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When I invent a time machine, what is the earliest period in Earth’s history in which I could comfortably survive? (continued)

Hillary Shaw
Newport, Shropshire, UK

Maybe no more than a millennium or two earlier. It is easy to miss the effects of the unseen microbiome, the panoply of bacteria and viruses around us. Some can make us ill, but many are crucial to our health. The further back (or forwards) you go, the more different this microbiome will become, and you risk being exposed to novel diseases your immune system has no experience of.

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