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People talk about an individual’s genome as if it was a single consistent entity — but it isn’t. Every one of us actually contains a cosmopolitan melting pot of different genomes. Our 37 trillion or so cells all arose from a single fertilized egg, and as this progenitor divided again and again, its daughters picked up mutations in their DNA that distinguished them, and their descendants, from their neighbors.
Scientists can now detect the subtle differences between these dynasties and trace their lines of ancestry. That is, they can reconstruct the genealogy of a single body. And they’re finding that these family trees are more convoluted than anyone suspected.Take the brain. You might expect that neighboring neurons would be closely related to one another, or that entire regions would arise from the same ancestral cells. But that’s not the case. Christopher Walsh from Boston Children’s Hospital has now shown that in one region, the prefrontal cortex, any given neuron is more closely related to cells from the heart than it is to three-quarters of its immediate neighbors.Walsh has been building up to this since the early 1990s, when he charted the migration of neurons in rat brains, and found that closely related cells would often end up in very different places. Now, using powerful techniques that can sequence the DNA from individual cells, he can trace these lineages with much more precision, and in human brains.
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