Even the earliest known species of land animals, known from fossils dating from the late Silurian period, 400 million years ago, show highly evolved adaptations to life on land. Since neither aquatic nor amphibious animals exhibit these adaptations, early species of land animals must have evolved very rapidly after leaving an aquatic environment. Which one of the following is an assumption on which the argument depends? ['Known fossils of early land animals include fossils of animals that lived relatively soon after the first emergence of land animals.', 'No plants were established on land before the late Silurian period.', 'No present-day species of aquatic animal is descended from a species of animal that once lived on land.', 'Fossils from the late Silurian period represent only a small number of the animal species that were alive at that time.']