1.did unicellular or multicellular life appear first? how do you know? 2. how did cyanobacteria contribute to the evolution of multicellular organisms like the plants and animals we see today ?

1. Unicellular life appeared first. Billions of years ago, life crossed a threshold. Single cells started to band together, and a world of formless, unicellular life was on course to evolve into the riot of shapes and functions of multicellular life today.

For more information about this, visit:
The momentous transition to multicellular life may not have been so hard after all

By Elizabeth Pennisi

2. The oxygen atmosphere that we depend on was generated by numerous cyanobacteria during the Archaean and Proterozoic Eras. ... The other great contribution of the cyanobacteria is the origin of plants. The chloroplast with which plants make food for themselves is actually a cyanobacterium living within the plant's cells. Sometime in the late Proterozoic, or in the early Cambrian, cyanobacteria began to take up residence within certaineukaryote cells, making food for the eukaryote host in return for a home. This event is known as endosymbiosis, and is also the origin of the eukaryotic mitochondrion.

For more information about this, visit: http://www.ucmp.berkeley.edu/bacteria/cyanointro.html