In the conditions of earths early atmosphere, oxygen was limited and most of the atmosphere would be filled with noxious methane, carbon monoxide, hydrogen sulfide, and ammonia.
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The answers are yeast and penicillium.
Some yeasts are used in medicine, for example, some Saccharomyces species, Candida species, etc. They are used for the production of some amino acids, vitamins (C, B2), some proteins (insulin). Penicillium is used for production of antibiotics.
Cancer is a mutation that occurs during the cell cycle, usually when it develops and grows it means the mutation was able to go through the cycle undetected at checkpoints so the cell was not destroyed by the lysosomes.
D) Stem cells are usually harvested from embryos.
