Encyclopedia of Evolution.pdf - Online Reading Center

0 Origin of Species (book)
ribose); it works in a fashion similar to DNA and RNA;
and it can bind with DNA and RNA, which means that it
could have transferred its genetic information to RNA during
the evolution of a new RNA-based life-form. In 2000,
Israeli scientists proposed that the first genetic system may
have consisted of lipid-like molecules that can not only
form cell-like structures that grow, but also pass information
into the new cells.
• It is widely accepted, following the lead of biochemist
Manfred Eigen, that an RNA-based genetic system would
have preceded a DNA-based system, for several reasons.
First, modern cells make DNA bases out of RNA bases.
Second, all modern cells use modified ribonucleotides as
the basis for some of their essential metabolic chemistry,
such as ATP and NADH, and some modern enzymes need
small RNA molecules to help them carry out their reactions.
Third, biochemists Thomas Cech and Sidney Altman,
who won the Nobel Prize for Chemistry in 1989, showed
that RNA can act as an enzyme (they are called ribozymes).
That is, RNA can be both genotype and phenotype. Some
organisms, such as the protist Tetrahymena thermophila,
use ribozymes. Many scientists consider the ribosome,
which is built of both RNA and protein, and in which it is
the RNA that has catalytic activity, to be a remnant of the
time when RNA was the blueprint of life, and in the laboratory,
RNA can catalyze its own reproduction. Strings of
RNA containing guanine result when RNA molecules containing
cytosine are used as a template. Leslie Orgel calls
this the “molecular biologist’s dream.” More than 20 different
ribozymes have been produced from random RNA
mixtures followed by a selection experiment, including a
ribozyme that synthesizes a nucleotide from a base, and
another ribozyme that makes more RNA.
Chemist Walter Gilbert proposed the RNA world scenario
in 1986. In this scenario, the primordial seas were filled
with RNA molecules that replicated themselves and therefore
constituted a primitive form of life. Later, according to
evolutionary biologists John Maynard Smith (see Maynard
Smith, John) and Eörs Szathmáry, these RNA molecules
were assisted by amino acids, which resulted in the origin of
the genetic code. Still later, complex DNA replaced simple
RNA as a more stable form of genetic information—but living
cells never got rid of the RNA completely. This process
would occur more efficiently if the RNA molecules consisted
of bases that were all left- or all right-handed, which would
explain the origin of handedness in the genetic molecules. In
RNA selection experiments, smaller RNA molecules replicate
faster than larger ones, which makes it difficult to explain
how complex molecules could have arisen in an RNA world.
The last universal common ancestor (LUCA) of all cells that
are alive today stored genetic information in DNA, but the
original life-form may have used RNA or an even earlier
chemical basis.
C. Formation of the first cells. The final step in the origin
of life would be to explain how the life reactions could have
been isolated into cells—a step necessary to keep the waves
of the ocean from separating and diluting them. Most mod-
ern cell membranes are made from phospholipids, which are
molecules that can bridge the gap between fatty and watery
molecules. Lipid-like molecules are today found in sea foam.
Some scientists suggest that this sea foam, in shallow primordial
ponds, may have formed the first cell membranes. Other
scientists have formed micelles, which are clusters of molecules
that carry out chemical reactions and replicate themselves,
in the laboratory, and they propose that the first cells
may have resembled these micelles.
At some point, the origin of cells must be explained.
Life may have been in operation for a long time before it was
compartmentalized into cells, according to Carl Woese, who
has described a life state that preceded life-forms.
In conclusion, scientific research has illuminated many
possibilities for the origin of life, in particular answers to
the questions of when and where; but for now scientists will
have to be satisfied with not knowing a definite answer to the
questions of how.
Further Reading
Busemann, Henner, et al. “Interstellar chemicals recorded in organic
matter from primitive meteorites.” Science 312 (2006): 727–730.
Chen, Irene A. “The emergence of cells during the origin of life.” Science
314 (2006): 1558–1559.
Davies, Paul. The Fifth Miracle: The Search for the Origin and Meaning
of Life. New York: Simon and Schuster, 1999.
Dick, Steven J., and James E. Strick. The Living Universe: NASA and
the Development of Astrobiology. New Brunswick, N.J.: Rutgers
University Press, 2004.
Hazen, Robert M. Genesis: The Scientific Quest for Life’s Origin.
Washington, D.C.: Joseph Henry Press, 2005.
Kasting, James F. “When methane made climate.” Scientific American,
July 2004, 78–85.
Lazcano, Antonio. “The origins of life.” Natural History, February
2006, 36–41.
Lunine, Jonathan. Astrobiology: A Multi-disciplinary Approach.
Upper Saddle River, N.J.: Addison-Wesley, 2004.
Marty, Bernard. “The primordial porridge.” Science 312 (2006):
706–707.
Schopf, J. William. “Microfossils of the early Archaean Apex chert:
New evidence of the antiquity of life.” Science 260 (1993): 620–
646.
———, ed. Life’s Origin: The Beginnings of Biological Evolution.
Berkeley: University of California, 2002.
Shapiro, Robert. Origins: A Skeptic’s Guide to the Creation of Life
On Earth. New York: Bantam, 1986.
Tian, Feng, et al. “A hydrogen-rich early Earth atmosphere.” Science
308 (2005): 1,014–1,017.
Wald, George. “The origin of life.” Scientific American, August
1954, 44–53.
Warmflash, David, and Benjamin Weiss. “Did life come from another
world?” Scientific American, November 2005, 64–71.
Origin of Species (book) On the Origin of Species by
Means of Natural Selection was Charles Darwin’s famous
1859 book that is considered the founding document of modern
evolutionary science. It is considered one of the most
important books ever written. Evolutionary scientist Ashley