See also: Is Homo floresiensis really that strange? – Zinjanthropus@ A Primate of Modern Aspect
A new, detailed and freely accessible paper, Reconstructing the Ups and Downs of Primate Brain Evolution: Implications for Adaptive Hypotheses and Homo floresiensis (provisional PDF) has just come online at BMC Biology, in which Stephen H. Montgomery et al discuss the reduced brain-size of Homo floresiensis, and suggest she is unlikely to have descended from Homo erectus, for which this is the abstract:
Brain size is a key adaptive trait. It is often assumed that increasing brain size was a general evolutionary trend in primates, yet recent fossil discoveries have documented brain size decreases in some lineages, raising the question of how general a trend there was for brains to increase in mass over evolutionary time. We present the first systematic phylogenetic analysis designed to answer this question.
We performed ancestral state reconstructions of three traits (absolute brain mass, absolute body mass, relative brain mass) using 37 extant and 23 extinct primate species and three approaches to ancestral state reconstruction: parsimony, maximum likelihood and Bayesian Markov-chain Monte Carlo. Both absolute and relative brain mass generally increased over evolutionary time, but body mass did not. Nevertheless both absolute and relative brain mass decreased along several branches. Applying these results to the contentious case of Homo floresiensis, we find a number of scenarios under which the proposed evolution of the Homo floresiensis brain appears to be plausible, dependent on body mass and phylogenetic position.
Our results confirm that brain expansion began early in primate evolution and show that increases occurred in all major clades. Only in terms of an increase in absolute mass does the human lineage appear particularly striking, with both the rate of proportional change in mass and relative brain size having episodes of greater expansion elsewhere on the primate phylogeny. However, decreases in brain mass also occurred along branches in all major clades, and we conclude that, while selection has acted to enlarge primate brains, in some lineages this trend has been reversed.
Further analyses of the phylogenetic position of Homo floresiensis and better body mass estimates are required to confirm the plausibility of the evolution of its small brain mass. We find that for our dataset the Bayesian analysis for ancestral state reconstruction is least affected by inclusion of fossil data suggesting that this approach might be preferable for future studies on other taxa with a poor fossil record.
There’s a pretty good write-up over at A Primate of Modern Aspect, from which this is excerpted:
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.
The diminished brain size of LB1 has been remarked upon ever since the initial discovery, at is generally supposed that the stone tools found in context would have required a hominid with a larger brain in order to deploy the cognitive capacity needed for such behaviours, leading some to suggest that they were copies of others made by unknown AMH others present on the island of Flores. This in turn raised the question of from what or whom Liang Bua 1 had descended – according to the interpretation by zinjanthropus, if LB1 is descended from either H.georgicus found at Dmanisi, or H.habilis, the size of her brain is much more in accordance than had the descent been from the H.erectus from Ngangdong. Here’s a related note from the paper, which I’m sure will be the subject of extended discussion in the near future:
From our analyses of evolution of H. floresiensis brain size under different phylogenetic hypotheses, we conclude that the evolution of H. floresiensis is consistent with our results across the primate phylogeny if it either evolved from populations of H. habilis or Dmanisi hominin by insular dwarfism, or under Argue et al.’s  proposed phylogenetic scenarios, and if H. floresiensis had a body mass towards the lower end of the range of estimates obtained from skeletal remains. In this respect we note that Brown et al.  suggested the lower body mass estimates are probably most appropriate, assuming H. floresiensis shared the lean body shape typical of Old World tropical modern humans.
If this were true we estimate the evolution of H. floresiensis involved a reasonable decrease in absolute brain mass, but an increase in relative brain size. Our analysis, together with studies of brain size in island populations of living primates[41, 42], therefore suggests we should perhaps not be surprised by the evolution of a small brained, small bodied hominin, although further clarification of the relationships between H. floresiensis and other hominins are required to confirm this observation. Finally, our analyses add to the growing number of studies that conclude that the evolution of the human brain size has not been anomalous when compared to general primate brain evolution [59, 61 91-94].
Reconstructing the Ups and Downs of Primate Brain Evolution: Implications for Adaptive Hypotheses and Homo floresiensis
Abstract – Provisional PDF
Stephen H Montgomery , Isabella Capellini , Robert A Barton and Nicholas I Mundy
BMC Biology 2010, 8:9doi:10.1186/1741-7007-8-9
||27 January 2010