Because life in Europa's oceans evolved around thermal vents that received no sunlight, the Europs' biochemistry is based on sulfur respiration. This is much less energy efficient than the Terran biosphere's oxygen-based respiration, resulting in the Europs being very slow: it's remarked that "even a sloth could outrun a Europ". This limit on energy efficiency also results in Europs being much smaller than humans. Overall, without the protection of the Monoliths, the Europs could have easily been wiped out by humanity with little effort.
