The compound that is responsible for the earthy smell after a spell of rain(‘petrichor’) falling on dry soil is called geosmin. It is produced due to gram positive bacteria Streptomyces, belonging to genus Actinomyces. They then release the compound to soil, which dissolves in water as rain soaks the soil and produces the earthy flavour and aroma. In acidic environment, the compound decomposes to odourless derivatives which explains the absence of the smell after acid rain. Chemically, geosmin is a bicyclic alcohol having formula C12H22O. This same compound is also responsible for earthy taste of beet.
25 Who hath divided a watercourse for the overflowing of waters, or a way for the lightning of thunder;
26 To cause it to rain on the earth, where no man is; on the wilderness, wherein there is no man;
27 To satisfy the desolate and waste ground; and to cause the bud of the tender herb to spring forth?
28 Hath the rain a father? or who hath begotten the drops of dew?
— Job 38
Too simple an example? Well, there’s always Myxotricha paradoxa:
From Lives of a Cell by Lewis Thomas
It is the protozoan Myxotricha paradoxa, which inhabits the inner reaches of the digestive tract of Australian termites.
It is not as though we would be starting from scratch. We have a fair amount of information about this creature already–not enough to under- stand him, of course, but enough to inform us that he means something, perhaps a great deal. At first glance, he appears to be an ordinary, motile protozoan, remarkable chiefly for the speed and directness with which he swims from place to place, engulfing fragments of wood finely chewed by his termite host. In the termite ecosystem, an arrangement of Byzantine complexity, he stands at the epicenter. Without him, the wood, however finely chewed, would never get digested; he supplies the enzymes that break down cellulose to edible carbohydrate, leaving only the non-degradable lignin, which the termite then excretes in geometrically tidy pellets and uses as building blocks for the erection of arches and vaults in the termite nest. Without him there would be no termites, no farms of the fungi that are cultivated by termites and will grow nowhere else, and no conversion of dead trees to loam.
The flagellae that beat in synchrony to propel myxotricha with such directness turn out, on closer scrutiny with the electron microscope, not to be flagellae at all. They are outsiders, in to help with the business: fully formed, perfect spirochetes that have attached themselves at regularly spaced intervals all over the surface of the protozoan.
Then, there are oval organelles, embedded in the surface close to the point of attachment of the spirochetes, and other similar bodies drifting through the cytoplasm with the particles of still undigested wood. These, under high magnification, turn out to be bacteria, living in symbiosis with the spirochetes and the protozoan, probably contributing enzymes that break down the cellulose.
The whole animal, or ecosystem, stuck for the time being halfway along in evolution, appears to be a model for the development of cells like our own. Margulis has summarized the now considerable body of data indicating that the modern nucleated cell was made up, part by part, by the coming together of just such prokaryotic animals. The blue- green algae, the original inventors of photosynthesis, entered partnership with primitive bacterial cells, and became the chloroplasts of plants; their descendants remain as discrete separate animals inside plant cells, with their own DNA and RNA, replicating on their own. Other bacteria with oxidative enzymes in their membranes, makers of ATP, joined up with fermenting bacteria and became the mitochondria of the future; they have since deleted some of their genes but retain personal genomes and can only be regarded as symbionts. Spirochetes, like the ones attached to M. paradoxa, joined up and became the cilia of eukaryotic cells. The centrioles, which hoist the microtubules on which chromosomes are strung for mitosis, are similar separate creatures; when not busy with mitosis, they become the basal bodies to which cilia are attached. And there are others, not yet clearly delineated, whose existence in the cell is indicated by the presence of cytoplasmic genes.
There is an underlying force that drives together the several creatures comprising myxotricha, and then drives the assemblage into union with the termite. If we could understand this tendency, we would catch a glimpse of the process that brought single separate cells together for the construction of metazoans, culminating in the invention of roses, dolphins, and, of course, ourselves. It might turn out that the same tendency underlies the joining of organisms into communities, communities into ecosystems, and ecosystems into the biosphere. If this is, in fact, the drift of things, the way of the world, we may come to view immune reactions, genes for the chemical marking of self, and perhaps all reflexive responses of aggression and defense as secondary developments in evolution, necessary for the regulation and modulation of symbiosis, not designed to break into the process, only to keep it from getting out of hand.
If it is in the nature of living things to pool resources, to fuse when possible, we would have a new way of accounting for the progressive enrichment and complexity of form in living things.
I take it on faith that computers, although lacking souls, are possessed of a kind of intelligence. At the end of the decade, therefore, I am willing to predict that the feeding in of all the information then available will result, after a few seconds of whirring, in something like the following message, neatly and speedily printed out: “Request more data. How are spirochetes attached? Do not fire.”