In 1832, while HMS Beagle was anchored off San Salvador, Brazil, Charles Darwin went ashore to explore. There he saw rocks that glittered, as if burnished, in the sun. Darwin hypothesised that the shining was caused by a thin coating of metallic oxides but could not account for how it had been made. The strange phenomenon has since been observed in many places around the world, especially in deserts, and is now called desert varnish. But its origin – whether the result of complex but non-living chemical reactions or a residue left by unknown living forms – remains utterly mysterious. 

In the 181 years since Darwin stumbled across this enigma, our understanding and appreciation of living forms and life itself have deepened and expanded to a stupendous degree, perhaps beyond anything he could have imagined. But the great naturalist’s central insights have been triumphantly vindicated, and none more so than