Fall 2003

Dr. Largen

• characteristics of life
• origin of life

Characteristics of Life

• What is life?
• What qualifies something as "living"?

In-Class Activity #5: Characteristics of Life

• Work in groups of 3 - 5
• prepare a list of characteristics that qualifies something as "living"

What is Life?

• What is life?
• What qualifies something as "living"?
• consider necessary versus sufficient criteria
• necessary
• possessed by all life?
• sufficient
• possessed only by life?

• four possible criteria of life
• movement
• sensitivity (responding to stimuli)
• death
• complexity

What is Life?

• four possible criteria of life
• movement
• not necessary
• not sufficient
• sensitivity (responding to stimuli)
• not necessary
• not sufficient
• death
• necessary
• not good criterion because of circular definition
• complexity
• necessary
• not sufficient

All living things share key characteristics

• All organisms on earth exhibit these 7 fundamental properties
• cellular organization
• sensitivity
• growth
• development
• reproduction
• regulation
• homeostasis
• heredity

• cellular organization
• all organisms consist of one or more cells
• sensitivity
• all organisms respond to stimuli
• not to all stimuli in same way
• growth
• assimilation of energy, use of it to grow
• via a process called metabolism
• development
• multi-cellular organisms
• systematic, gene-directed changes through growth, maturity
• reproduction
• passing on traits from one generation to the next
• regulation
• coordination internal processes
• homeostasis
• maintaining relatively constant internal conditions, different from their external environment
• heredity
• genetic system based on replication of DNA
• allows for adaptation and evolution over time

Ideas about the origin of life

• ideas about the origin of life
• many, from different cultures, religions
• can’t definitively answer question of how life originated
• three possible explanations for the origin of life
• special creation
• extraterrestrial origin
• spontaneous origin

• special creation
• hypothesis that life forms may have been put on earth by supernatural or divine forces
• at core of most major religions
• considered an "unscientific" explanation
• cannot be tested and potentially disproved
• extraterrestrial origin
• hypothesis that life did not originate on earth
• carried to earth by meteors or cosmic dust as an extraterrestrial "infection"
• cannot be rejected based on evidence currently available to science
• possible fossils in Mars rocks
• liquid water under surface of Jupiter’s ice-shrouded moon Europa
• considered an "unscientific" explanation
• cannot be tested and potentially disproved
• spontaneous origin
• hypothesis that life evolved from inanimate matte
• associations among molecules became more and more complex
• molecules increased their stability
• persisted longer, initiated more and more complex associations
• culminated in the evolution of cells
• this view does not preclude the other two possibilities
• divine agency may have acted via evolution
• life may have infected earth from some other world and then evolved
• considered the only "scientific" explanation
• could potentially be tested and disproved (or supported)
• only explanation routinely focused on by "science"
• goal is attempting to understand whether
• forces of evolution could have led to origin of life and, if so
• how might the process have occurred

Figure Some major episodes in the history of life

What was the early earth like?

• exact composition not agreed upon by all scientists
• some fundamental characteristics
• "reducing" atmosphere

Figure A painting of early Earth showing volcanic activity and photosynthetic prokaryotes in dense mats (Campbell & Reece)

• some fundamental characteristics
• reducing atmosphere
• early atmosphere contained
• principally CO₂ , N₂ gas
• significant amounts of water
• H atoms, bound to light elements (S, N, C)
• H₂ gas
• little O₂ gas
• no layer of ozone (O₃) to protective from ultraviolet light
• called "reducing" atmosphere due to
• ample availability of H atoms and their electrons
• facilitated gain of electrons by certain molecules (reducing charge of atom from neutral to -)
• today’s atmosphere
• considered an "oxidizing", contains app. 21% oxygen
• high temperatures
• 4.6 - 3.8 billion years ago (BYA), surface of earth was kept molten hot
• bombardment from rubble
• ~ 3.9 BYA, bombardment stopped, temperatures dropped, ocean temperature was 49 to 88 ° C (120-190° F)
• ~4.0 - 3.5 BYA life appeared

Figure Lightning

Testing the spontaneous origin hypothesis

• Miller-Urey experiment
• by reproducing early conditions
• assembled similar atmosphere
• excluded gaseous oxygen
• placed atmosphere over liquid water
• maintained mixture just below 100 ° C
• simulated lightning
• bombarded mixture with energy (sparks)

Figure The Miller-Urey experiment (Campbell & Reece)

• results (within 1 week)
• 15% of C (originally methane gas (CH₄)) - converted to other simple C compounds
• simple C compounds
• combined to form formic acid, urea, amino acids glycine and alanine (building blocks of proteins)
• in similar, later experiments, complex ring-shaped molecule adenine – a base found in DNA and RNA – was formed

Figure Volcanic activity and lightning associated with the birth of the island of Surtsey near Iceland; terrestrial life began colonizing Surtsey soon after its birth (Campbell & Reece)

The End