This week1 saw quite a few interesting papers in biology. Here's a few that caught my attention:
- Mammalian brain evolution driven by smell - One of the biggest characteristics that distinguish mammals from other animals is that we have relatively large brains in relation to body size. Just why mammals evolved rather large brains has been a point of speculation, but a paper published in Science this week suggests that a reliance on our sense of smell resulted in our bulging brains. The research by T. Rowe et al. involved examining the fossils of two species of Jurassic cryodonts, the group of ancient reptiles which would diverge to become the mammalian lineage. They used a technique called X-ray computed tomography to reconstruct endocasts of the cryodont brains, and compared them to those of earlier, Triassic cryodonts. They first looked at the endocast from Morganucondon oehleri, and noted that the brain was 50% larger than in earlier cryodonts. But there were other important differences: the olfactory bulbs were larger, there was expansion in the cerebral hemisphere, and the cerebellum extended to cover the midbrain. They next looked at the endocast from Hadrocodium wui, the closest known fossil to living mammals. They observed another 50% increase in brain size, with even larger cerebral hemisphere and olfactory bulbs. These observations lead the team to speculate that increased dependence on olfaction drove the evolution of larger brain sizes in early mammals. Given that early mammals were likely nocturnal, and had to rummage around in the dark for food, this idea does make sense. The researchers are now looking for evidence that might indicate that early mammals were indeed nocturnal.
- Another strike against the Central Dogma - Every student in biology is brought up to know the "Central Dogma" - the idea that can be summed up as "DNA encodes RNA encodes protein". This is not a had and fast rule, though, and the discovery of things like ribozymes have shown the Central Dogma to be less dogmatic. A new paper published in Science this week furthers this point. A team of researchers lead by M. Li from the University of Pennsylvania in Philadelphia compared the DNA sequences from 27 different individuals to their corresponding RNA sequences. In a very large number of cases, they discovered that the RNA sequences that had been transcribed from the DNA sequences were different than would be expected; that is, the RNA sequences contained sites where the nucleotides had been changed. These changes in RNA sequences were shared across many of the individuals studied, indicating that the changes were not likely due to random mutation. Furthermore, using mass spectroscopy, they found peptides whose sequences reflected the RNA variant sequences rather than the original DNA sequences. What all of this suggests is that the DNA-RNA-Protein relationship is not as strict as previously thought. The DNA sequence of a gene might not dictate the exact composition of it's gene product after all.
- Extinction rates may be overestimated - Estimating extinction rates is an important part of ecological conservation, but unfortunately, there is really no reliable way of directly determining such rates. Instead, researchers often rely on indirect methods, but this can run into problems. One popular indirect method is to observe the number of different species found as your area of study gets larger (called a species-area accumulation curve), and then extrapolating backwards to successively smaller and smaller areas to determine the rate at which species number decreases. A new paper in Nature by Fangliang He and Stephen Hubbell argue that this method routinely overestimates extinction rates (sometimes by as much as 160%!). This comes as both good news and bad news; it means that species loss due to habitat destruction in some areas might not be as high as previously estimated, but it also means a more accurate method for estimating extinction rates needs to be devised in order to develop optimal conservation projects.
(F. He and S. Hubbell. "Species-area relationships always overestimate extinction rates from habitat loss". 2011. Nature 473: 368-371. doi:10.1038/nature09985)
1. Yeah, I am aware I'm posting this a little late. So sue me.