A New History of Life

by Peter Ward

  But perhaps this is not the whole story. If it could be shown that some new kind of plant life suddenly and radically increased in numbers across the globe, once again the possibility arises that the sudden reduction of carbon dioxide by photosynthesis, rather than chemical weathering, was involved. In fact this may have been the case. Some of the newest of all understandings about the history of life is that land plants, still only single celled but nevertheless potentially extending over vast areas of land, appeared around 750 million years ago. This would have done the trick.

  A SNOWBALL MASS EXTINCTION—AND SNOWBALL-PRODUCED STIMULUS FOR THE ORIGIN OF SO MANY KINDS OF ANIMALS?

  What would have happened to the life on Earth of about 750 to somewhat more than 600 million years ago by the change from a world of ocean and land to one of snow, ice, and bare rock? A simple thought experiment suggests that both the abundance and the diversity of life on Earth found just before these Proterozoic-era snowball Earth events would have diminished. The life then was largely of the single-celled variety, although by this time multicellular plants such as the common kelp and alga (green and red) that adorn so many seashores of our world would have been present too. But much of life was composed either of single-celled protozoa, all eukaryotes, or vast sheets of bacterial slicks and growths both as near-shore stromatolites and other masses of cyanobacteria, and also huge biomasses of single-celled, photosynthetic microbes in the seas. On land, we speculate that single-celled, perhaps even more complex assemblages of photosynthetic organisms, including great sheets of microbes, would have inhabited freshwater, and would perhaps appear liberally on damper land surfaces. Soils as we know them would not have yet existed, but certainly the chemical weathering of rock surfaces, incorporating the dead and rotting bodies of what plants there were would have added organics to the clays and sand of the surface of the land. And then onto both the surface of the sea and that of the land came ice for the former, and for a while ice and certainly cold to the latter.

  The extinction potential in terms of biomass is easy to imagine and fathom. Kilometer-thick ice covering the sea surface would have greatly reduced sunlight. While there is microbial life in ice, and in fact some sun does filter through sea ice, surely the biomass of plant life would have plummeted. The loss of sunlight was one part, but perhaps as significant would have been the loss of important nutrients, the all-important iron, nitrates, and phosphates of our world. As the land surface cooled and in many parts came to be covered in snow and ice, chemical weathering slowed, as did the vigor and abundance of land “plants” of whatever kind there were (this is, of course, hundreds of millions of years before true, complex land plants with stems and leaves). But the land would have produced far less fertilizer getting to the sea. Ocean productivity plummeted, and as it did so, surely mass extinction not only of individuals but also of whole species followed.

  Yet from this scenario comes a model that perhaps answers the question of why there are so many kinds of animals. Although the entire ocean surface would have frozen with pack ice, in fact the world then had far more volcanic activity than it does now. There would have been many hot springs, geysers, and especially active volcanoes blasting heat into oceans, and in so doing producing small warm bodies of open water, free of ice. Surrounded by icebergs and finally frozen sea, these small “aquaria” would have been isolated, and being scattered around the world, subject to many kinds of different environmental conditions. Evolution works best on small, isolated populations. Thousands of these small marine and even freshwater refuges would have been evolutionary incubators, using the principle of “genetic bottlenecks” (where tiny populations, when isolated, can quickly evolve because of their small number of genes). In this way, protozoa, those small single-celled eukaryotes, may have evolved into many different kinds of metazoans—animals. With the release of the snowball conditions, caused by the eventual buildup of greenhouse gases from all of those active volcanoes, there would have been rapid melting of the ice, as well as a rapid release of these thousands of new evolutionary experiments.

  Earth came out of its last snowball 635 million years ago, a place very different from the planet we know today. But forces—both evolutionary and physical—were under way that would make our Late Proterozoic Earth much more Earthlike, in the sense that we know it. The oceans were teeming with life: most were single celled, but largely composed of the complex Protozoa, such as amoeba, paramecia, and the enigmatic half plant half animals such as multicellular Volvox and single-celled Euglena. The shores and sea bottoms were festooned with various kinds of kelp, more formerly the large, multicellular red and green algae so common on Earth, still so common on Earth. The stage was set for the evolution of the first animals. Around 635 million years ago that process began. We think. The newly named Ediacaran period began at the end of the last snowball and ended with the appearance of creatures that were unquestionably animals. It also is the last formal time interval immediately before the start of the Paleozoic Era. The time is named after its most important denizens, then the most complex organisms to have ever evolved. We call them Ediacarans.6

  These iconic fossils of this latest Precambrian time—the last part of the Proterozoic era—reveal a wide variety of peculiar body types unlike anything alive today. Once known only from the Ediacaran Hills of South Australia, there are now numerous places on Earth where these enigmatic fossils are known to be found. But the best remains the low hills north of Adelaide.

  The Ediacaran Hills are part of the largest mountain range in the southern part of Australia, the Flinders Ranges. Like so much of Australia away from the more verdant coast, much of the Flinders Ranges is composed of sand, rocky outcrops, and scattered vegetation adapted to a semiarid environment. Here and there larger trees dot the landscape, including sugar gum, cypress pine, and black oak. Year-round water holes are scarce, but when found, a rich assemblage of the iconic Australian fauna is abundant; red and western gray kangaroos have flourished in the area since the eradication of the carnivorous dingoes, their most dangerous predator at one time. Even the once-endangered yellow-footed rock wallabies can now be seen with regularity. But it is not the kangaroos and the other smaller marsupials that make this place special; it is the ancient fossil fauna.

  Along with the Burgess Shale of Canada, Solnhofen Limestone of Germany, and the Hell Creek Formation of North America, the Ediacaran Hills is arguably one of the four most famous fossil sites in the world. Ranging between 560 and 540 million years in age, these hills contain the record of what most paleontologists agree are the first-known body fossils of animals.

  Late Precambrian imprint of a segmented worm-like animal, called Spriggina, an Ediacarian fossil from South Australia. This is thought to be a primitive annelid and possibly an ancestor of the trilobites.

  The discovery was made when geologist Reginald Sprigg was examining old mines in the Ediacaran Hills region of South Australia. Sprigg was a government geologist for the state of South Australia. He was walking through a desolate area countryside of eroded hills as part of his state’s reassessment of the mineral resources. His job was to decide whether this particular area should have been a focus for new mining activity. However, Sprigg had been an ardent amateur fossil collector during his student days and was able to recognize that the strange markings he encountered by chance within the slabs of coarse sandstones scattered across the rolling Ediacaran Hills had to have been produced by some life. But what kind?

  Sprigg was confronted with what looked like the casts and impressions of jellyfish. But he knew jellyfish are rarely if ever fossilized, with “rarely” a euphemism at best. The strata that Sprigg was looking through were extremely old, and in fact he correctly surmised that the strange fossils he collected had to have been among the oldest direct records of animal life in the world: which was his statement when he first announced the discovery a year after first finding them. Sprigg noted that the fossils appeared to represent animals of varied affinities.7

 
Soon after this first announcement, Sprigg collected more bizarre fossils, this time accompanied by Professor Douglas Mawson of the University of Adelaide and his students. In 1949, Sprigg released a full account of the discovery from a very much larger collection at the same locality, as well as the first detailed description of these curious fossils.8 They all came from the Pound Quartzite, a geological formation that had never had a satisfactory age determination. If Cambrian, they would be nothing of great interest. But if Proterozoic, the strange fossils would indeed be the oldest-known animal remains ever found on Earth. Subsequent work indeed showed that they were older than the classical Cambrian fossils (the trilobites) that were then used to define the Cambrian (a definition that has since been revised).

  When examined in detail these fossils are indeed different from any known living animal, and according to some scientists in the late twentieth century, in fact came from animals with body plans no longer living, with no known descendants, a view first espoused by the great and sadly late Dolf Seilacher.9 But it was their nature as fossils that was perhaps the oddest aspect of their mystery. First of all, organisms without hard parts rarely produce fossils. When they do, it is generally only in very fine-grained rocks, such as mudstones or shale, sedimentary rocks that have been deposited on the bottoms of quiet, stagnant bodies of water. But Sprigg’s clearly skeletonless creatures were preserved in sandstone, rather than in a finer-grained kind of rock.

  To determine whether Sprigg’s fossils indeed came from the closest-living match, jellyfish, sea anemone, and soft colonies of anemone-like creatures called sea pens, experiments and tests were conducted to see whether such fossilization could happen at all. One such test was conducted by Martin Glaessner, Australian geologist and author of The Dawn of Animal Life: A Biohistorical Study.10 He describes a series of experiments using newly captured, very large jellyfish, placed on thin beds of sand. He noted that the jellyfish indeed left impressions within the sand. But there’s still the problem of the sand itself. Sprigg’s fossils should never have been preserved.

  Sand grains are deposited in places with relatively high energy. Sandstones today are found near shore localities, in rivers, in sand dunes, all places where moving water can carry these fairly heavy grains. In such environments the finer mud and clay particles are never deposited; they are just too light to settle and not be picked up again by currents, waves, or wind and carried to some other locality. Yet the Ediacaran fossils are both large and numerous, and are found in such sandstone settings.

  To further test this dilemma, in the summer of 1987 coauthor Peter Ward invited students enrolled in an advanced paleontology class at the University of Washington’s Friday Harbor Marine Labs on San Juan Island in Washington State to attempt to re-create the conditions that led to the formation of the Ediacaran fossils. Several kinds of experiments were conducted. The rich inland sea around the San Juan Islands contains a large diversity and abundance of cnidarians, the phylum seemingly most similar to the apparent Ediacaran body plan. To mimic a 600-million-year-old shallow water bottom, large buckets filled with sand of various screen sizes were then covered with seawater. These experiments were similar to those earlier conducted by Martin Glaessner, but in this case the bodies used were larger, and body types other than jellyfish were studied as well.

  Species in Ediacaran zones.

  Bodies of newly dead sea pens, anemones, and some of the world’s largest jellyfish were placed on the sand. More sand was then put over the top to the bodies, and then the experiments were left for a time, and after some days the top layer of sand was removed. In fact, none of these experiments left any sort of mark in the sand; the cnidarians would rot away, leaving nothing.

  Finally, one student had a quite different idea. A square piece of very fine mesh nylon, from a nylon stocking, was placed over the top of the sandstone, and then a very large jellyfish was gently placed on top of the nylon. More fine sand was then added over the top of it all, with the entire jellyfish. Sea pen and anemone sandwiches were then covered with seawater. After several weeks, when the top layer of sand and the nylon sheet were removed (the soft parts of the animal put there had already rotted away), it was discovered that just underneath the nylon stocking there was a beautiful impression of the animal that was put there, including extremely detailed morphology that matched the underside of the animals used in these experiments.

  Perhaps these experiments mean nothing. But what if the world of that time were covered with something of similar thickness and material properties to nylon stockings, properties that allowed sand grains that would otherwise be picked up by the slightest current to be held in place. We can envision a world where the shallow marine environments became covered with a thin sheet or multiple sheets of microbial life. Although fragile and easily destroyed by storms, these sheets would stabilize sediments and also leave soft-part impressions in the sand below when animals would die, fall onto the bottom, and then be covered by more sand, which would allow for the sanitation of new beds of sand.

  We no longer have such marine environments today, ones that can preserve the outlines and impressions of tissue-rich but skeleton-free organisms. The evolution of mobile animals, which both tore and ate the resource-rich microbial sheets, would destroy these. Just as the stromatolites all disappeared with the evolution of animal herbivores, so too would many of the microbial mats and sheets of perhaps all of the world’s shallow-water environments have been eaten out of existence.

  THE WORLDWIDE EDIACARAN FAUNA

  Today, the “Ediacaran biota” is known from about thirty localities on six continents, and its fauna is classified into seventy different species, all restricted in age to the latest Neoproterozoic11 (although there might be a few of the species that do survive into the earliest Cambrian). The Ediacaran organisms seem to have evolved toward their full diversity in an evolutionary event called the Avalon diversification of 575 million years ago, which would have been as much as 50 million years following the cessation of the last of the Proterozoic snowballs.

  From that time they seem to have thrived, whole communities of them in fact. Then, at about 550 to 540 million years ago, when the first evidence of animal locomotion appears in the fossil record of this age as trace fossils (activity fossils of animals, including locomotion and feeding marks preserved in sediment), the Ediacarans rather suddenly disappeared. A large, diverse group of organisms disappeared just as the first animals rapidly appeared on Earth, in an event known as the Cambrian explosion.12 This disappearance is really the first major mass extinction marked in the fossil record (although certainly not the first mass extinction). While first thought to have been isolated on the Australian continent, it is now clear that the Ediacarans had a worldwide range.

  There has been no end of suggestions about how energy flowed through the Ediacaran’s ecological communities.13 In modern ecosystems, photosynthetic plants make up the base of the food chain, and these are then grazed upon by several levels of consumers, which in turn are the prey of several levels of predators. The biomass of each of these steps is only about 10 percent of the “trophic” level below it. The Ediacarans, to some, showed a very different type of community structure. No jaws have ever been found, and no indication of predation at all, yet the most common assignment of most of the Ediacarans is to the phylum Cnidaria—which are all predators! There have been suggestions that the Ediacarans might have contained microscopic symbiotic algae (dinoflagellates) in large numbers, just as modern corals do. But no proof of this exists. Because of the seeming lack of predators, one of the most memorable descriptions of this long-ago time was that it was the garden of Ediacara, the last time larger life lived in a predator-free world. By 540 million years ago this garden was gone, its serpents being a wide diversity of crawling, swimming, predaceous (and herbivorous) animals.

  Why did it take so long for these first mobile animals to evolve on Earth? External environmental factors such as low atmospheric oxygen may have been at fault, or
very high temperatures of air and sea. What we do know is that in the time between about 635 and 550 million years ago, a whole new category of organisms had evolved, ones with internal water-filled spaces that could act as an internal or hydrostatic skeleton, as well as creatures with muscles, nerves, specialized sensory cells, germ cells, connective tissue cells, and the ability to secrete precipitated skeletal hard parts. Animals or not, the Ediacarans were the first on Earth to evolve skeletons, albeit nonmineralized. Skeletons allow for the attachment of muscles, and muscles allow locomotion. Locomotion then creates other needs that continue to drive the evolution of ever more complexity. Once moving, an animal needs sensory information to find food and mates, as well as to avoid predators. Sensory information needs a brain to process it. All of these developments were intertwined, and were triumphs of the eukaryotic metazoan revolution, which is really what happened near the end of the Proterozoic.

  We can now hypothesize about the appearance of what we might call the “stem metazoan,” the single ancestor of all the complex organisms now on Earth. It would have been small, composed of relatively few cells. Internally there would be no cell walls. It would have an epithelium sealed highly against the external environment, but here would be internal cavities filled with collagen, giving stiffness to the organism. It would also have a “genetic toolbox” allowing it to increase in size and complexity. Large, ecologically specialized, sexually reproducing, multicellular eukaryotes: these were the organisms producing life’s greatest adaptive radiation, resulting in the crawling, writhing, swimming, walking, and sessile animal biodiversity marking today’s Earth. Numerically dominant among today’s animal kingdom are animals, like us, with bilateral symmetry. In the Early Cambrian, however, these were few in number but aligned to take over the Earth.