They began farming, which extended their own life spans and increased population on Earth. They built cities and roads, made art, developed languages and laws. They formed empires and nations.
Eventually, they invented machines, landing themselves in the industrialized age, driven by engines and turbines, which are powered by combustible fuel.
Thus began man-made greenhouse gases.
The world tomorrow
The main culprit is carbon dioxide, and its levels have jumped in the last 100 years, Marcott said. In the 11,000 years prior, it only changed "very slowly," he said.
Marcott is concerned about people's ability to adapt to a perhaps drastically changed climate.
"As civilization has grown, we're kind of set up for things not to change too much," he said.
The last time Earth has been as warm as it is projected to be by 2100 was before the last Ice Age started -- over 130,000 years ago. That's too long ago to gather reliable data on, he said.
He didn't want to speculate on what the world will look like, if global warming continues.
"I certainly hope we can pull ourselves out of it," he said.