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Way back in the late Precambrian, some 670m years ago, something happened that changed the course of history: the first worm evolved. Until that moment, the most sophisticated animals were jellyfish—which, being radially symmetrical, have neither a front nor a back. Worms have both. Food goes in the front. The back is where what is left over once the digestive system has done its work comes out again.
That new layout mattered because, very rapidly, the front was equipped by evolution with sensors such as eyes and taste receptors to increase the amount of food entering, and clusters of nerve cells to handle the information from those sensors. These were the precursors of brains.
Brains are the most complex natural objects known, and the most complex of all of them is
the human brain.
In the long-gone days when I studied zoology and first learned these facts, the idea that researchers would be able to unpick brains on a neuron-by-neuron basis was fanciful. Now, as we report in this week’s edition, it has become reality. An international project based at Princeton University in America has formally published a “connectome” of a fruit-fly brain, showing all its constituent neurons as well as the synaptic connections between them. In the individual fly whose brain was used, say the researchers, there were 139,255 nerve cells, with almost 55m connections between them. | 
