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Is Homo floresiensis really that strange?
January 28, 2010Posted by on
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!
Montgomery, 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