Encyclopedia of Evolution.pdf - Online Reading Center

origin of life
they refer to life today. The Book of Genesis, for example,
said nothing about bacteria; and the Genesis term breath of
life referred to animal life. Even today, people use the word
many different ways. In the debate about whether cloned
human embryos are alive, the disagreement is not over biological
life (which they certainly have) but human life. This
confusion is even greater when investigating the origin of life
on the early Earth.
The following are some of the characteristics that most
scientists have in mind when they refer to life (see biology):
A. Life consists of complex carbon-based molecules. All
life on Earth is based upon the element carbon (C). Is it possible
that life in a different part of the universe might be based
upon a different element—or perhaps on pure energy, rather
than matter? The second possibility cannot now be investigated.
However, the element silicon (Si) has some chemical
properties similar to carbon. Might life somewhere in the universe
be based upon silicon instead of carbon? This is unlikely
to occur, because:
• Silicon is much heavier than carbon, therefore an active,
moving organism made out of complex molecules with silicon
would need to live in a world with much less gravity.
Such a planet would probably also have insufficient gravity
to retain an atmosphere.
• Carbon is able to cycle through our planet, from CO2 in
the air, through photosynthesis, through the food chain,
through decomposition, and back into the air, because CO2 is a gas. SiO2, however, is a mineral (quartz); a food chain
probably could not be based upon it.
• Silicon-based molecules would have limited complexity.
Although some molecules involved in biological processes
are simple (such as nitrous oxide that functions in the human
body), most biologically active molecules are complex.
Perhaps, therefore, carbon-based life is the only possibility.
Fortunately, there is a lot of carbon in the universe.
B. Life operates in a water medium. A solid medium
would be too slow, and a gaseous medium too chaotic, for
life processes. Water appears to be the only liquid abundant
enough in the universe and suitable as a medium for biological
processes, although some scientists speculate that liquid
methane (as on one of Saturn’s moons) may be a suitable
medium.
C. Life-forms obtain energy and matter from the environment
and process it into new forms, releasing waste products.
D. Life-forms have the ability to organize themselves,
rather than depending on externally imposed structure.
E. Life-forms have the ability to respond to environmental
conditions and information.
F. Life-forms have a genetic system, which stores information
for structures and functions:
• The genetic information controls the growth and maintenance
of the organism.
• The genetic information is copied when reproduction
occurs.
• Mutations in the genetic information allow evolution to
occur.
G. Life-forms are enclosed within membranes that keep
their processes distinct and separate from the environment.
Today, all life processes are encapsulated within cells. Cell
membranes prevent biological systems from being disrupted
and dispersed into the environment around them.
One of the simplest definitions of life, yet still complex,
was offered by biochemist Leslie Orgel. He refers to life-forms
as Citroens (making humorous reference to the French automobile
Citroën): Complex Information Transforming Reproducing
Objects That Evolve by Natural Selection. Within a
century, therefore, the scientific community has changed from
considering natural selection as a hypothesis to making natural
selection a part of life’s very definition.
When
In his famous 1954 article, biochemist George Wald said that
time “is the hero of the plot.” Given enough time (billions of
years between the formation of the Earth and the first complex
life-forms), the impossible becomes possible, the possible
becomes likely, and the likely becomes inevitable. Scientists
now know that this is not what happened. Life could not
have originated slowly, over the course of billions of years; it
had to originate quickly:
• Cells were probably in existence by 3.5 billion years ago,
which was not very long after the oceans themselves
formed. Evolutionary scientist J. William Schopf has found
evidence, albeit controversial, of cells in rocks of the 3.5billion-year-old
Apex chert of Australia. From these, and
other rocks of similar age from around the world, Schopf
claims to recognize at least 11 different kinds of cells,
which resemble modern cyanobacteria (see bacteria, evolution
of).
• The oldest sedimentary rocks in the world—3.8 billion
years old—come from the Isua formation of Greenland.
These rocks contain no fossils but do contain carbon compounds
with an isotope ratio that suggests that it is of biological
origin (see isotopes).
• Until 3.9 billion years ago, the solar system was filled
with errant asteroids and comets that crashed into
the planets. On Earth, evidence of this bombardment has
been largely erased by erosion; however, on the Moon,
the craters have been preserved. Most of the craters were
produced more than 3.9 billion years ago, although a few
craters such as Tycho were produced more recently. Collisions
with asteroids may have vaporized any oceans, and
any life they might have contained. The collision between
the Earth and another planet, 4.4 billion years ago, which
ejected part of the Earth’s crust (which became the Moon),
would certainly have done so. The water now in the
oceans was delivered later, perhaps by comets, which consist
mostly of ice.
It appears that the origin of life is bracketed between 3.9
billion years ago, before which life would have been exterminated,
and 3.8 billion years ago, by which time life was
already in existence.