Early Life Revisited

That the evolution of organisms depends in large part on the evolution of their environment is something paleontologists have been noting since the early nineteenth century, and indeed, it is so inherent in Darwinian theory as to seem almost banal. That this dependency might have been two-way—that the earth’s minerals, atmosphere, oceans, and climate have been in large measure determined by the evolution of different life-forms—was somewhat harder to document and accept, partly because the most dramatic evidence was hidden, at the molecular level, in the elusive Precambrian rocks. The concept of the coevolution of Earth and life saw its first cohesive and most provocative expression when James Lovelock presented his Gaia hypothesis in the early 1970s, but not until the end of the twentieth century were the basic tenets of the hypothesis accepted as a valid theory. Lovelock began conceiving the Gaia hypothesis when he was designing instruments for NASA’s first extraterrestrial explorations and it occurred to him that, unlike the moon and Mars, the earth had an atmosphere composed of gases that couldn’t and wouldn’t coexist without life’s intervention. At the same time, a handful of paleontologists and geochemists had been conceiving similar if less provocatively formulated hypotheses based on their studies of the earth’s most ancient rocks and sediments. In 1979, a decade after Geoff, Thomas Hoering, and Keith Kvenvolden had more or less given up on the prospect of garnering clues about early life-forms from the fossil molecules in Archean and early Proterozoic rocks, one of those paleontologists inadvertently inspired a certain Australian chemist to give it another go. Roger Summons met the paleontologist Preston Cloud when Cloud was on sabbatical at the Australian Institute of Marine Science. Summons was working in the biology department at Australian National University and had been assigned to play guide and chauffeur for Andrew Benson, a visiting American plant physiologist who was staying out at the marine institute. “There was a couple living in the guesthouse next to us,” Summons tells me. “And this guy was a jogger. He’d leave every morning at 5:00 A.M. and run past the house, clump clump clump clump, and I’d wake up.”