Some 4 to 2 million years ago globally predominant dense forests thinned out initiating the evolution of extensive savanna grasslands due to a drier climate.
This change in plants (flora) was accompanied by development of grazing animal species (fauna and megafauna) with their grazing patterns central to the formation of modern landscapes and soils. Further climate change, with hunting and later domestication by humans, led to adaptions favouring smaller and more mobile species and our current domesticated species.
Climate change, both natural and human influenced, have substantially modified landscapes, plant and animal species over many millions of years. Archaeological and anthropological evidence indicates that at least from 8 to around 4 million years ago moist and dense high canopy rainforests predominated, providing foods for evolving species favouring arial and ground-based habitats and with gut structures that could process highly fibrous material such as leaves and stems in addition to energy rich fruits and tubers.
These forests receded due to a drying climate with the open space and additional sunlight favouring the expansion of grasses and proliferation of species adaptions that were mobile and able to process grass and shrubs. Early species included megafauna, large scale herbivores with bison and elephants contemporary descendants.
Evidence from Africa shows how megafauna kept landscapes open by efficiently consuming massive quantities of vegetation, especially by seasonal migrations that follow the peaks in plant productivity. The huge herds of bison in the Americas, credited with building some of the richest soils on earth, represent a recent observed example. This intense but mobile grazing activity incorporated dung and urine into soils and also reduced invasive woody species that have deleterious effects on ecological function.
Humans evolved within the same timeframe using fires to contain woody vegetation and promote grassy biomass, increasing the productivity of the ecosystem. The resulting grassland landscape with interspersed trees and shrubs typifies current savannah grass and rangelands that constitute around 70% of all global agricultural land.
Hunting of megafauna by humans and further rainfall reduction favoured smaller and more mobile herbivores leading to extinction of many ancient forms and evolution of modern species including both game species, abundant in Africa, and domesticated horses, sheep, goats and cattle in addition to less populous camels, lamas, alpacas and others. Evidence suggests that hunting evolved to herding with domestication enabling milk production for caloric security of pioneering farmers and dairy products facilitating the expansion of farming through the ability to reside in a localised area.
Meat supply also evolved from nomadic hunting toward domesticated production with both modern extensive grazing and intensive systems representing degrees of management intensity, diet management and animal confinement.
Agricultural expansions involved differential combinations of animal and crop types in response to environmental conditions. Livestock have consequently been intimately associated with cropping for millennia converting the roughly 80% of non-human edible crop biomass and processing wastes to high quality human food in addition to the unique role of ruminant grazing in maintaining and improving the 3.2 million ha of global rangeland environments that cannot support crop production.
