Final answer:
Water served as an excellent nursery for early animal evolution due to its stable environment, nutrient richness, and support for organism structures. The transition to land required significant changes, including adaptations for resisting desiccation and efficient locomotion. Land offered benefits such as abundant sunlight and new ecosystems to explore, fostering further evolution.
Step-by-step explanation:
Water proved to be an excellent nursery for the evolution of animals for several reasons. Initially, the ocean provided a relatively stable environment with a consistent temperature and a bounty of nutrients. The oceans were filled with a variety of invertebrates, leading up to the point where the first fish evolved. Furthermore, the medium of water supports the bodies of aquatic creatures, reducing the stress on their structures compared to land-dwelling organisms that require stronger limbs and support to counteract gravity. Also, the early oceanic atmosphere helped in developing respiratory systems that would later adapt to terrestrial life.
Moreover, the evolutionary journey from water to land involved significant physiological changes. Animals needed ways to resist drying out, the development of cavities for breathing atmospheric oxygen, and the adaptation of limbs for movement on land, which first occurred with invertebrates like arthropods, and later with vertebrates evolving from fish. The shift was gradual, with plants colonizing land first, followed by animals that seized the opportunity for new food sources and less competition from other predators.
The transition onto land offered new advantages: more abundant sunlight, more readily available carbon dioxide for photosynthesis, and initially, no predators for the first terrestrial plants. As land-dwelling forms evolved, they began to take advantage of these benefits. This led to a boom in diversification and evolution, as seen in mammals like whales, which have their origins in land animals before adapting to life in the ocean.