II Cold War
The war between Europa and Callisto had lasted almost 4 billion years. Of the Galilean moons, Callisto was the fourth most distant from Jupiter and Europa the second. Their deep, dark Oceans had always nurtured the bulk of the organic matter in the solar system. Evolution on these relatively small bodies was powered by tidal stresses arising from their orbits around their enormous primary. This allowed for a steadier, if slower, growth in biological sophistication than had been possible in the ferocious sun-driven ecologies of the inner solar system. The biospheres of Earth or Mars or Venus were accident-prone, since they needed to mediate the climatic interaction of the land and the sea and the atmosphere. In two of these cases, the effort resulted in irreversible catastrophes of ice and fire early in the solar system's history. On Callisto and Europa, however, there was just the Ocean. Stability required only that life so influence the thermal budget of these worlds as to keep them just warm enough to allow a thin, protecting film of ice to form on the surface.
Survival, however, required that one biosphere destroy the other. Callisto and Europa were different worlds, with different histories, but they were not quite isolated. Their low escape velocities ensured that even modest meteoric impacts would splatter a significant amount of oceanic ice across the Jovian system. On a few occasions, even multicellular life-forms made the journey from one world and established themselves in the other. The constant in the evolution of both bodies, however, was the repeated invasions of microorganisms that each inflicted on the other every few hundred thousand years.
The earliest exchanges were the most catastrophic. New strains of infection more than once brought the native life of Callisto or Europa close to extinction, only to be beaten back when the indigenous biology found a way to circumvent the alien's advantages. Gradually, though, each biosphere became accustomed to the major biochemical themes in the evolution of the other. Each then modified its own evolution to take advantage of its most recent experience of infection, thus preparing a more sophisticated counterstroke for when its own biological material traveled to the other body. The escalating feedback eventually brought a kind of stability: the contenders grew too resilient to be seriously damaged even by the most sophisticated biochemical innovation. The effect of each new exchange was felt, of course, but the damage it did was usually subtle. A reasonable observer might have concluded that this struggle was actually a kind of symbiosis, a relationship destined to last as long as the Jovian system.
A reasonable observer would have been wrong. Europa finally destroyed Callisto. There was nothing altogether novel about the last infection: it expanded only incrementally on the major strategy of the last 500 million years of the conflict. This was the "translation" of the host-organism's genetic code to that of the invader, rather than any immediate gross changes in the information being translated. The effect was rarely to kill the host. In fact, it usually imparted a peculiar new vitality. It did, however, thoroughly disrupt the way that infected organisms reacted to each other, since its point of reference was the maintenance of the metabolic integrity of the individual, rather than the ecology in which it lived. In effect, it turned its hosts into counterfeits.
Though both Callisto and Europa had been evolving these mechanisms, Europa was the first to break some invisible barrier in the speed of infection. Soon after the last eruption of Europan matter arrived, Callisto was overwhelmed before it could develop countermeasures. In a matter of a few centuries, the Callistan biosphere collapsed and 60% of its Ocean froze. The residual biota was a Europan biochemical colony, "dressed" in caricatures of the extinct native life.
An so it seemed that Europa would be left in peace. In effect, its history was over. Its biosphere luxuriated in the dark, salty bliss of perfect isolation. Paradise lasted only a dozen million years, however. Then, once again, alien life intruded below the sheltering ice. This time, though, the invaders were multicellular entities whose evolution had been grossly anatomical. The biochemistry of these entities was so crude that a dozen Jovian years passed before the sophisticated life of Europa found a way to counterattack.