Sponges
For almost three billion years, the only living things on earth were one-celled creatures, either prokaryotes or eukaryotes. Even today, the vast majority of the living things on earth are one-celled creatures, or microbes. But about 600 million years ago, a few one-celled creatures began to group together and cooperate to make multi-celled creatures. Sponges are the earliest and simplest creatures with more than one cell.
Probably the first creatures with two cells formed by accident, when one cell was dividing to make more cells and the new cell happened not to detach completely from the old cell. Then these cells found that there were advantages to sticking together - maybe they were too big for other cells to eat, or they were able to use a root to stay in one place, near a good source of food.
One problem that any multi-celled creature has to solve is how to get food to all of the cells. All of the sponges lived in the ocean, like all other living things at this time, so the food normally just floated by and the cell pulled it inside through its lipid membrane. If you made a long chain of cells, each cell could still touch water and get food. A flat surface two cells thick also would work, or a cylinder of cells two cells thick.
Learn by doing - Sponges
Worms
Plants
Animals
To find out more about cells, check out these books from Amazon.com or from your library:
