The Emergence of Complex Life - University of Michigan



 

 

|The Emergence of Complex Life |

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|What is life?  |

|It is the flash of a firefly in the night.  |

|It is the breath of a buffalo in the wintertime. |

|It is the little shadow which runs across the grass and loses itself in the sunset.  |

|Crowfoot, Blackfoot warrior and orator, 1890 |

|It is an error to imagine that evolution signifies a constant tendency to increased perfection. That process undoubtedly involves a constant remodeling of the |

|organism in adaptation to new conditions; but it depends on the nature of those conditions whether the direction of the modifications effected shall be upward |

|or downward.  |

|- Thomas Henry Huxley, English Biologist/Evolutionist  |

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|[pic]Format for printing |

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|We wish to learn: |

|What evolutionary advances have taken place at the level of the cell?  |

|What are the major events in the history of life?  |

|What causes extinctions, and how are extinctions related to opportunities for new evolutionary advances?  |

|Are rates of extinction and rates of evolution uniform, or variable? |

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|[pic] |

|The Beginning |

|How life emerged from non-life is an extremely challenging question. The experiments of Oparin, Miller and others now lend weight to the hypothesis that energy |

|in the form of ultraviolet light from the sun, or lightning discharges, could have created complex organic molecules. Over the immensity of time, cell-like |

|aggregates of these molecules, called coacervates, somehow gave rise to the first primitive cells. Major, additional steps are needed - the origin of |

|photosynthesis and respiration, and the ability to self-replicate. We know little about these.  |

|Geological evidence suggests that the first cells arose at least 3.5 billion years ago.  Fossil remains of  2-billion-year old stromatolites - large structures |

|formed by blue-green algae - demonstrate that much biological activity was taking place then, and probably much earlier.  Similar structures can be seen today |

|along the coast of Australia. Geological evidence also tells us that photosynthesis appeared on the scene roughly 2.5 billion years ago. Initially this oxygen |

|was taken up by easily oxidized rocks, producing "banded rock" and "red bed" formations. About 1 billion years ago, oxygen began to accumulate in the |

|atmosphere. This had two important consequences. First, it set the stage for the advent of aerobic (oxygen-based) respiration. Second, as ultraviolet light |

|split oxygen molecules, ozone was formed, resulting in the ozone layer that now serves as a shield against UV light.  |

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|The Eukaryotic Cell |

|The eukaryotic cell is more structurally organized than the prokaryotic cell of bacteria and blue-green algae. The prokaryotic cell has no internal membranes |

|and little internal organization. In contrast, the eukaryotic cell contains a nucleus and other sub-cellular organelles. The nucleus is surrounded by a |

|membrane, and contains the genetic information in chromosomes and related organelles. The eukaryotic cell shows other evidence of sub-cellular organization for |

|more efficient function. Various energy-releasing enzymes are organized within mitochondria. Many more differences exist than we can consider here.  |

|For our purposes it is important to appreciate that the origin of the eukaryotic cell some 2 billion years ago was an important evolutionary step. The bacteria |

|are prokaryotes. All other life forms -- protozoa, fungi, plants and animals -- are eukaryotes. It is now thought that at least two organelles found only in |

|eukaryotes -- mitochondria (the location of energy transformations) and chloroplasts (the location of photosynthesis) originated as prokaryotic cells that took |

|up residence within "hospitable" eukaryote precursors. This endosymbiotic hypothesis may explain the evolution of more complex cell structures from simpler cell|

|precursors. Later, the evolution of multicellularity was a further significant advance toward higher life.  |

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| How did Life Arise ? |

|The atmosphere of primitive earth, created by volcanic out-gassing, lacked free oxygen.  It is believed to have contained a mixture of H20, CO, CO2, N2, H2S, |

|CH4, NH3 and possibly H2.  Energy in the form of ultraviolet light or lightening discharges may have been responsible for the creation of some of these |

|compounds.  In the 1920s the Russian scientist Oparin put forth a hypothesis for the origin of amino acids, the building blocks of proteins.  He suggested that |

|energy from lightening might have formed complex organic molecules, which somehow clumped together, taking on the characteristics of primitive cells.  |

|Coacervates, amoeba-like objects that can contain and release compounds, divide, and yet are purely physical in origin, provide a clue to how cell-like |

|properties might have evolved. |

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|In the 1950s, Miller and Urey successfully tested Oparin's hypothesis.  Using a simulated "primitive atmosphere" of methane, ammonia, and hydrogen, and an |

|electric spark, they observed the formation of amino acids in their apparatus.  Further experiments have substituted CO2 for CH4 and NH3, and ultraviolet light |

|for the electric spark.   |

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|These experiments suggest how life may have evolved from non-life.  The endosymbiotic hypothesis suggests a mechanism for the evolution of cell complexity.  It |

|is important to appreciate that an amazing amount of evolution precedes the point in time, roughly 600 million years ago, when the fossil record that we |

|recognize from our visits to museums begins to chronicle the evolution of higher life forms. |

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|Major Events in the History of Life |

|Earth history is divided into four eons; the most recent eon, the Phanerozoic, is divided further into eras and periods. The fossil record, and story of the |

|diversification of life as we know it, is largely the story of the Phanerozoic, which begins "only" 600 million years ago (mya). But this is a story based on |

|fragments -- for the most part, only organisms with hard body parts, and that happened to be buried in just the right way for fossil formation, and then have |

|been discovered, enter into this story. Burial with fine sediments, and the absence of oxygen so that decomposition is minimized, are important factors that |

|favor fossil formation.  |

|The Precambrian world was relatively rich in life, but unfortunately we have an extremely poor fossil record from that ancient time. However, we know that |

|protozoans, fungi, and animals had evolved - only higher plants and vertebrates had yet to appear. Many invertebrate phyla were already represented, and all the|

|kingdoms of life existed.  Steven Gould, in The Burgess Shale, paints an exciting picture of the diversity of life at the dawn of the Cambrian, and of the |

|"might-have-been's" that never advanced further, due either to chance or inferior design. This great diversification roughly 600 million years ago is the "big |

|bang" of animal evolution.  |

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|The four eons of earth history. Ga = billion years ago, Ma = million years ago. After Purves et al. |

|EON |

|ONSET |

|MAJOR EVENTS |

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|Hadean |

|4.5 Ga |

|formation of earth and continents, chemical evolution |

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|Archean |

|3.8 Ga |

|origin of life, procaryotes flourish |

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|Proterozoic |

|2.5 Ga |

|eukaryotes evolve, development of oxygenated atmosphere, some animal phyla appear |

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|Phanerozoic |

|540 Ma |

|most animal phyla present, diverse algae; explosive evolution of higher life forms |

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|The eras and periods of the Phanerozoic Eon, after Purves et al. Ma = million years ago.  |

|ME = period ended with a mass extinction. |

|ERA |

|PERIOD |

|ONSET |

|MAJOR EVENTS |

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|Paleozoic |

|Cambrian |

|540 Ma |

|most animal phyla present, diverse algae |

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|Ordovician |

|490 Ma |

|first jawless fishes, animal diversification |

|ME |

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|Silurian |

|445 Ma |

|first bony fishes, colonization of land |

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|Devonian |

|420 Ma |

|first insects and amphibians, fish diversify |

|ME |

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|Carboniferous |

|355 Ma |

|extensive forests, first reptiles, insects radiate |

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|Permian |

|290 Ma |

|continents aggregate into Pangaea, reptiles radiate, insects are diverse |

|ME |

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|Mesozoic |

|Triassic |

|250 Ma |

|continents begin to drift, early dinosaurs, first mammals, marine inverts. diversify |

|ME |

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|Jurassic |

|200 Ma |

|continents drifting, first birds, diverse dinosaurs |

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|Cretaceous |

|145 Ma |

|most continents widely separated, flowering plants and mammals diversity, dinosaurs continue diversification |

|ME |

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|Cenozoic |

|Tertiary |

|65 Ma |

|continents nearing present locations; radiation of mammals, birds, flowering plants, pollinating insects |

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|Quaternary |

|2 Ma |

|repeated glaciations, humans evolve, extinctions of large mammals |

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|Unfortunately, we do not have time to undertake a detailed examination of this 600 million years of evolutionary diversification and extinction. For our |

|purposes it is helpful to make a few major generalizations:  |

|The history of life involves enormous change. Major life forms have appeared, flourished, and died out. Reptiles ruled the earth for nearly 200 million years. |

|Yet, like most species and many life forms (families, orders, even phyla), the dinosaurs are gone, replaced by life forms that either were biologically |

|superior, or just luckier. At some points in earth history many species went extinct in a short time. These are called mass extinctions, a topic we will revisit|

|shortly.  |

|Over time, life has become more diverse and more complex (although it can be argued that complexity lies in the eye of the beholder). The increase in the number|

|of families of marine vertebrates and invertebrates throughout the Phanerozoic Eon illustrates this clearly (see figure below). The number of families of marine|

|organisms has increased slowly over geological time. Occasional mass extinction events are shown by lightning flashes.  |

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|Image from Wilson, "The Diversity of Life."  [pic] |

|Extinction is commonplace. The vast majority of taxa have gone extinct. On average, a species lasts about 2 - 10 million years, and on average, 1-2 species go |

|extinct per year.  |

|The Earth's geological and biological histories are intertwined. Plate tectonics (the movement of plates within the earth's crust, which carry continents with |

|them) played a major role in isolating taxa from one another, promoting geographic speciation. Volcanism and meteorite impacts apparently contributed to mass |

|extinctions. Climate change has been a constant of earth history, and also is a causal factor in both speciation and extinction.  |

|Mass Extinctions |

|A mass extinction is defined as a relatively brief period in which more species become extinct than at other times. Five main mass extinctions are recognized |

|(Table 2), but a number of additional "peaks" in the extinction rate also are candidates. The following figure portrays the five main mass extinctions, and a |

|sixth, recent extinction of large mammals and birds at the end of the Pleistocene (50,000 to 10,000 years ago, which is attributed by many to human hunting).  |

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|[pic] |

|The K-T extinction marking the end of the Cretaceous and beginning of the Tertiary, some 66 mya, is the best known event to most people. The mass extinction at |

|the end of the Permian is the single largest mass extinction. It is estimated that up to 95% of terrestrial and marine species became extinct during this |

|event.  |

|The K-T extinction has attracted much interest not only because it marks the end of the age of reptiles and the radiation of birds and mammals, but because some|

|remarkable scientific detective work suggests that the cause was the collision of a large meteorite with earth. A thin but abnormally rich band of iridium - a |

|metal common in meteorites but rare in the earth's crust - marks the boundary between Cretaceous and Tertiary rocks. The theory was put forward that a meteorite|

|as large as 10 km in diameter collided with earth at a speed of 72,000 km/hr. This thin layer of iridium found around the world is thought to be the signature |

|of a colossal impact that likely produced an immense dust cloud that cooled the earth, greatly reduced photosynthesis, and created acid rains for a period of |

|years. Fires, tidal waves and volcanic eruptions might have resulted as well. The subsequent discovery off the coast of the Yucatan Peninsula, Mexico, of a |

|crater 180 km in diameter, makes this theory all the more compelling.  |

|Other mass extinctions have not been associated with such a specific and short-term catastrophic event. Instead, climate cooling seems to be the best current |

|explanation for other, more ancient mass extinctions. For example, the Permian extinction coincides with the coalescing of the continents into the |

|super-continent Pangaea. The interior of Pangaea, far from the moderating influence of the oceans, would have experienced harsh, continental climates and |

|massive glaciation.  |

|The Pleistocene extinction also is of great interest. Between 100,000 and 10,000 years ago, depending on location, a large fraction of the world's large mammals|

|went extinct. The loss of the mammalian mega-fauna in North America is particularly spectacular, rapid, and recent. Over at most a few thousand years, |

|coinciding with the retreat of the last (Wisconsin) glaciation, a rich diversity of large mammals went extinct. This event also coincided at least approximately|

|with the arrival of humans in North America. Crossing the Bering Land Bridge from Eurasia, the first humans on this continent spread southward, eventually |

|colonizing the South America as well.  In a brief flash of earth history, the mammalian mega-fauna of North America changed dramatically, and it is tempting to |

|ascribe this to human hunters.  Human hunting may have caused mega-faunal extinctions in other regions of the world as well. The evidence is still too thin to |

|say: in particular, the timing of human arrival in North America is still uncertain. Possibly climate change and over-hunting acted in combination, and loss of |

|certain (keystone) species set off a chain of events in which further losses took place. On the other hand, glaciers advanced and retreated many times during |

|the Pleistocene - why did the great extinctions occur only with the last period of warming, unless humans played a role?  |

|Every extinction carries within it an opportunity that may work to the advantage of a new species or body plan. Indeed, many extinctions are simply the gradual |

|evolutionary change in which descendent species replace their ancestors because they are better adapted to then-prevailing conditions (see natural selection). A|

|mass extinction is an opportunity for adaptive radiation. Perhaps the most dramatic example is the rise of the mammals. Our ancestors shared the earth with |

|dinosaurs for tens of millions of years. Ancestral mammals were small, undifferentiated scavengers. After the demise of the dinosaurs, within another ten |

|million years all of the major orders of mammals (and of birds as well) had differentiated.  |

|Many observers believe we are now entering a modern period of mass extinction. This is a topic which we will revisit during the second part of this course.  |

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|Punctuated Equilibrium |

|Evolutionary change is often portrayed as gradual and steady. Darwin overcame his detractors partly by arguing that very small changes, over the immensity of |

|time, would gradually result in major new life forms. Until recently, most evolutionary biologists accepted that evolution might proceed at different rates in |

|different lineages, or the rate might vary from time to time, but these were seen as minor "hiccups" in an overall gradual process. An alternative idea, known |

|as punctuated equilibrium, has emerged over the past 10 or more years. This idea argues that species undergo long periods of stasis, interrupted by occasional |

|episodes of rapid evolution. These bursts of rapid evolution are thought to be triggered by changes in the physical or biological environment ?perhaps a period |

|of drought, or the appearance of a new, more challenging predator.  |

|This argument remains unsettled, in part because imperfections in the fossil record can give the appearance of alternating periods of stasis and rapid change. |

|Perhaps a more complete fossil record would support one or the other theory; perhaps evidence exists for both theories, but is insufficient to help us decide |

|which is more likely to be correct, and under what circumstances. Regardless, the notion of a rate of evolution can be quantified, and that it might fluctuate, |

|are important ideas.  |

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|The Causes of Extinction |

|The vast majority of species that have ever lived are extinct. As dramatic as are mass extinctions, many species go extinct seemingly independent of one |

|another. This background rate of extinction is roughly 1-2 species per year. We can discuss extinctions of species, genera, families, even phyla have gone |

|extinct. An extinction at the species level may simply mean that one named form has evolved into another named form (its descendent). When a family or other |

|higher taxonomic lineage disappears, clearly something more is going on. Even that most interesting claim, that (certain) dinosaurs gave rise to the birds, |

|doesn't explain the demise of so many different types of dinosaurs. The explanations generally fall into two main categories: 1) changes in the physical |

|environment, and 2) the appearance of biologically superior life forms (eg, more effective predators, better competitors).  |

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|Summary |

|The history of life, best documented only for the past 600 my, is a record of enormous change. Most species that have ever lived have gone extinct, new species |

|have arisen, and major new body plans have originated, often following soon after a mass extinction. There is a background rate of extinction, but mass |

|extinctions also have occurred, due to catastrophic events, rapid climate change, or other as yet undiscovered causes. The resulting great diversity of life |

|poses fascinating scientific challenges: how can we best organize this enormous diversity, and how can we trace in detail the history of its diversification? |

|That will be the subject of our next lecture.  |

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|Suggested Readings: |

|Gould, S.J. 1989. Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton, New York. |

|Wilson, E.O. 1992. The Diversity of Life. W.W. Norton and Company, New York. |

|Purves, W.K., G.H. Orians and H.C. Heller. Life: The Science of Biology. Sinauer, Sunderland MA.  |

|Take the Self-Test for this lecture. |

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|All materials © the Regents of the University of Michigan unless noted otherwise. |

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