Digestion

A hydra eating: the mouth is where all the tentacles
come together,near the center of the picture.
The hydra is eating the round bug in its stomach.

The first digestive systems, like the first living creatures on Earth, were very simple. That was fine for the first three billion years of life on Earth, and it's still fine for most living creatures today. But about 600 million years ago, when the earliest creatures formed that had more than one cell, their digestion became more complicated. How could these bigger creatures, like hydras and sponges, get food to all of their cells?

Hydras solved this problem by forming their cells into a hollow cup, like a tall water glass. The ocean water, and the hydrocarbon molecules in the ocean, could reach both the inside and the outside of the cup. That cup is the beginning of our own digestive system, if you think of the opening of the cup as the mouth. Hydras send out enzymes into this cup, where they begin to digest the food, and then the smaller molecules pass into the cells through the cell membranes, where the lysosomes and mitochondria break them down even more. Leftover molecules that can't be digested - like our poop - go back out of the cell and eventually float back out of the hydra's mouth.

Flatworm

Sponges, which also evolved around 600 million years ago, have a slightly different system: they have lots of tiny holes all through them, so that all of the cells of the sponge can touch the ocean water and get their food directly from the ocean. Jellyfish and flatworms, which evolved around 500 million years ago, use the same digestive system as the hydras.