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
Format for printing
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?
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.
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.
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.
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.
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.
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.
The four eons of earth history. Ga = billion years ago, Ma = million years ago. After Purves et al.
formation of earth and continents, chemical evolution
origin of life, procaryotes flourish
eukaryotes evolve, development of oxygenated atmosphere, some animal phyla appear
most animal phyla present, diverse algae; explosive evolution of higher life forms
The eras and periods of the Phanerozoic Eon, after Purves et al. Ma = million years ago.
ME = period ended with a mass extinction.
repeated glaciations, humans evolve, extinctions of large mammals
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.
Image from Wilson, "The Diversity of Life."
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.
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).
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 (seenatural 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.
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.
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).
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.
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.