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Defining Life
What is life?
Evolutionary adaptation and evolution, natural selection
Phylogenetic tree
Metabolism
Defining life
1. 
2. 
3. 
4. 
5. 
6. 
Order
Reproduction
Growth and development
Energy utilization
Responsiveness to environment
Evolutionary adaptation
1. Order
! 
Molecules in living organisms
exhibit order.
◦  Arranged in patterns that
make cell structures
! 
The presence of order in living
organisms is a necessary, but not
a sufficient condition for life.
2. Reproduction
! 
Living organisms reproduce or are products of reproduction.
◦  All organisms have finite lifetimes and eventually die. Reproduction
corrects for this.
! 
Types of reproduction
organism
◦  Between human beings - sexual
◦  Bacteria reproduce by cell division – asexual
! 
Exceptions:
◦  Mule is sterile and cannot reproduce even though it is a reproduction
between a horse and a donkey
◦  Viruses are incapable of reproducing by themselves, need a living
organism
◦  Prions are infectious proteins (mad cow disease)
Amoeba – single cell
3. Growth and development
! 
Living organisms grow and
develop in patterns in part by
heredity, traits passed to an
organism from its parents
4. Energy utilization
!  Energy utilization in living
organisms is one of the most
basic requirements of life,
without which organisms could
not maintain order, growth and
reproduction.
5. Responsiveness to environment
! 
Life interacts with the surroundings and responds to environmental
changes
◦  Exception: human-made devices (thermostats)
6. Evolutionary adaptation
!  Life evolves as a result of the interactions between organisms and the
environment, leading over time to evolutionary adaptations that make
species better suited for the environment
◦  When the adaptations are significant, these organisms may be very
different from their ancestors – they may constitute a new species
Evolutionary adaptation
! 
Species: groups of organisms that are genetically distinct
◦  Precise border between species not always clear (especially in e.g.,
microorganisms)
! 
Once a species is identified it is given a name consisting of two parts:
Genus (generic) and Species (specific)
◦  Homo Sapiens, Homo Neanderthalensis
◦  Horses and donkeys: Equus Caballus and Equus Asinus
Of the six basic properties of life, biologists consider the most fundamental
to be evolutionary adaptation.
Evolution is an ancient idea
!  Anaximander (610-547 BC): life arose in water and evolved in more
complex forms
! 
Empedocles (429-432 BC): creatures poorly adapted will die
! 
Jean Baptiste Lamark (early 1800s): relationship between fossils and living
organisms.
◦  Life forms evolved by gradually adapting to perform successfully in
their environment; not able to explain how evolution worked
! 
Charles Darwin: The origin of species, 1859
Darwin’s The origin of species:
Overproduction and
competition for survival
1. 
◦ 
2. 
Population has potential to produce
more offspring than the
environment can support (food,
shelter) => competition for survival
Individual variations
◦  Individuals vary in many heritable traits, some traits make them better
to compete for survival
Conclusion: Unequal reproductive success = natural selection
Individuals whose traits best enable them to survive and reproduce will (on
average) leave the largest number of offspring that in turn survive to
reproduce
Evidence for evolution by natural selection:
Darwin backed up his claim by documenting cases of related organisms
adapted to different environments of lifestyle.
The different species of finches from Galapagos evidence of Darwin’s theory
of natural selection because they have evolved adaptations from a common
ancestor to suit the environmental conditions on different islands.
! 
Darwin discovered fossils of extinct organisms related to modern
organisms (giant armadillos)
! 
Realized that natural selection explains differences not only between
closely related modern species (finches), but larger changes can occur
over a long time => extinction of some species and appearance of
others.
! 
Artificial selection: selective breeding of domesticated plants and animals
by humans over few thousands of years
◦  Dog breeds with common ancestors: Rottweilers and Chihuahuas
! 
Evolutionary adaptations are related to changes that occur through time
in DNA
! 
Most basic definition of life which takes accounts of the relative
importance of the six fundamental properties of living things is that life is
something that can reproduce and evolve through natural selection.
! 
http://www.nasa.gov/vision/universe/starsgalaxies/
life's_working_definition.html
! 
Above link nice reading about whether it is suitable to define life at all –
general theory of living systems may be more useful
◦  Doesn’t yet exist
! 
Brings us to the question: Do we need to define life in order to detect it?
! 
Life may not be definable but there are characteristics of life that might
be canonical of all life and they would leave “biosignatures” .
◦  Carbon‐based
◦  Self‐replicating
◦  Uses chemical and light energy
◦  Undergoes Darwinian evolution
◦  Requires water and more than 20 elements
All studies on the origin of life rely on these characteristics as the basic
starting points.
Which of these characteristics of Earth life has the largest
likelihood of being different in extra-terrestrial carbon-based
life?
! 
Commonly believed that life can be recognized even if it is very different
from life as we know it
◦  Life can be recognized by what it does: living organisms create hallmark
molecules and create chemical disequilibrium (Nealson, University of
Southern California)
! 
The Earth-centric approach to search for life on other planetary
bodies:
◦  Follow the water, follow the energy, etc.
◦  Look for biosignatures:
"  Organic compounds that follow the rules of C chemistry found only in
organisms (living or fossils)
"  Ratios of CHON indicating life (‘redfield ratios’)
"  Isotopic fraction of C,S, N indicating metabolism
◦  Look for intact ‘cells’ enclosed in a membrane
Earth-centric approach expressed in media…
“The astounding March 11 discovery made at the Ross‐Waterhaus Observatory in
Johannesburg, South Africa comes just weeks after University of Arizona scientists announced
their findings that a huge flood swamped the Red Planet in ancient times. At least one expert
believes this proves that the Noah story is true – only it didn’t happen on Earth.The tale of
Noah may have been brought to Earth by Martian visitors in prehistoric times”
What about ‘weird’ life (different from Earth life), could we detect it?
! 
Weird life can be either modifications of Earth’s carbon based life or life
that exists out of the habitability conditions on Earth (we are not sure
about the full limits of C based life)
◦  Slightly weird: Same biochemistry but different amino acids; novel
metabolisms; different genetic code
◦  A bit more weird: Same biochemistry but utilizing novel energy
sources other than light or chemical energy
◦  Even more weird: different biochemistry, not-protein catalysis; C based
but in non-water solutions; Si based life
◦  Very weird: non-cellular life