"Life itself modified the Earth system. As the system changed, more complex life forms became viable. Eventually diverse multi-cellular organisms fluorished. But not without first being smacked in the face by a few snowballs."
-- T. Lay
The tree of life diverged from some original primitive form, which amongst many possible lineages snuffed out by chance, came to be the common ancestor of all life on Earth, the Cenancestor. The Cenancestor was a single-cell organism exercising some form of chemosynthesis in the Earth environment, possibly deep in the ocean near upwelling magma centers that supplied heat energy and through which nutrients circulated in hydrothermal systems. Over time, the action of selection and adaptation left to diversification of the Cenancestor into two major distinct branches, leading to organisms identified as Eubacteria (bacteria) and Archaea. The primary evidence for this is provided by analysis of nucleotide sequences in RNA, which retains a history of prior life forms precursory to current organisms. Eubacteria and Archaea were all single-cell organisms with no nucleus, using either chemo-synthesis or photo-synthesis to provide energy for their life functions and replicating asexually by mitosis. Archaea occupied some of the most extreme environments on Earth, toxic to other life forms. Some survive in very hot waters, some in very alkaline or methane rich waters, some in very sulfur rich waters. Various forms of energy production adapted to these extreme environments. Bacteria diversified, with photosynthetic forms exploiting light energy in shallow water environments, while others remained isolated in deeper anoxic environments. The Earth's surface was barren; it would be 4 billion years before plants appeared, but the rocks were forming from volcanic eruptions, from rainfall causing erosion that deposited sediments in basins, and from the internal thermal engine of the Earth, constantly shuffling the continents around on the surface as ocean floor was created and destroyed. For billions of years the surface was in a reducing state, where excess hydrogen would bond to elements readily, and metals such as iron would not rust, but would react with sulfur or H. There was a nitrogen dominated atmosphere, with more CO2 than at present and more CH4 (methane) as well. Solar radiation helped to break up some of the methane at high atmospheric altitudes, and some H would escape; helping to lower the reducing state of the atmosphere. All that was needed now was some free oxygen to bring about massive changes; and over 3 billion years, tiny photosynthetic bacteria pumped it out.
Despite the ongoing photosynthesis in blue-green algae and other bacteria since at least 3.5 Bya, oxygen levels remained very low until between 3-2 billion years ago. This is documented by the rocks that survive from those times. In rocks of the early Proterozoic (2.5-0.6 Bya), we find extensive pyrite (iron sulfide) and uranite (Uranium oxide) deposited in river sediments. These minerals require low oxygen levels to form. So, what was happending to the oxygen from photosynthesis. The oldest rocks preserved in continents that were formed in shallow seas show curious layering, and are called Banded Iron Formations (3.5-2Bya). These involve layers of chert and iron oxide. Chert is Silicon Oxide, and is deposited under low oxygen levels in the sea water. When the oxygen level builds up, previously soluble iron reacts to form iron oxide, which then deposits. It appears that the shallow seas underwent cycles of oxygen enrichment (iron oxide formation/sedimentation) and oxygen depletion (iron solution in water and chert formation/sedimentation). Pulses of stromatolite growth and expansion may account for this. It is also possible that some early bacteria or archaea had evolved aerobic respiration. But eventually stromatolite production of oxygen overwhelmed the sinks, and oxygen levels in the ocean, and then in the atmosphere began to build up.
Sub aerial rocks deposited on continents show an onset of iron oxide deposits by 2.3 Bya. These are called red beds, due to the rusty color. This requires that there be oxygen available at the Earth's surface; Fe was no longer reacting with S, but it was rusting! As free oxygen built up in the atmosphere, some of it would form Ozone (O3), and unstable molecule which can persist in the high stratosphere. This began the development of the ozone layer that absorbs ultraviolet radiation from the sun. The surface was beginning to be a big nicer place to visit. You would still die in just a few minutes if transported back 2 billion years to visit, for oxygen levels were a small fraction of today, the surface was barren, with no plants or animals, and radiation was still intense.
Between 2.1 and 1.7 billion years ago cells began to have nuclei, and are called Eukaryotes. A relatively large single-cell organism called Grypania left fossils from this time. The third major form of life had appeared, and we believe it involved a merger of preexisting bacteria and archaea. The notion of a merger is called the Endosymbiotic Hypothesis, and it postulates that a single-cell organism, possibly an Archaean served as a host to various bacteria, for mutual benefit. The mitochondria, plastids, and flagella of Eukaryotes appear to have once been free-living bacteria. Purple bacteria merged into the host cell to produce the energy factories of mitochondria. Photosynthetic cyanobacteria merged into host cells to establish cholorplasts in the first plant cells. Spirochetes bacteria became attached flagella on cells. These organelles were separated from the cytoplasm by complex membranes and they retained their own DNA. For ensuing life forms (including you), the organelles have genetic material that more resemble bacteria than the host cell). The new cells shrouded their genetic material in a nucleus, prompted perhaps by the increasing size of the cells that accommodated diversified components.
This evolution of Eukarya allowed more complex specialization of cells and resulted in larger cells. This, in turn, required new methods for generating, storing, and transporting energy around in the organism. Aerobic respiration, utilizing free oxygen to exploit its favorable chemical reactivity, was adapted by almost all of the larger Eukaryotic cells. Thus, the availability of free oxygen, itself produced as a waste product by the early bacteria, allowed life to seek new ways of tapping chemical energy. Mitochondria can do aerobic repiration at atmospheric oxygen levels 2% of modern values. Cholorplasts can do photosynthesis at 10% of current O2 levels.
Rocks 1.7-1.2 billion years old reveal common fossils larger than 60 microns, with low diversity communities. From 1.2-0.7 billion years there was increasing diversity of unicellular eukarya (along with sustained populations of bacteria and archaea; although many were driven out of the oxygenated environment favored by the aerobic respirators. Near the end of this time we find fossils of acritarchs, which are resting cysts of protozoans (multi-cellular organisms), and the invention of sexual reproduction (meiosis). Sexual reproduction, by which genetic material from gametes of two different parents were merged into the offspring, radically changed the opportunity for evolution to occur. In contrast to the relatively reliable fissioning of individual cells, yielding nearly identical genetic material in the offspring, there was opportunity for much greater mixing, and errors, in the genetic transfer. With increasing complexity of the organisms, the genetic code was lengthening as well, enhancing the opportunity for mutation. Life was now ready to explode into rapidly evolving multi-cellular forms.
But something bad happened to Earth. Throughout the Archean and Proterozoic, the Earth system had evolved by largely gradual processes; progressive build-up of atmosphere and ocean by volcanic emissions, slow evolution of biochemical processes and the gradual transformation from a reducing to an oxygenated atmosphere; gradual development of the the ozone shield. About every 100,000,000 years during this long 3 billion year history, a large asteroid or meteorite would hit the Earth (we find craters in ancient continental rocks that confirm this), possibly causing massive disruption of photosynthesis or local extinction of some life forms; but the dominant single-celled life forms of the time were wide-spread and resilient to annihilation. The course of evolution was not reset as it may have repeatedly been during the Hadean, as the impacts did not vaporize the entire hydrosphere any longer. Luckily, nothing akin to the Mars-size impactor that formed the Moon was still around to hit the Earth.
But, after all this history of fiery birth of the planet, it did something strange. It froze. It appears that several times (2-4) times between 800 Mya and 600 Mya (and possible once or twice billions of years ago), the Earth ICED OVER. The oceans froze. To all latitudes. We call this the SNOWBALL Earth. The evidence for this is found in the rocks from this time. These indicate that there were glaciers at sea level at the equator (glacial deposits, scarred rocks with glacial striations, and rocks dumped by glaciers that float to sea are all found). The rocks deposited in oceans at the time (now uplifted into continental margins), show that no photosynthesis was occurring in the oceans. A thick layer of calcium carbonate overlies the glacial deposits, indicating a massive change in the carbon cycle from the freezing time to an ensuing warming.
How did this happen? The climatic catastrophe was likely a run-away process of ice spreading from the poles. Ice reflects sunlight, so it helps to chill the planet, and computer models show that if you can get ice to spread as far as 30°N and S from the poles, the planet will quickly freeze over. This is further than ice advanced in any of the more recent ice ages, so it needs something unusual to make it happen. This may have been a period of lower solar intensity (stars vary in radiation with time due to evolution of their internal fusion system), or it could result from loss of CO2 in the atmosphere (CO2 absorbs long wavelength energy coming in from the Sun and reradiated by the Earth, helping to keep the planet warm). Too much CO2 gives a hot greenhouse, too little leads to dropping temperatures. We are not sure what would make the CO2 drop, but CO2 is consumed in rock weathering as well as by photosynthesis. Some global sink is needed.
It is clearer how one escapes from the Snowball. Even if the oceans and land freeze over, shutting off photosynthesis in the shallow seas and continental areas, volcanic emissions will continue. With not photosynthesis consuming the CO2, and ice and cold, dry climate inhibiting rock weathering, the CO2 content of the atmosphere will rebuild. Greenhouse warming will heat up the planet, melting back the glaciers, and allowing a resumption of photosynthesis. The retreat of ice allows rapid erosion (rock weathering to scrub CO2 out of the air, depositing it in the calcium carbonate layer overlying the glacial deposists. While massive life loss would have occurred, life could survive in ice cracks, near volcanic vents and hot springs, and for anoxic forms, deep in any free water or soil under the frozen ocean and land surface. But nothing big could have made it through; around 700 Mya, the Earth had only single-celled, albeit sexy (well, sort of) critters on it.
Around 700 million years ago there was an abrupt great diversification of more and more complex organisms, involving many cells that now differentiated to achieve separate functions, with oxygen transported through the system. These are the METAZOANS. Multicellular organisms are bigger and longer lasting than signle cells. The genetic material embedded a huge number of on/off switches for progressive development of the organisms, and for specialization of structures from a root cell. Most were soft-bodied creatures, but there were now plants, fungae and beginning around 544 million years ago, animals.
Many of the life forms that have existed from this point on are totally unknown to us; they leave no fossils. Soft body forms are particularly poorly known, but some were preserved in special geological rock formations such as the Burgess Shale. Some were so weird, they were called Hallucigenia by the paleontologists that discovered them. But many forms of plants and animals do leave portions of their bodies in the form of fossils. This allows us to track the exploding diversity of life forms that began 544 mya. We must use radioactive dating to assign times to the early life forms of the Archean and Proterozoic times, because the fossils have little diversity. But with great diversification of life and attendant extinction of species, distinct floras and faunas that succeed one another provide us with a detailed history of life (at least a significant sampling of it) from 544 million years on. We call the time since then, the Phanerozoic, and the first block of time is called the Cambrian. The explosions of life that commenced in the Cambrian led to the first lifefoms with exoskeletons, such as trilobites, and within the last 544 million years the vertebrates and land plants appeared.
The last 700,000,000 years have been particularly interesting for life, after several billion years of boring bacteria, algae, and single-cell organisms living in hot springs. We will see in the next lecture that the very changes in lifeforms have defined the geological time scale, with major boundaries in the scale associated with massive extinctions.
![[return icon]](../dinoheadt.jpg)
Return to the Earth Sciences
80A Lectures Page