They had been faced by a difficult cold, dry Ice Age environment for nearly 100,000
years, and had developed a range of subsistence strategies through social or
collective learning.
Then, after the last glacial maximum ended some 18,000 years ago,
the climate began to warm.
It improved dramatically after about 12,000 years ago when the world entered
the current interglacial period known as the Holocene.
Applying and extending the hard-won knowledge of the Pleistocene to a more
amenable environment, people around the world intensified and
diversified their foraging practices.
As biomes became more productive,
people learned to extract more food from the same amount of territory.
An unintentional outcome of that intensification repeated many times around
the world was the advent of agriculture Agriculture is
more than just the cultivation of plants or the raising of animals.
Pre-agricultural societies manipulate their environments in profound ways,
such as burning large areas to foster the growth of desirable plants and animals.
They even manually reseed plants in, for example, Australia or the Middle East.
These activities are not, however,
considered agriculture because they did not involve plant domestication.
Domestication is defined by changes in plants and animals at the genetic level.
Domesticated plants are not simply cultivated, and
domesticated animals are not merely tame.
Their genes differ from their wild relatives.
They are adapted to thrive with human assistance and
most survive poorly without it.
At least in the beginning,
these genetic changes were not the result of intentional manipulation, but
rather the result of symbiosis between certain plants or animals and humans.
Indeed, association with domesticated species influenced human evolution and
changed our genetics.
Lactose tolerance, for example, is a mutation that became widespread on more
than one occasion in response to animal domestication.