Back to the trees

There is still doubt, but there seems to be a good chance that the Flores Hobbit was a member of a distinct hominid species, rather than some homo sap with a nasty case of microcephalic dwarfism.   If this is the case, the Hobbits are likely descended from a small, Australopithecus-like population that managed to move from Africa to Indonesia without leaving any fossils in between, or from some ancient hominid (perhaps homo erectus) that managed to strand themselves on Flores and then shrank, as many large animals do when isolated on islands.

Island dwarfing of a homo erectus population is the dominant idea right now.  However, many proponents are really bothered by how small the Hobbit’s brain was.  At 400 cc, it was downright teeny, about the size of a chimpanzee’s brain.  Most researchers seem to think that hominid brains naturally increase in size with time. They also suspect that anyone with a brain this small couldn’t be called sentient – and the idea of natural selection driving a population from sentience to nonsentience bothers them.

They should get over it.  Hominid brain volume has increased pretty rapidly over the past few million years, but the increase hasn’t been monotonic.  It’s decreased about 10% over the past 25,000 years. Moreover, we know of examples where natural selection has caused drastic decreases in organismal complexity – for example, canine venereal sarcoma, which today is an infectious cancer, but was once a dog.

There is a mechanism that might explain what happened on Flores – partial mutational meltdown.  Classic mutational meltdown occurs when a population is too small for too long. Selection is inefficient in such a small population: alleles that decrease fitness by less than 1/N drift fairly freely, and can go to fixation.  At the same time, favorable mutations, which are very rare, almost never occur.  In such a situation, mutational load accumulates – likely further reducing population size – and the population spirals down into extinction. Since small population size and high genetic load increase vulnerability to disaster, some kind of environmental catastrophe usually nails such doomed, shrinking populations before they manage to die off from purely genetic causes.

In principle, if the  population is the right size and one adaptive function is considerably more complicated than others, presenting a bigger mutational target,  you might see a population suffer a drastic decline in that function while continuing to exist. There is reason to think that intelligence is the most complex adaptation in hominids. More than half of all genes are expressed in the brain, and it seems that a given degree of inbreeding depression – say cousin marriage – depressesIQ more than other traits.

Flores is not that big an island and the population density of homo-erectus type hunter-gatherers must have been low – certainly lower than that of contemporary hunter-gatherers, who have much more sophisticated tools.  Thus the hobbit population was likely small.  It may not have been possible to sustain a high-performing brain over the long haul in that situation.  Given that their brains performed poorly – while the metabolic costs were as high as ever – selection would have shrunk their brains.  Over hundreds of thousands of years, this could well have generated the chimp-sized brain we see in the LB1 skeleton.

Of course, this could only have happened if there was an available ecological niche that did not require human-level intelligence.  And there was such an opening: Flores had no monkeys.

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23 Responses to Back to the trees

  1. jb says:

    I thought there were tools associated with the Flores Hobbits. Doesn’t that require more intelligence than a chimp?

    Also, how big is the brain of a young human child? I clearly remember being three years old and being quite sentient. I remember taking toys apart and putting them back together, trying to see how they worked. I remember talking with adults, and at least feeling like I had held up my end of the conversation. I certainly wasn’t three feet tall at the time. Given more strength, and perhaps some sort of instinctive inclination towards toolmaking, I can easily imagine my three year old self making spears and hunting with them. Would this level of sentience be beyond the reach of a 400cc brain?

    • gwern says:

      > Given more strength, and perhaps some sort of instinctive inclination towards toolmaking, I can easily imagine my three year old self making spears and hunting with them. Would this level of sentience be beyond the reach of a 400cc brain?

      Sure, but could you maintain the technology, assuming you ever invented it? I think Tasmania is the relevant example here (and Tasmania was bigger than Flores…)

    • weavercht says:

      If I recall correctly, even some birds can learn (from other birds) to use tools, and big cats spend a great deal of time teaching their young how to hunt.

      Brains can be specialised, so some abilities might be retained.

  2. gcochran says:

    At age three, human brains are > 80% of adult size – probably two and a half times bigger than
    the brain of a chimpanzee or Flores Hobbit.

    I doubt if a hominid with a 400 cc brain could make better tools than a chimpanzee.

    • jb says:

      Greg, what is your explanation for the tools that were reportedly associated with the hobbit remains, and also evidence for cooperative hunting and the use of fire?

      I think the question of brain size versus intelligence is extremely interesting. I’m impressed that birds can show the level of intelligence they do (e.g., Alex the gray parrot), despite their tiny brains, and I wonder why elephants and whales aren’t more intelligent than they seem to be. You noted in a comment somewhere that it isn’t obvious why intelligence should scale according to brain/body ratio rather than brain size alone, and I wonder about that too. Does controlling the body of a blue whale really require more computation than that of a dolphin? (Or a squirrel, for that matter!)

      It has been claimed that the hobbit brain shows signs that it might have been reorganized to be more intelligent than one would expect from size alone. I understand your argument that if this were possible it should have happened everywhere else as well (and faster than on Flores, given higher populations). So what do you make of those claims?

  3. Abelard Lindsey says:

    Of course researchers are uncomfortable with the idea that natural selection may favor a loss of sentience. It suggests that that sentient intelligence may not be evolutionarily favorable in the long term. Sentient intelligence is very resource intensive and comes with some disadvantages. Our brains use about 20% of our metabolism. They also make for a longer gestation period and a much longer maturation period than other mammals. Our big brains make child-birth a more risky process as well.

  4. teageegeepea says:

    Has Flores always been monkey-free? I know that chimpanzees hunt colobus monkeys, and humans still eat the “bush meat” of apes sometimes (hence, AIDS). And when humans move into new territory they often wipe out existing species that haven’t coevolved with them.

    • gcochran says:

      Always monkey-free. It’s east of the Wallace line: even at the lowest sea levels in glacial times, there was no connection with Asia. None with Australia/New Guinea, either.
      Back in the day, Flores has a very limited fauna: “komodo dragons and other smaller monitor lizards, crocodiles, several species of Stegodon (an extinct close relative of modern elephants), giant tortoises, and several kinds of small, medium, and large-bodied rats.”

  5. Nanonymous says:

    At 400 cc, it was downright teeny, about the size of a chimpanzee’s brain. Most researchers seem to think that hominid brains naturally increase in size with time. They also suspect that anyone with a brain this small couldn’t be called sentient

    I wouldn’t worry about sentience based on a size alone. Miniature pinscher is a lot smaller than Labrador retriever and while there are many obvious differences, fundamentally the smaller guys are still perfectly functional dogs.

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  7. dearieme says:

    “Of course researchers are uncomfortable with the idea that natural selection may favor a loss of sentience”: why “of course”? Do they favour some sort of Whig interpretation of evolution?

  8. gudenuf says:

    “Moreover, we know of examples where natural selection has caused drastic decreases in organismal complexity – for example, canine venereal sarcoma, which today is an infectious cancer, but was once a dog.”

    Are you telling me that a complex mammal evolved into a tumor? Where did you learn this?

  9. gcochran says:

    Yes, I am telling you that. I’ve read a few papers about CVT. There’s a similar disease in Tasmanian devils. There are analogous events. One reason that we irradiate whole blood is to prevent TA-GvHD (Transfusion-Associated Graft versus Host Disease). This occurs when a transfusion introduces immunological stem cells that are not eliminated by the native immune system. Sometimes those foreign white cells proliferate and kill you. Along the same lines, melanoma has been known to cross the placenta and infect a baby.
    The technical phrase is “host cell line infections”. There are probably more that we don’t know of – there might be some in humans.

  10. hbd chick says:

    @greg – “Hobbits are likely descended from a small, Australopithecus-like population that managed to move from Africa to Indonesia without leaving any fossils in between….”

    my impression is that, since wwii, most of the fossil hunting for hominids has been carried out in africa. not so much has been done at all in south asia. there hasn’t been enough looking for any possible “fossils in between,” afaics.

  11. MikeP says:

    The association with what may be described as “appropriately-sized” tools with H. floresienses over a long period does not suggest a monkey-like biology. Cave dwelling and fire making are obviously not characteristic of monkeys either. The brain of floresiensis is not only very small, but is also markedly unusual in morphology. Because of the extremely high energetic cost of the brain, it is quite plausible that this island dwarf experienced a strong selective pressure for a uniquely “efficient” brain, one with very high intelligence for its size.

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