Part 26: The diversity and mechanisms of life on earth, as well as it rough history, from mass extinctions to evolutionary profusions.
These short videos were created in August 2007 by Dr. Christopher D. Impey, Professor of Astronomy at the University of Arizona, for his students. They cover a broad range of terms, concepts, and princples in astronomy and astrobiology. Dr. Impey is a University Distinguished Professor and Deputy Head of the Astonomy Department. All videos are intended solely for educational purposes and are licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. The full list of collections follows below:
01. Fundamentals of Science and Astronomy
02. Ancient Astronomy and Celestial Phenomena
03. Concepts and History of Astronomy and Physics
04. Chemistry and Physics
05. Quantum Theory and Radiation
06. Optics and Quantum Theory
07. Geology and Physics
08. Solar Neighborhood and Space Exploration
09. Outer Planets and Planetary Atmospheres
10. The Solar System
11. Interplanetary Bodies
12. Formation and Nature of Planetary Systems
13. Particle Physics and the Sun
14. Stars 1
15. Stars 2
16. Stars 3
17. Galactic Mass Distribtuion and Galaxy Structure
19. Galaxies 2
20. Galaxy Interaction and Motion
21. Deep Space and High-Energy Phenomena
22. The Big Bang, Inflation, and General Cosmology
23. The Big Bang, Inflation, and General Cosmology 2
24. Chemistry and Context for Life
25. Early Earth and Life Processes
26. Life on Earth
27. Life in the Universe
28. Interstellar Travel, SETI, and the Rarity of Life
29. Prospects of Nonhuman Intelligences
Stability of Earth's Atmosphere
Transcript: The stability of the Earth’s atmosphere over billions of years has been impacted in part by the carbon dioxide cycle which has acted as a thermostat to regulate the temperature of the Earth, even over time spans when the Sun was changing its brightness or the amount of tectonic activity was varying. The basic chemical principle is that the rate at which carbonates form depend on temperature, and carbonates from faster when it’s hotter. Thus when the temperature increases, the rate of carbonate formation increases which means the rate at which they dissolve in the oceans increase, and the dissolution of carbon dioxide in the oceans removes it from the atmosphere thereby lowering the greenhouse effect which causes the temperature to fall. On the other hand, if the temperature of the Earth is reduced for any reason, the rate of carbonate formation also falls as does the rate of its dissolution in ocean water, which leads to more greenhouse gas, which leads to a higher temperature. Thus, the process is self-regulating.
Transcript: In addition to gradual change over billions of years, Earth’s climates has been fluctuating and subject to instabilities that have taken it to extremes that are hard for us to imagine. In a period of six hundred to seven hundred and fifty million years ago, Earth was subject to a series of deep ice ages when glaciers reached nearly to the equator, and the oceans froze to a depth of about a kilometer. This is hard for us to imagine. It got this way because a normal fluctuation made the Earth cooler at which point the glaciers advanced, and the oceans began to freeze. Ice reflects ninety percent of light incoming rather than five percent for water. As the oceans reflect more light, they get colder still accelerating the freezing process in a runaway fashion. However, during this time volcanic activity, driven by energy sources deep in the Earth, does not diminish, and so eventually carbon dioxide is released which is not absorbed in the oceans because they are frozen. So it builds up in the atmosphere, warming the planet and melting the ice, and more sunlight gets absorbed. This also becomes a runaway process, and the large amount of carbon dioxide leads to an overshoot, an enormous warming of the temperatures. Thus, there are instabilities in the Earth’s atmosphere that can lead to climate extremes which must have affected life on Earth at the time.
Tectonics and Life
Transcript: There is over 100,000 times more carbon dioxide locked up in the ocean and the rocks of Earth than there is in the atmosphere. If even a tiny percentage of this carbon dioxide were released into the atmosphere, it would lead to a runaway greenhouse effect that would raise the temperature of the Earth to the level Venus and beyond, and make life extremely difficult. Thus, there appears to be a fundamental connection between the regulation of carbon dioxide and the carbon dioxide cycle, and the stability of the Earth’s atmosphere long enough to allow complex life to evolve, and the key aspect of this is the presence of plate tectonics, because plate tectonics leads to the subduction of CO2-bearing rocks, and the eventual release of the gas back into the atmosphere through volcanism; thus, it’s a key part in the carbon dioxide cycle. We do not know if planets elsewhere in the solar system or in the universe must have plate tectonics to have well-regulated atmospheres, but appears to be no coincidence that on Earth, plate tectonics help to make the Earth a habitable place.
Climate and Earth's Orbit
Transcript: Ice ages are periods of global cooling by a few degrees, up to ten or so degrees, that occur at irregular intervals of tens of thousands to hundreds of thousands of years. The causes of ice ages are complex, but over the past few million years, there is good evidence that variations in the Earth’s orbit has contributed to the cause of ice ages, in particular the Earth’s tilt on it’s axis, which is varied from twenty-two to twenty-five degrees, influenced primarily by Jupiter. Other influences in the ice age could be the geomagnetic reversals that occur every few hundred thousand years in the Earth’s core which are also influenced by orbital dynamics. Astronomers are only beginning to unravel the effects of subtle changes in the Earth’s orbit and their impact on global climate.
Transcript: Simple organisms such as bacteria reproduce by making copies of themselves. Once cells had developed the capability of having nuclei, sexual reproduction became possible, and this is very important for genetic diversification. In sexual reproduction, the offspring get half their genetic material from each of the parent organisms. Genes can combine in different ways from generation to generation which facilitates experimentation and adaptation to a changing environment.
Transcript: About six hundred million years ago in the oceans of the Earth the first multicelled organisms developed. These are distinct from colonies of simple cells like stromatolytes which existed much earlier. A multicelled organism is a large number of cells acting in concert within a single creature. In the evolution of life, the transition from prokaryotes to eukaryotes, cells with nuclei, spurred a huge amount of biological diversity. The transition from single-celled nuclei, eukaryotes, to multicelled organisms spurred an even larger amount of diversification. The diverse functions of cells led to specializations of their function. The cells were independent, and the organism was able to adapt better to the environment. The ceaseless experimentation of multicelled organisms lead to new capabilities. As just one example, consider the cells that evolved on the surface of a creature that were able to detect light. Eventually those cells could aggregate and form, over hundreds of millions of years, eyes.