Answer:
It is a member of the extensive family of terpenoid oils that are natural odours, flavours and signalling molecules (more than 70,000 naturally occurring terpenoids have been discovered). The sensitivity of diverse animals to geosmin is astonishing: it is reported that humans – not famous for their olfactory virtuosity – can smell it at levels as low as 100 parts per trillion.
By the time that geosmin was chemically characterised it was well established that its major source in soil was bacteria of the genus Streptomyces. These abundant and complex bacteria grow like fungal moulds as a mycelium of branching thread-like hyphae, playing a very important part in the recycling of vegetable matter. Echoing fungal moulds, they reproduce by sending up aerial hyphal branches that bear spores. Geosmin is associated with Streptomyces spores, which are present in huge numbers in many soils. We can safely assume that the time-traveller visiting the planet as it was about 440,000,000 years ago would recognise the familiar smell of soil, as the earliest land plants collaborated with the first streptomycetes to generate protocompost.
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