Life on a Young Planet: The First Three Billion Years of Evolution on Earth

  • Andrew H. Knoll
Princeton University Press: 2003. 304 pp. $29.95, £19.95
Hot springs eternal: cyanobacteria exist in conditions that may not have changed for billions of years.

There is a 'great divide' in the fossil record, known as the Cambrian explosion. Above it is a plethora of creatures large and small; below it are mostly microbes and slime. Palaeontologists spend 99% of their efforts working on fossils above it — but what do we know about what's below? Is the great divide what we think it is?

The prominent Harvard palaeontologist–geobiologist Andrew Knoll is an ideal guide through this early phase of life's history on the Earth. ('Early' is a relative term here, for when his narrative ends, the 'young' planet is four billion years old and has 'only' half a billion years to go before the Beagle sets sail from Devonport.)

Knoll starts and ends (barring a final trip into astrobiological space and a pleasing philosophical epilogue) with the Cambrian period. The reason is pedagogic: first introduce the reader to a world that is at least vaguely familiar, then plunge into deep time to find the beginnings of things, gradually returning to the more familiar territories. Just as the subject is broad and varied, so too are the demands on the reader: the text moves between impressionistic images of fieldwork and technical accounts and explanations. The reader has to be prepared to change gear, and if you don't know what a Rube Goldberg machine is, you could always try the Internet.

The dearth of data in Precambrian palaeobiology has inspired lateral thinking and multidisciplinary approaches — but it has also made the field vulnerable to speculation and wishful thinking. Even though the number and quality of Precambrian fossil sites have grown significantly during the past half-century, the available information is still scanty and scattered compared with that above the great divide. How representative is it?

Science is the art of not fooling yourself (as Richard Feynman put it), but the lure to present a coherent story by connecting dots is strong. None of us is immune to this, and one of the strengths of Knoll's book is that it presents science as the open-ended endeavour that it is. In one of the final chapters he writes: “the absence of a definitive punch line ... is why I get up in the morning.” Ah, yes, what could be more depressing than knowing that all of the problems are solved! Knoll sees the beauty of science as a never-ending story, and transmits his vision well to the reader.

So, despite the remarkable progress of the past 50 years, some of the most crucial questions in Precambrian palaeobiology are still unanswered. When, where and how did life begin? The evidence for a very early establishment of life on Earth is currently under heavy fire; whether life started hot or cold, in soup or stone, on Earth or beyond, is undecided. Two other long-standing enigmas are why it took a billion years or more for eukaryotes to start making their mark after the prokaryotes appeared, and why another billion or two passed before multicellular life finally erupted in the Cambrian explosion. Did evolution really need so long to build complex creatures, or was something keeping the lid on those cans of worms during eons of enforced tranquility?

The Knoll hypotheses regarding evolution's dallying and rallying are 'oxygen' and 'permissive ecology'. The ideas are not new, but Knoll has been instrumental in developing them. The first great oxygenation event in the Earth's atmosphere identified by geochemists took place about 2.2 billion years ago, with a second just less than 1 billion years ago. Knoll believes that the latter event fuelled the Cambrian explosion.

A prerequisite is permissive ecology — an initial lack of fierce competition, allowing for rampant diversification before developmental pathways and tightening ecological interactions restricted the possibilities. Although Knoll believes that preceding extinctions thus paved the way for the Cambrian explosion, he is lukewarm about the Snowball Earth hypothesis (the idea that the whole Earth froze over), which would otherwise be a pretty good extinguisher in this scenario. Slushball, yes; snowball, no.

What did the earlier oxygenation event result in? Knoll argues that this, too, led to an expansion of the ecology, where oxygen-dependent organisms, including eukaryotes, caught the whiff of a grander future. Eukaryotic evolution after this event seems remarkably lethargic, however. Maybe the reason for this is what Knoll calls the 'Canfield' ocean — the global Black Sea that geochemist Don Canfield has proposed to be the result of moderately low ambient oxygen levels following on from the earlier boost. If so, life for the early eukaryotes may have been a harsh struggle against limiting conditions for more than a billion years. And if organisms and rocks are scarce, we need to search long and hard to acquire a data set that we can trust to give us a complete picture of the Precambrian biotas.

The mark of a healthy research field is that there is never a good time to write a book about it. Like other attempts to spell out the history of early life on Earth, Life on a Young Planet is a time document, but it expresses better than most the bumptious vitality and sheer fun of open-minded research.

More on the Cambrian explosion

In the Blink of an Eye

by Andrew Parker

Perseus, $24.95, Can$ 38.95; Free Press, £18.99