Brain Size Is Not A Filter

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. … Encephalization [vs. time] trends are associated with sociality in extant species. These findings … highlight the role sociality may play in driving the evolution of large brains. (more; HT Razib Khan)

The biggest brains have consistently gotten bigger over the last half billion years since multi-cellular life appeared. Big brains seem to be a necessary precondition for human level intelligence and civilization, and human size brains appeared only very recently. These facts strongly suggest that achieving human level intelligence is just not a big component of the great filter.  It appeared quickly after big brains, and big brains seem likely given enough time and sociality, and sociality seems likely.

This unfortunately means that it is very difficult to collect data on all steps of the great filter.  It is big and real and matters enormously, but we can hardly see it.

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  • I want to believe this argument, partly because the idea of a galaxy brimming with human-like intelligence sounds nice to me. But I have some doubts.

    If sociality is what drove human brain development, the particular kind of sociality that drove it has been fairly rare in Earth’s history. If it happened for other branches of the evolutionary tree, but if it did, something stopped it from producing creatures with human-like brains.

    It sounds like you’d like this to be a matter of “sociality varies at random, so if life evolves on an Earth-like planet, at some point you’ll get a evolutionary branch with enough sociality to drive human-like brains.”

    But it may be kind of sociality that matters. For example, if Geoffrey Miller’s hypothesis is right, brains evolved as a very complicated way of signaling fitness. Yet the number of species who signal fitness in less complicated ways (like showy feathers) vastly outnumber the one that signals fitness with big brains.

    Why think it’s especially likely that on any planet that supports the evolution of Earth-like life, some evolutionary branch will hit on that approach to signaling eventually?

  • Michael Kirkland

    Big brains alone won’t give you a technological civilization – you also need dexterous manipulators, and probably an omnivorous or carnivorous diet to benefit from the tools the former allows.

    The combination could be a minor filter.

    • Gorillas, which are folivores, do show primitive tool use. They also satisfy the more social meaning of manipulator.

      There’s a feedback effect in the social brain hypothesis. Mammalian sociality itself may have (partly) risen because postnatal development of the brain offered survival value and postnatal development in turn required care. The new cognitive space allowed for more sociality, which had its own benefits, but also in turn allowed for increasingly more postnatal development of the nervous system.

      I think that once mammals appear on stage, sociality and higher cognition are virtually inevitable.

  • Isn’t the biggest constraint geochemistry? Photosynthesis as we know it would have had problems with lower pH, without photosynthesis we might not have had the Great Oxygenation and without the oxygenation available redox potentials might just not have supported multicellular life, let alone intelligence.

    It’s just a hunch – it seems there’s too much reinforcement towards intelligence in multicellular life.

  • Gil

    I believe that in the days of Phrenology men knew brain size alone does not equate to intelligence. It is known that physical complexity in a small brain will lead to a smarter person than a large but simple brain. It’s like the evolution of computers – faster yet smaller.

  • I think that VT is right in the most basic way (though since my background is geochemistry I have a strong bias there).

    There is also evidence that “sociality” or social behavior / other awareness occurs in a wider range of species than we once believed. Tortoises and other reptiles appear to be able to read and interpret social gaze cues and there’s also evidence of morality (or at least proto-morality) in bees and mole rats.

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  • > It appeared quickly after big brains, and big brains seem likely given enough time and sociality, and sociality seems likely.

    Or sociality only arises with brains; even ants have nervous systems. The causation could run either way here…

    I’m struck that they find no encephalization slopes in several mammal orders like Carnivores and Insectivores. (Carnivores is apparently a very big order; they list a skeleton _n_ of 20 for Primates, and 129 for Carnivores.) If there were no niches for Primates, then the next most encephalized groups are Cetaceans (obviously they’re not going to work for building a tech civilization) and then Perissodactyls (horses, rhinoceroses, etc.)