The first life on Earth evolved around 3.8 billion years ago, but multicellular life - macroscopic algae, fungi, plants and animals - evolved just 0.8 billion years ago. For three billion years, life on Earth consisted of single-celled prokaryotic microorganisms: bacteria and archaea. Energy-hungry multicellular eukaryotic organisms were able to evolve and diversify only after one group of bacteria, the cyanobacteria or blue-green algae, developed a form of photosynthesis that produced free oxygen as a waste-product. Even now, prokaryotic microorganisms are still found everywhere in Earth's biosphere, from deep inside the Earth's crust (bacteria discovered near a gold mine 2.8 kilometres underground thrive on sulphur in anaerobic groundwater and hydrogen produced by decay of radioactive elements) to the stratosphere. Sulphur-reducing bacteria form the basis of rich ecosystems around deep sea vents; thermophilic bacteria tint the water of hot springs in Yellowstone Park and elsewhere.
One species entered into symbiosis with early eukaryotic cells and its descendants survive as the mitochondria that produce ATP, the chemical that's the basis of our cells' energy economy. Other species inhabit our skin and guts: the human microbiome accounts for between 1 and 3% of our body mass, outnumbers our cells by 10 to 1, and may contain more than a hundred times the number of genes in our own genome. We're each a bacterial microcosm. Living spaceships patchworked with dozens of ecosystems, carrying trillions of passengers.
While we search for signals from alien civilisations, for charismatic megafauna like us, the first aliens we discover may be weird microorganisms lofted on the plume of a geyser rooted in the world ocean of Jupiter's moon Europa or the polar sea of Saturn's moon Enceladus, thrifty sulphur-reducing extremophiles deep in the Martian crust, or tar-eating microbes in a Titanian hot spring. Or maybe we'll spot the characteristic chemical signature produced by methanogenic bacterai in the atmosphere of an exoplanet around a distant star. And if we do ever find creatures like us and the alien ambassador shakes the hand of the President of Earth, it won't just be a meeting of minds, but an exchange between two ancient and indescribably diverse empires.