by Peter Ward
Example of a striated cobble from the first “snowball Earth” event in earth history, the Makganyene glaciation of South Africa. This rock has several sets of parallel striations, in different orientations, carved on all surfaces. Patterns like these are known to form only on cobbles that are dragged along the basement rock at the bottom of actively moving glaciers. The sets of differently aligned groves form each time the rock acquires a different orientation at the bottom of the ice. Most such stones are ground down into glacial dust; this one was lucky enough to survive.
That was the situation until 1987, when detailed analysis of new samples directly from glacial rocks in Australia proved the low-latitude magnetic direction had been there before the sediments turned from mud to rock. This was the first bulletproof result demonstrating an equatorial position for a sea level, widespread glaciation. And if Earth was frozen on the equator, it must have been even colder toward the poles. With this impetus, a change of scientific view took place. Once there was acceptance that perhaps it was possible to have world-covering ice in the deep past, the available information from fossil distributions, rock types, and even the paleomagnetic data made more sense, but it kept putting the major continental masses on the equator. The commonly accepted model of glaciers creeping along the continents (and never covering the oceans) from high latitude till they reached the equator simply did not agree with the data.
As the various possibilities of how the world had produced glacial deposits at the equator were reexamined, it became clear to at least some of the scientists studying this time that the Earth actually had frozen over. Once that great leap of faith was made, the rest fell in line. Floating pack ice would seal off the ocean surface, curtailing photosynthesis, stifling gas exchange with the entombed ocean beneath it, and causing the sea bottoms to go anoxic. Hydrothermal vents at the seafloor would then gradually build up concentrations of iron and manganese in solution, which would supply the metals needed for deposition of the banded iron stones mentioned above. Without access to sunlight, photosynthesis would be restricted to a few hydrothermal areas that would manage to break through the ice, as is done today in Antarctica and Iceland. Photosynthetic life could survive there. In a short seven-paragraph chapter in a 1,400-page book published by a project at UCLA in 1992 (four years after it was written), coauthor Joe Kirschvink marshaled this data for the first time, and gave it a new name: snowball Earth. At the same time, he took an additional step, by hypothesizing that the aftermath of one or more of the snowball Earth episodes of the Proterozoic era might have produced environmental conditions that would have resulted in rapid evolution—what we now accept as the evolutionary drive for the radiation of animal phyla.
So what was wrong with the climate models, all of which gave solutions suggesting that once in this kind of global glaciation, the Earth would never escape from global ice? The problem was that they had not incorporated the increase of carbon dioxide over geological time that would gradually increase the greenhouse effect. Climate scientists, particularly James Walker and Jim Kasting, had noted ten years earlier that CO2 could eventually cause an escape from the ice catastrophe because of a pressure broadening of its infrared absorption spectrum. However, their suggestion was only one paragraph in a long paper and it had never been included in global climate models, simply because no one ever suspected that this has actually happened!
In the two decades that have followed publication of this idea, numerous geologists, geochemists, and climate scientists have conducted intense debates on, and tests of, this hypothesis, expanding the concept and clarifying predictions of models. Paul Hoffman and colleagues at Harvard, for example, contributed an enormous amount of stable isotope data showing that the elevated carbon dioxide concentrations in the atmosphere most likely wound up being converted into the limestone and carbonates that smother the glacial deposits. Geochronologists using high-resolution uranium-lead dates were able to show that both of the major low-latitude glacial intervals in the Neoproterozoic ended synchronously, a clear prediction of the model.
Here, again, we see a major refutation to the principle of uniformitarianism. A snowball Earth would inevitably cause a severe decline in marine organic production because the sea ice would block out sunlight. A succession of snowball glaciations and their ultragreenhouse terminations must have imposed a severe environmental filter on the evolution of life. The pre-Ediacaran fossil record provides few clues, but the diversity of microfossils in the sea known as acritarchs (planktonic organisms of small size, but definitely eukaryotes) waxed and waned dramatically. Many living organisms are known to respond to environmental stress by wholesale reorganization of their genomes. The developmental and evolutionary significance of such genomic changes are hot topics of research in molecular biology. The fact that diverse Ediacaran fossils first appear in the immediate aftermath of the snowball glaciations supports the hypothesis of an ecological “trigger” for their abrupt appearance. However, molecular sequence comparisons of extant organisms imply that the major metazoan clades evolved prior to some or all of the snowball events, but such “molecular clocks” assume uniform rates of genetic change. If the climatic shocks associated with snowball events caused greatly accelerated rates of gene substitution in most ancestral metazoan lineages, then the molecular and fossil evidence may be reconciled.
A frozen ocean, however, is a bad place for surface-dwelling organisms, and thus it was that the great oxygenation event could not have started, ironically, until the signal event that would allow it to melt away. During this snowball Earth, the cyanobacteria survived, probably in local hot springs. Earth was lucky that it was close enough to the sun and had enough volcanic activity releasing greenhouse gases to let it eventually escape the snowball state or else we might be frozen still, and not get liquid oceans until some time in the future when our ever-heating sun finally melts through the ice. If it had been slightly farther from the sun, CO2 could have frozen at the poles as dry ice, robbing Earth of the snowball escape and making it more like Mars. Surface life might have died completely.
The Earth with its new oxygen atmosphere was a bizarre place, at least in terms of what was happening, or not happening, to life. It is clear that aerobic respiration, our biochemistry that allows us to breathe oxygen, could only have evolved after oxygen was present. There had to have been a time gap between the presence of oxygen and the first organisms capable of breathing it. In fact, evolution would have immensely favored any organism that could use oxygen, since no other molecule lets the chemical reactions we call life take place faster, with more precision, and liberate as much energy as those where oxygen is used.
The time gap between the evolution of oxygen release and the presence of organisms in the biosphere that could breathe it is identifiable in the geological record. The cyanobacteria that suddenly found themselves in a world no longer covered in ice would quickly have invaded the new and warm surface waters of every ocean, and because the amount of land area more than 2.2 billion years ago was vastly less than now, and the planetary ocean would have had millions of years to load up on raw nutrients from hydrothermal vents, they would have multiplied to numbers almost incomprehensible, rapidly increasing the amount of oxygen. They would have been floating in the marine ecosystem on shallow subsurface horizons where light could reach, and even on what little land area was present. While these organisms would be madly excreting this molecular oxygen, they would also be rapidly depleting the carbon dioxide that had built up in the air during the snowball Earth event they caused, producing a wealth of hydrocarbons in the ocean environment. For every molecule of O2 released by photosynthesis, one atom of carbon is incorporated into the stuff of life. Today, light hydrocarbons of that sort are eaten by oxygen-breathing organisms and converted back into carbon dioxide. But if organisms had not yet evolved the ability to breathe oxygen, the question arose as to where all of this floating organic material would have gone. There would have been so much of it that there would have been major
changes to the surface of the Earth’s chemistry and its oceans and air.
Oil and oxygen when mixed together in the air form an explosive cocktail; a single spark of lightning would cause a reaction to go without stopping. But oil dispersed in water, as little particulates, can only be degraded by the action of microbes. Without efficient recycling, Earth should have experienced a huge imbalance in the carbon cycle. In particular, a large amount of oil should’ve been produced, and an equal amount of oxygen should’ve been pumped into the atmosphere. At this time, we do have evidence for a massive oxidation event at 2.1 GA; it formed one of the world’s largest deposits of pure hematite (Fe2O3) iron ore—the Sishen mine in South Africa.15 Earth’s atmosphere must have been supercharged with oxygen at that time, to levels not encountered since, and probably impossible to reach without some deviant biosphere driving it there. If planets orbiting other stars went through the same process, the hyperbaric oxygen in their atmospheres would be waving a spectral flag proclaiming, “We are here, and we solved the photosynthesis problem!”
In fact, the record of carbon isotopes for the period of time between 2.2 and 2.0 billion years ago is so wildly out of balance that geochemists have given it its own name, with the jaw-breaking name “Lomagundi-Jatuli excursion,” and it is the biggest and longest such event yet found in the entire history of our planet. Most of the carbon being emitted from volcanoes was being sequestered as organic material, releasing oxygen to the air; today this ratio is only about 20 percent. This is the evidence of an Earth with oxygen but without organisms capable of breathing it: wild swings in the carbon cycle caused by cyanobacteria excreted lots of carbon compounds as waste, but with no organisms using these chemicals as food. In fact, the remnants of this sludge appear to exist in the Russian province of Karelia, as a weird rock type called shungite. Today, most of these oil-like compounds would have been quickly biodegraded by microorganisms that breathe oxygen, like the fate of most of the Deepwater Horizon spill. This is direct evidence that the environment choked on hydrocarbons, rather than recycling them directly. As a result oxygen kept rising in content until it was so abundant that it produced an atmosphere supersaturated in oxygen, existing at pressures much higher than today. Had there been any forests, the first spark from lightning would have caused a global forest fire of heat and scope beyond anything that has ever occurred on Earth in the time of forests.
This weird episode in the history of life ended abruptly when evolution produced the first organisms that could breathe oxygen efficiently. Special copper-based enzymes evolved to do this, but copper deposits themselves require oxygen-rich environments to form. An entirely new kind of intracellular body came into existence, and it exists still, the organelle called the mitochondria, the major source of energy for eukaryotic cells, which are cells that are larger than their prokaryotic (bacterial) ancestors, as well as being cells that contain walled-off interior “rooms” in the giant (compared to all that came before) cells. The mitochondria has its own little piece of DNA, left over from when it was once a free-living bacterium, a microbe that learned to breathe oxygen efficiently. As a result, it has been enslaved for the past 2 billion years.
It is intriguing to note that the best estimate for the age of the last common ancestor of all eukaryotes is about 1.9 billion years, and that may mark the time that eukaryotes finally evolved to restore balance to the global carbon cycle. It would seem that the biosphere required over 200 million years of evolution to come up with an adequate response to the presence of the intrinsically poisonous oxygen.
CHAPTER VI
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The Long Road to Animals: 2.0–1.0 GA
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The time between the great oxygenation event (culminating at ~2.3 GA) and the first appearance of common multicellular life has been called the boring billion. The reason is that (supposedly) virtually nothing happened in terms of major biological change. It is as if the history of life took a snooze. A billion years is a long time for almost nothing to happen. But like so much else, the boring billion has recently been shown to be not so boring. New discoveries are showing us that life was not resting at all. But at the same time, in spite of repeated suggestions to the contrary, there are no animals a billion years in age. Instead, this long interval begins with the first significant oxygen in the atmosphere, and by 2 billion years ago a major revolution in life had occurred—the common occurrence of eukaryotic life, our kind of life, large cells with a nucleus. And while the greatest diversity of these new creatures during this period of time were protozoa, familiar to us as the still-living amoeba, paramecia, euglena, and their cohorts, there appeared some strange larger fossils as well, including one of the most bizarre fossils ever recovered.
The various experts agree that there was probably not enough oxygen between 2.2 and 1.0 billion years ago to support animal life.1 (This is a good time to quickly summarize the difference between animals, (metazoans) and protozoans. All three are eukaryotes—organisms with large cells that contain a nucleus as well as other smaller organelles, such as mitochondria. Animals and “metazoans” are the same thing. All are composed of more than a single cell during their lives except at fertilization. Protozoans can seem animal-like, in that many are capable of movement and relatively complex behavior. But all of them are composed of only a single cell. Nevertheless, they are far larger and far more complex than bacteria.) Yet if that was agreed on, the reason for this was not. Life was capable of oxygenic photosynthesis, but there should have been far more life than all evidence suggests. Animals need a good 10 percent of the atmosphere to have oxygen (we are at 21 percent today) and the “photosynthesizers” were not doing their job. The answer, when it finally came, was once again the element that runs through the history of these pages in an ever-repeating pattern: sulfur, usually in the guise of its most toxic and at the same time life-giving form—hydrogen sulfide, molecule of life and death. In a 2009 paper published in the Proceedings of the National Academy of Sciences,2 Harvard paleobiologist Andy Knoll and his colleagues showed that oxygen levels should have been higher during the boring billion, but were not. Something was holding them back. The long interval devoid of any kind of real intermediary between the single-celled organisms of the 2.3-billion-year-old great oxidation event and the appearance of larger, multicellular creatures of far, far later in time was real.
There were no life forms that we might call complex in this long interval of time (although we hope it is clear from preceding chapters that even the simplest life forms on Earth are unbelievably complex when viewed at the molecular and chemical scale!). And the reason was an overabundance of single-celled sulfur-using bacteria that were competing with the oxygen-releasing forms. Thus it was that two very different life forms competed for resources coveted by all life—space and nutrients. The sulfur-requiring microbes, called green and purple sulfur bacteria, are still alive today, but only in the most toxic of places—shallow-water lakes and some seaways that have no oxygen but are shallow enough so that sunlight can penetrate to the levels of the bacteria, allowing photosynthesis. But the problem is that this kind of photosynthesis does not split water apart, and thus does not produce oxygen as a by-product.
Our new model for the rise of atmospheric oxygen and some of the related events.
Fundamentally, it seems that life was lazy. Splitting water is actually a difficult task, which generates all sorts of nasty, toxic compounds. Using H2S for photosynthesis instead of H2O results in less toxic sulfur compounds, and even many strains of cyanobacteria—if given the choice—will shut down their oxygen-generating machinery and use H2S rather than water.
For most of the boring billion the oceans were stratified, with a thin top layer of oxygenated, clean surface water where single-celled green algae took sunshine and used that energy for cell growth, all the while releasing oxygen. But beneath them, perhaps only ten or twenty feet below, was a totally different layer of seawater, and this layer would have extended all the way to the deepe
st ocean bottom. It would have been purple in color in its uppermost, shallowest regions, stained this color by the untold numbers of the purple sulfur microbes. The water they lived in would have been a fatal poison for most ocean life of our world, as it was filled with toxic hydrogen sulfide all brewed in a near-boiling miasma of liquid brimstone. Even in death they would have helped rob the world of oxygen (unconsciously, of course, although some microbial specialists actually seem convinced that the microbes have always been some kind of sneaky smart). After death, their tiny bodies would have sunk to the bottom, or even stayed in place in water salty or sediment filled enough, and in rotting would have taken even more of the precious few oxygen molecules being produced by the thin layer of oxygen-producing microbes in the surface layer above them. Precious oxygen molecules, destined for the atmosphere and clear oceans, were used up instead in the rotting of a purple demon.
While rare on Earth now, this same stratified system still exists in a few places. One of the most famous is in the Micronesian island of Palau, in the famous “jellyfish” lakes. Here, large freshwater lakes are filled to bursting with enormous, abundant jellyfish, swimming gracefully through aquamarine and well-oxygenated water. Yet some tens of feet below this crystal lens of clean, oxygen-life-filled water rests a second and deeper stratum, which is dark, and to us creatures of light and oxygen, vile to the extreme. It has little or no oxygen, but is saturated with hydrogen sulfide. And it is dark purple in color, stained by untold numbers of the same purple sulfur bacteria that kept the world unsafe and unavailable for anything needing abundant oxygen for what was to them probably not boring at all.
