Algae saved our planet by transforming our atmosphere to oxygen, allowing life to exist. Algae saved us again by providing the Earth's first food.
Not coincidentally, in many languages the words humus and human share the same etymological root: Earth. Just as human intestines have friendly bacteria and microbes, so too does the soil of the Earth, functioning as the digestive tract of all organic material that ever lived. As plants and animals die, microorganisms begin their job decomposing and returning the nutrients to the Earth. This life cycle continues to create new life in the soil, in turn feeding the plants and the animals eating those plants.