PROTOZOA (first animal)
Protozoans are one-celled animal-like protistans and are among the simplest life forms. Most are microscopic, but some of these unicellular forms reach five centimeters (2 inches) in diameter. Food is secured by engulfing other microscopic animals or plants in a temporary depression formed in the cell membrane. This food is digested, and the residue is expelled through another temporary opening made in the membrane. All protozoans reproduce sexually (they have two parents). Generally, however, asexual (i.e., nonsexual) reproduction also takes place, and the cell splits in two, with each cell being equal to the other but smaller than the parent.
Protozoans are usually water-dwelling animals that live as individuals or in colonies. Though they are diverse in fresh-water, marine, and wet soil habitats, few types are fossilized. Two orders, the Foraminifera and Radiolaria, secrete a mineralized shell that is often fossilized. Protozoans made their appearance on earth shortly before the first fossil-bearing rocks of the Cambrian Period (543-489 million years ago). Some foraminiferans were so abundan t that they make up entire rock units. The Egyptian pyramids are made of blocks of limes tone that are almost entirely composed of giant foraminiferan fossils.