Early evolution of neurons.

How did a structure as complex as our own brain ever evolve? Although biologists have pondered this question since Charles Darwin, the explosion of molecular information in recent years has provided new insights into this question, particularly its first step: the evolution of neurons. Meshing information about genomes with insights from more classical anatomical, physiological, and developmental approaches has led to some remarkable insights and surprises. Because 'phylogenomics' is still a young field, however, there are arguments about which genes to include in comparisons, how much to weigh genetic versus 'classical' features, and which algorithms to use in making such comparisons. One source of serious discussion is the explanation for a feature being present in one clade (a group of animals with a common ancestor) but absent in a second clade. Does the feature's absence in clade 2 mean that the feature was never present in the ancestors of clade 2, or was it present in clade 2's ancestors but subsequently lost? A second phylogenomic problem is posed by convergent evolution (or 'homoplasy' in genetic terminology): a feature or a molecule that is present in two clades might have evolved independently in each clade. Both of these problems, secondary loss and homoplasy, confound the interpretation of evolutionary relationships. For the moment, the only solution to these problems is to compare more genes in more animals to see whether the features that are missing from one species, for instance, can be found in other closely-related species. The purpose of this primer is not to consider the evolution of brains, however, but the more modest goal of determining the evolution of neurons, the information processing cells that compose brains. Even this more limited goal is, at this juncture, beyond our reach, but the journey to this goal has already uncovered some remarkable relationships and has made clearer what are the key questions and how they can be approached.