Read the given paragraph and answer the given questions.
The first cellular forms of life appeared on earth about 2000 million years ago (mya). The mechanism of the evolution of cells was not known. Few cells had the ability to release O2. The reaction could have been similar to the light reaction in photosynthesis, where water is split with the help of solar energy captured and channeled by appropriate light-harvesting pigments. Gradually, single-celled organisms became multicellular life forms. By the time 500 mya, invertebrates were formed and active. Jawless fish probably evolved first, followed by the evolution of seaweeds and a few plants. We are told that the first organisms to invade land were plants. They were common on land before animals invaded it. Fish with stout and strong fins could move on land and go back to the water.