A New History of Life (13 page)

We apologize for the complex chemistry necessary in the preceding section. But to get this story right requires complexity. As we see now, the world was unalterably changed from this point onward.

In all of Earth history, we have rarely seen ocean stratification (where the ocean has a thin upper layer that is oxygenated, but a much thicker layer underneath that is not) when the Earth’s high latitude poles are glaciated. Cold water sinks at the poles, driving circulation. On top of that, the glaciers themselves are very good at grinding up continental rocks into powder and throwing them back into the oceans, where the tiny particles of rusted iron and phosphorus are two of the same key ingredients of fertilizer we use on our lawns and gardens today. Satellite images of melting icebergs show a plume of photosynthetic activity in their wake, confirming the powerful effect on oceanic productivity that a little ground-up rock can have. And a great debate is raging even today about the effect of an illegal iron-dumping experiment in the Pacific Northwest in 2012, commissioned by Haida Gwaii (formerly
the Queen Charlotte Islands), which was followed only two years later by a massive increase in salmon.

During Archean and Early Proterozoic time there were several major glacial intervals before the great oxidation event, including three minor episodes from ~2.9 to 2.7 billion years ago, and several more between 2.45 and 2.35 GA. A simple calculation suggests that the amount of iron and phosphate dumped into the oceans during any of those glacial advances would have been more than enough for cyanobacteria—if they had evolved by then—to completely overwhelm the anoxic surface environment, and flip the planetary atmosphere and surface ocean into a stable oxygen-rich situation like today; it would have taken less than 1 million years to do so.
¹¹
The fact that it did not happen then is another strong line of reasoning that oxygenic photosynthesis had not yet evolved.

The
youngest
, and firmest, constraint on the presence of copious oxygen in the atmosphere comes from the presence of a vast deposit of the mineral manganese, known as the Kalahari manganese field in South Africa, dated at 2.22 GA, in the same basin where the Agouron drilling project sampled. This deposit is enormous, a blanket fifty meters thick that covers nearly five hundred square kilometers, deposited on a continental shelf. There is no trace of detrital pyrites, uraninite, or weird sulfur isotopes. It could only have formed in an oxygen-rich atmosphere, and thus this gives us the oldest date at which we are sure that the world of cyanobacteria, the ozone shield, and oxygen in both the sea and air existed.

Between this deposit and the underlying manganese overlap interval is another peculiar beast—a glaciation so severe that it marched into the tropics
¹²
and most likely froze the entire ocean surface, producing the first of the snowball Earth episodes.
¹³

This first snowball Earth episode, actually named by coauthor Kirschvink, may have lasted nearly 100 million years.
¹⁴
So what is a snowball Earth? In fact they were first discovered in younger rocks.

We now know that glacial deposits were produced between 717 and 635 million years ago, and can now be found on virtually all the continents. Two geologists working in the first half of the twentieth
century, Brian Harland of the UK and Douglas Mawson of Australia, recognized early on that there was a great infra-Cambrian ice age that seems to have had an unusually large, global extent. Although they recognized clear features of unambiguous glacial origin—like drop stones, tillites, and glacially striated pavements at the bottom of the units—there were several features of these deposits that were puzzling. Many of the clasts were composed of shallow-water limestone, much as if the glaciers had marched out over the carbonate platforms like those in the Bahamas (which today form only in the tropics), ripping up pieces and carrying them away. They were also associated with an unusual occurrence of banded ironstones, similar to those that had disappeared from Earth nearly a billion years earlier, and the glacial sediments were usually covered by layers of limestone (again, a “fingerprint” of low-latitude formation). In a 1964 review article published in
Scientific American
, Harland argued that the glaciers must have reached the equator because some of the deposits would have been at low latitudes no matter where Earth’s rotation axis had wandered. Harland also specifically rejected the idea that the oceans might have frozen over, as it would invoke the “ice catastrophe” from which climate modelers assured him the planet could never have escaped.

Measuring the latitude of continents in the past is a specialty of a branch of geophysics called paleomagnetism, which studies the fossil record of Earth’s magnetic field. Earth’s field is vertical at the poles, but horizontal at the equator. Hence, measuring the angle of the magnetic field at the time a rock formed with respect to the (horizontal) bedding planes provides an estimate of the latitude at the time the rocks formed. Unfortunately, it is necessary to actually prove that the magnetism one measures is as ancient as the rock, and was not acquired during recent weathering or some metamorphic event. (To be meaningful, we must study things that really and truly date to the time that the rocks formed. This is the flaw with the Precambrian biomarker studies noted earlier.)

The possibility of testing this low-latitude glaciation hypothesis attracted many early attempts at paleomagnetic analysis. However, in
1966 a new paradigm for the geological sciences was proposed—that of plate tectonics. If the continents could move relative to each other, it was then possible that all of the infra-Cambrian glacial sediments actually formed at the poles, and plate tectonics could have moved them down to their present position in low latitudes. The idea of low-latitude Precambrian glaciation basically dropped off the geophysical radar screen. It just seemed too far-fetched to the scientists studying the early Earth.

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 CO
₂
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, CO
₂
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 O
₂
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.