I study the genomes of past organisms by directly analysing the DNA (“ancient DNA”) from their preserved remains, often from archaeological or geological contexts. Remains like subfossilised bones and teeth, or environmental substrates like buried sediments, can often be a genetic palimpsest: a mixture of ancient DNA from many different microbes, plants and animals. I use this data and approaches from population genetics and metagenomics to understand how ecosystems change through time, with a particular interest in past zoonotic diseases (transmitted from animals to humans), like plague.