by Peter Ward
Our revised model for oxygen in the air and sea.
The purple sulfur bacteria and their world needs were finally sent to dank, poisonous back rooms of our world. But they were always there, always ready to take back the world they lost when oxygen finally broke through to higher levels, some 600 million years ago. They can be thought of as the evil empire. And in the Devonian, Permian, Triassic, Jurassic, and middle Cretaceous, this empire struck back, as we will see in subsequent chapters.
Eventually, the balance of sulfur photosynthesizers to oxygen producers changed in oxygen’s favor, possibly triggered by a gradual increase in the area of subaerially exposed continents. Iron eroding from the continents and washing down into the ocean would react rapidly with the sulfur, precipitating it into a heavy, sinking solid mass of pyrite, keeping it out of the system. This loss would have starved the sulfur bacteria of the one element they could not do without. In addition, continental weathering and erosion generates clay minerals, which bind strongly to organic molecules and bury them in the sediments. If an atom of organic carbon is buried before something can eat it, the molecule of oxygen that was produced when it formed hangs around in the environment, raising the oxygen level and destroying H2S. Prompted by two snowball Earth events, each of which seemed to jack up oxygen levels by the postsnowball algal bloom, the environment reached a tipping point of some kind. After the last event, 635 million years ago, the first traces of big animals appeared. It did not take so long to evolve them after all, once hell on Earth was banished.
THE BIZARRE, FIRST MULTICELLULAR CREATURES
Most life during the now not-so-boring billion was composed of the long-running champions of life on Earth, the longest-running show of all—stromatolites. Microbes still held sway, just as they had from their first appearance on Earth. But appearing about 2.2 billion years ago a strange new kind of life form appeared. It looks like a thin black spiral, but is certainly not microscopic. Its name is Grypania, and its appearance demonstrates that life had made an important advance: the ability to live as “colonies” of cells, held together and bound by membranes. These were the first multicellular organisms.
Grypania has long been known. But in 2010 a strange series of fossils from Gabon, Africa, changed our view of things.3 While Grypania might be a colony of prokaryotes (in this case probably bacteria), the new fossils, still unnamed, look too large and too complicated. Whatever they were, we know what they were not. They certainly were not the first animals.
The first true animals are much younger than Grypania and its ilk. Animals are less than a billion years old, and while the exact age of the first animal keeps getting put back into older-aged rocks, based on ever more sophisticated means of detecting their presence, there is still no known fossil evidence of animals much older than that last snowball event. But in a way this is an argument over rather small chunks of time, compared to the vast interval that life has been on this planet. There are, of course, many types of multicellular organisms, including a considerable diversity of prokaryotic forms, and there is no doubt that the evolutionary invention of life with more than a single cell goes back to more than 2 billion years ago. But in most cases these multicellular prokaryotes are composed of only two cell types, and none would be mistaken for an animal.
Cellular slime molds are multicellular, as are some cyanobacteria and one group of magnetotactic bacteria. In a way, however, these are evolutionary dead ends (unless, of course, you are a slime mold; however, this group ultimately gave rise to little else but slime molds). They have existed on Earth for more than several billion years, and are highly conservative in an evolutionary sense. More complex are the multicellular plants that appeared more than a billion years ago, species probably looking very much like the green and red algae found on any seashore, from the intertidal zone down to the levels that light can penetrate. Animals, however, are younger yet.
The size of organisms seems to show some relationship to the appearance of oxygen in the atmosphere. Oxygen has allowed larger size than times prior to oxygen, and biological adaptations increasing the rate and/or volume of oxygen acquisition have often lead to gigantism.4 The best example of this will be described in a later chapter, showing how the gigantism in dinosaurs was caused by a new kind of highly efficient lung and respiratory system design.
Fossils of true animals first appear in abundance about 600 million years ago. About this time the rock record shows the first evidence of “trace fossils,” the trackways or feeding records of ancient animals preserved not as body fossils in sediment, but as activity fossils—the record of ancient behavior. By that time oxygen levels were approaching (but not yet reaching) modern levels. Not only free oxygen but ozone levels had also reached relatively high concentrations, and thus much of the hard ultraviolet and other radiation reaching the Earth’s surface in earlier times was muted.
Geobiologist Andy Knoll of Harvard University on Neoproterozoic rocks exposed in East Greenland on an anomalously sunny day. (Copyright Andy Knoll, used with permission)
THE CURIOUS ORGANISMS KNOWN AS ACRITARCHS
In any discussion of Precambrian life, acritarchs actually make up a fair bit of the conversation. They appeared early on Earth: some of the oldest seem to appear around 3.2 billion years ago, and they then continue all the way into the time of animals. Yet the fact that they are a “garbage can” taxon, meaning that any number of not only different species but even different kingdoms and domains of organisms get placed in this catchall name, is just one more indication of how poorly we know the history of life before fossils became common in the time of animals and higher plants.
While being among the oldest-known multicellular fossils, first appearing at the almost unfathomable time period of 2 billion years ago, they remained relatively rare. But halfway through the Proterozoic era, or about a billion years ago, they started to increase in diversity, size, abundance, and morphological complexity in shape. The increase in complexity was generally marked by an increase in the number of spines extending from their small, spherical bodies. From 1 billion to 850 million years ago they remained common, and then the Cryogenian period began, with the enormous global changes that gave this time interval its name, the onset of the Greek word “cryo,” all right: a great freeze. The result of the Proterozoic snowball Earth episodes was that there must have been a great mass extinction in the oceans, and perhaps on land as well. Their populations crashed during the snowball Earth episodes—when all or very nearly all of the Earth’s surface was covered by ice or snow—but they proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic.
It is a given that any young aspiring paleontologist will gravitate to dinosaurs above all fossils. Since professional paleontologists always start as fossil-mad kids, in fact the supposedly less exciting fossil groups attract far less attention, even among the eventual professionals. Few indeed are the young scientists wanting to study tiny microfossils. And yet some of the most important of all scientific questions can be answered with them. So it is with larger questions about the history of life, as the acritarchs and other microfossils of a billion years ago are rich in question-answering information, and only recently have provided whole new insights into what was, in fact, a hugely important time period in the history of life, beginning at a billion years ago.
From 2 to 1 billion years ago, the microfossils of Earth were simple and long ranging through the rock record. They must have been formed by both prokaryotic and small (compared to later) eukaryotes of the single-celled variety, such as the still-living protozoans. But about a billion years ago a strange thing happened. The formerly unornamented microfossils began to acquire ornamentation.
The increased spinosity of acritarchs, starting about 1 billion years ago but then continuing through the Cambrian period, could have several causes. First, spines on a small sphere would increase surface area to volume relationships, and thus slow the rate of settling of these tiny spheres in the ocean. Many pl
anktonic species extant today use this method of staying high in the water column rather than sinking onto a deep bottom and sure burial under the constant snowfall of sediments that typifies most ocean bottoms. But a second use of spines is defense against predators. Perhaps the oceans of a billion years ago began to harbor an ever-greater rogues’ gallery of carnivores (or for acritarchs, these might technically be considered herbivores). In any event, eaten is eaten, no matter what one calls the eater. Yet the new work of Knoll and his group now show that spiny microfossils became ever more diverse and abundant soon after the end of the last snowball Earth, some 635 million years ago, but then utterly disappeared about 560 million years ago, a time when the evolution of animals was well under way. In the next chapter we will see how an understanding of what we might call the Ediacaran revolution is importantly fleshed out by the record of spiny microfossils, as well as by their disappearance. We will return to the story of these spiny microfossils in the next chapter.
Changes in the morphology of acritarchs, enigmatic microfossils that were composed of a number of distinct kinds of small, marine, floating organisms. Notice the change from Proterozoic (A), which are smooth, to the very spiky forms of the late Neoproterozoic (B) and Cambrian (C).
THE END OF THE BORING BILLION
Here is a view of a shallow sea bottom, some 1 billion years ago: Kelp-like plants and green algae wave in the currents, as do shimmering mats of rainbow-hued microbial life, multicolored sheaths of the softest chiffon covering all of the sunlit portions of the bottom.5 Stromatolites peak out from the bottom sheaths, large to small domes and hummocks punching upward out of the microbial sheaths. The water is thick with life, single celled to multicellular. There is nary an animal anywhere on the planet. But a genetic and atmospheric clock is ticking down toward catastrophe and icy crèche.
In the oceans a revolution was brewing a billion years ago, while on land there may already have been a vast biomass of life: the ever-resourceful microbes, invading first ponds and swamps, but ultimately covering wetlands, bogs, and anywhere that was exposed to sun, had a least a modicum of water, and might get windblown dust with enough phosphates and nitrates to allow these tiny, single-celled plantlike microbes to grow their land-covering tarps of green snot. Life, colonizing the land in exuberance. And in so doing ultimately nearly extinguishing itself from the Earth.
CHAPTER VII
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The Cryogenian and the Evolution of Animals: 850–635 MA
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The Australian city of Adelaide is a well-kept secret. Isolated on this island continent from the rest of the world, and even isolated from the rest of Australia, this coastal city has evolved its own culture artistically and scientifically. The latter has been importantly influenced by an enormous paleontological discovery, made right after the end of World War II—the discovery in the arid hills inland from Adelaide of the first acknowledged larger animal fossils, the Ediacarans. Adelaide pays homage to this fossil record in many ways, including the naming of buildings and institutes for two of the scientific giants who brought clarity to the time period of a billion to 600 million years ago: Douglas Mawson, a hardy Australian who survived harrowing Antarctic expeditions and the killing fields of World War I France, a man who also went on to discover proof of late Precambrian glaciations in Australia, a concept highly doubted at the time, and Reg Sprigg, who discovered the fossils,1 as we will recount below, followed, like Mawson, by another geology professor at the University of Adelaide, where coauthor Ward now lives and works—Martin Glaessner.2 But new generations of workers have kept this tradition of the study of the origin of animals alive, and one of the most important is Jim Gehling of the South Australian Museum, which sits next door to the University of Adelaide. Gehling has overseen a new exhibit of Ediacaran fossils in a newly refurbished, large, and modern room of the museum, and there, unlike so many new museums where actual fossils are kept away from the public, substituting plaster casts or other reproductions instead, the Ediacaran exhibit that Jim Gehling3 oversaw has real fossils, real Ediacarans on display. The surprise is how large and complex they are. But another surprise is how they are interpreted. Until recently the party line has long been that these were sedentary, strange, and mainly flat creatures, like stuffed pillows sitting on the seafloor (and some as large as a large, if flat, pillow). But overhead, on television screens, the animated reconstructions are anything but sedentary. Some even swim; others move robustly. Herein lies the controversy. This view is new. But is it correct?
The time interval of this chapter is the long period beginning about a billion years ago and ending with the start of the Cambrian period around 540 million years ago (MA). In that interval, far more than great changes in life’s history took place. Just as it had in the period of around 2.5 to 2.4 billion years ago, at around 717 million years ago the Earth cooled. It cooled so much that—as it had near the end of the Archean era—the oceans began to freeze, starting at high latitudes, but continuing toward lower and lower latitudes until the entire ocean from pole to equator was ice-covered. Once again the Earth had become a snowball. The first time that singular event caused a great revolution in the history of life by leading to an oxygen-rich atmosphere. This second time, the Proterozoic snowball Earth also produced momentous if very different effects. This time the snowballs led to animals—but not without danger to all life on Earth. Once again life was in the balance. The overriding question is whether the snowball Earth episodes of this time interval were the key reasons for the sudden rise of animals, a case we will make.
LIFE AND SNOWBALL EARTH EVENTS
As we saw in an earlier chapter, the first snowball Earth episode (beginning at about 2.35 billion years ago) seems to have been caused by life: the explosive rise of cyanobacteria caused a reduction in the greenhouse effect of the atmosphere’s methane and carbon dioxide content. The start of this second and final series of snowball Earth events of Earth’s long history to date occurs within the Cryogenian time period described in chapter 1. Due to recent work on calibrating the Cryogenian, we know now that there were most likely two major events beginning 717 million years ago and ending 635 million years ago. The start of this second and final series of snowball Earth events is essentially in the middle of what is now formally defined as the Cryogenian period in the geological time scale (it begins prior to a pair of sharp isotope shifts slightly older than 800 million years ago, which are the result of a true polar wander oscillation).
Both of the differing snowball Earth episodes (each made up of ocean freezing and then thawing events) caused a severe decline in marine organic production, because the sea ice would block out sunlight. Thus, the amount of life on Earth, as measured by its overall mass (known as biomass), shrunk to tiny values compared to both before and after the events themselves. The succession of snowball glaciations and their ultragreenhouse terminations during both the periods from 2.35–2.22 billion years ago and from 717–635 million years ago must have imposed a severe environmental filter on the evolution of life. The fossil record provides few clues, but the acritarchs first described in the last chapter (planktonic organisms of small size) waxed and waned in both diversity and abundance.
Many living organisms are known to respond to environmental stress by wholesale reorganization of their genomes, and any snowball Earth event would have been stressful, to say the least. The developmental and evolutionary significance of such genomic changes are hot topics of research in molecular biology. The fact that diverse fossils of more complicated organisms than were there before the onset appear in the immediate aftermath of the snowball glaciations supports the notion that the snowball events created some sort of an ecological trigger for vast changes in the complexity of life and its diversity.
Diagram showing the rate of temperature increase and decrease over time in the Snowball Earth episodes.
One of the most profound of all questions relating to the snowball Earth events relates to their cause. Earlier we noted th
at the first snowball Earth episode might have been triggered by life itself: the invention of oxygenic photosynthesis, which would have caused a rapid depletion of greenhouse gases. But there may have been a quite different reason for the onset of the second episodes, occurring well more than a billion years after the first. The second snowballs may have been triggered by the movement and tectonic activity of continents of the time.4
The so-called Neoproterozoic snowball events, the most recent of the two grand snowball Earth episodes, occurred around 40 million years after the great continental amalgamation—called the supercontinent Rodinia (an amalgamation of every continent into one continuous landmass)—began to disintegrate. Supercontinents tend to have arid climates because most of their land area is far from the ocean. Conversely, when continents and especially supercontinents separate, maritime climates displace formerly arid regions, creating the potential for increased chemical weathering. Chemical weathering of silicate rock minerals causes a rapid reduction of carbon dioxide levels in the atmosphere. As CO2 drops, so too does temperature. This second time it may not have been life so much as inorganic chemical reactions. Interestingly enough, the onset of the second snowball event (called the Sturtian after exposures in Australia) coincides rather precisely with the eruption of a massive volcanic province in Canada, at 716.5 million years ago.5 Although some CO2 is emitted from eruption of these large igneous provinces, when they erupt on land the drawdown of gases far exceeds the volcanic input, bringing the system closer to a planet getting so white that most sunlight is reflected back into space. And that produces ever more cold.
