Lecture Topics ‐ Biology • Defining life and life’s characteris:cs (1 lecture) • The Top‐Down approach to understanding the origins of life (2 lectures) – The limits of life – Extreme environments • The origin of life (3 lectures) – The top‐down and boFom‐up approaches – “replicator‐first” versus “metabolism‐first” – The characteris:cs of early life: The Lost City hydrothermal vent environments as a possible model • The origin of eukaryotes (2 lectures) – Endosymbiosis theory – The possible role of virus’s in the origin of eukaryotes • Summary
NW Rota Brimstone (pH 2 with S°) Depth 540 m
41 st Saas‐Fee course from Planets to Life 3‐9 April 2011 Lecture 1: The ‘Biology’ of Astrobiology – An overview (John Baross) • Can we define life and is that important in our search for life elsewhere? • Is “life as we know it” the only form of life possible, and/or the best of all possible designs for life? • What are the limits of Earth life – can it evolve to grow outside the limits of Earth environments? • What do the characteris:cs and requirements of Earth life tell us about poten:ally habitable planetary bodies – And the origin and early evolu:on of life
The origin and early evolu0on of life: Background lectures • “Top‐Down” approach: What we can learn from extant organisms – Possible environmental seangs for the origin of life – Ancient metabolisms and their origin – The origin of a replicator: “The RNA world” – The Last Common Universal Ancestor (LUCA) – The origin of eukaryotes (the importance of symbiosis) • “BoFom‐Up” approach: What we can learn from organic chemistry, Earth history, geology and geochemistry?
Lecture 1 ‐ outline • Can we define life? • Canonical characteris:cs of Earth life – would they apply to life‐forms different from Earth life? • Diversity and the three domains of life
Can we define life and is this an important issue? There are multiple definitions that can be separated into several categories 1. Darwinists (many examples) 2. Metabolists 3. Energists/Thermodynamists (e.g. Schrödinger, 1944) 4. Biosphereists (Shapiro and Feinberg, 1990) 5. Complexists 6. Others
Evolutionists definitions of life –” Descent with modification” (mutation and natural selection) (a few) • Life is a self-sustained chemical system capable of undergoing Darwinian evolution (Joyce, 1994) • Life is a self-replicating, evolving system based on organic chemistry (Pace, 2002) • System capable of evolution by natural selection (Sagan, 1970) • Material system that undergoes Darwinian evolution (C. McKay) • The minimal living system must be self-duplicating and mutable, and it must have the capacity for hetero-catalysis for bringing about chemical changes in the environment that support the self-duplication function (Hartman)
The “metabolists” defini:on Generally, a series of chemical reac:ons that produce energy and increasing complexity of organic compounds (replica:on and evolu:on are not part of this defini:on) This idea has become more important as a model for the origin of life than as a defini:on Metabolism is the sum of all chemical reac1ons in a living organisms 1. Catabolism – breakdown of complex organic molecules into simpler compounds and releases energy 2. Anabolism – building of complex organic molecules from simpler ones and requires energy
The Biospheric Defini:on of Life Feinberg & Shapiro, 1980 • Life is fundamentally the ac:vity of a biosphere. A biosphere is a highly ordered system of maFer and energy characterized by complex cycles that maintain or gradually increase the order of the system through an exchange of energy with the environment Image of Serius A and Serius B – This defini:on would include taken by the Hubble Telescope. as life, “plasma life” in the Serius B, which is a white interiors of stars including dwarf, can be seen as a faint pinprick of light at the lower white‐dwarf stars and lei of the much brighter Serius interstellar gas clouds A (Wikipedia)
Definition of life: Cleland and Chyba (2002; 2007) discuss the philosophical dilemmas on the nature of definitions (“Definitions specify meanings by dissecting concepts that we already possess”) Two kinds of definitions: 1. Words or terms whose existence depends solely upon human interests and concerns, such as “bachelor” or “fortnight” are readily defined 2. “Natural kind” terms such as “life”, “water” and “heat” cannot be defined by describing properties because there is more to their meanings than the features we identify (e.g. water is defined as H 2 O and not by characteristics such as “cooling”, “tasteless”, “odorless” etc) 3. Defining life from properties of life has problems because there are “non-living” analogues. (e.g. replication - clays and other minerals replicate; ability to evolve - mineral growth involves changes “mutations” that replicate; energy and fire, etc)
Definition of life: continued What we don’t know is how all of the components that make up a living en:ty become life. Cleland and Chyba (2007) approach this dilemma by poin:ng out that “to answer the ques0on ‘What is life?’ we require not a defini0on but a general theory of the nature of living systems.” This theory does not yet exist.
Moreover……. A defini:on of life would have to include any living en:ty even if it were radically different from Earth life. This implies that a defini:on of life is not a list of canonical characteris:cs possessed by earth life, such as being carbon based, self‐ replica:ng and having the ability to undergo Darwinian evolu:on.
However….. A case can be made that the ability to replicate and undergo Darwinian evolu:on are essen:al characteris:cs of all life even if significantly different from earth life, but they would not be part of a defini:on of life but instead essen0al mechanisms to produce progeny and to create diversity and complexity
But – what about…..
Life: A chemical system capable of Darwinian evolu0on This defini:on holds a model of what is possible, vs. what is conceivable. If we were to encounter Calamarain or Q on a real, not conceptual Star Trek, we would be forced to change our defini:on to include them as living systems. We do not change now because we don’t believe that the weirder life conceived in the Star Trek scrip:ng room is possible outside of that room (from Steve Benner) Q, requiring neither maFer Calamarain, a life form built nor energy from pure energy
We exclude other kinds of life because we do not believe that it could have arisen naturally, even though we believe it can exist and certainly will exist in the future (from Steve Benner) Nanites and Data are examples of ar:ficial life. We don’t doubt that androids can be created, including androids who (note the pronoun) desire to be human. Ar:ficial life form A biosignature, not biology
Back to the ques1on : Do we need to define life in order to detect it? • Life may not be definable but there are characteris:cs of life that might be canonical of all life and they would leave “biosignatures” – Carbon‐based – Self‐replica:ng – Uses chemical and light energy – Undergoes Darwinian evolu:on – Requires water and more than 20 elements All studies on the origin of life rely on these characteris:cs as the basic star:ng points
Which of these canonical characteris:cs of Earth life has the greatest likelihood of being different in extraterrestrial carbon‐based life? (note my bias for carbon‐based life) We don’t know ‐ our search for life elsewhere remains primarily earth‐centric
Many believe that life can be recognized even is radically different from life as we know it “Life can be recognized by what it does: living organisms create hallmark molecules and create chemical disequilibrium” Ken Nealson (University of Southern California)
The Earth‐centric approach to our search for life on other planetary bodies • Follow the water, follow the energy, etc • Look for Biosignatures: 1. organic compounds that follow the rules of carbon chemistry found only in organisms (living or fossils) 2. ra:os of CHONPS etc that are indica:ve of life (Redfield Ra:os), chemical disequilibrium, etc 3. isotopic frac:ona:on of C, S, N as indica:on of metabolism • Look for intact “cells” enclosed in a membrane The earth‐centric approach to life elsewhere is very oien expressed in the popular media, for example….
One of many examples of Earth‐ centric views about life on other planets and moons Illustra(on from the New York Tribune, February 8, 1920 en(tled “Scien(sts, Agreeing Mar(ans Are Super‐Race, Believe That Planet May Be Signaling to Us” (detail) Cap:on: “Scien:sts agree that the people of Mars differ from us in many ways. The Mar:ans are believed to have very large noses and ears and immense lung development because of rarified atmosphere. Their legs are poorly developed, because maFer on Mars weighs less than here and sturdy legs are not needed to bear their weight. Birds and buFerflies are very large and beau:ful.
Recommend
More recommend
