Breaking news that recent discoveries may push back traces of the earliest forms of life on earth to a remarkable 3.77 billion years ago brings new relevance to Cradle of Life: The Discovery of Earth’s Earliest Fossils, by J. William Schopf, a dazzling exploration of how science was able to overcome substantial obstacles to look back to the very edges of the dawn of life on our planet. Schopf, professor of earth sciences at UCLA, is a paleobiologist with a long, impressive resume that includes service as NASA’s principal investigator of lunar samples during the golden age of manned moon landings. More significantly, it was Schopf himself who in 1965 was the first to discover Precambrian microfossils of prokaryotic life in stromatolitic sediments in Australia’s Apex chert dated to 3.5 billion years ago, the oldest confirmed evidence for life at the time. Because new breakthroughs in this field are rare and the science moves forward at a somewhat glacial pace, Cradle of Life–first published in 1999–remains fresh and fascinating.
If references to “prokaryotic life” and “stromatolitic sediments” sound intimidating, the reader should take some comfort in that Schopf’s fast-moving and well-written chronicle is largely accessible to the non-scientist. At the same time, do not overlook the implied caution in “some comfort” and “largely accessible,” but neither should that serve as roadblock to his rewarding if sometimes complicated account. I frequently challenge myself to read above my level, and these forays typically orbit around science and mathematics, where my strengths in historical analysis are of little service to me. I will long remember how humbled I was by The Particle at the End of the Universe, Sean Carroll’s brilliant exposition on the hunt for the Higgs Boson. Such books are a reminder of how smart you aren’t! I came better equipped to Schopf than to Carroll, largely because of my long self-taught study of evolution and paleontology, yet still I found myself tested as I trod through the evidence for ancient microbial life and how it functioned in an ecosystem so distant in time to us that it compels the reader to brave the contours of both science and imagination.
The earth was formed some 4.54 billion years ago, but as recently as the late nineteenth century it was believed to be only 100 million years old. Darwin, who advanced his theory of evolution before the earth’s antiquity was established, was troubled by what he viewed within that framework as the lack of requisite time for the slow process of natural selection to occur. Later, with the age of the planet at least approximated, it yet remained rather fuzzy when organic life first appeared, today believed to be as early as 4.1 billion years ago. Until relatively recent times, it was accepted wisdom that whatever life–simple and microscopic–may have existed before the fossil-rich Cambrian Period (the first geological period of the Paleozoic Era, which began 541 million years ago) left no verifiable remains.
In the opening chapters of Cradle of Life, Schopf neatly summarizes for the non-specialist the fundamentals of earth science, geology, and plate tectonics, as well as the key concepts of evolutionary biology, before moving on to recount the various attempts to locate evidence of Precambrian life. It turns out that such traces are often ambiguous, and vestiges of life in the layered mineral record bear a striking resemblance to similar nonbiologic designs in sedimentary rock. Experts who fell victim to such errors brought disrepute to the field when shown to be mistaken, which engineered a strong resistance to authentic microfossils when these came to light. Fortunately, for all of its wrong turns, science is self-correcting, and the bulk of this fine book relates the discovery of indisputable ancient fossil life and how that has impacted what we know about evolutionary paleobiology.
Schopf, as noted, was at the forefront of this dramatic breakthrough, which was manifested in his discovery of microfossils of cyanobacteria preserved in stromatolites–accretionary layers produced by the activities of mat-building microorganisms–in the sedimentary rock of some of the oldest pristine continental crust still extant on the planet, in northwestern Australia’s Pilbara Craton. Cyanobacteria, formerly and incorrectly tagged “blue-green algae,” are an ancient single-celled organism that, lacking a nucleus, are characterized as prokaryotes. Cyanobacteria still exist, but were of outsize significance to an early earth that was both starved for oxygen and subject to punishing UV radiation unshielded by an ozone layer that was yet to exist. Critical to the evolution of much of life as we know it, it was cyanobacteria’s pioneering strategy of oxygen-producing photosynthesis that literally created the environment suitable for the explosion of the diversity and complexity of organisms that later populated the planet. As cyanobacteria flourished, vast quantities of oxygen were pumped into the atmosphere that not only created conditions salubrious to evolving life but led to the formation of the protective ozone layer essential to the latter’s survival. In the meantime, cyanobacteria demonstrated an astonishing level of adaptability to a whole host of habitats, a testament to how it is that this “living fossil” remains with us today.
While I see no inherent conflict between science and religion, as a self-styled “dogmatic skeptic” the evolution of life strikes me as even more wondrous and miraculous without the presence of a creator god. Schopf underscores that every form of life on earth is linked to one another, that: “Whether large or small, living or fossil, life comes in just two varieties …” the nonnucleated single-celled prokaryotes like cyanobacteria, and the eukaryotes, which “have chromosomes packaged in a saclike nucleus.” Eukaryotes encompass both single-celled creatures as well as all of the more complex multicellular organisms–like azaleas, ants, frogs and humans–that exist on our planet. The author rhetorically chides us for our “big-organism bias” that puts frequent focus upon plants, fungi and animals–the only three branches of life that “include large, many-celled organisms,” as well as microscopic ones. [p237] Cradle of Life reminds us that the most noteworthy creatures can be those that are invisible to the naked eye. Cyanobacteria, measured in micrometers–a millionth of a meter–are not nearly as wide as a human hair. Life’s four most critical biogenic elements are carbon, hydrogen, oxygen and nitrogen (CHON), so the contributions of oxygen producing cyanobacteria–a microbial single-celled prokaryote–to the evolutionary processes unleashed cannot be overstated. Schopf argues that cyanobacterial oxygenic photosynthesis, and the much later innovation of eukaryotic sex that fueled genetic variation, are the two “surpassingly important” components of evolution ever devised. [p249]
While much of Schopf’s delightful book is accessible to the non-specialist, portions nevertheless present some difficulty to those outside of this field. The complexity is, however, much mitigated by photographs, illustrations, timelines and a comprehensive glossary of terms that I found most helpful. Moreover, Schopf’s prose is both articulate and engaging. While I found some of this read challenging, I came away from the work much rewarded for the effort. For those little familiar with the origins of life on earth and the line that can be drawn from that hazy ancient past to my very fingers typing out this review, Cradle of Life comes highly recommended.