Is Homo floresiensis really that strange?

BMC Biology has recently published a paper (It’s Open Access!) which explores trends in brain size in the Primates.  A trend toward a larger brain is usually considered one of the “hallmarks” of the Primates, but Stephen Montgomery and his colleagues have shown that in many lineages, there is a trend towards secondarily “shrunken” brains.

The authors looked at three different traits- absolute brain mass, absolute body mass, and relative brain mass (a derivitive of brain mass and body mass) in 37 living species, and 23 extinct species.  They reconstructed the ancestral state using three different phylogenetic methods:  Parsimony, maximum likelihood, and a Bayesian Markov-chain Monte Carlo.  They found that there are increases in both absolute and relative brain size in the Primate lineage, but not necessarily in body size.  So primates, in general, have more brain per pound of body than most other mammals.

In a few branches, once an initial increase in brain size occurred, there was a secondary decrease in brain size.  On the surface, it would seem that this doesn’t make sense.  More brain= more smarts= more behavioral flexibility= more food, more mating, more survival.  Right?  Usually.  But in some species, the energy it costs to maintain all of that extra brain costs more than it’s worth.  If you are a bat who hunts for insects, you may not need to remember the location of all of the fruiting trees in the area, and the extra weight incurred by having a big brain weighs you down when you’re trying to fly.  In that case, it might be advantageous to reduce your brain size.

There’s also the issue of a “phyletic dwarf” or “phyletic giant.”  These are species which are very closely related to each other, but one is very big or very small.  Something like the Aye-aye and the Giant Aye-aye.  Brain size generally exhibits negative allometry because it’s such a specialized organ- so, as the body gets larger, the brain doesn’t keep up and is, as a result, proportionately smaller.

It’s extremely important for most of your organs to increase with body size.  For example, a bigger animal needs to pump more blood, so it needs a bigger heart.  A bigger animal eats more food and needs a bigger liver.  There are certain areas of the brain that increase allometrically with body size- usually areas that are in charge of motor skills.  If you’ve got bigger legs, you’ve got bigger muscles, and you need more neural projections in order to control them.  But does a larger animal need to think more?  Will it benefit from an extra few cubic centimeters of neocortex?  Probably not, so it’s not worth the extra time and energy it takes to develop that neocortex.

And that sort of brings us to an important question in evolutionary neurobiology: Does absolute brain size matter, or is it solely brain size relative to body size?  Brains that are absolutely larger have more neurons, which could have important cognitive implications.  But how many of those extra neurons are just being used to control the physiological functions of the body?

Does size even tell us anything at all?  Any way you look at it, brain size is a crude measurement of cognitive ability.  In an ideal world, we would know the proportion of each of the different regions of the brain in each species and go from there.  But, those kinds of measurements are hard to obtain in living species, and impossible in fossils.  Ralph Holloway has been saying since 1967 that there has got to be a better way than just plain ol’ cranial capacity… but other than noting the relative position of different sulci and gyri on endocasts, there isn’t too much else to be done.

Anyway, those considerations aside, Montgomery and friends found that there were initial increases in proportional brain size- one at the node between ancestral primates and strepsirrhines, another between ancestral primates and haplorrhines, and then another between the ancestral haplorrhines and anthropoids.  At the terminal ends of the branch, tarsiers, galagos, aye-ayes, and humans all show large increases in relative brain size.

Cool!  But what about the decreases in brain size?  They found that absolute brain size decreased in about 14% of the branches, in clades like Mangabeys, tamarins and marmosets, and some of the small lemurs.  In every one of those cases, the decrease in brain mass was accompanied by a decrease in body mass.  And body mass decreased a lot more often than brain mass- 46% of the branches showed a decrease in body mass.  So what that means is that brains tend to stay the same size in lineages where the body size is decreasing.

Only 4% of the branches showed a decrease in relative brain size.  Most of these are lineages where the body size increased disproportionately to brain size- the negative allometry discussed above.  Think about gorillas:  They have a small proportion of brain mass to body mass.  But is it because they have tiny brains, or because they have huge bodies?  This study seems to support the idea that it’s their large bodies that are influencing the numbers.

Okay, so we’ve got lots of increases in brain size, and a few decreases.  In the cases where we have decreases, we usually have body size decreases as well.  More often than not, we have body size decreases which result in a disproportionately large brain size, but occasionally we have a body size increase which results in a disproportionately small brain size.  And all of that brings us to the Hobbit.

The authors looked at Homo floresiensis in relation to the Dmanisi hominids, Homo habilis, and a Homo erectus from Ngangdong and found that if we use Dmanisi or habilis as an ancestor, the decrease in brain size and body size isn’t exceptionally weird when compared to other primate groups.  The mouse lemur decreased in both to a greater degree, for example.

But if you use the Ngangdong erectus as the ancestor, it is a really weird decrease.

So, I guess the question is, is it reasonable to use Dmanisi or Homo habilis as the ancestor and not Homo erectus?  And of course, we don’t know that yet!

ResearchBlogging.orgMontgomery, S., Capellini, I., Barton, R., & Mundy, N. (2010). Reconstructing the ups and downs of primate brain evolution: implications for adaptive hypotheses and Homo floresiensis BMC Biology, 8 (1) DOI: 10.1186/1741-7007-8-9

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16 responses to “Is Homo floresiensis really that strange?

  1. rich lawler January 28, 2010 at 9:20 pm

    I was a bit surprised that these authors didn’t discuss their results in the context of Lande’s 1979 study. Using quant. gen. models, he showed that changes in brain size was a correlated response to directional selection on body size at low taxonomic levels (and drift can also produce this result). At higher taxonomic levels there was more direct selection on brain size, hence breaking up the correlation. This is what he posited to have occurred in primates. What’s great about this paper is that Lande is looking at a particular question about brain-body allometry but he just happened to derive one of the most cited equations in evolutionary quantitative genetics while answering it.

  2. afarensis, FCD January 28, 2010 at 9:33 pm

    @ Rich – are you refering to this study?

  3. afarensis, FCD January 28, 2010 at 10:26 pm

    I thought so, just wanted to be sure.

  4. zinjanthropus January 29, 2010 at 12:33 am

    Good point, Lawler! I had forgotten all about that paper until now.

  5. Eric January 29, 2010 at 11:09 am

    Well there is this one rule, which sais that animals with smlaller body mass have higher energy requirements per unit body mass, than bigger ones.

    If we are assuming, that there was a decrease in body size of Homo floresiensis, than that would also mean, that their relative energy requirements would increase.
    And because a large Brain is quite expensive, it would become one of the first candidates when it comes to “energy saving”.

    Ok, I dont’t know if the decrease in body size of Homo floresiensis would be sufficient enough to trigger this kind of event but I think it’s something we should keep in mind.

  6. Stephen Montgomery January 29, 2010 at 2:58 pm

    Hello, I’m one of the authors on this paper, a friend of mine passed a link to your blog onto me. Just wanted to say Rich Lawler is quite right, we probably should’ve discussed Lande but I guess we missed out on that one. Thanks for noticing the paper!

    ps congrats on the blog, seems like a good one

  7. rich lawler January 29, 2010 at 3:11 pm

    Hi Stephen, your paper is a great study on its own. I was just making an observation, so perhaps my use of the word “surprised” was too strong. I’d been thinking of Lande’s brain-body size model recently so when I read this post (as well as your paper) that thought just popped into my head and I left it as a comment.

  8. Robert February 25, 2010 at 5:58 am

    In China, some ancient monasteries have doors that are only 4 feet high. That means that, at the time, the human population there averaged very short.

    As recently as 1950, Japanese persons averaged only about 5 feet tall.

    It’s not hard for me to imagine a human species only 3 feet high several thousand years ago. Body size evolves very rapidly, because it responds to changes in one’s environment.

    • zinjanthropus February 27, 2010 at 10:15 am

      Hi Robert-

      I agree that it’s not particularly hard to imagine. But that’s why we need to test the hypothesis that floresiensis is a new species even more rigorously. Alternatives that are also not very hard to imagine include that flo might be a pathological human, or she might be a non-pathological, but very diminutive human, or…

    • Juergen Posse September 1, 2010 at 2:36 pm

      Hi Robert, that’s really interested. Do you have more information about such old monasteries? I’m just searching for any kind of stories, legends, artefacts with relation to very small people in Asia and Australia.

      Thanks Juergen

  9. Robert February 25, 2010 at 6:03 am

    50,000 years ago, the genus homo had at least four living representatives: homo erectus, homo floresiensis, homo neanderthalensis, and homo sapiens.

    I think it is a great shame that only one species survived in this world.

  10. Charles D.A. Savy June 11, 2010 at 5:30 pm

    Following on Robert’s comment on the subject from Feb 25,
    we know that nature has naturally eliminated unsustainable species over rhe millenia, and that today Homo sapiens assist, in an accelerated ellimination of other species. Maybe such “assisted” elimination of other species, by homo sapiens started much earlier than we dare to think!
    Yet today we look to space for other intelligent life – a little sad. We no longer have “a little little brother/sister or cousin to look after let us at least look after the rest of the natural world that still surrounds us.

  11. Hanneke Meijer June 15, 2010 at 12:36 pm

    This one is just out, may be of interest to you: The fellowship of the hobbit: the fauna surrounding Homo floresiensis. http://www3.interscience.wiley.com/journal/123378314/abstract

  12. Yitz June 29, 2010 at 4:24 pm

    Those people are tiny.
    But they would’ve killed humans

  13. Brett November 3, 2010 at 8:32 pm

    great insight. Thanks.

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