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Vancouver, BC, V6T 1Z4, Canada. 9. Sloan, D.B., Nakabachi, A., Richards, S., Qu, J., characteristics separating ‘symbiont’ Murali, S.C., Gibbs, R.A., and Moran, N.A. E-mail: [email protected], from ‘organelle’ have become less (2014). Parallel histories of horizontal gene [email protected] clear [3,4,19,20]. There is an transfer facilitated extreme reduction of endosymbiont genomes in sap-feeding insects. understandable desire to draw a Mol. Biol. Evol. 31, 857–871. http://dx.doi.org/10.1016/j.cub.2014.05.073 distinct line between the two for simplicity, but first we must ask, does this line exist? If so, it is best drawn by evolutionary and mechanistic distinctions, not by perceived Animal Evolution: Looking for the differences born of tradition, definitions, or historical contingency. First Nervous System Organelles were discovered first, have been studied for decades, and their bacterial origins dominated the The human brain is easily the most baffling bit of biology on the planet. How did discussion about endosymbiosis and the nervous system evolve? What came first: neurons or synaptic proteins? A new Trichoplax evolution for many years. They enjoy paper studying the pancake-shaped suggests it was not the neurons. a status apart from other biological entities: derived from bacteria, but Erik M. Jorgensen rise of complex, unfamiliar looking so different as to be given their own multicellular marine life forms, ended name. But the list of their ‘unique’ Something bad must have happened with an extinction that wiped out most characteristics is shrinking: stable around 542 million years ago: the of these creatures. Unfortunately, this endosymbioses promote extensive Ediacaran period, which had seen the event obscured our view on the Current Biology Vol 24 No 14 R656 evolution of the first nervous systems. speck of an animal. It was originally has a protein closely related to the We know that some Ediacaran discovered in 1883 crawling on the synaptic synaptotagmin and thus animals must have had nervous glass of a fish tank containing perhaps only Trichoplax will exhibit systems, as they left tracks and seawater. Its simple circular body plan, true synaptic behavior, i.e. tunnels, and thus could move in an only 1 mm in diameter, was described calcium-activated vesicle fusion. active, organized fashion. But at the time but the organism was largely Surprisingly, only ctenophores because they left no directly forgotten thereafter until its possess panexin gap-junction recognizable successors, we can’t rediscovery in the 1970s. proteins, which allow rapid infer how those first nervous systems Can this dog do tricks? Sure, the communication between cells by direct arose. The Cambrian explosion that essential ones: the pancake that is electrical conductance. All three phyla, followed gave rise to the familiar Trichoplax can glide along a surface, placozoa, porifera and ctenophora, animal phyla. Examination of their which implies coordination of beating lack pentameric ligand-gated ion genomes suggests that the nervous cilia. Moreover, when it comes across a channels altogether, and therefore system arose once, in a perfect state, patch of algae, it pauses over the food show no evidence of fast as all of the key molecular components source, presumably digesting them by neurotransmission by acetylcholine, of the nervous system are conserved secreting enzymes onto its ventral serotonin, dopamine, GABA or glycine. in the genomes of animals: the ion surface, for absorption via microvilli. It is possible that variants will channels used by excitable cells, Trichoplax also changes shape from a eventually be found when these draft an extensive repertoire of circular to an elongated disk genome sequences are fully neurotransmitters and synaptic sometimes. So is there evidence of a assembled. But the lack of most machinery. In fact at the level of nervous system underlying these classical neurotransmitter receptors neurotransmitter systems, the human behaviors? Now, through the work of is further supported by a lack of nervous system looks like a simplified Smith et al. [1], a full understanding of the vesicular transporters for version of the primordial nervous the anatomy of the w50,000 cells in acetylcholine, monoamines, GABA, system.