Created oxygen in the early atmosphere. photosynthetic bacteria, asteroids hitting the earth, plankton, volcanic eruptions, forests,

The correct answer for the blank is- photosynthetic bacteria.

Photosynthetic bacteria ( also called cyanobacteria or blue green algae) are the microbes, which are primarily found in seawater. These were the first bacteria that created oxygen in the early atmosphere of the earth planet.

These bacteria contain chlorophyll pigment, which help in synthesizing food ( in the form of carbohydrate) using carbon dioxide and water in the presence of sunlight. Oxygen is released in surrounding in this process. This process is called photosynthesis.      

this protocol provides a detailed procedure for co-immunoprecipitation (co-ip) to identify protein–protein interactions.

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.