Metazoans were the next stage of animal life after protozoans.
Though still very simple, metazoans were multi-celled creatures rather than single cells. They developed systems of respiration, circulation, waste elimination, and sexual reproduction.
Metazoans began to rely more and more on oxygen, which was becoming more plentiful in both the water and the air as a by-product of photosynthesis.
Metazoans also began to feed on plants and each other.