William Ruddiman, a professor of environmental sciences at the University of Virginia, is extremely worried about climate change, but he nonetheless hopes the committee votes against both questions. For the past two years, he has lobbied its members to think of the Anthropocene not as a sudden upheaval, but as a gradual change, a slow transformation of the planet that began 5,000 years ago. “Where could you possibly pick a single start date in this ever-evolving story?” he once asked me in an email.

Last week, he and 23 other researchers argued the topic at length in the scientific journal Progress in Physical Geography. At stake is a seemingly simple question: When did human influence over the environment reach a tipping point?

For Jan Zalasiewicz, a professor of geography at the University of Leicester, the answer is clear. Zalasiewicz chairs the Anthropocene Working Group, the committee that will soon vote on the existence of the epoch.

“If you look at the main parameters of the Earth-system metabolism, then … things only began to change sharply and dramatically with industrialization,” he told me. He believes that the most significant event in humanity’s life on the planet is the Great Acceleration, the period of rapid global industrialization that followed the Second World War. As factories and cars spread across the planet, as the United States and U.S.S.R. prepared for the Cold War, carbon pollution soared. So too did methane pollution, the number of extinctions and invasive species, the degree of surface-level radiation, the quantity of plastic in the ocean, and the amount of rock and soil moved around the planet.

It was “the Big Zoom,” he said, borrowing a phrase from the journalist Andrew Revkin. There is “nothing really comparable” to that shift in any other period of Earth history. Even setting carbon pollution aside, he said, the spike in fertilizer use led to the largest jump in surface nitrogen levels in 2.5 billion years. Zalasiewicz hopes the committee will start the Anthropocene in the middle of the 20th century.

Ruddiman isn’t so sure. He believes that humanity’s effect on the planet is spread throughout time and is driven primarily by agriculture. Before the year 1750, he argues, humans had already cleared so much forest as to produce 300 billion tons of carbon emissions. Since 1950, deforestation has only led to 75 billion tons of emissions.

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Humans remade the planet in other ways, too. About 12,000 years ago, we drove a huge swath of American mammals, including the giant ground sloth, into extinction. About 11,000 years ago, we entered into unprecedented relationships with crops and some livestock, domesticating them and taming their genome. Between 6,000 and 7,000 years ago, humans began clear-cutting forests to create new agricultural land; they may have transformed much of Europe by doing so. And by about 1,000 years ago, as humans embraced tilling and made rice paddies, they began moving more dirt and rock around the surface of the planet than is moved naturally.