Science picks: Death of dinosaurs, brains of bugs, and the ancestor of animals
by KrauzeDeath and contingency in the game of life
Penny D. & Phillips M.J., "The rise of birds and mammals: are microevolutionary processes sufficient for macroevolution?", Trends in Ecology and Evolution 19(10):516-22 (2004) [PDF]
The picture of mammals hiding in caves and crevices until a meteor wiped out the dinosaurs and allowed our furry ancestors to populate the planet has long been the consensus, both in popular science books and in the halls of academia. But in this article David Penny and Mathew J. Phillips, two evolutionary biologists from the Massey University in New Zealand, warn against conflating different questions. In particular, they say, the proposition that a large meteor struck the planet about 65 million years ago is distinct from the proposition that it was this event that led to the extinction of dinosaurs. As Penny and Phillips note, "there is widespread querying among the geological and paleontological communities of any 'sudden and unexpected' demise of dinosaurs at the K/T boundary."
This is particularly relevant when reviewing Stephen Jay Gould's argument for contingency in evolution, in which he claimed that re-running the tape of life would result in a wildly different biota. One of the examples he used was the extinction of the dinosaurs and the rise of mammals. Mammals weren't any fitter than dinosaurs and wouldn't have prevailed, had it not been because a meteor crashed into Earth and shook things up. But, as Penny and Phillips point out, "[t]here is as large body of earlier work about the potential competitive advantages of mammals and/or birds over earlier reptilian groups", based on such traits as lactation, specialized teeth, brain size, and endothermy. In short, "[o]nly if we disregard mammalian and avian physiology could we maintain that there were no potential of mammals over, for example, the smallest dinosaurs."
Deep homology in insects and vertebrates
Hirth F., et al., "An urbilaterian origin of the tripartite brain: developmental genetic insights
from Drosophila", Development 130(11):2365-2373 (2003) [PDF]
The fact that human are different from fruit flies is something you don't need a microscope to see. However, if you do have a microscope, you'll notice that the difference between these groups also extend to their development. At the blastopore stage, the embryo is composed of a ball of cells that has been folded into itself, creating an opening, which is called a blastopore. What the embryo does with the blastopore differs from humans and flies; in flies, the blastopore becomes the mouth, whereas in humans it's used to form the anus. (Stop giggling. What are you, eight years old?) This difference marks a big division in animals, with vertebrates, starfish, sand dollars, and a worm-like group called hemichordates forming the deutorostomes, and arthropods, mollusks, and annelids forming the protostomes.
The deuterostome brain is divided into three parts, known as the forebrain/midbrain, the hindbrain and the intervening midbrain/hindbrain boundary region, hence the "tripartite" of the title of this article. Deuterostome brains are morphologically different from protostome brains, and it has traditionally been thought that the two brain types evolved independently.
However, in this study, a group of biologists from Germany and Switzerland show how the Hox genes used in the development of the deuterostome brain have homologues in fruit flies, and that the flies also use these genes to construct their brains. As the authors note: "This suggests that a tripartite organization of the embryonic brain was already established in the last common urbilaterian ancestor of protostomes and deuterostomes."
From so complex beginnings
Larroux C, et al., "Developmental expression of transcription factor genes in a demosponge: insights into the origin of metazoan multicellularity", Evolution & Development 8(2):150-173 (2006) [Abstract]
The sponges sit near the trunk of the family tree of animals, commonly considered mere colonies of cells with little organization. This belief receives a dash of cold water in this article by a group of Canadian and Australian researchers. They identify a range of transcription factor gene classes in the sponge Reniera, which were thought to be unique to the "higher animals". As the authors note, these observations "suggest that the last common ancestor (LCA) to all living animals was developmentally more sophisticated than is widely appreciated", which fits in perfectly with the front-loading perspective. Another interesting aspect of this article is the fact that some of these genes are absent in invertebrates, leading the researchers to suggest that the genes have been lost in these lineages, a trend I've blogged about before (in fact, Larroux, et al. reference the same paper as that post of mine was based on).
