What Planet Earth
teaches you

Earth's major ecosystems aren't random — they're determined by latitude, altitude, and ocean currents. The equator gets the most solar energy, creating tropical rainforests. Higher latitudes get less energy, producing temperate forests, then boreal forests, then tundra. Deserts form where dry air descends (around 30 degrees latitude). Mountain tops replicate this pattern vertically — climb a mountain and you travel through biomes that would take thousands of kilometers to cross horizontally.

Almost all life on Earth depends on photosynthesis — plants convert sunlight to energy, and everything else eats plants (or eats things that eat plants). But at hydrothermal vents on the ocean floor, bacteria use chemical energy from hydrogen sulfide instead. This is chemosynthesis — life powered by Earth's internal chemistry, not the Sun. The discovery revolutionized astrobiology: if life can exist without sunlight on Earth, it could exist in dark oceans under the ice of Europa or Enceladus.

Every atom in a living organism was once part of something else and will be part of something else again. When organisms die, decomposers (fungi, bacteria, insects) break them down into nutrients that plants absorb, starting the cycle again. Carbon, nitrogen, phosphorus, and water all cycle through living and non-living systems. A forest is not a collection of trees — it's a nutrient recycling system where death is as essential as birth. Remove the decomposers and the forest dies within a decade.

Natural selection favors survival. Sexual selection favors reproduction — and sometimes they conflict. The peacock's tail makes it harder to survive (heavier, more visible to predators) but easier to mate (females prefer elaborate tails). Darwin recognized sexual selection as a separate force from natural selection. Birds of paradise have evolved some of the most extravagant displays in nature because female choice drives males to ever-more-elaborate performances. The trait persists not because it helps survival, but because it gets chosen.