Symbiosis

Some living things share their strengths and make a perfect team. A classic example is the clownfish, which hides among the stinging tentacles of a sea anemone. The clownfish stays safe from scary predators, and the anemone gets to eat leftover food scraps the fish drops.

Plants and tiny microbes you can't see can be great partners too. Nitrogen-fixing bacteria settle into the roots of legumes like beans and peas. They turn nitrogen from the air into nutrients the plant can absorb. In return, the plant gives the microbes sugars and other carbon compounds made by photosynthesis, which they use for energy.

When both partners clearly gain from living together, we call it mutualism. It's nature's smart strategy: two working together have a much better chance of surviving than one struggling alone.

When people hear "symbiosis," they often picture a fairy-tale partnership where everyone helps everyone. But in biology, the broad meaning of symbiosis covers any lifestyle in which two different species live in close contact. How the relationship looks depends on who gains and who loses.

Picture a remora swimming stuck to the belly of a big shark. The remora gets a free ride and eats the scraps the shark leaves behind, but the shark is neither helped nor harmed. When one side benefits and the other is basically unaffected, it's called commensalism.

Go one step further: when one side takes the benefits and the other is harmed (lost nutrients, slower growth, and so on), that's parasitism, and it's also a branch of symbiosis. A tick latching onto an animal to suck its blood, or mistletoe sinking its roots into a tree to steal water, both count as symbiosis in the broad sense.

Symbiosis goes beyond sharing meals. It has changed the whole course of evolution. A very long time ago, a small cell that used oxygen to produce energy efficiently was swallowed by a larger, more primitive cell.

That little cell should have been digested and gone. Instead, it survived inside the big cell and began to multiply along with it. This was the beginning of the mitochondria, the power plants that make energy inside most plant and animal cells today. This union and coexistence at the cell level between different organisms became a turning point in the evolution of complex life.

Lichens, which grow on rocks and tree bark, are another amazing example. A fungus and an alga or cyanobacterium (a photosynthesizing microbe) are so tightly woven together that they live like a single body. In nature, survival isn't only about beating your rivals in a dog-eat-dog contest. It can also come from becoming part of each other.