Astronomy Chapter 15: Life on Other Worlds
THE NATURE OF LIFE
• Life is a process by which an organism extracts energy from the surroundings,
maintains itself, and modifies the surroundings to foster its own survival and
reproduction
• All living things on Earth, share certain characteristics in how they perform the
process of life
The Physical Basis of Life
• The physical basis of life on Earth is the element carbon
• They can form long, complex, stable chains that are capable of storing and
transmitting information
Information Storage and Duplication
• Almost every action performed by a living cell is carried out by chemicals it
manufactures
• The “DNA: The code of life” identifies three major points:
1. The chemical recipes of life are stored in each cell as information on DNA
molecules that resemble a ladder with rugs that are composed of chemical
bases, providing instructions to guide chemical reactions within the cell
2. DNAinstructions normally are expressed by being copied into a
messenger molecule called RNAthat causes molecular units called
proteins. Proteins serve as the cell’s basic structural molecules or as
enzymes that control chemical reactions
3. The instructions stored in DNAare genetic information passed along to
offspring. The DNAmolecule reproduces itself when a cell divides so that
each new cell contains a copy of the original information
Modifying the Information
• If the information stored in DNAcould not change, then life would quickly go
extinct
• The process by which life adjusts itself to its changing environment is called
biological evolution
• When an organism reproduces, its offspring received a copy of its DNA
• Sometimes external effects such as radiation alter the DNAduring the parent
organism’s lifetime, and sometimes mistakes occur in the copying process
• Offspring born with random alternations to their DNAare called mutants
o Amutation can actually help an organism survive
• Differing rates of survival and reproduction are examples of natural selection
• Over time, the beneficial variation becomes more common and a species can
evolve until the entire population shares the trait
• Natural selection adapts species to their changing environments by selecting, from
the huge array of random variations, those that would most benefit the survival of
the species
• Evolution is not random
• Natural selection is not random LIFE IN THE UNIVERSE
• The key to understanding the origin of life lies in the process of evolution
• This means that life on Earth could have begun very simply, even in as simple a
form as carbon chain molecules able to copy themselves
The Origin of Life on Earth
• The oldest fossils are all the remains of sea creatures, and this indicates that life
began in the sea
• Individual organisms were microscopic from WesternAustralia that are more than
3 billion years old contain features that experts identify as fossil stromatolites,
which are remains of colonies of single-celled organisms
• Simple organisms lived in Earth’s oceans 3.4 billion years ago
• The Miller experiment consisted of a sterile, sealed glass container holding
water, hydrogen, ammonia, and methane
o An electric arc inside the apparatus created sparks to simulate the effects
of lightning in Earth’s early atmosphere
o They found that the interaction between the electric arc and the simulated
atmosphere had produced many organic molecules from the raw material
of the experiment, including such important building blocks of life as
amino acids
o Earth’s early atmosphere probably consisted of carbon dioxide, nitrogen,
and water vapour instead of the mix of hydrogen, ammonia, and methane
assumed by Miller and Urey
o The Miller experiment is important because it shows that complex organic
molecules form naturally in a wide variety of circumstances
• Lightning, sunlight, and hot lava pouring into the oceans are just some of the
energy sources that can naturally rearrange simple common molecules that make
life possible
• If you could travel back in time, you would expect to find Earth’s first oceans
filled with a rich mixture of organic compounds called the primoridial soup
• Amino acids can link together proteins by joining ends and releasing a water
molecule
• Because some molecules are more stable than others, and some bond together
more easily than others, scientists hypothesize that a process of chemical
evolution eventually concentrated the various smaller molecules into the most
stable larger forms
• Eventually, a molecule formed that could copy itself
• At that point, the chemical evolution of molecules became the biological
evolution of living things
• Radio astronomers have found a wide variety of organic molecules in the
interstellar medium, and similar compounds have been found inside meteorites,
such as the “Murchison”
• The first reproducing molecules could have been formed by natural processes • The first cells must have been simple single-celled organisms much like modern
bacteria
o These kinds of cells are preserved in stromatolites, mineral formations
formed by layers of photosynthetic bacteria and shallow ocean sediments
• Over the course of eons, the natural processes of evolution gave rise to stunningly
complex multicellular life forms with their own widely differing ways of life
Geologic Time
• Life has existed on Earth for at least 3.4 billion years
• There is no evidence of anything more than simple organisms until about 540
million years ago
• This sudden increase in complexity is known as the Cambrian explosion, and it
marks the beginning of the Cambrian period
• Life in a year:
st
o Jan 1 – world began
o March/earlyApril – signs of life
o November – complex organisms of the Cambrian explosion
o November 28 – life on land
th
o December 12 – dinosaurs walking the continents
o December 26 – dinosaurs extinct, mammals and birds on the rise
o December 31 – first humanoids, making the first stone tools
• Tremendous amounts of time were needed for the first simple living things to
evolve in the oceans
Life in our Solar System
• Liquid water seems to be a requirement of carbon-based life, necessary for both
vital chemical reactions and as a medium to transport nutrients and wastes
• Water is a cosmically abundant substance with properties such as heat capacity
that set it apart from other common molecules that are liquid at the temperatures
of planetary surfaces
• The moon and Mercury are airless, and water would boil away into space
immediately
• Venus has traces of water vapor in its atmosphere, but its is too hot for liquid
water to exist on the surface
• The Jovian planets have deep atmospheres, and at a certain level water condenses
into liquid droplets
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