Chapter 15: Life on Other Worlds
15.1 The Nature of Life
• The physical basis of life on Earth is the element carbon
• Carbon atoms bond to each other and to other atoms. Therefore, the can form long,
complex stable chains that are capable of storing and transmitting information.
• Large amounts of information are necessary to maintain the forms and control the
functions of living things
• Although, carbon may not be crucial to life
• Science fiction authors have speculated that silicon can be substitutes for carbon
because the two elements share similar properties. But this is unlikely because silicon
chains are harder to assemble and disassemble than carbon and can’t be as long.
• Other speculations: electromagnetic fields and ionized gas.
Information Storage and duplication
• Almost every action that is performed by a living cell is carried out by the chemicals the
• The chemical recipes of life are stored in each cell as information on DNA molecules that
resemble a ladder with rungs that are composed of chemical bases. They provide
instructions to guide chemical reactions within the cell
• DNA instructions normally are expressed by being copied into a messenger molecule
called RNA that causes molecular units called amino acids to be connected into large
molecules called proteins. Proteins are the cell’s basic structural molecules or are
enzymes that control chemical reactions.
• Genetic information stored in DNA is passed along to offspring.
Modifying the Information
• Biological evolution: The process of mutation, variation, and natural selection by which
life adjusts itself to its changing environment
• Mutant: Offspring born with DNA that is altered relative to parental DNA
• Natural selection: The process by which the best genetic traits are preserved and
accumulated, allowing the fittest organisms and species to survive.
• A copy of an organisms DNA is received by its offspring
• External influences such as radiation can alter DNA during the parent organisms lifetime
and mistakes can then occur in the DNA copying process.
• Most mutations make no difference but some are fatal. But on rare occasions, a
mutation could actually help and organism to survive. • These changes produce variation among the members of a species (eg. Squirrels could
have varying tail lengths or faster growing claws)
• Variations make nearly no difference until the environment changes (eg. Squirrels with
thicker coats would be more likely to survive in colder climates)
• Natural selection
15.2 Life in the Universe
The Origin of Life on Earth
• Stromatolite: A layered formation caused by mats of algae or bacteria combined with
• The oldest fossils are all the remains of sea creatures, indicating that life began in the
• Fossils that are billions of years old are hard to recognize because the earliest living
things contained no bones or shells.
• Scarce evidence indicates that simple organisms lived in the ocean 3.4 billion or more
years ago (that’s less than 1.2 billion years after the Earth formed!)
• Miller Experiment: An experiment that attempted to reproduce early conditions on Earth
and showed how easily amino acids and other organic compounds can form
• The experiment was important because it showed that complex organic molecules form
naturally in a wide variety of circumstances.
• Lightning, sunlight, and lava pouring into the ocean – energy sources that can naturally
rearrange simple common molecules into the complex molecules that make life possible.
• Primordial Soup: The rich solution of organic molecules in Earth’s first oceans
• Many of the organic compounds would have been able to link up to form larger
• The complex organic molecules were still not living things. They did not reproduce but
instead linked together and broke apart at random
• Chemical evolution: The chemical process that led to the growth of complex
molecules on primitive Earth. This did not involve the reproduction of exact molecules
• According to hypothesis, after a sufficient time, a molecule formed that could copy itself
somewhere in the oceans.
• An alternate theory is that the origin of life could have arrived here from space.
• Organic molecules in the interstellar medium have been found and similar compounds
have been found inside meteorites • The Miller experiment showed how easy it is to create organic molecules in a hydrogen-
right environment so it is not surprising to find them in space.
• Experiments have shown that microscopic spheres the size of cells containing organic
molecules form relatively easily in water.
• The first cells must have been simple single-celled organisms much like modern
• These kinds of cells are preserved in stromatolites.
• Stromatolites formed in rocks with radioactive ages of 3.4 billion years.
• Multicellular: An organism composed of more than one cell
• 540 million years ago, life suddenly branched into a wide variety of complex forms.
• Cambrian explosion: A geologically brief period about 540 million years ago during
which fossil evidence indicates Earth life became complex and diverse. Cambrian rocks
contain the oldest easily identifiable fossils.
• Humanoid creatures have walked the Earth for about four million years.
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.
• Therefore, it is not surprising that life formed