A quote / paraphrase from part 1 of “A history of world agriculture, from the neolithic age to the current crisis” by Marcel Mazoyer and Laurence Roudart:
Life began to develop around 3.5 billion years ago, in a solar system and on a planet formed 4.6 years ago, in a universe whose origin is unknown but whose oldest rays of light reach us from such a distance that we conclude it has been expanding for 15 billion years.
Plants are autotrophs: they are capable of synthesizing, by means of solar energy, their own organic sustenance from water, carbon dioxide, and the other elements they find in the air and soil. By contrast, humans and animals are heterotrophs, living upon organic matter provided directly or indirectly by plants. That is why the fertility of an ecosystem, that is, its capacity to produce biomass, is ultimately measured by its capacity to produce plant biomass.
Our species, Homo Sapiens Sapiens, modern humans, appeared 50,000-200,000 years ago. The first period of its prehistory, the Paleolithic, saw a profusion of increasingly varied and specialized stone tools. This new equipment allowed humans to hunt new species of large and small game, develop fishing, more effectively gather certain vegetable products, build artificial shelters, and hence occupy and exploit new environments. Thus 20,000 ago, even before the advent of agriculture, humans were already present in almost every part of the world.