Cradle of Life: The Discovery of Earth's Earliest Fossils

  • J. William Schopf
Princeton University Press: 1999. 367pp $29.95, £17.95

Of all the fossils on Earth, some have to be the earliest. So what's the big deal, except for an entry in The Guinness Book of Records ? Quite a big deal, in fact. The Phanerozoic — the ‘time of visible [animal] life’ of the past 550 million years — is now known to have been preceded by three billion years or more of Precambrian life, almost exclusively microbial. It seems that life appeared as soon as conditions at all permitted it. Life may well be (to use Christian de Duve's words) a cosmic imperative.

Bill Schopf has been a prime mover on this frontier, in his quest to find microfossils by slicing up uncountable chunks of the Earth's older crust and in his messianic efforts to bring together scientists of all creeds and talents to ask: ‘What does it all mean?’ His persistent questioning, arguing, pleading, shouting, bullying, persuading, fund-raising, entertaining, writing and editing have probably promoted and inspired more interdisciplinary work on the history of life than any other factor.

In the well-written Cradle of Life, Schopf tells his story of how Earth's early microbial biosphere was discovered. He ranges from biochemistry to natural history, science history and personal anecdotes, and although the path can be tortuous, it is not too convoluted. The many diagrams, however, usually lack information about data sources; this is particularly troublesome, as it is apparent that some plots are based on idealized data.

Schopf's treatment of the earlier players in the field is balanced and entertaining, but currently active players hardly get a mention. One could argue with some of the judgements. A. C. Seward is (dis)credited with stifling research in the field for almost 40 years through his 1931 critique of the known or asserted Precambrian fossil record. If Seward's call for “more critical examination of the evidence” indeed had such an effect, surely the blame lies with those who might have taken up the challenge but didn't.

Nor does Schopf spare himself; the account of how, in 1965, he helped his professor take advantage of a peer-review commission to get the edge on a competitor would raise more than eyebrows if the act were committed today. The 1983 paper by Awramik, Schopf and Walter, reporting the world's oldest fossils from the Warrawoona Group in western Australia, is called a fiasco, because the original sampling spot could not be found; Schopf's own 1993 paper is credited with the real discovery. However, the earlier work showed that these fossils existed, and so was no more a fiasco than Christopher Columbus's voyage (about which there was also uncertainty over exact locations). In this vein, Schopf's paper rather compares to Amerigo Vespucci's voyages. Isn't that good enough?

The book is not free from bloopers, which could have been avoided by more careful reviewing. The map of North America on page 37 should be deeply offensive to those of a Canadian persuasion in New Brunswick and Nova Scotia; the Bolsheviks are credited with seizing power from the czar (who had actually abdicated more than half a year before the Bolshevik coup); and atmospheric carbon dioxide is said to act “like the windows of a greenhouse, it holds in heat, storing it in the chemical bonds that knit its atoms together” (no, it absorbs infrared radiation and converts it to heat in the atmosphere; greenhouse windows mostly just keep warm air in).

A central message of the book is that Precambrian biology differs fundamentally from Phanerozoic biology. This is indeed so; the transition between the two stands out as the greatest revolution in the history of the biosphere. Schopf makes a claim for ‘hypobradytely’ — extremely slow evolution during the Precambrian, owing to the predominance of asexual reproduction, “a new fundamental insight that ⃛ stands out as one of the most striking findings ⃛ since Darwin”. Whether or not this bold claim will stand the test of time, the measurement of evolutionary rates in fossil microorganisms faces formidable difficulties, because in general only morphology is preserved, and poorly so.

Another obstacle to understanding early fossils is the bias imposed by the small subset of life-forms that have happened to survive until today. By using living organisms as our models (for example, by identifying the earliest fossils as cyanobacteria, as Schopf does), we may blind ourselves to the host of extinct organisms, and unwittingly introduce hypobradytely as a taxonomic artefact.

This problem follows us into astrobiology; we need methods to recognize life-forms, fossil or living, that may be totally unrelated to those on Earth. In an epilogue on that theme, Schopf mischievously places a chapter on two of the most famous howlers in palaeontology — Scheuchzer's ‘Homo diluvii testis’ and Beringer's ‘Lügensteine’ — side-by-side with a critical discussion and effective debunking of the claim for ancient life in the Martian meteorite ALH84001 (Schopf was the invited sceptic at the NASA press conference in 1996). In no way does he deride the field, however. Like Seward 65 years earlier, Schopf has advocated constructive scepticism in a field of research where even the existence of the subject is in doubt. Like Seward, he deserves praise for this.