 And in the midst of this tick-tock cycle, suddenly emerges humans, homo sapiens, on the plains of the Rift Valley in Eastern Africa. And now, 200,000 years later, they're affecting the Earth system so profoundly that we're moving beyond the Earth system envelope of the Holocene and, in fact, moving beyond the envelope of these interglacial cycles into uncharted waters. What is the Great Acceleration?