Moments in human history are etched into the Earth. Now researchers are piecing together evidence of our impact on the planet – through the marks we've left on nature.

The microbes living in this French harbour have never recovered from World War Two. Between 2012 and 2017, Raffaele Siano pulled sediment cores from the seabed at Brest harbour, wondering what he was going to find. When he and his colleagues at the French Institute for Ocean Science (Ifremer), analysed the fragments of DNA captured in those cores – they discovered something remarkable.

The oldest, deepest layers of sediment – dating to before 1941 – had traces of plankton called dinoflagellates that were strikingly different to the genetic traces of plankton left in the shallower, more recent layers. "There was a group, an order of these dinoflagellates, that was very abundant before World War Two – and after World War Two it almost disappeared," says Siano. He and his colleagues published a study detailing their findings in 2021.

Siano mentions that Brest harbour had been bombed during the war. Then, in 1947, a Norwegian cargo ship exploded in the Bay of Brest. The disaster killed 22 people and spread ammonium nitrate, a toxic chemical used to make fertiliser and explosives, into the sea. Even younger sediment in the 1980s and 1990s showed further changes in the plankton community in the harbour. "We correlated this from another type of pollution coming from intensive agricultural activities," says Siano.

Nature has a way of remembering things. Echoes of certain human activities, especially highly-polluting ones, sometimes show up in tree rings, coastal sediments and ecosystems. Arguably, these traces are hints of the Anthropocene, a proposed geological epoch in which humanity is said to have irrevocably, and drastically, altered Earth. Human history, it turns out, is written into the very fabric of our planet – and the life that co-exists here with us.

Siano and his colleagues are predominantly ecologists but they also work with historians. "The land changed because of human impact and also because of historical events," Siano says. When the team analysed the sediment cores from Brest, they also detected a gradual rise in heavy metal pollution as time passed. Younger layers of sediment contained higher volumes of mercury, copper, lead and zinc, for example.

The report notes there were similar levels of some of these metals – especially lead and chromium – in Pearl Harbour, a major US naval base in Hawaii that was heavily bombed by Japanese warplanes in 1941. However, Siano adds that he can't be sure whether these metals came directly from the bombs themselves. Either way, there is a signal in both Brest and Pearl Harbour of a calamitous, and polluting, moment in human history.

Other researchers have........

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