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Reading: Chapter 13 pages 384-389 Chapter S6 pages 695-698 |
From the on-line book Life at High Temperatures about organsims living in Yellowstone Park's hot springs - thought to be typical of places where life could have originated.
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Reading: Chapter 13 pages 384-389 Chapter S6 pages 695-698 |
From the on-line book Life at High Temperatures about organsims living in Yellowstone Park's hot springs - thought to be typical of places where life could have originated.
Inevitably, we are going to find many more questions than answers in this speculative section.
What do we mean by "life"? This question can seem trivial since we know life when we see it - it is obvious that a tree is alive but a chair is not. But when we try to write down firm rules for defining life it is not so easy. Many scientists have defined life in terms of characteristics that all recognized forms of life seem to have in common. By extrapolation, perhaps currently unknown forms of life may be recognized by these traits, as well.
Specifically, things that are 'alive' all appear to have the ability to:
(1) React to their environment.
(2) Take in nourishment, producing energy and growth.
(3) Reproduce, passing their characteristics on to their offspring.
(4) Evolve through small changes, from generation to generation.
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(1) (a) Using the 4 characteristics of life, consider how they might apply to (i) computers or robots and (ii) grass growing on a lawn. (b) Now consider the textbook's argument that complexity is a better guide for defining life than such rules. Which has greater complexity, a computer (or robot) or lawn grass? |
At a very simple level life is "just" chemistry - but complex chemistry. The BIG MYSTERY is how such complex chemistry as that causing humans and their behavior could have arisen naturally.
Making the building blocks of life seems to be relatively simple - one can imagine such a process happening all over the universe, wherever there is sufficient abundance of elements such as carbon, hydrogen, oxygen, nitrogen ("CHON" - as these comment life-building elements are sometimes called). The next steps of building single-cell organisms and, eventually, mammals, are much trickier.
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(2) (a) What were the ingredients of the "primordial soup"? Which elements? Look up their abundance. (b) What was the Urey-Miller experiment? What did they do and what did they find? (Incidently, here is an interview with Miller himself). (c) Electical discharges were used in the Urey-Miller experiment to provide energy in a rapid fashion (to speed up the chemical reactions). What other sources of energy were available for such chemical reactions in the early solar system? (Not necessarily as intense as lightening). |
The elemental building blocks (CHON) and the amino acids of the Urey-Miller experiment are just the very beginnings - they from a wide variety of simple chemicals ("Primordial Chemical Diversity"). From these simple ingredients chemistry evolved (via "Prebiotic Chemical Selection") to Self-Organizing Chemical Systems. See the "Hourglass of Life" diagram below.
The big mysteries are how these chemical systems evolve into biology - that is, how does life begin? Were there many different early biological systems and only one system "won"? ("Biochemical Diversity" means many different biochemical systems but these were still very simple in nature - having "Morphological Simplicity"). If we found life elsewhere would the same biological system have inevitably arisen?
All life on Earth shares the same basic biochemistry ("Biochemical Uniformity") - using the ATP molecule to store energy and DNA/RNA to carry genetic information. As biology evolved, this same materials were the foundations of a wide variety of kinds of organisms ("Morphological Diversity") - from microbes to slime molds, to plants and animals.
The planetary bodies listed on the left of the diagram are the places where we might explore for evidence of processes along this evolutionary path from chemistry to biology. Farther out in the solar system we might expect to find earlier and earlier stages - primordial or pre-biotic processes.
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(3) (a) How many years after the formation of the solar system (4.5 billion years ago) did the first one-celled organisms appear? (See page 385) (b) What are "stromatolites"? (c) What geological factors "frustrated" early life on Earth? |
The figure below is often called "The Tree of Life" - it illustrates how ALL forms of life on Earth originate from the same "Universal Ancestor" and are based on the same DNA chemistry.
This figure is based on gene sequences in rRNA molecules. The length of the line segments represents the genetic distance that each organism has evolved since splitting off from the rest of the tree. The underlined words Bacteria, Archaea and Eukarya are the tree domains of life that have all believed to have arisen from a Universal ancestor. (Figure 13.23 is a similar version).
There are 3 particularly interesting things to note about the above diagram:
(1) Notice, as animals, our nearest neighbors are slime molds and fungi!
(2) Notice all the words that begin with "thermo-". These are organisms that live in hot environments such as deep sea volcanic vents. There is a strongly-growing school of thought that complex life first evolved from the simple building blocks in such "hydrothermal" vents - our universal ancester arose from hot springs.
(3) Notice all the words beginning with "methano-" which means these organisms involve reactions that use methane (CH4) and without oxygen - they are sometimes called "methanogens".
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(4) (a) Remembering the climate evolution of Earth, Venus and Mars, why might methanogens have been prevanlent in early Earth but not so common now? (b)Which of these forms of life (if any!) would you expect to find on Mars and Venus? (c) How many years after the multi-cellular organisms did humans evolve? (Humans separated from the other apes about 6 million years ago). |
While all current lifeforms on Earth share this common origin, the probability of the same - or even similar - evolutionary proceses occuring on other planets, under different (or even similar) conditions prevail are exceedingly unlikely. In other words.....
Thus, we are likely to find very different lifeforms should life have arisen
on other planets.
Having explored the issue of life on Earth, we now move on to life elsewhere in the solar system.
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(5) (a)Most textbooks say "Life as we know it" is generally taken to mean carbon-based life that originated in a liquid water environment. Why "carbon-based" life? (b) What are the physical/chemical reasons for considering a liquid water environment to be important for the origin of life? (c) What are the factors that determine whether a planet has liquid water on its surface? |
By now you are probably getting the impression that there are very few places where we might find life - existing or ancient - in the solar system:
(1) Mars - while Mars is probably currently too cold for life to survive, life may have prevailed for a billion or so years, about 3 billion years ago.
(2) Europa - IF there is a liquid ocean under the surface ice, heated by deep volcanism, perhaps Europan life forms have evolved and survived.
(3) Titan -under Titan's thick atmosphere could life have evolved in liquid pools of hydrocarbons? Has the greenhouse effect produced warm areas where water remains liquid?
The Goldilocks paradox says "Venus is too hot, Mars is too cold, Earth is just right". But there is substantial evidence that Mars was once much warmer and that water flowed on the surface. Could life have once flourished on Mars and fossil remnants might be left on the surface? Could life still be surviving on Mars? We do not know the answers to these questions - yet. But there are some hints.
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(6) (a) What is the evidence for liquid water on the surface of Mars in the past? (b) What did the Viking missions tell us - and NOT tell us - about life on Mars? ( Page 394) (c) The discovery of ALH84001 was a landmark for exobiology. What is ALH84001? How do we know ALH84001 came from Mars? (d) What is the evidence of Martian life in ALH84001? (7) (a) Mars cooled down about 3 billion years ago. If life developed on Mars at the same rate as on Earth (big "if"), then how complex would life have developed to on Mars when the "big freeze" hit? (b) What kinds of fossils could we perhaps expect to find on Mars? (c) Where on Mars should we go to look for such fossils? |
The issues of life on Mars are a major theme of NASA plans to explore Mars. But it is not so "simple" as sending a probe to "find" life - here are some complicating issues:
(1) What were/are the past/present climatic conditions under which life may have survived (or failed to survive)? We need to measure past and present climate.
(2) Where is/was the water? Where has it gone?
(3) If we send spacecraft from Earth to Mars should we be concerned about carrying terrestrial organisms which might contaminate Mars?
(4) Where are the best places to find evidence of life? If we do find life, where did it exist and where did it not exist?
(5) If we bring back samples, do we need to be concerned about bringing back Martian life forms which might contaminate Earth? How could we prevent backward-contamination?
Issues (3) and (5) relate to the general topic of planetary protection. Did you know NASA has a Planetary Protection Officer? What a job title!
Another aspect of astrobiology that relates to the possibility of life on Mars, is the recent realization that life can exist on Earth under very extreme environments. Perhaps the Martian conditions are survivable.
| Astrobiology has become a rapidly-expanding area of research - with lots of material available on the web - check out these links: |
In case you are interested, here are NASA's plans for human exploration of Mars - it might just happen in your lifetime!
Finally, we delve into the far reaches of human imagination - the possibility of turning Mars into an Earth-like environment, the Search for Extra-Terrestrial Intelligence....
Terraforming Mars - the idea has been around for decades, discussed as much by planetary scientists as space buffs.
First of all, it should be kept in mind that there's a fallacy in thinking that if the Earth has become unlivable then the solution is moving to Mars. Currently human population doubles every 37 years. If this rate is maintained we will over-populate Mars just 37 years after we get there.
But it is interesting to think about the concept of terraforming. How could terraforming really be done? First, we need to warm up the planet to liberate the water frozen in the ground and polar caps. Some people propose sprinkling carbon on the polar caps to absorb sunlight. Others suggest adding very effective greenhouse gases (such as chloro-fluoro-carbons or new, specially-designed molecules) to Mars' tenuous atmosphere. Some have even proposed using nuclear bombs. Once liquid water is available the planting begins. As with early life on Earth, vegetation converts the atmosphere of carbon dioxide to oxygen. Eventually animals, even humans, might be able to breathe the air.
Terraforming Mars - check out these links:
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Search for Extra-Terrestrial Intelligence - at the end of the textbook Chapter S6 is about Interstellar Travel and includes a section on extra-terrestial intelligence (pages 695-7). Not many believe that there is what could really be called "intelligent" extra-terrestrial life in the solar system (why have they not contacted us if they are there?) - but, partly because it is fun to think about and partly because the Planetary Society is heavily into the subject, here are some fun links to end this journey through the solar system.
"The Drake Equation" How can we estimate the number of technological civilizations that might exist among the stars? An equation developed in the 1960's by Dr. Frank Drake conceived an approach to bound the terms involved in estimating the number of technological civilizations that may exist in our galaxy. ("Calculate" is being a little generous - guestimate is more appropriate)
SETI - check out these links:
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Happy Explorations!
Model answers to the comprehension questions.