Atmospheric concentrations of carbon dioxide were measured at 415.26 parts per million (ppm) on Saturday, believed to be the highest level since humans first evolved.

“This is the first time in human history our planet’s atmosphere has had more than 415 ppm CO2,” tweeted veteran meteorologist and climate hawk Eric Holthaus. “Not just in recorded history, not just since the invention of agriculture 10,000 years ago. Since before modern humans existed millions of years ago. We don’t know a planet like this.”

Scientists believe atmospheric CO2 last reached this threshold 2.5 to five million years ago. “During this period, global temperatures were 2.0 to 3.0°C higher than they are today, global sea levels were at least 25 metres higher, and sea ice at the Arctic had retreated and given way to forests, where summer temperatures regularly reached 15°C,” The Independent reports.

When the Scripps Institution of Oceanography’s Mauna Loa Lab in Hawaii began daily measurements of atmospheric CO2 in March 1958, concentrations stood at 313 ppm. They hit 400 ppm in May 2013.