For hundreds of millions of years, plants thrived in the Earth’s oceans, safe from harsh conditions found on land, such as drought and ultraviolet radiation. Then, roughly 450 million years ago, plants found a way to make the move to land: They evolved spores—small reproductive cells—and eventually pollen grains with tough, protective outer walls that could withstand the harsh conditions in the terrestrial environment until they could germinate and grow into a plant or fertilize an ovule.