Only a worm's 302-neuron brain has ever been fully mapped
The old claim turned out to be wrong and was rejected by evidence.
TaughtThe only complete wiring diagram of a brain (a 'connectome') belongs to the roundworm C. elegans, with its 302 neurons. Mapping every neuron and every connection in anything more complex is far beyond what anyone can do.
NowOn 2 October 2024, the FlyWire consortium published in Nature the complete connectome of an adult fruit fly brain: all 139,255 neurons and more than 50 million synapses, mapped and annotated. It is the most complex brain ever fully wired up.
What actually happened
The jump in scale is enormous: from 302 neurons to nearly 140,000, roughly a 460-fold increase in cells and vastly more in connections. Researchers sliced the fly brain into thousands of layers, imaged them with electron microscopes, and used AI to trace each neuron in 3D, with a global community proofreading the result.
The honest framing: a fly brain is still a very long way from a mouse's (~70 million neurons) or a human's (~86 billion). This is not 'we mapped a mind.' What was disproved is specific and real: the standing claim that no brain beyond the worm's had been completely mapped. Because most human disease genes have fruit-fly counterparts, the map is also a working tool, not just a trophy.
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Who was taught this
Still standard through 2024, so anyone who finished school between 1950 and 2024 learned the earlier version.