The Big Bang Theory is the most accepted theory regarding the origin of life on earth & different forms of life in it.
According to this theory, the universe is a result of a huge explosion, which occurred 20 billion years ago.
Once these multicellular organisms evolved, evolution also happened at a much faster pace.
The further series of development & evolution of new organisms like invertebrates, plants, and animals such as reptiles, amphibians, mammals, etc., took place within a time range of 500-300 million years ago (mya).
Environmental conditions seen like climatic changes, limited resources, predators, etc. generally play an enormous role in that.
During each phase of evolution, one life form dominated over the other.