* Cradle of Life by J. William Schopf, Princeton University Press, $29.95, 367 pages. Reviewed in the Raleigh News & Observer on August 22, 1999.

The News & Observer


August 22, 1999

Appreciating the scum of the Earth


By Phillip Manning

Most country dwellers have seen a clear lake turn into greenish soup almost overnight. The soup is dismissively called pond scum. But during the last few decades scientists have learned that the organisms in it ruled the planet for billions of years. In fact, pond scum made our own lives possible.

Pond scum, also known as blue-green algae, is a bloom of single-celled organisms called cyanobacteria. William Schopf, a professor of paleobiology at UCLA, has been investigating ancient cyanobacteria for three decades. In "Cradle of Life," he describes his own work and that of other scientists, whose research involves spalling ancient rocks and peering through high-powered microscopes at slices of them to find fossilized microbes. He brings to the subject a wealth of knowledge and an enthusiastic writing style that features the liberal use of exclamation marks! and an occasional lumbering sentence ("Balloonlike sphaeromorph acritarchs have been known since . . . ."). But on the whole, the book is an easy read, and it will captivate anyone who cares about the history of life on Earth.

Until recently, the fossil record extended back only 550 million years (to the Cambrian Period), when multicellular life developed. Yet, the Earth had coalesced from solar gases about 4.5 billion years ago. Thus, for almost 4 billion years, there was no record of life on Earth. This gap mightily perturbed Charles Darwin: "Why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer."

Scientists took a big step toward solving Darwin's dilemma in the early 1960s, when they studied stromatolites in the hypersaline waters of Shark Bay in Australia. Stromatolites are rocky domes about a foot or two in diameter that resemble a stack of thin pancakes. Geologists had assumed they were rocks created by geologic processes. However, part of the stromatolites at Shark Bay were undeniably alive. The uppermost layer of each stromatolite was a sticky mat of cyanobacteria. Over time, debris sticks to the cyanobacteria, cutting it off from sunlight. In response, the cyanobacteria grow upward, through the debris. But not all of them make it: Thus fossilized stromatolites contain fossilized cyanobacteria.

After this discovery, scientists concentrated their search for Precambrian fossils in rock formations containing stromatolites. In 1965, William Schopf's mentor, Elso Barghoorn of Harvard University, found the first cyanobacteria fossils. They were 1 billion years old. With a few strokes of a geological hammer, Barghoorn had doubled the length of the fossil record. Soon afterwards, Schopf found the oldest fossils yet discovered - an astounding 3.5 billion years old. Thus, life emerged on the planet within 500 million years from the time it became livable. (During its first 500 million years, the Earth was bombarded by meteors - some the size of Mars - which vaporized the oceans. And because water is essential to life, the planet was uninhabitable.)

The early Earth was a tough place. There was almost no oxygen in the atmosphere, and no ozone layer to absorb the deadly ultraviolet radiation that bombarded the planet. According to Schopf, the earliest life forms lived in the shallow seas, which protected them from radiation, and fed on glucose in the primordial soup. But glucose was scarce, so evolution favored organisms that could manufacture their own food by photosynthesis. Soon afterward, the first cyanobacteria appeared.

What are these tiny organisms that existed for 3 billion years before the first animals developed? Cyanobacteria are single-celled photosynthesizers that eat carbon dioxide and water and exhale oxygen. Their DNA - unlike modern plants and animals - is not encased by a nucleus. Even compared to other microbes, cyanobacteria are tiny, only about one-tenth the size of modern nucleated cells. But they are incredibly hardy, which enabled them to survive the extinctions that have regularly decimated life on our planet.

For billions of years, these single-celled organisms changed very little. The ancient fossils that Schopf dug out of the Australian chert closely resemble (and may be identical to) the cyanobacteria found today in farm ponds. Why didn't they evolve? After all, it took only a few million years for us humans to develop from our apelike precursors.

Schopf gives a two-part answer. First, cyanobacteria are "generalists able to survive and grow under the most varied conditions." The second reason is sex - or the lack thereof. Cyanobacteria reproduce by simple cell division, in which the DNA of the parent cell uncoils and is duplicated in a daughter cell. The genetic shuffling created by sexual reproduction does not occur in cyanobacteria; the only genetic variability is introduced by mutation, a rare event that happens when DNA from the parent cell is incorrectly duplicated. Of course, about 1 billion years ago, some bacteria must have felt something was missing from their lives, because mutations did occur and sexual reproduction did develop. The pace of evolution accelerated, and multicellular life followed. Some of the resultant menagerie - snails, for example - began to feed on cyanobacteria, which is why they no longer rule the Earth.

But for 3 million millennia, cyanobacteria were unchallenged. With no need to evolve and only limited means to do so, they stayed the same and - molecule by molecule - changed the world.

A human conceit that we hear often in these days of global warming goes like this: People are the only animals smart enough (or dumb enough, depending on your point of view) to alter the atmosphere. That, of course, is nonsense. Long before animals arose on the planet, cyanobacteria were transforming the atmosphere. Billions of years of photosynthesis by pond scum produced today's oxygen-rich atmosphere. And without it, the Earth would be devoid of animals - including the almost hairless two-legged ones who believe that only they can change the atmosphere.

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