Chapter 8 – Origins of Life
8.1 Life as a Geologic Force
 Rather than originating in a single event, life may have arisen through a series of
steps, each building upon the complexity established by the previous mechanism
 During its evolution, life may have undergone evolutionary selection for certain
metabolic pathways over others
o Some of these pathways (such as photosynthesis) would eventually start to
act as a geologic force by exchanging matter and energy with physical and
chemical systems (such as the rock and hydrologic cycles)
8.2 Early Theories of the Origin of Life: Spontaneous Generation and Panspermia
 The theory of spontaneous generation arose from still earlier concepts regarding
the distinction between living and nonliving things
o This theory holds that living entities are generated by the power of nature,
and new living forms are constantly being generated from nonliving ones
 The results of Pasteur’s microbiological experiments demonstrated that bacteria
could only arise from other bacteria
o This resulted in the demise of the spontaneous generation theory
 The ongoing quest to understand the origin of life led to panspermia, that life
came from chemical compounds or even bacterial spores from space (brought by
meteorites and comets)
o Panspermia shares some aspects of the steady state theory of the universe
8.3 What Is Life?
 Basic Traits of Life
o A living organism is an open system that exchanges matter and energy
with the surrounding environment
o Furthermore, living organisms are surrounded by a cell membrane, which
acts as a barrier to separate the cell contents from the surrounding
environment
o Living systems are also able to reproduce themselves from nonliving
matter and energy taken from their environment
o Living systems are more complex than nonliving (they consist of many
more compartments)
o Living systems store and process information and transmit it to their
offspring
o Living systems can evolve
 Composition of Life
o Life is based on carbon; many molecules of living organisms are based on
long chains of carbon atoms
o Carbon is prominent because it formed early during nucleosynthesis, so it
is abundant in the universe; it is also versatile, forming one or more
covalent bonds with hydrogen, nitrogen and sulfur
o Nitrogen resembles carbon in that it is abundant (the main component of
Earth’s atmosphere), forms multiple covalent bonds with other elements,
and thus can form a variety of complex compounds (such as amino acids
and proteins)
© 2013 Jones and Bartlett Learning, LLC
o This is also true of sulfur and phosphorous; these elements may have been
involved in prebiotic energy storage and transfer
o Hydrogen is the most abundant (and was the first) element in the universe;
it forms weak bonds with other molecules, making it ideal for many
biochemical reactions
8.4 Chemical Evolution
 Early Theories
o Darwin and Huxley were early supporters of chemical evolution: that
simple precursors could be assembled to produce life
o Two basic approaches to chemical evolution are bottom-up (building
blocks are synthesized and then assembled) and top-down (existing
biological systems and biochemical pathways are extrapolated backward
to simpler components and systems)
o Miller and Urey’s experiment lent support to earlier ideas regarding the
synthesis of organic molecules under conditions present on the early Earth
(the bottom-up approach); later studies indicated that different (less
reducing) conditions actually existed
o Recent research has focused on other mechanisms or sites where prebiotic
compounds might have been concentrated
o Clay minerals may have been involved in concentrating substances and
catalyzing their reaction
o Life could have developed in one of a number of different settings, such as
hot springs
o Many life forms in modern hot springs are extremophiles (survive under
conditions hostile for most life forms) and/or chemoautotrophic (rather
than use light to manufacture carbon compounds, they obtain energy from
chemical reactions)
 Hydrothermal Vents and the Pyrite World
o Deep sea vents are a promising site for the origin of life because they
incorporate a number of different aspects of early evolution into a single
hypothesis
o Early Earth was warm, so spreading centers would have been abundant
o The depth of hydrothermal vents would have made them less affected by
Hadean and Archean impacts
o The most primitive organisms are thermophiles
o Hydrothermal vents are characterized by reducing conditions
o Sulfide minerals such as iron pyrite occurred at hydrothermal events; cell
membranes, early metabolism, and the genetic code are all suggested to
have originated on the surface of pyrite or other sulfide minerals
 The RNA World
o The Central Dogma of biology is DNARNAprotein
o The RNA world scenario holds that RNA and protein synthesis appeared
first, and DNA evolved later as a means of storing and protecting
information, with RNA remaining as the intermediary between DNA and
protein.
© 2013 Jones and Bartlett Learning, LLC
o However, the RNA world requires the evolution of some form of primitive
metabolism before the RNA world could have appeared
 Autocatalysis
o In each of these “worlds” life may have driving its own formation through
the process of autocatalysis, in which reactants and products organize
themselves into a more complex system
o Autocatlysis is a type of self-organizing system; this type of behavior has
been observed repeatedly in laboratory settings, so there is no reason to
believe it does not occur in nature
8.5 Origin of Eukaryotic Cells
 The earliest cells were prokaryotic (lacking a nucleus)
 Eukaryotic cells (protists and higher organisms) have a distinct membranebounded nucleus and may have organelles
o Eukaryotes may have originated through endosymbiosis, the engulfment
of one prokaryotic cell by another cell
o Animal-like eukaryotic cells arose through acquisition of mitochondria,
and plant-like eukaryotic cells by engulfment of photosynthetic bacteria,
which became the forerunners of chloroplasts
8.6 Precambrian Fossil Record and Molecular Clocks
 The fossil record for the Precambrian is poor, making the question of when the
first prokaryotes and eukaryotes arose very difficult to address
o Results from protein sequencing suggest that the various groups of
prokaryotes and eukaryotes diverged no earlier than 2.0-2.5 billion years
ago (a billion years later than what the fossil record suggests)
o Scientists are attempting to use molecular clocks compare the sequences
of RNA and proteins of prokaryotes and eukaryotes to answer this
question
© 2013 Jones and Bartlett Learning, LLC