|
Items in cart [0] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 122
THE LIFE SCIENCES
diversity of a single crop is preferable to reliance on a single genetic strain);
to indicate the possibilities of utilization of new species of plants and animals
as major foodstuffs for man (e.g., the many ungulates of Africa, the Saiga
antelope, originally from Alaska, which is much more efficient than sheep
and goats at cropping the tundra, or red deer, which are more successful
on rocky islands than are sheep); to maximize the food harvest from the
oceans and larger bodies of fresh water (the manatee, consumer of water
hyacinth, one of the most productive of crops, looks particularly promis-
ing); to enable us to predict adequately the consequences of increase in
the consumption of fossil fuels as opposed to increase in the utilization of
nuclear power; and to enable us to protect naturally or deliberately im-
pounded bodies of fresh water-examples without end.
Man has claimed this planet as his own. In so doing, he must accept
responsibility for the multitudes of species that he has displaced and that
he husbands. The planet can never again return to the circumstances that
obtained when Homo sapiens was a small wandering clan of hunters. Nor
is there any reason to think that desirable. But it can be preserved in beauty
with an immense and diverse flora and fauna, while supporting its human
population, provided sufficient ecological sophistication is brought to bear.
It is regrettable that the need for such understanding has become imperative
so early in the life of this young science, which warrants all the support
our society can provide.
THE ORIGIN OF LIFE
The origin of life is the least understood aspect of biological evolution.
Significant progress in untangling this puzzle has been made in recent years.
This progress, stemming from advances in such diverse fields as cosmology,
geochemistry, and molecular genetics, together with the search for extra-
terrestrial life, which is one of the prime objectives of the national space
program, has now heightened interest in this central problem.
In the past, discussions of the origin of life tended to be entirely specu-
lative exercises, often tinged with superstition. But this topic has now
become a problem for legitimate inquiry, subject to the same intellectual
discipline as other attempts to understand evolutionary processes, including
the requirement for logical elaboration of hypotheses, avoidance of arbi-
trary assumptions, and recourse to observation and experiment. Unfortu-
nately, knowledge of the terrestrial environment in the remote past is
uncertain and, as the history of this question shows, is liable to drastic
OCR for page 123
FRONTIERS OF BIOLOGY
revision from time to time as new evidence accumulates. In any case, it is
impossible to duplicate or approximate the geological time scale, as well
as the variety of conditions and the secular changes in these conditions
that have occurred during the Earth's history. Because of such constraints,
the.most one can hope to claim for conclusions on this subject is a high
degree of plausibility. In this respect, however, studies of the origin of life
differ in degree but not in kind from other scientific investigations.
Life is not one of the fundamental attributes of the universe, like matter,
energy, or time, but is a manifestation of certain molecular combinations.
These combinations cannot have existed forever, since even the elements
of which they are composed have not always existed. Therefore, life must
have had a beginning. Current views of the origin of life differ funda-
mentally from those of preceding centuries in that they are concerned with
the origin of these molecular combinations rather than of organisms en-
dowed with mysterious properties. From this standpoint, the origin of life
must be viewed as a historical incident in the evolution of our planet,
i.e., as an event limited in place and time by prevailing physical and chem-
ical conditions.
The unique attribute of living matter, from which all its other remarkable
features derive, is its capacity for self-duplication and mutation. Living
systems reproduce, mutate, and reproduce their mutations. The endless
variety and complexity of living organisms and the seeming purposefulness
of their structure and behavior are consequences of their mutability. Any
system that has the capacity to mutate randomly in many directions and to
reproduce those mutations must evolve.
On the basis of various geological dating methods, it is estimated that
the earth was formed about 4.5 billion years ago. The first hard-shelled
animals in the fossil record appear at the beginning of the Cambrian, about
0.7 billion years ago. It is clear that life was present well into the Pre-
Cambrian, a period lasting 3.8 billion years, but one cannot yet say how
far back. The time when life started is an important parameter because,
by difference, it provides the time scale for the organic synthetic reactions
leading up to the origin of life. Paleontological evidence comes from ex-
amination of various Pre-Cambrian rocks for their fossil remains and organic
content. These include the Nonesuch shale of northern Michigan, the
Gunflint chert of Ontario, the Soudan shale of northern Minnesota, Bula-
wayan limestone of Southern Rhodesia, and the Fig-Tree chert of the
Transvaal. All contain isoprenoid hydrocarbons, particularly pristane and
phytane, both of which are breakdown products of chlorophyll, and per-
haps of other biological molecules. The oldest, the Fig-Tree chert, is three
billion years old and contains small amounts of these hydrocarbons as well
123
OCR for page 124
THE LIFE SCIENCES
as microfossils of bacterial and algal size and form. If these are genuine
fossils and residues of biological activity and are not the consequence of
subsequent contamination, then life started between 3 and 4 billion years
ago.
The conditions on the earth's surface at that time are not entirely certain.
But all the indications are that the atmosphere was unlike that of the mo-
ment, that in place of oxygen, nitrogen, water, and carbon dioxide, there
were methane, ammonia, carbon monoxide, hydrogen, and lesser quan-
tities of hydrogen cyanide and formaldehyde. This is a reducing atmos-
phere, in contrast to the oxidizing atmosphere of the moment, and was
probably generated largely by outgassing of the initial solid matter of the
earth. On this assumption, a variety of experiments have been conducted
to ascertain what circumstances might have led to the kinds of organic
compounds characteristic of living material. In the first such successful
experiment, an electric discharge was passed through a mixture of ammonia
(or nitrogen), methane, and water vapor above boiling water. Aldehydes
and hydrogen cyanide were formed under these conditions, and these in
turn reacted to form detectable amounts of the amino acids, glycine,
alanine, serine, and aspartic and glutamic acids. Remarkably, these are
the very amino acids that, to the present day, are the most abundant amino
acids of proteins. By increasing the amount of hydrogen cyanide in such
preparations, and by varying the energy source to simple application of
heat or ultraviolet radiation, the number of products formed has been
extended significantly. Among these products are hydrogen cyanide and
its polymers such as dicyandiamide, which in turn have led to the forma-
tion of the major purines, guanine and adenine, and, under the correct
conditions, to guanylic and adenylic acids. It was more difficult to find
conditions that would lead to the formation of pyrimidines, but both
cytosine and uracil have been obtained in impressive yields. In yet other
experiments, at 100° C clays such as kaolin catalyze formation of such
sugars as glucose and ribose in good yields from dilute solutions of form-
aldehyde. Moreover, cyanide and its polymers have been found to
catalyze synthesis of random polypeptides from relatively concentrated
solutions of amino acids, and of random polynucleotides from mononucleo-
tides. In sum, therefore, conditions that may or may not mimic those of
the prebiotic era on this earth, but that are as close as current theory can
suggest, result in the formation of a large number of the primary building
blocks of biological macromolecules as well as primitive macromolecules,
thereby setting the stage for the origin of life.
Beyond those facts, all is speculation. The problem is to ascertain how
these noninformational proteins and polynucleotides combined to form a
OCR for page 125
FRONTIERS OF BIOLOGY
self-duplicating system. No point is served by repetition here of current
highly speculative hypotheses. It must suffice to indicate that the necessary
raw material would have accumulated somewhere on earth as a consequence
of the physical and chemical conditions on the earth's surface.
If, over the passage of 1 or 2 billion years, molecules with a very low
order of accidental catalytic activity served to catalyze further syntheses
either on their own surfaces or on the surfaces of other molecules, the
earliest beginnings would have been made. According to this concept, life
is not necessarily a highly unlikely event, but rather the almost obligatory
consequence of the zero-time conditions on the earth's surface.
One further set of concepts warrants recital. From the notion described
above, once primitive life began, and the crudest membrane surrounded
such macromolecular packages, the materials available for further trans-
formation, like those that contributed to the primitive genetic apparatus,
must have been those that were present in the original "primordial soup."
It is in that context that one finds explanation of the fact that the nucleo-
tides not only serve as the building blocks of all forms of nucleic acids, but
also participate in intermediary metabolism as the coenzymes for hundreds
of metabolic processes presumably because "they were there." ATP be-
came the energy source for cellular reactions because it was there. In time,
as the original supply of organic materials began to dwindle, selective ad-
vantage would come to those primitive cells that "learned" to synthesize
what they required from other organic materials still present in the medium.
In a metabolic pathway such as those we have considered, there is invari-
ably a set of intermediates that serve no purpose but as substrate for the
transformations that lead to the desired end product. Patently, a cell that
had "learned" to convert what we currently recognize as a starting com-
pound e.g., glucose to one of the first stages in the current biosynthetic
pathway would not have benefited at all by such an event. Synthetic path-
ways as we now know them must have evolved backward in the sense
that the next capability when guanine and adenine began to disappear
should have been an enzyme that could make use of the hypoxanthine also
formed in the primordial reactions. When that was gone, utilization of
some other substrate to make hypoxanthine would again confer high sur-
vival value. It is of interest, therefore, that aminoimidazole carboxamide,
even today an intermediate in purine biosynthesis, was also formed under
the same rudimentary circumstances that have been shown to lead to the
formation of adenine, guanine, and hypoxanthine.
Patently, the information at hand is a far cry from genuine understanding
of the origin of life, but it may well be that an important beginning has
been made.
Representative terms from entire chapter:
amino acids
