ties of the Precambrian fossil record would pay off. The four keys of the strategy, as valid today as they were three decades ago, are to search for (i) microscopic fossils in (ii) black cherts that are (iii) fine-grained and (iv) associated with Cryptozoon-like structures. Each part plays a role.
Megascopic eukaryotes, the large organisms of the Phanerozoic, are now known not to have appeared until shortly before the beginning of the Cambrian—except in immediately sub-Cambrian strata, the hunt for large body fossils in Precambrian rocks was doomed from the outset.
The blackness of a chert commonly gives a good indication of its organic carbon content—like fossil-bearing coal deposits, cherts rich in petrified organic-walled microfossils are usually a deep jet black color.
The fineness of the quartz grains making up a chert provides another hint of its fossil-bearing potential—cherts subjected to the heat and pressure of geologic metamorphism are often composed of recrystallized large grains that give them a sugary appearance whereas cherts that have escaped fossil-destroying processes are made up of cryptocrystalline quartz and have a waxy glasslike luster.
Cryptozoon-like structures (stromatolites) are now known to have been produced by flourishing microbial communities, layer upon layer of microscopic organisms that make up localized biocoenoses. Stromatolites permineralized by fine-grained chert early during diagenesis represent promising hunting grounds for the fossilized remnants of the microorganisms that built them.
Measured by virtually any criterion one might propose (Fig. 5), studies of Precambrian life have burst forth since the mid-1960s to culminate in recent years in discovery of the oldest fossils known, petrified cellular microbes nearly 3,500 million years old, more than three-quarters the age of the Earth (Schopf, 1993). Precambrian paleobiology is thriving—the vast majority of all scientists who have ever investigated the early fossil record are alive and working today; new discoveries are being made at an ever quickening clip —progress set in motion by the few bold scientists who blazed this trail in the 1950s and 1960s, just as their course was charted by the Dawsons, Walcotts, and Sewards, the pioneering pathfinders of the field. And the collective legacy of all who have played a role dates to Darwin and the dilemma of the missing Precambrian fossil record he first posed. After more than a century of trial and error, of search and final discovery, those of us who wonder about life 's early history can be thankful that what was once “inexplicable” to Darwin is no longer so to us.