EARTH HISTORY (NOTE: this material covered in lecture on Origin of Life & Fossil Record)

Much of evolutionary biology involves the history of organic diversity. Organic diversity has been shaped and affected by the origin and history of planet earth. To appreciate this history we need to acquire some knowledge of the geological processes that have shaped the earth. One general theme to consider in this and the next lecture is: if we were to start the history of earth over again from the "primeval soup" would the results be the same? Almost certainly not (see Gould, 1989. Wonderful Life for a detailed discussion). History is unique and events are contingent on what has occurred previously. Much of the contingency of organic evolution is dependent on the unique series of events that shaped the earth, this is why we need to understand some basic geology.

How was the planet formed? What is its relationship to other matter in the universe? A popular hypothesis for the formation of the earth is the nebular hypothesis. This idea dates back to the philosopher Immanuel Kant (1755) and Laplace (1796) and has been modified as empirical evidence and theory mount. Recent incarnations (chemical -condensation-sequence model) start with the solar system forming from a rotating, diffuse cloud of dust and gasses (a nebula). As the nebula cooled the matter condensed into "planetesimals", near the sun where temperatures were highest elements with the highest melting points (metals and heavy minerals) condensed first. Lower melting temperature elements and compounds (water, methane, ammonia) condensed more readily in the cooler areas further from the sun. This helps to explain the density gradient in the solar system, the closest planets to the sun are terrestrial while those further away are gaseous.

How did the earth form in the condensing nebula? The earth may have formed through the accretion of many planetesimals and as the mass increased through gravitational attraction and compression (overhead). The earth was probably initially a homogeneous ball that heated from three sources: 1) energy of planetesimal impacts, 2) gravitational compression lowered potential energy releasing heat, and 3) heat from radioactive disintegration (20 cals is released for 1 cm3 of granite over 500 million years). As the earth heated it began to differentiate into various zones of matter with different properties (overhead). Differentiation was possible because molten material could rise or sink depending on density, be moved by convective currents, and localize due to chemical zonation (overhead). As the earth cooled outgassing of the mantle released compounds (water vapor, carbon dioxide, hydrogen, nitrogen) into a primitive atmosphere.

Early geologists tried to determine how old the earth was from observations about the features of the earth. Age = Thickness of sedimentary rock/rate of sedimentation. Old (<1.5 billion years) but not old enough. Age = salinity of sea/rate of salt deposition in seas. Again old, but not old enough. Lord Kelvin (of absolute zero fame) calculated the age of earth from its temperature, assuming it was molten at its formation. Gave 100 million years (and gave Darwin a bit of a problem: was this enough time?? Radioisotopes cleared things up (see below)

We can divide the processes that alter the earth's surface into two categories: 1) igneous processes (volcanism and mountain building) construct features by increasing the average elevation of the land, 2) Sedimentary and erosive processes (deposition and weathering) act as forces wearing down features created by volcanoes and creating new horizontal features (e.g. river delta). The theory of Plate tectonics provides a synthetic model for understanding how the dynamics of the earth work. The plates move around, collide, move over or under one another. Divergent boundaries are where plates move apart, convergent boundaries are where plates move toward one another, transform boundaries (e.g. San Andreas fault) are where plates move by each other. The continental plates (lithosphere) float on molten inner layer (asthenosphere). Where plates meet there can be uplifting or subduction. Uplifting results in mountain building through igneous activity and at the boundaries between plates and actual scraping off of material from the subducted plate. Subduction results in plates being forced downward and is seen is formations such as ocean trenches.

The rock material of continental plates can be viewed as going through a rock cycle that can be related to plate tectonics. Magma (molten rock) e.g., released from volcanoes, crystallizes and forms igneous rocks ("fire formed rocks"). Through weathering and transport sediment is formed which by lithification become sedimentary rock. Through exposure to high temperatures and pressure, sedimentary rock (or any rock) can be changed into metamorphic rocks. If this rock is exposed to extreme temperatures it can become molten again and form magma, and if released through volcanic activity be reintroduced as igneous rock.

In what kind of rock would we expect to find fossils? Sedimentary rocks. Their structure can tell us a lot about earth history. Laid down in strata of sedimentary layers. Bedding planes generally mark the boundary between the end of one sediment and the beginning of another.

Several logical rules can be used to determine the sequence of events: Relative dating. generally one follows several principles: superposition the older rock is below and the younger rock is above; original horizontality: the strata are laid down originally in a horizontal position (gravity is what lays them down). Thus nonhorizontality must have occurred after the deposition. The cross cutting relationship states that the cut formation is older that the formation doing the cutting.

Another prominent feature is an unconformity which occurs when the rate of deposition has been interrupted, the sediments eroded and deposition renewed. A clear break in the sequence of events is apparent. One type of unconformity is an angular unconformity where strata with originally horizontal bedding planes now have bedding planes that intersect. Significant because it reflects a major episode of geologic change.

All well and good for a given formation, but one would like to be able to make general statements about larger regions. This can be done by correlation of strata from different formations separated by some distance. Stratum "X" may lie near the top of one formation and many miles away, X may be found near the bottom of a new formation, at the top of which is a different layer "Y". Several miles further on, "Y" may lie at the bottom of a third formation, and in this way one can link or correlate the different strata.

This may work for a large region but one would like to do this for the entire earth. It turns out that there are diagnostic fossils found in different formations around the world. These Index fossils help correlate different formations on each of the major land masses. This was recognized by William Smith (see lecture 2). The phenomenon is more pronounced than an occasional fossil here and there: entire biotas go through successive changes in sequential strata, illustrating the principle of faunal (biotic) succession. We thus have the "age of trilobites" seen early in the fossil record. Later the age of fishes, age of reptiles, age of mammals are clear in formations around the world indicating the comparable ages of formations separated on different continents.

These fossil beds lead to the formation of the Geologic time scale, the names of each period deriving from the locality where the characteristic formation was found. The major divisions (eons, eras) are defined by the presence or absence of fossils: proterozoic, phanerozoic (visible life or animals). Geological dating is often problematic because geologists use fossils to date rocks and biologists use rocks to date fossils. A measure independent of stratigraphy and fossil remains is necessary. With the discovery of radioactive decay it became apparent that one could use the ratio between the parent isotope and the daughter product (e.g., U238 decays through several steps to Pb206). By measuring the amount of isotope and daughter product and knowing the half life of the isotope one can estimate the absolute age of a rock formation. Problems: when the daughter material escapes and hence produces an inaccurate estimate. Additional tests with different isotopes can corroborate one another.