An 800,000-year-old footprint. Image: British Museum
That is an 800,000-year-old footprint; the oldest trace of humankind yet discovered outside of Africa. Four more sets of nearby prints—left by both adults and children, perhaps a family—were found amongst the preserved sediments of Happisburgh, outside of Norfolk, England in 2013.
It's hard to imagine what the world would have looked like to them; our ancestors some 800,000 years ago. They would have lived amongst mammoths, rhinos, and hippos, and England would still have been connected by land to Europe. But one thing was the same then as it is now: the composition of the atmosphere.
For the first time since those prehistoric humans took their totemic stroll, carbon dioxide accounts for more than 402 parts per million of the planet's atmosphere. Observations at Mauna Loa have confirmed that CO2 levels have soared past the record-breaking 400 ppm marker we breached last year.
“Carbon dioxide is higher now in the atmosphere than it has been for millions of years and the rate of increase has not been seen, certainly not in the ice core record going back 800,000 years," climatologist Pieter Tans told climate reporter Andrew Freedman. "This is truly exceptional compared to times when geology or the natural system prevailed.”
The National Oceanographic and Atmospheric Administration can help you visualize this landmark, in its video, "Time history of atmospheric carbon dioxide from 800,000 years ago until January, 2012." The action really starts to pick up halfway through.
A result of a centuries-spanning, industrial-scale burning of carbon for fuel, this atmosphere we're breathing is a first for human civilization. But not, necessarily, for humans.
The Stone Age began 2.5 million years ago. But there were humanoids scattered across the planet 800,000 years ago, like our nomadic, tool-using forebears that left their mark in Happisburgh. These were Australopithicus; Homo sapiens wouldn't appear for another 500,000 years or so.
"At this time Britain was linked by land to continental Europe and the site at Happisburgh would have been on the banks of a wide estuary several miles from the coast. There would have been muddy freshwater pools on the floodplain with salt marsh and coast nearby," Science News explained at the time of the footprint discovery.
The last time there was so much carbon in the atmosphere, these early humans were surrounded by spectacular megafauna.
"Deer, bison, mammoth, hippo and rhino grazed the river valley, surrounded by more dense coniferous forest," Science News notes. "The estuary provided a rich array of resources for the early humans with edible plant tubers, seaweed and shellfish nearby, while the grazing herds would have provided meat through hunting or scavenging."
Research indicates that humans finally emerged from the Stone Age in a burst of innovation 40,000-80,000 years ago, and that the surge coincided with an abrupt change in the climate.
"In turn, the end of certain stone tool industries of the period coincides with the onset of a new, drier climate," researchers wrote in a study published in Nature Communications last year. "The findings confirm one of the principal models of Paleolithic cultural evolution, which correlates technological innovation with the adoption of new refuges and with a resulting increase in population and social networks."
That change wasn't necessarily linked to the concentration of carbon in the atmosphere, but it illustrates how deeply our existence is tethered to the climate. Today, that climate is again growing wetter and warmer, drawing away from the comparatively stable and dry one that was so kind to human development.
It's common now to think about our "carbon footprint" when we weigh our impact on the environment. For well over 800 millennia, we have been meandering forwards, leaving our imprint on an increasing multitude of natural things—on muddy beaches, prairies turned to farmland, rainforests razed for cattle ranches, and, finally, an atmosphere and ocean increasingly saturated with carbon dioxide. As Bill McKibben wrote, "We have produced the carbon dioxide—we are ending nature."
That's the 800,000-year-old footprint of man.