Chapter 13 Biodiversity Through Earth History Underlying assumption is that the process of evolution is occurring evolution: creation of new species random mutation: genetic changes natural selection: environmental pressures favor certain characteristics adaptation: ti favored characteristics ti prevail extinction: death of all members of a species The text starts off with two ideas about how the number of species changes over time that do not seem to pan out One view is that, perhaps, diversity is a dynamic process: logistic growth S-shaped growth observed for populations, common model used in population dynamics rate of change in number = origination - extinction of species on earth rate rate This does not seem to be the case. These observations do not appear to be logistic, but do they indicate exponential growth in # of species? Problem with this: # of species may simply be a function of the amount of sediment available. Does not appear to be either logistic or exponential 1
The taxonomic tree for homo sapiens Data on the number of new appearances of marine invertebrates of a given age (namely, as they appeared) by phylaclasses-orders taxonomic levels. Compare to previous data - no exponential growth is observed when looking at the evidence this way 440 my 360 my 251 my 206 my 65 my Marine invertebrate fossils indicating diversity through geological time At least five major extinctions over the past 500 m. y. Largest mass extinction at end of Permian, 251 mya. Best researched extinction (dinosaurs) is the Cretaceous- Tertiary (K-T), 65 mya. Fossil record indicates high taxonomic levels are less affected, and that lower taxonomic levels recover rapidly from mass extinctions Mass extinctions cause great losses but stimulate origination. (see chapter 1 also) Extinction of dinosaurs, many other marine and terrestrial species (both animal and plant) Iridium: normally deposited at a slow, known, rate from extraterrestrial sources. Iridium concentrations in the layer at the K-T boundary are so high that it could not have been deposited by normal deposition, must have been a massive, sudden extraterrestrial source. Most iridium on earth is in core, concentration in asteroids are much higher than on earth s surface 2
(see chapter 1 also) Estimated size of meteorite, from amount of iridium, = 10km diameter Example of a shock to the earth system. The climate, biosphere, recovered, but at different rates. Biosphere recovery was very different from prior biosphere. What caused it, and how did the earth system respond? This has been the most studied, and perhaps most controversial, extinction event. Big debate in the scientific literature in the 1980s 90s Realistic possibilities: Sudden sea level or climate change Volcanic eruptions Meteorite (comet or asteroid, extraterrestrial) impact Sudden sea level or climate change Not likely, because some of the times of greatest climate and/or sea level changes have not coincided with mass extinctions Volcanic eruptions or extraterrestrial impacts the idea is that the heat and explosion from the impact would kill everything regionally, and that the subsequent cooling from dust / soot would kill many things globally. Volcanic eruptions could result in global cooling from sulfur aerosols in the stratosphere. Impact would also send hot particles across the globe starting fires; destroy the ozone layer for several years; caused tsunamis; food would be scarce for survivors; CO2 would have been released increasing temperatures for thousands of years; toxics across the globe; slow recovery time of biosphere. Luis Alvarez s 13 predictions which indicate that the geological evidence is consistent with the meteorite impact theory (only some listed here) Iridium layer should be at the K-T boundary worldwide Frequency of large meteorites is sufficient to explain the record frequency of collisions of large objects with Earth. 3
Luis Alvarez s 13 predictions that show that the geological evidence is consistent with the meteorite impact theory Both plants and animals should have been affected clays in the iridium layer should be identical worldwide, be different than the layers above and below, and have an extraterrestrial signature Luis Alvarez s 13 predictions that show that the geological evidence is consistent with the meteorite impact theory There should be evidence of high temperature and high pressure at impact. Sphericals (from cooling of molten droplets in air) and shocked quartz (only forms when quarts is under high pressure) are there. Evidence of wildfires Layer should be between a change in fossils Evidence for mass extinction of plants Evidence for mass extinction of plants Luis Alvarez says that the geological evidence is inconsistent with the volcano theory Vl Volcanoes would not distribute the spherules globally Shocked quartz has never been found in volcanic deposits, but is often found in meteorite impact craters Volcanic deposits tend to have low iridium concentrations Additional evidence: Helium isotopes found in the layer are extraterrestrial The Chicxulub Crater: - associated with cenotes - contain spherules and shocked quartz - enriched in iridium - in the right age range Location of the Chicxulub impact crater Evidence for mass extinction of plants Size of impact crater 4
Did the biological pump shut down for hundreds of thousands of years? Currently, and prior to K-T, there is a difference in C 13 /C 12 ~2o/oo Difference is caused by the biological pump, algae preferentially take up C 12, then sink to bottom and decompose After K-T, much smaller difference. Asteroids: rocky material composed of minerals and metals from dust to 100km in size found mostly in the asteroid belt (between Mars and Jupiter) Comets: materials that are volatile on earth but solid in space, plus some minerals and metals found mostly in the Oort Cloud Both asteroids and comets can be deflected, by collision, into different orbits that might cause them to collide with a planet, including earth. Jupiter acts as a major gravitational ti attraction ti to deflected d asteroids, affecting their paths and possibly sending them towards other planets 1. Galactic plane hypothesis 2. Companion star hypothesis 3. Planet X hypothesis 4. Artifact of the way the data are collected Vertical lines drawn every 26 million years 5
Support for meteorite impact as a cause for the K-T mass extinction: frequency of collisions of large objects with Earth. Location of the Chicxulub impact crater 1. Galactic plane hypothesis: our solar system moves up and down in a 26 million year cycle through h the plane of the galaxy. When we pass through the center, we encounter more gas/dust, higher chance of disturbing the orbit of an asteroid or comet Size of impact crater Evidence supporting meteorite impact and its effect on plants at the K-T boundary 1. 2. Companion star hypothesis: our sun has a companion star (small brown dwarf, <.08 size of sun) in an orbit that passes through the Oort cloud every 26 million years 1. 2. 3. Planet X hypothesis: there is a planet that we have never seen, located in an orbit beyond Pluto, that disturbs the Kuiper Belt 6
1. 2. 3. 4. Artifact of the way the data are collected: data are collected in stages, perhaps this results in some funny cyclicity in the data Vertical lines drawn every 26 million years 7