Chapter 48
BIOCHEMICAL ORIGIN OF TERRESTRIAL LIFE

Lecturer—J. LEDERBERG

PRE-LECTURE ASSIGNMENT sally,
1. Quickly review notes for the previous lec- b. DNA is the carrier of genetic informa-
ture. tion.
2. Suggested readings: 2. At a higher level of organization, the chro-
a. General genetics textbooks mosome and the remarkably uniform pro-
Altenburg: Chap. 27, pp. 487-488. cess of mitosis are found both in plant and
Srb and Owen: Chap. 1, pp. 1-7. animal cells.
b. Additional references 3. It is concluded, then, life has had a com-

 

Lederberg, J. 1959. A view of genetics. mon plan ever since plants and animals be-
Stanford Med. Bull., 17: 120-132. This came separate nearly a billion years ago.
Nobel Prize lecture is published also in 4. Nucleic acid and protein are, perhaps, the
"Science", most durable geochemical. features of the
Lederberg, J., and Cowie, D. B. 1958. earth.

Moondust. Science, 127: 1473-1475. C. Minimum requirements for the first organism
Miller, S. L., and Urey, H. C. 1959. 1. It must be self-reproducing, and capable of
Organic compound synthesis on the prim- mutation (heritable changes).

itive earth, Science, 130: 245-251. 2. The mutants must be subjected to a natural
Oparin, A. I. 1957. The origin of life selection-that retains the fitter forms.

on the earth. 3rd Ed. New York: Aca- 3. The free-living bacterium, too complex to
demic Press. have arisen spontaneously, must be the re-
Sinton, W. M. 1959. Further evidence sult of a previous evolution of complexity.
of vegetation on Mars. Science, 130: 4. The replication of DNA in vitro (Kornberg)
1234-1237, would be one of the simplest systems sub-
ject to evolution. However, complex nu-
LECTURE NOTES cleoside triphosphates and other accessor-
A. Terrestrial life and its origin ies are required for this synthesis.
1. The universe is about 10 billion years old. 5. Accordingly, the primary living material
2. The earth is about five billion years old, may have been DNA synthesized by some

and has a fossil record only for the past one
billion years or so.

3. The absence of fossils before this time re-

quires a synthetic attempt to reconstruct

a. the features of the earth when it origi-
nated.

b. the most likely features from which or-
ganisms developed.

B. Common plan of present organisms
1, Comparative biochemistry of present higher

plants and animals, bacteria, and many vi-
ruses shows that
a. the same amino acids are found univer-

mechanism even simpler than is presently
known, or may have been even simpler than
DNA.

. Attempts should be made, aided by the

polymer industry, to construct linear poly-
mers which show some degree of self-repli-
cation.

Organic synthesis prior to organisms

1.

Not very much is learned about the evolu-
tionary steps leading to the first organism
from our present habitat, for this is, in
large measure, the result of the metabo-
lism of living forms.
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nie aa

Figure 48-1

. Living things have been responsible, for

example, for large deposits of carbon in
coal sediments and for the release of
oxygen to the atmosphere consequent to

photosynthesis.

Organisms tend to destroy large accumu-

lations of organic compounds.

For these reasons the distribution of or-
ganic compounds might have been more
complex before life started than it is

now.

2. Organic compounds
Such compounds of carbon are not made E.

only by the metabolism of organisms.
Wohler synthesized the organic com-
pound, urea, by heating the inorganic
compound, ammonium cyanate (Fig. 48-

a.

b.

1, center).

3. Oparin, about 1928, proposed that certain
organic syntheses could occur naturally in
the absence of life (Fig. 48-1, right top
portion).
a, Carbon or methane reacting with metals

produces carbides (CaCy), which upon

hydrolysis yields acetylene (CoHg9),
which when hydrolyzed yields acetalde-

hyde (CH3: CHO).
ing with hydrogen.

ammonia, would produce high

. Nitrogen can produce ammonia by react-

. Other reactions, between aldehydes and

molecular

weight polymers of considerable interest

for biological development.

4, Miller and Urey subjected mixtures of

(cacanic)

GG as H,0 Wate 7.

nies marae

eras

gases, predicated as being in the primitive

earth atmosphere, to ultraviolet light and

spark discharges. In this way, they have
produced large amounts and large varieties
of amino acids.

a. The synthesis of glycine, indicated in
Fig. 48-1 (right lower portion), is an
example.

b. The detailed chemical steps in these
syntheses are unknown, but supposedly
involve the production of highly-reactive
free-radical intermediates.

Comparative chemistry of earth and universe

(Fig. 48-1, left)

1. Most of the universe is hydrogen and heli-
um,

If all the other atoms are totaled, the ele-
ments of unique importance in organic
compounds (O, N, C)

a. comprise 80% of this total in the case of
the universe, while

b. the earth has, relatively, only traces of
carbon.

3. Whereas the universe is a good place, the
earth is a very poor place for starting an
organic chemistry which would be of bio-
logical interest.

4. Yet, despite the unlikelihood, life did de-
velop on earth!

5. Comets have been shown to contain CH,
CN, CC, and CO radicals. Such radicals
are common in organic compounds.

6. The primitive earth could have accumulated
large amounts of different complex organic

 

2.

269

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270

materials which remained undegraded until

the advent of organisms.

a, As the first organisms used up these re-
sources, there would be a selection in
favor of mutants capable of synthesizing
these organic materials from simpler or
from inorganic components.

b. In this way organisms would acquire syn-
thetic capabilities.

Search for organic compounds and life on other

planets

1. Mars

a. Astronomers have reported variations in
apparent color and texture of its surface.

b. Using the Palomar 200-inch telescope,
Sinton found infra-red spectroscopic evi-
dence for the presence there of organic
molecules of an asymmetric type.

ce. While there is, therefore, evidence for
appreciable quantities of organic mate-
rial on Mars, these may or may not be
of organismic origin.

d. Missions to or near Mars will be neces-
sary in order to determine definitely its
organic contents, the presence of DNA,
and the presence of life.

2, Accidental transplantation of terrestrial

genotypes to other planets must be avoided.
a. If a single bacterium, like Escherichia
coli, were placed on suitable medium it
would occupy a volume the size of the
earth in about 48 hours.
b. Such a premature transplantation would
be disastrous for our study of
1) the indigenous forms of life, or,
2) in the absence of organisms, the pre-
organismal evolution of organic com-
pounds,

3. Venus

a. While estimates of its temperature vary
widely, some are compatible with the
existence of life.

b, Its surface is unknown, being hidden
completely by an opaque highly-reflect-
ing cloud layer.

c. It cannot be assumed biological activity
is impossible there,

4. Moon

a. Having no atmosphere and probably no
water, the presence there of earth-like
life is out of the question.

b. It has been suggested that the moon
might act as a gravitational trap for fos-
sil spores which may have drifted be-
tween planets.

¢. Although improbable, the very possibility
of an interplanetary gene flow is too im-~
portant to ignore in our plans to exploit
and explore space.
G. The genetics of the minutest organisms and
the replicative powers of DNA have cosmic
importance.

POST-LECTURE ASSIGNMENT

1. Read the notes immediately after the lec-
ture or as soon thereafter as possible,
making additions to them as desired.

2. Review the reading assignment.

3. Be able to discuss or define orally or in
writing the items underlined in the lecture
notes.

4. Complete any additional assignment.
QUESTIONS FOR DISCUSSION

48. 1. Discuss the geochemical evolution of the
earth from its origin to the time the first
gene was formed.

48. 2. Discuss the gene as the basis of life.

48. 3. What properties would you predict for
genes present on other planets ?

48. 4. What evolutionary processes do you imag-
ine took place on earth between the origin of
the first gene and the occurrence of the first
free-living organism?

48. 5. Do you suppose other planets have forms
of life superior to ours? Explain your an-
swer.

48. 6. What information might we obtain about
life on other planets without leaving our own?

48. 7. What genetic predictions would you make
for marriages between earth humans and
planetary "humans"?

48. 8. What specific suggestions did Lederberg
make with regard to future research on the
origin of life?

48. 9. Do you believe planetobiological research
should be supported regardless of cost? Ex-
plain.

48.10. Would superhumans on some other planet
be likely to beam radio signals specifically
to the earth? Why?

48.11. Are our present human genotypes adapted
for living on Mars? Explain.

48.12. What would you predict about an organism
which drifted to the moon from another plan-
et?

48.13. Is RNA likely to be a basic component of
the first organism on our own or on other
planets? Explain.

48.14, In what respects do the earth's minutest
organisms and DNA have cosmic importance ?

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