less encephalization over time.

– The correlation between proportion of species which exhibit sociality and encephalization of the taxon is strong. From the text:

Encephalization slopes were correlated with both the proportion of species with stable groups (order R = 0.92, P = 0.005, n = 6; suborder R = 0.767, P = 0.008, n = 9; Fig. 2 A and B) and the proportion in either facultative or stable social groups (order R = 0.804, P = 0.027, n = 6; suborder R = 0.63, P = 0.04, n = 9).

The last figure makes it is clear that the correlations are high, so the specific values should not be surprising. Don’t believe these specific figures too much, how one arranges the data set or categorizes may have a large effect on the p-value. But the overall relationship seems robust.

A highly encephalized “alien”

What to think of all of this? If you don’t know, one of the authors of the paper, Robin Dunbar, has been arguing for the prime importance of social structure in driving brain evolution among humans for nearly twenty years. The relationship is laid out in his book Grooming, Gossip, and the Evolution of Language. Robin Dunbar is also the originator of the eponymous Dunbar’s number, which argues that real human social groups bound together by interpersonal familiarity have an upper limit of 150-200. He argues that this number arises because of the computational limits of our “wetware,” our neocortex. Those limits presumably being a function of biophysical constraints.

One interesting fact though is that the median cranial capacity of our species seems to have peaked around one hundred thousand years ago. The average human today has a smaller brain than the average human alive during the Last Glacial Maximum! (see this old post from Panda’s Thumb, it’s evident in the charts) This may be simply due to smaller body sizes in general after the Ice Age. Or, it may be due to the possibility that social changes with the rise of agriculture required less brain power.

Ultimately if Dunbar and his colleagues are correct, if social structure is the most powerful variate in explaining differences in brain size when controlling for phylogenetics and body size, then in some ways it is surprising to me. After all, it does not seem that ants have particularly large brains, despite being extremely social and highly successful. Clearly the hymenoptera and other social insects operate on different principles from mammals. Instead of
developing “hive minds,” it seems as if in mammals greater social structure entails greater cognitive structure.

Citation: Susanne Shultz, & Robin Dunbar (2010). Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality PNAS : 10.1073/pnas.1005246107

(Republished from Discover/GNXP by permission of author or representative)
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  1. After all, it does not seem that ants have particularly large brains, despite being extremely social and highly successful.

    Well, the assumption here is that ants have developed a sociality in which they don’t need to know each individual member of the nest. They just need to follow basic communication signals. If I detect behaviour X or pheromone Y, I don’t need to know which particular member of the nest originated it – I just react to it. Altruism is enforced by strong genetic control, which itself is maintained by kin and group selection.

    By contrast, in humans, altruism has a strong reciprocal component. I’ll scratch your back if you scratch mine. This implies that I can actually remember that you, as an individual, did scratch my back, or that you are expected to in the future. Simply reacting to basic signals would leave me open to exploitation. Keeping track of reciprocal gestures (and punishing cheaters) requires personal knowledge of each member of the group that you interact with. This requires some cognitive resources, which are presumed to grow with the number of interactors you need to keep track of.

    In some circumstances, having large cooperative groups can be a selective advantage. In this case, there will be a pressure to evolve larger brains to cope with the larger relational network. Hence the uber-brainy humans. Or so the “social brain” hypothesis goes.

    A nice thing about Dunbar is that he was always careful to support his assertions with actual number-crunching. You can have a look at the original “social brain hypothesis” paper.

  2. bio – nearly eight years:) I assumed 800?

  3. I saw a little blurb pop up on Fox News yesterday morning promoting an upcoming segment: “Coming up in the next half hour: Are dogs smarter than cats? A new study claims they are!! It has to do with their being more social!” I’m guessing this paper is that study – pretty funny.

  4. It’s not clear to me that the ability to identify/remember many individuals is more evident with higher encephalization in mammals. It’s also not clear to me how the defined “sociality” which I would tend to think of as a continuous variable rather than a discrete one…you could get pretty far toward a convincing data set by cherry picking in the grey area.

    Assuming the correlation is true, why the causality assumption that sociality–>big brains rather than vice versa?

    Most vertebrates are teleost fish, which vary tremendously in sociality. They don’t present data on birds. Talking about mammals we’re left with a relatively small sample size…if there is an evolutionary principle at work here why is it specific to furry lactators?

    Finally, I’ve never understood why brain size relative to body size means anything when we are talking about objective cognitive abilities (number of individuals you can remember, where you left several thousand acorns, etc). Is a .22 as powerful as a .45 if it’s shot by a tiny person? I know the argument, I’ve just never seen it supported by anything.

  5. What about smart non-mammals, like birds or cephalopods? Do we even have knowledge about these?

  6. @miko – “Assuming the correlation is true, why the causality assumption that sociality–>big brains rather than vice versa?”

    Because the reverse would require a behavioral change without an evolutionary change to cause it. An evolutionary change can come out of nowhere because of random mutation, but a behavioral change not associated with biology has no source to spring from, in general.

  7. On the effect of farming on human brain size. There are still human groups who have never been farmers, so the comparison could be done. I see your point, though, it would be as a sort of ‘human domestication’ in a similar way that dogs have smaller brains than wolves.

  8. This reminds me of this paper, by Paul M Bingham:;2-X/abstract;jsessionid=458B8B3AD45A49BF43D93D21C75C70FF.d03t01

    The basic idea is that brain development evolved subsequently to the evolution of a social cooperation enforcement mechanism (mainly, the ability to throw).

  9. “After all, it does not seem that ants have particularly large brains, despite being extremely social and highly successful.”

    Perhaps there is difference in the cognitive skill needed to navigate eusocial environments common to ants and the more properly “social” environments of most mammals. A good case study to examine would be the naked and Damarland mole rats, the two eusocial mammal species now in existence. While parts of the rats brain seem to be larger than that of a typical rodent, the size of the entire brain does not seem to be any larger.

  10. Too bad we don’t have brain samples through the last 20,000 years. It’d be interesting seeing changes not just in overall brain size determined from skulls but also in the relative regions.

    I suspect the types of cognitive activity required in a very primitive society under stress were different from what one found in less stressful hunter/gather situations and especially under agriculture and the rise of civilization. One could do that sort of analysis by study of brains in existing primitive or recently primitive societies. But I suspect that variation due to environment not to mention genetic variation in the region might undermine that. (i.e. would it be fair to compare the brian of someone from the Amazon with a white European while ignoring the other evolutionary processes during the separation of the two)

    Still I bet were this done we’d find some regions getting bigger while other regions, probably dealing with physical movement and instinctive environmental analysis, changing. Just a completely off the cuff guess, but I suspect we’d see more evolution towards language use among those in civilized or quasi-civilized groups (which could include hunter/gatherers where division of labor takes place)

    What would be most interesting to know would be whether the driver was female or male roles in the culture. i.e. if there was selection based upon female power via social/language use in the community or if it was more driven by male social use both in terms of mate selection, number of mates as well as age of death.

    I should pick up Robin Dunbar’s book. Sounds interesting although many others have discussed similar issues. I enjoyed Tomasello’s book on the evolution of language even if empirical data has undermined a few of his arguments. The real issue (to me) is given this evolution what prevented civilization from developing earlier. I think Tomasello really engages with that question. However the more interesting question is the evolution since the rise of solid language/social cognitive development.

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