If the theory [of evolution] be true, it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed…and the world swarmed with living creatures. [Yet] to the question why we do not find rich fossiliferous deposits belonging to these earliest periods…I can give no satisfactory answer.
—CHARLES DARWIN, ON THE ORIGIN OF SPECIES
DARWIN’S DILEMMA
When Darwin published On the Origin of Species in 1859, one of the weakest lines of evidence was the lack of undisputed fossils before the Cambrian Period, when complex multicellular animals like trilobites first appeared. Darwin had personal familiarity with Cambrian rocks and fossils, because in 1831 he had been a field assistant in the Cambrian rocks of western Wales for his Cambridge mentor, the legendary geologist Adam Sedgwick, the first man to hold the title “professor of geology.” But even when Darwin wrote about the topic in 1859, some 30 years later, the lack of fossils was still a mystery.
Darwin and most geologists since have known that one of the problems was that old rocks from before the Cambrian were usually transformed by enormous heat and pressure into metamorphic rocks, so all fossils were destroyed. In addition, the older the rocks are, the more likely they have not only been metamorphosed, but simply eroded away. Finally, really ancient rocks are usually at the bottom of the stack, and thus are buried by younger post-Cambrian sediments, so they are only exposed in a few places on the earth where the ancient “basement” rocks have been uplifted and erosion has stripped away their cover.
Nevertheless, scientists took up Darwin’s challenge and kept on looking. There were many blind alleys and false leads. A weird branching structure that looked like a primitive plant was named Oldhamia. It turned out to be formed by the burrows of a worm, but it was not a body fossil. A slimy “organism” found in jars of mud from the deepest ocean and touted as a new creature, Bathybius, by Darwin’s “bulldog” Thomas Henry Huxley turned out to be a product of chemical reactions of calcium sulfate with the alcohol used to preserve the sample. In 1858, the year before Darwin’s book was published, pioneering Canadian geologist Sir William E. Logan discovered some layered structures on the banks of the Ottawa River just outside Montreal (figure 13.1). Most scientists were not convinced, because there are many nonbiological ways that layered structures can form in rocks. One of Logan’s protégés, Canadian paleontologist J. W. Dawson, was convinced it was produced by life, and named the structures Eozoon canadense (“dawn animal of Canada”). He called it “one of the brightest gems in the crown of the Geological Survey of Canada.” But soon other geologists looked closer at the “fossil” and where it had come from and concluded that it was a banded metamorphic structure formed by layers of calcite and serpentine minerals.
Figure 13.1
Layered structure known as Eozoon once thought to be a fossil, but now known to be inorganically grown pseudofossils. (A) Illustration in J. W. Dawson’s book, Dawn of Life. (B) The holotype specimen in the Smithsonian Institution. Scale bar = 1 cm. (Photo courtesy of J. W. Schopf)
PSEUDOFOSSIL OR REAL FOSSIL?
After so many false alarms over pseudofossils, geologists were justifiably skeptical of any specimen touted as evidence of Precambrian life. It’s an easy mistake to make. There are lots of ways that natural rocks and minerals can form structures that look (to the inexperienced collector) like real fossils. Many rock hounds split open shales and find delicate branching lacy black structures on them and think they have a fossil plant. But this is a well-known pseudofossil, known as a pyrolusite dendrite, produced by the branching growth of crystals of manganese oxide. Many a paleontologist has been handed an odd-shaped rock by an amateur and been told it’s a “fossil egg” or “fossil brain” or “fossil heart” or even a “fossil phallus.” Most of the time, these are simply sediments that have been cemented together into suggestive shapes, known as concretions.
But in 1878, another interesting structure was proposed as evidence of Precambrian life. The young Charles Doolittle Walcott was working as an assistant for the legendary James Hall, the first official geologist and paleontologist for the state of New York. (Walcott later became America’s foremost paleontologist, and at various times he also served as the head of the Smithsonian Institution, director of the U.S. Geological Survey, and president of the National Academy of Sciences.) While visiting the resort and horse-racing mecca of Saratoga in the upper Hudson Valley, Walcott stopped at a place now called Lester Park, about 3 miles west of Saratoga Springs. There he found a large outcrop of layered structures that looked like the heads of a bunch of cabbages that had been sliced through (figure 13.2). These structures had been discovered by geologists before and named “stromatolites” (Greek for “layered rock”). However, the specimens in Lester Park were extraordinary. The 28-year-old Walcott sat down to write and publish his first scientific paper about them, giving them the name Cryptozoon (Greek for “hidden life”) and touting their biological origin.
Figure 13.2
The layered cabbage-like stromatolites known as Cryptozoon from Lester Park, New York, with their tops sliced off by a glacier. (Photo by the author)
Naturally, he met with a cool reception from the scientific community. They had been burned before with the layered structure Dawson called Eozoon, and now they were gun-shy. The world’s most famous paleobotanist, Sir Albert Charles Seward, spent many years dismissing Walcott’s Cryptozoon, and he had a lot of influence. He rightly argued that they had no organic structures or detailed plant tissues or anything that would rule out simple layers caused by mineral growth.
Nevertheless, more and more different kinds of stromatolites were being found and described. Some were not shaped like simple sliced cabbages, but were tall domed pillars. Still others had shapes such as tall cones (Conophyton) or convex layers flattened in the center (Collenia). Well-preserved stromatolites of many shapes were common in the late Precambrian rocks of Siberia, so Soviet geologists set about naming and describing many such structures. Still, the proof that they were truly organic and not some sort of geological structure was missing.
SHARK BAY
The only convincing way to show that stromatolites were really fossils was to find them living and growing in the present day. Yet nearly all known stromatolites were from the Precambrian, more than 550 million years ago, with a few minor exceptions. They had once been the most common visible fossil on the planet but had mysteriously vanished during the Cambrian, when the evolution of multicellular animals took off.
The breakthrough occurred during routine geological exploration of a relatively unknown region. Geologist Brian Logan of the University of Western Australia and other geologists were mapping the northern coast of Western Australia in 1956. They traveled up the coast and reached a salty lagoon called Shark Bay, about 800 kilometers (500 miles) north of Perth. As they explored the bay at low tide, they found a shallow area called Hamelin Pool that was covered by pillars almost a meter or more tall with domed tops (figure 13.3). They were a dead ringer for Cryptozoon and other structures that had been called stromatolites.
Figure 13.3
Modern domed stromatolites growing in Shark Bay, Australia. (Courtesy of Wikimedia Commons)
Once they looked more closely and took samples, they found that the Shark Bay structures were indeed made of millimeter-scale layers of fine sediment, just as were found in fossil stromatolites. And now they could discover what had made these mysterious layered structures. The top surface of each pillar was covered by a sticky mat of blue-green bacteria, or cyanobacteria, growing in the sun. Older books called them “blue-green algae,” but they are not algae, which are true plants with eukaryotic cells with a nucleus and organelles. Cyanobacteria are prokaryotes without a discrete nucleus but with the internal chemistry needed for photosynthesis.
Once these structures were analyzed further, it was apparent how they got their layered structure. The mat of cyanobacterial filaments is sticky, so as fine sediment washes over it and settles, it is bound into a layer of filaments. Then the filaments grow up through the coating of sediment to seek the light again, making a new sticky mat that accumulates even more sediment. This goes on every day, so if the conditions are right, you could have a structure with hundreds of daily layers. When the cyanobacteria die, they leave behind a structure of layered sediment, without the organic material or more plant-like structures that the paleobotanists had long been demanding.
PLANET OF THE SCUM
So why did stromatolites, which had dominated the earth for 3 billion years, seem to vanish about 500 Ma? It turned out that Shark Bay is exceptional in more ways than just having a colony of living stromatolites. It has a very narrow mouth with a sandbar blocking it, which restricts the flow of water in and out of the bay during the tidal cycle. In addition, it is located in a tropical region, so the rate of evaporation is very high. This means that the water in the bay is very salty (7 percent salt, twice the salinity of the ocean). This is too salty for most of the snails and other organisms that would otherwise eat up the films of algae and cyanobacteria that naturally grow on intertidal rocks around the world.
Once the Shark Bay discovery was published in 1961, the tide of opinion turned quickly, and soon most paleobotanists and geologists agreed that stromatolites were indeed true fossil structures. Over the years, they have been found growing in a few more places, and these localities all have one thing in common: the water is inhospitable to any other organisms, especially snails and others that might eat the sticky mats. I’ve walked on them growing in salty lagoons along the Pacific Coast of Baja California. They grow in the salty waters of the west coast of the Persian Gulf, and huge dome-topped pillars like those at Shark Bay also grow in the salty lagoons of Lagoa Salgada (“salty lagoon” in Portuguese) in Brazil. Among the few that grow in normal marine salinity are those found in Exuma Cays in the Bahamas, where the water currents are too strong for even marine snails to hang on.
So why were they the most common visible fossil from Precambrian rocks? Remember, when cyanobacteria first evolved over 3.5 Ga, they were the only form of life on earth. We have their fossils from a locality dated to 3.5 Ga in the Warrawoona Group in Western Australia, and other kinds of bacterial fossils also dated to 3.5 Ga in the Fig Tree Group in South Africa, so they got an early start. Just as this book was finished, there were even reports of possible stromatolites from the Isua Supracrustals in Greenland, which have been dated to 3.8 Ga (see chapter 12). But for more than 3 billion years, the fossil record shows that nothing bigger than single-celled microbes evolved, so these mats of cyanobacteria had no grazers to crop them through 80 percent of life’s history. They ruled the planet, and as my friend Professor J. W. Schopf of the University of California–Los Angeles puts it, Earth was the “Planet of the Scum” (figure 13.4).
Figure 13.4
The world was dominated by simple layered stromatolites for most of its history. (Drawing by Carl Buell)
Not until the Early Cambrian did snails and other organisms evolve that could crop these mats of cyanobacteria and algae that had blanketed the shallow seafloor uncropped for 3 billion years. Once they began to do so, the stromatolites nearly vanished. Meanwhile, the seafloor was no longer covered by a mat of sticky algae and cyanobacteria, so other animals could burrow into the sediment for the first time, and this opened up a whole new range of niches for life to exploit.
This “lack of cropping” explanation is supported not only by the places where they live today (with no snails or other croppers) but also by where they occasionally reappeared in the geologic past. Microbial mats are always ready to spring back and flourish anytime their croppers are suppressed. After three of the earth’s great mass extinctions (the end-Ordovician, the Late Devonian, and the biggest mass extinction of all at the end of the Permian Period), stromatolites returned in abundance during the post–mass extinction “aftermath” times when there were few survivors of the animals that were clobbered by mass extinction. In each case, stromatolites grew like weeds, taking advantage of the wide-open landscape with few opportunistic survivor species and flourishing whenever the creatures that ate them had been wiped out.
Earth was the Planet of the Scum for over 80 percent of life’s history. There were no other visible life forms that could leave fossils, only microbial mats building condos of stone. All the evolution was taking place in microbes, which only leave fossils in the best of circumstances. What held back the development of multicellular life? The very fact that the seafloor was blanketed by microbial mats might have also been a barrier, but once grazing snails evolved, many other animals could find niches as well. A seafloor without a film of slimy bacteria can be exploited and burrowed into by trilobites and many deeper diggers, and we see just that evidence in the burrows of the Early Cambrian.
There are lots of other ideas out there, but most geologists agree that the low levels of atmospheric oxygen prevented multicellular organisms from getting very large. Ironically, the low level of oxygen (discussed in chapter 14) was overcome by the photosynthetic activities of those same cyanobacteria that made stromatolites. It took them almost 3 billion years to do it, but bit by bit they pumped out so much oxygen that it finally overwhelmed all the crustal rocks that absorb it and eventually this gas became abundant in the oceans and atmosphere. Once it did so, it killed off most of the anaerobic bacteria that can only live in low-oxygen settings, triggering an “oxygen holocaust.” Eventually, oxygen levels got high enough that oxygen-breathing multicellular animals, like worms and trilobites, could evolve. In fact, most of the oxygen you are breathing now comes not from trees in the forest but from the huge bloom of photosynthetic algae and bacteria in the oceans. So the next time you see some algal scum on a beach rock, thank it. You would not be here, nor could you breathe, without that scum.
FOR FURTHER READING
Chambers, John, and Jacqueline Mitton. From Dust to Life: The Origin and Evolution of Our Solar System. Princeton, N.J.: Princeton University Press, 2013.
Gargaud, Muriel, Hervé Martin, Purificacíon López-García, Thierry Montmerle, and Robert Pascal. Young Sun, Early Earth, and the Origins of Life: Lessons for Astrobiology. Berlin: Springer, 2013.
Hazen, Robert M. The Story of the Earth: The First 4.5 Billion Years from Stardust to Living Planet. New York: Penguin, 2013.
Knoll, Andrew H. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton, N.J.: Princeton University Press, 2003.
Schopf, J. William. Cradle of Life: The Discovery of Earth’s Earliest Fossils. Princeton, N.J.: Princeton University Press, 1999.
Shaw, George H. Earth’s Early Atmosphere and Oceans, and the Origin of Life. Berlin: Springer, 2015.
Ward, Peter, and Joe Kirschvink. A New History of Life: The Radical New Discoveries About the Origin and Evolution of Life on Earth. New York: Bloomsbury, 2015.
