Darwin maintained that the different forms of life had developed gradually from a common ancestry, and that the driving force of this process was the struggle for existence and the survival of the fittest. Life-forms proliferate exponentially and compete for the same living space. Since resources are limited, only the fittest survive. It ignores the microscopic world whose biomass is much greater than that of macroscopic life. It has been estimated that the top 6 in of fertile soil may contain more than 2 tons of fungi and bacteria per acre.
Microorganisms dominate the oceans where the macroscopic world is even less abundant than on land.
Their outstanding achievement was the maintenance of oxygen and carbon dioxide in the atmosphere, without which life would not have evolved. Cellulose is degraded solely by microorganisms. Microbes and degradation products are ingested by protozoa. These in turn are ingested by higher life-forms and so on. At the high end of this food chain are the animals which serve as our food.