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OCR for page 81
4
Evolution and the
Biosphere
Rae
_ _ he topics coverer] in the first three chapters of this
book genetics, clevelopment' and neuroscience can all be seen in
two complementary lights. On the one hand, they are biological pro-
cesses, complete and consistent in themselves. Biologists may not totally
unclerstand these processes, but they can stucly them and try to explain
them in biological terms. This is the how of biology: how is a given
biological system constructed, how does it function?
At the same time, biological processes are the products of evolution,
with antecedents billions of years oIcI. Biologists studying these pro-
cesses from an evolutionary perspective are trying to answer the why
of biology: why has a given system evolved as it has, what historical
forces account for its nature?
It would be a mistake, however, to partition biological topics into the
categories of Show it works" ant! Why it evolved." While the actions
of molecules may be the most immediate explanation of a biological
process, evolution is the ultimate explanation of that process. This is
what makes evolution the primary unifying theme of biology. It accounts
for the simultaneous diversity and unity of life, for the differences and
similarities observed in organisms. No other modern idea has done so
much to change our view of the biological world and our place in that
world. To cite the title of a well-known essay by the prominent twentieth-
century geneticist Theoclosius Dobzhansky, Nothing in biology makes
sense except in the light of evolution."
Darwin's theory of evolution has been refined and strengthened since
he first proposed it in 1859. Yet the central concept of Darwin7s theory
8
OCR for page 82
remains the cornerstone of modern evolutionary theory natural selec-
tion. Darwin recognized that natural selection requires two contrasting
forces. The first is a mechanism to generate the tremendous amount of
variation that he observed among members of a species. The second] is
a process whereby some individuals succeed in passing on their genes
to the next generation and others do not. in this way, nature selects
traits more fitted to the environment, and those traits tent! to be per-
petuated. The steady accumulation of changing traits over long periods
of time, combined with changes in the environment, is the substance
of evolution.
Darwin did not know the mechanisms responsible for indiviclual var-
iation among members of a species. But the development of the science
of genetics in the twentieth century has i',rtifi~H and rl~rifi~H may n
v -- ~ ~ ~ ~ _ ~ w ~ ~ ~ ~ ~ ~ ^ ~ ~ ~ ^ ^ ] ~ ^
1 ~ 1 1 1 · ~
nits assumptions. today we know that Individual variation arises from
mutations and rearrangements of the genes, the sequences of nucleotides
that cocle for proteins. This variability is so extensive that no two human
beings (with the exception of identical twins) are likely to have ever
hac] identical genomes.
With the explosion of molecular biology since the 1950s, the study
of evolution has progressed to the molecular level. This advance has
added a new ant! largely unanticipated level of complexity to evolu-
tionary theory. Sequencing of genes and proteins has revealed much
more variation at the molecular level than biologists hac] expected.
Genes have been found to rearrange themselves and transfer between
organisms in ways that were previously unknown. Molecular biology
has revealed that relatively little of the genome in complex organisms
codes for proteins' raising the question of what role the noncoding
portions of the genome play in evolution. It has also demonstrated the
evolutionary importance of the regulatory regions of DNA, since many
species are distinguished not so much by the proteins they produce as
by the amount of those proteins and the timing of their production.
Molecular biology has also had a large impact on more classical
studies of evolution. For instance, it has maple important contributions
to the fielct of systematics-the classification of organisms and de-
scription of their relationships. Analyses of DNA or protein sequences
can reveal differences between organisms too subtle to discern in out-
ward appearances or behavior.
Molecular biology has also come to the aid of field studies. It has
been user! to study the structure ant! activities of populations, by tracing
the flow of genes through interbreeding groups. The formation of new
species still a prominent concern in evolutionary biology-can now
be stucliec! on a genetic level. The evolutionary history of microorgan
82 SHAPING THE FUTURE
OCR for page 83
isms, once a murky area in biology, has become much clearer because
of the application of molecular techniques.
There are many aspects of evolutionary biology that cannot be ex-
plainec] on the molecular level. Natural selection, for example, is not
a molecular process, it is rather the result of interactions among different
organisms within a complex environment. in general, explanations in
evolutionary biology cannot all be reduced to a single organizational
level. They have to call upon many different levels.
These levels of organization in biology extent! up to the broadest level
of all the entire earth and its collection of living things. Organisms
do not only adapt to the environment; they also change the environment,
and in doing so they change the course of evolution. Human beings are
not the first creatures that have had the ability to remake the entire
planet and its biosphere. it has been done before, by organisms much
more humble than us. As shown later in this chapter, we would not be
here if not for these organisms. And as shown in the essay that follows
this chapter, human beings have not taken this interdependence of the
biosphere to heart in their treatment of the earth's other species.
The Evolution of Proteins
One of the most remarkable accomplishments of molecular biology
has been to trace the course of evolution in single molecules. According
to Francisco Ayala, professor of ecology and evolutionary biology at the
University of California at Irvine, this technique Was truly revolution-
ized the reconstruction of evolutionary history."
One of Darwin's boldest and most controversial conjectures was that
all organisms are clescende~l from common ancestors. in other wools,
if any two organisms living today could be traced back through evo-
lutionary time, at some point their lines of descent would converge.
The closer two organisms are in evolutionary terms, the more recent
their ancestor. The most recent common ancestor of humans and chim
panzees, for example, seems to have been an ape-like creature, now
extinct, that lived in Africa about 5 million years ago. The most recent
ancestor of humans and mushrooms was probably a single-celled or-
ganism living in or near the water over a billion years ago.
Because of their common origins, all organisms share certain met-
abolic processes. We have proteins in our bodies that serve the same
functions as proteins in mushrooms. However, these proteins are not
identical. Over time, the DNA that codes for proteins undergoes random
mutations, which supply the variation needecl for evolution. These mu
EVOLUTION AND THE BIOSPHERE 83
OCR for page 84
tations can change the sequence of amino acids in a protein replacing
a glycine molecule, for instance, with one of the other 19 amino acids
that commonly make up proteins. Therefore, as two evolutionary branches
emerge from a common ancestor, the proteins in those branches also
evolve. But because these changes are random, the proteins change in
(lifferent ways. As time goes on, the proteins become more and more
dissimilar.
Proteins cannot change unrestrictedly. If a protein is to serve a given
function, certain amino acicis must stay the same, and certain rela-
tionships among the amino acids must be maintained. If a mutation
floes alter a critical amino acic! in a protein that is essential to an
organism, the organism will not survive ant] the mutation will not be
passed on. On the other hand, if a mutation alters the function of a
protein in such a way that the fitness of the organism is enhanced, that
mutation can be selected for anc! the mutation will spread.
Some biologists have proposer! that relatively few mutations have
such a positive effect. Among those mutations that do not cause a
decrease in fitness, they argue, the great majority are simply neutral
in their effects. Such a neutral mutation may change the sequence of
amino acids in a protein, but it will not affect the protein's overall
function. Other biologists disagree, contending that natural selection
plays a much larger role in guiding protein evolution than the believers
in the so-callLe(1 neutrality theory claim.
A Molecular Clock
If the neutrality theory were correct, Ayala observes, it would have
profound implications for the stu(ly of evolutionary relationships. It
would mean that random changes are incorporate(1 in DNA at a more
or less constant rate. The interval between changes in a given protein
would vary, in keeping with their random nature. But over a lone enough
time these intervals would average out.
In this way, changes in specific proteins would serve as a sort of
evolutionary molecular clock. By measuring the differences in DNA or
protein sequence between two different organisms, it would be possible
to determine how much time has elapsed since those organisms diverged
~A _ A A ~ ~ A ~ ~ ~ ^ ~
from a common ancestor.
A classic example of such a molecular clock, Ayala notes, is the
protein cytocurome c. consisting of 104 amino acicis in vertebrates and
a few more in some invertebrates, plants, and fungi, cytochrome c is
a protein that evolved over a billion years ago to help organisms break
. · 1 ~· -
84 SHAPING THE FUTURE
OCR for page 85
down organic molecules and supply themselves with energy. In the
1960s, Walter Fitch and Emanuel Margoliash studied cytochrome c
molecules from 20 different organisms incluclina the fungi Neurospora
1 ~7 · 7 . 1 . C' 7
~ (_7 c~ 1
and Cand~da, the yeast baccharomyces, insects, a fish, reptiles, birds,
and mammals, including man. They determined the differences in amino
acids among the different proteins and calculated the minimum number
of substitutions that would be needed in the DNA of the organisms to
account for those differences. They then used this information to arrange
the organisms in evolutionary time, as shown in Figure 4-~.
The results astonished biologists. From a single molecule, Fitch and
Margoliash had reproduced centuries of work by biologists in tracing
evolutionary relationships among different organisms. `'1 can well re-
member in 1967 reading this paper in Science and being literally
dumbfounded, 7' says Ayala. ``Here they were looking at a very small
molecule, 104 amino acids, and in the midst of a single molecule the
whole of evolutionary history could be reconstructed by and large cor-
rectly. Furthermore, there are tens of thousands of genes coding for
proteins, and each one of these genes or each one of these proteins
could provide us with a molecular clock, and therefore with an inde-
pendent reconstruction of evolutionary history. This is what caused the
enormous enthusiasms of evolutionary biologists and others with this
hypothesis."
However, the diagram produced by Fitch and Margoliash is not
perfect. It shows turtles, a reptile, being closer to birds than to the
rattlesnake, another reptile. Within the birds, chickens appear to be
more closely related to penguins, whereas they are in fact more closely
related to ducks and pigeons. Finally, the primates, including humans,
seem to have branched away from the mammalian limb before the
kangaroo did, whereas in fact just the opposite occurred. Could it be,
Ayala asks, that these mistakes point toward more serious problems
with molecular clocks?
An Erratic Clock
One way in which Ayala and his colleagues have been studying this
problem is by analyzing a protein known as superoxide dismutase. A
protein involved in protecting cells from the reactive effects of oxygen,
superoxide dismutase consists of two identical subunits of 153 amino
acids in humans, horses, yeast, and mold and 151 amino acids in rats,
cows, swordfish, and fruit flies. A total of 55 of the 153 possible amino
acid sites are identical in all eight organisms indicating that parts of
EVOLUTION AND THE BIOSPHERE 85
OCR for page 86
the protein need to be conserves! to maintain its function. The other
98 sites vary from species to species.
The amino acic! differences between humans and the other three
mammals-rats, cows, and horses are fairly uniform, ranging from
25 to 30 amino acids (Figure 4-2~. This makes sense, since the fossil
record indicates that primates diverged from rodents about 63 million
o
5
Al
z
~ 10 _
z
o
-
z
20
25
30
-
~ ~ 8 ° ° '9 ~ ~ °' ° "~ ~ ~ -' d!
:6~.517//~
_
~ _
I2V 7l
3~165//172
3.3\
0.2\
In
0
as ~
o o
Q -
O
C ~ g
O ID ~
By In
57/
7.4
~ 1,
28.
Is 2\
as
._
c
19.6
234
FIGURE 4-1 Differences in the amino acid sequence of the enzyme cytochrome
c can be used to order the evolutionary relationships among organisms. The numbers
between each branch point are the minimum number of changes in the nucleotide
sequence of DNA needed to account for the observed amino acid differences. Re-
printed, with permission, from F. I. Ayala, The Journal of Heredity 77:226-235,
1986. (it) 1986 American Genetic Association.
86 SHAPING THE FUTURE
OCR for page 87
years ago, that the lineages leading to cows and horses diverged around
the same time, and that the common ancestor to the four mammals liver]
about 75 million years ago.
The difference between humans anc! fish is about 48 amino acids,
or nearly twice as much as between humans and the other mammals.
if superoxicle dismutase were an accurate molecular clock, this would
mean that fish and humans cliverged about twice as long ago as humans
and the other mammals, or approximately ~ 50 million years ago. How-
ever, the fossil record indicates otherwise: the most common ancestor
to humans and fish seems to have liver! about 450 million years ago.
The situation is even worse for the other organisms. Human super-
oxide dismutase cliffers by 69 amino acids from the superoxide dis
us
o
200
`,, 400
6
LL
~ 600
o
co
O 800
1 ,000
1 ,200
~ ~ 0 0 en 2 ~ °
I ~ (a) LL ~ ~ ~
~ |
FIGURE 4-2 The evolutionary relationships derived from the fossil record for
eight organisms can be compared with the differences in amino acid sequences for
the superoxide dismutase enzymes they contain. For instance, only 25 of the 153
amino acids in the superoxide dismutases of humans and rats differ, whereas 69 of
them are different in the respective enzymes of humans and yeast. The fungi lineage
split from the line leading to human beings about 1.2 billion years ago, the insects
about 600 million years ago, the fish about 450 million years ago, the ungulates about
75 million years ago, and the rodents about 63 million years ago. Reprinted, with
permission, from F. J. Ayala, The Journal of Heredity 77:226-235, 1986. @) 1986
American Genetic Association.
EVOLUTION AND THE BIOSPHERE 87
OCR for page 88
mutase of yeast and molds, or about three times as much as within the
mammals. Yet the lines leading to humans and to yeast became distinct
an estimated T.2 billion years ago, 20 times more distant than the
common ancestor among the mammals.
The raw numbers of amino acid differences are not a completely
accurate measure of evolutionary distance, Ayala points out. For in-
stance, if an amino acid mutates] at some point during evolution and
then mutated back to the original amino acid, the clouble substitution
would be hidden. Various statistical corrections can be appliecI to the
numbers to correct for such events. But these corrections are not nearly
enough to account for the discrepancies in the substitution rate of
superoxide clismutase.
Another possibility is that the amino acid substitutions in superoxide
(lismutase are constrained in some way that is not yet thoroughly un-
derstood. For instance, there are 98 variable sites in the superoxicle
clismutases of the eight organisms studied. Perhaps this value approx-
imates a maximum number of differences and the rate of amino acid
.
substitutions slows down as it approaches that value. However, the
greatest number of substitutions between any two organisms is only 69,
nowhere near the maximum. Also, a maximum number of substitutions
does not seem to be a factor with comparable proteins, like cytochrome
c. Overall, Ayala says, one is forced to conclude that superoxicle dis-
mutase his not a very good molecular clock."
The Value of Molecular Clocks
"What is more common," Ayala asks, C`the relative regularity of
cytochrome c or the capriciousness of superoxide dismutase? ~ think
the answer is that we don't know. The amount of information that we
have is very limitecI. if the situation that we have with superoxide
dismutase prevails, we are not going to be able to use the evolutionary
clock very readily, unless we become more sophisticated about it and
learn some things."
Nevertheless, there are certain circumstances in which molecular
clocks can still provide valuable results, Ayala contends. For instance,
some proteins, such as cytochrome c, appear to be better clocks than
others. By comparing many such clocks, biologists can learn which
proteins provide the most valid results. Such information will become
increasingly available as DNA sequencing projects for humans anal
other species get under way.
Individual molecular clocks can also be valuable within certain lim
88 SHAPING THE FUTURE
OCR for page 89
l
its. By looking at relatively similar groups of organisms, in which
evolutionary processes are presumably similar' ant] by considering rel-
atively long periods of time' in which variations in the clock can average
out, molecular clocks can yield accurate results. Even superoxide dis-
mutase is a good molecular clock if one looks just at the evolution of
mammals. But to apply such clocks more wiclely, Ayala concludes,
biologists need to learn more about the factors that shape protein evo-
lution.
Two Periods of Life
Based on the geologic record, the history of the earth can be clivicled
into two very unequal periods (Figure 4-3~. About 600 million years
Man Appears
Invasion of Land by Animals
Invasion of Lund by Plants <`e<~°~___tion of Earth
Oldest Known Multicellular \ /11
Organisms ~ /
q<~ /1 0
~ /
1 Billion Years Ago
_ -9 a.m.
Oldest Known Eukaryotes>< 8
~
OCR for page 90
ago, fossils suddenly appear in great profusion in sedimentary rocks-
first marine plants and animals, and later their terrestrial descendants.
The period since then is known as the Phanerozoic, from the Greek
word for visible or manifest.
The first geologic period in the Phanerozoic is known as the Cam-
brian. Everything before the Phanerozoic is therefore known simply as
the Precambrian. The earth seems to have coalesced from a cloud of
matter circling the sun 4.5 billion years ago. So the Precambrian period
on earth spans about seven-eighths of its history.
For well over a century, biologists searched without success for
1 · · ~ ~ ~ ·]
conclusive ev~ctence ot fossils In Precambrian rocks. The sudden ap-
pearance of living things 600 million years ago posed a serious problem
for the creators of evolutionary theory who believed that all organisms
arose through a process of gradual change from other organisms. in Ore
the Origin of Species, Darwin wrote, `'To the question why we do not
find rich fossiliferous deposits belonging to . . . periods prior to the
Cambrian system. T can give no satisfactory answer.... The case at
present must remain inexplicable; and may be truly urged as a valid
argument against the views here entertained."
Only in the past few decades has the puzzle finally been solved,
according to I. William Schopf, professor of paleobiology at the Uni-
versity of California at Los Angeles. It is not the case that fossils do
not exist in Precambrian rocks. The problem was that people were
looking for the wrong kind of fossils.
The key to the puzzle of Precambrian life begins with a certain kind
of rock deposit known as a stromatolite (Figure 4-4~. Shaped like a
stack of mattresses (or stromas in Greek), stromatolites were first de-
scribed in the early 1800s. Almost immediately, some biologists began
to speculate whether they might have been formed by living organisms.
But they contain no visible fossils, and most biologists concluded that
stromatolites were caused by nonbiological processes. ``This debate went
on and on, and many geologists simply refused to investigate these
structures, largely because they had been taught by their professors
that they weren't worth the time,~' says Schopf. ``But they turn out to
be worth the time.'
Several things came together to change biologists minds about the
origins of stromatolites. For one thing, biologists began to find and
examine colonies of bacteria that produce structures remarkably like
the stromatolites seen in the geologic record. In a few dry. salty.
J. At, and
sunny places in the world, such as the coasts of Baja California and
northwestern Australia, bacteria grow in columns rising from shallow
water (Figure 4-5~. Typically these columns are not hard; they can be
90 SHAPING THE FUTURE
OCR for page 91
FIGURE 4-4 Stromatolites in the fossil record typically resemble stacked pan-
cakes rising in pillars or mounds. These specimens occur in limestone deposits about
1.3 billion years old from Glacier National Park, Montana. Photograph courtesy of
I. William Schopf.
~ . . . _.^ ~ A. . . ~
~ I: ' ' ~ ' ~ ~ ~_-~
it, ~==
_- ~^ -41.'::
. B111k ,-~ ~
_ ~. Hi- ~
' 'I.. 'A
_F'~-''I
~ '^" ~ ~
_.~
FIGURE 4-5 One of the few places in the world where stromatolites still live is
in Shark Bay, Western Australia. For much of its history, the earth's surface probably
looked something like this. Photograph courtesy of J. William Schopf.
EVOLUTION AND THE BIOSPHERE 91
OCR for page 92
cut with a machete, Schopf says. But in some places, conditions are
such that calcium carbonate, the mineral constituent of limestone,
adheres to their surfaces, forming a hare] pillow-shaped structure. If
covered with sediments and compacted, these deposits woulc! be vir-
tually indistinguishable from geologic stromatolites.
Living stromatolites are complex ecosystems composed of several
different kinds of bacteria. The top layer consists of a type of bacteria
known as cyanobacteria' the same kind of bacteria responsible for pond
scum on bodies of stagnant water. Like all bacteria, the cyanobacteria
are prokaryotes that is, they do not have a distinct nucleus containing
their DNA. For this reason, a more common name for cyanobacteria-
blue-green algae is not entirely accurate, since all true algae have
nuclei and are therefore eukaryotes. The cyanobacteria are also pho-
tosynthesizers' like green plants. Thev use light from the Olin to c.onvf~rt
_ _ 1 1 - · 1 1 . · . . 1
~ ~ _ ~ ~ _ _ _ . ~ ~
carDon a~ox~ae anc' water Into the organic compounds that they use to
grow.
These compounds in turn support different kinds of bacteria that live
beneath the cyanobacteria in modern stromatolites. These bacteria get
their energy as animals do, by feeding off the organic molecules pro-
duced by photosynthesizers.
Schopf points out that modern stromatolites might seem to lead a
rather precarious existence. C`They grow near the secliment-water in-
terface," he says' Which is not a goof} place for organisms that rely
on the sun to exist. They can get buriec] when the spring rains come
ant] detritus covers them up. They can no longer see the sun and should
1- ''
ale.
Modern stromatolites get around this problem by growing. The cy-
anobacteria making up the top layer of the column are phototactic-
they move in response to light. C`So indeed they do get buried' but as
they do they glide up through the accumulated detritus and make a
new layer," Schopf explains. C`Anc! there they sit, happy as clams, or
happy as blue-green algae, until they get buried again and again, layer
after layer. An 1 that's how a stromatolite forms."
The discovery of modern stromatolites was a suggestive piece of
1 .
1 1 . · . . 1 ~ 1 ~
evidence, but It was not enough to prove that the stromatolites in the
geologic record were also made by bacteria. To do that, biologists had
to find evidence of the organisms that built the geologic stromatolites.
They did this by examining stromatolites preserved in silicon dioxicle-
the mineral quartz. If such stromatolites are sliced with a diamond-
impregnated saw anc] ground down by hand, it is possible to make thin
layers that can be seen through with a microscope. By exhaustively
searching through such translucent layers, researchers managed to find
92 SHAPING THE FUTURE
OCR for page 93
the remains of fossilized cells (Figure 4-64. Sometimes the cells are
joined into rows' or flattened into disks' just like the bacteria in modern
stromatolites. The fossils of the Precambrian era had finally been found.
It was immediately obvious why earlier biologists hac] found no ev-
idence of Precambrian life. ``The Precambrian was the age of micro-
~.onio. life-" Schonf noints out. ``People were asking the wrong question.
They were looking for macroscopic organisms, for the equivalent of
trilobites and clams. But such organisms had not yet evolvecI. The
planet was dominated for three billion years by microbes, which set
the stage for all subsequent evolution."
---rid 7 ~ r- r
Bacteria and the Atmosphere
Stromatolites first appear in the geologic record about 3.5 billion
years ago, and Dreary they contain fossilized cells (this is the earliest
$~ '. S~ =
!i'i~ :~ ~ :~ ~:~ ~ ~ Ail... Act.
· i: ~ ~ I:: i:: :: T :~:.:
i::: ::::~::::: ~ i:: :: i: ~ .3
:. .:: ~ i:::: ~ i.:. i: i: it:::::,
~ -.- ~ :. ,,. :~:, :,~2~22,..".
:: :.,::
::: i:.:::: ::.::.::::::, :.:::.
_..
::
:'':::
:. -::::::: ::~ ::: ~ :':: ~:.~"~.2~,"
.:: i:
.
FIGURE 4-6 Fossilized cells from 3.5-billion-year-old deposits in northwestern
Australia still exhibit cell walls and a filamentous form. These fossils are among the
oldest now known. Photomicrograph courtesy of J. William Schopf.
EVOLUTION AND THE BIOSPHERE 93
OCR for page 94
1-1 .1 .1 . r
direct evidence for life on earth, as described in the box below). But
what kin(ls of cells were they? Were they photosynthesizing bacteria,
like tne ones tnat torm stromatolites today? Or did some other kind of
organism form these early stromatolites?
To search for evidence of photosynthesis, paleobiologists have stucliec]
the rocks in which these early cells are embedded. Carbon clioxide,
which photosynthesizers absorb to create organic compounds, has al-
ways been abundant in the earth's atmosphere' because it spews into
the atmosphere ant] oceans from volcanoes and deep-sea vents. But not
all carbon dioxide is alike. The carbon in carbon dioxide consists of
two different isotopes-carbon-12 (with six protons and six neutrons)
and carbon-13 (with six protons and seven neutrons). When organisms
photosynthesize, they tend to slightly prefer carbon-12 to carbon-13.
Extinction and the Fossil Record
Molecular clocks can only be used with
living creatures and those few organisms
for which perishable DNA or protein sam-
ples have been preserved. For Features that
are now extinct, biologists must rely on the
fossil record to derive evolutionary rela-
tionships.
Fewer than one percent of all the species
that have ever lived on the earth still exist
today. As described in the essay following
this chapter, there are somewhere between
5 and 30 million species now living on the
earth, of which somewhat more than a mil-
lion have been named and described. It is
difficult to estimate the number of species
that have lived on the earth throughout its
4.5-billion-year history, but it is thought to
be somewhere around 4 billion. For rea-
sons that are not yet clear, major extinction
events seem to occur about every 26 to 28
million years. Some biologists believe that
periodic changes in climate or sea level are
responsible for these events, whereas oth-
ers blame them on collisions with extrater-
restr~al objects.
Paleobiologists have described some
250,000 extinct species of plants, animals,
94 SHAPING THE FUTURE
and microorganisms occurring in the 3.5
billion years during which living things are
known to have existed. Museums and uni-
versities have collections containing tens of
millions of fossilized specimens, but rela-
lively few have been thoroughly exam-
ined.
co
z
~ .06
z
o
z
~ol, , ,
.10
.04
i
02F ~v
l
250 200 150 100 50 0
S
'1 . ~ ~ ~ ~
GEOLOGICAL TIME (millions of years)
Mass extinctions seem to occur about every
26 million years, although the reasons for this
periodicity are still obscure. Reprinted, with
permission, from D. Kerr, "If There Was One
Killer Impact, Were There More?" Science
242:1381, 1988. @) 1988 American Association
for the Advancement of Science.
OCR for page 95
Therefore, when these organisms die and are fossilized, the carbon in
those fossils tends to be enricher! in carbon-12 compared with the carbon
in nonbiological deposits.
Researchers have measured the ratio of carbon-12 to carbon-13 in
early stromatolites, and the results indicate a definite enrichment in
carbon-12. '`The carbon isotopes are consistent with the presence of
photosynthesis going back three and a half billion years ago,', says
Schopf.
The question then becomes what kind of photosynthesis the early
bacteria engaged in. Plants and cyanobacteria combine carbon dioxide
with water to get organic molecules, giving off oxygen in the process.
But other kinds of photosynthesis are possible. In particular, certain
bacteria that evolved before cyanobacteria use hydrogen sulfide rather
than water in photosynthesis, releasing sulfur as a waste product. The
bone] between sulfur and hydrogen is easier to break than the one
between oxygen and hydrogen, and the biochemical machinery needed
is less involved. Perhaps these bacteria were responsible for the early
stromatolites.
Schopf and his colleagues have attacked this question by examining
the size and shape of the fossils found in the earliest known stroma-
tolites. They are larger than modern sulfur-pro~lucing bacteria, he points
out, being more along the lines of modern cyanobacteria. They are also
organized into rows or globular colonies encased by thick, layered
sheaths' a common feature among cyanobacteria and rare among other
prokaryotes. Altogether, he says, the evidence supports the idea that
oxygen-proclucing bacteria hac! evolved by 3.5 billion years ago, but it
. .
Is not cone issue.
To examine the issue of when oxYzen-oroducina ohotosynthesizers
r. ~ 1 · ~
first appear, pa~eon~o~og~sts again turn to the geologic record. When
photosynthesizers give off oxygen, it enters the atmosphere and oceans.
There it influences the formation of certain minerals' leaving a trace
of its presence. By examining these minerals, it is possible to get a
fairly good idea of when oxygen first appeared! in abundance in the
earth's atmosphere.
The best evidence for atmospheric oxygen comes from a geologic
feature known as banded-iron formations. Between about 3 billion and
2 billion years ago, a series of red bands formed by the mineral hematite,
or iron oxide, appear in the geologic record. '`No matter where You no
1 1 ~r ~. ~1 11
J V
on the earth s surface, ~ to ~ billion years ago, you're going to find this
type of rock," says Schopf. These bands appear to have been former]
by (reposition in the earth's oceans. Before the earth's atmosphere
contained oxygen, the oceans were saturates! with ferrous iron, which
EVOLUTION AND THE BIOSPHERE 95
OCR for page 96
can exist dissolved in water. But as photosynthetic bacteria began giving
off oxygen in large quantities, this oxygen combined with ferrous iron
to produce ferric oxides, which are insoluble in water and dropped to
the ocean floor. There they were compacted by other sediments to form
the banded-iron formations.
Today, most of the world's commercially important deposits of iron
come from these formations. '~Why were there steel mills in Pittsburgh,
or an automobile industry in Michigan?" S~honf kc 'I shot'=
_. 1_ _ ~ 1 1 '~ ~1 · -
wnere one Iron was processed. thus Iron was deposited as banded iron
formations 2.2 billion years ago, along the shores of an ancient sea.
As Schopf says, '`The world rusted."
o._ . 1 -. · 1 1
oIromalollles appear In abundance in the geologic record about 2.8
billion years ago. It is reasonable to assume, Schopf asserts, that they
were built by the same oxygen-producing photosynthesizers responsible
for the banded-iron formations.
For several hundred million years, iron in the oceans and other
geologic sinks absorbed the oxygen given off by these early photosyn-
thesizers. But eventually these sinks for oxygen got used up. At that
point, oxygen began to accumulate in the atmosphere. The result was
a biological revolution that would forever change the course of evolution.
An Oxygenic Atmosphere
The introduction of oxygen into the atmosphere was one of the most
momentous events in earth's history. Today the earth's atmosphere is
about a fifth oxygen, with virtually all of that oxygen produced biolog-
ically by green plants and cyanobacteria. But this abundance of oxygen
disguises the fact that it can be a deadly toxin to life. Oxygen reacts
with organic molecules, destroying the functions of proteins, nucleic
acids, and other essential molecules. Essentially, oxygen burns these
substances' removing their biological activity.
The development of an oxygenic atmosphere undoubtedly drove many
organisms to extinction. Others fount! ways to avoid oxygen by retreating
to anaerobic, or oxygenIess, environments. Still others managed to
develop biochemical defenses against the reactivity of oxygen. For
instance, superoxide dismutase, the subject of Ayala's molecular clock
studies, evolved to protect intracellular ~v~tem~ from torrid ~-ri`Tot;~7~=
of oxygen.
~ ~wow ~wit ~- ~ ~ Call ~1~ ~11 V wL1 V ~O
But evolution is endlessly opportunistic, and while some organisms
suffered because of oxygen, others thrived. In the upper atmosphere,
oxygen atoms formed a layer of ozone, which absorbs ultraviolet light
96 SHAPING THE FUTURE
OCR for page 97
and keeps it from reaching the earths surface. Previously, organisms
had been forced to protect themselves from the biologically damaging
effects ot this radiation by sh~eict~ng themselves from clirect sunlight or
evolving elaborate biochemical defenses. With the ultraviolet light all
but gone' they court! spreac! into ecological niches that had been closed
to them. (The shutting off of ultraviolet light also eliminated the plentiful
source of energy that many biologists believe contributed to the for-
mation of life, as described in the box below.)
Even more important, organisms began to develop ways to use oxygen
to their advantage. They evolvec] biochemical mechanisms that used
oxygen to break down foodstuffs, resulting in a much more efficient
use of organic molecules to get energy. They clevelopec} biochemical
pathways in which oxygen was an essential participant, resulting in
such molecules as steroids, carotenoids, ant! unsaturated fatty acids.
These organisms conic! not only tolerate the presence of atmosphere;
they used its growing abundance to establish their biological dominance.
The development of an oxygenic atmosphere by about T.7 billion
years ago set in motion an entirely new era in evolution. By as early
^^ ~· ~ · . ~· 1 ~ · 1 1
The Origins of Life
The fossil record contains many gaps,
but none is longer or more exasperating
than the very first one. The oldest known
rocks date from about 3.8 billion years ago,
when the earth was just 700 million years
old. But the first sedimentary rocks, in which
fossils might be found, are about 3.5 billion
years old. Already these rocks contain fos-
silized bacteria, so earlier forms of life must
have arisen sometime in the earth's first
billion years.
To learn more about evolution during
earth's early history, biologists have tried
to understand the chemical conditions that
then existed. The earth's environment was
much different then than it is today. A1-
most all of the planet was probably covered
by water, and the atmosphere consisted of
gases recently expelled by the earth's cool-
ing rocks. Most important, the atmosphere
contained little or no oxygen. Without ox
ygen there was no ozone layer to block the
sun's ultraviolet radiation, and it fell freely
upon the earth. This energetic radiation
could have provided the energy to syn-
thesize many organic compounds from
molecules like water, carbon dioxide, am-
monia, and methane in the earth's early
atmosphere and oceans. These organic
compounds, in turn, could have associated
inside droplets encased within a mem-
brane-like skin and engaged in a rudimen-
tary biology.
It is a long way from associating organic
compounds to self-replicating nucleic acids
and proteins, and biologists have few clues
as to how this transformation occurred. But
a broad account of how life developed from
inanimate compounds on the primitive earth
is not unattainable. Reconstructions of the
earth's early environment, biochemical
studies of existing organisms, and even ob-
servations of other planets in the solar sys-
tem could all shed light on life's origins.
EVOLUT10N AND THE BIOSPHERE 97
OCR for page 98
as T.5 billion years ago, cells with nuclei and other internal structures
began to appear. To this clay' almost all of these eukaryotic cells are
aerobic, requiring the presence of oxygen' and the exceptions are clearly
descenclec] from earlier aerobic eukaryotes. About 700 million years
ago, these cells began to form integrated multicellular colonies, and
individual cells acquirer] specialized functions. At first these multi-
cellular organisms had soft bodies that were rarely preserved as fossils.
But about 600 million years ago organisms began to evolve hare} skel-
etons and other body parts, which when buries! by sediments left easily
visible traces. Today, all multicellular organisms, including all plants
and animals, are composed of eukaryotic cells.
A Modern View
The anaerobic portion of earth's history from its creation 4.5 billion
years ago until about 2 billion years ago may seem a rather sedate
time in the history of evolution. But that is because evolution tends to
be equated with changes in the shape of organisms. During the first
half of earth's history, the dramatic events in evolution occurred inside
Levis. The period! was characterized by what Schopf calls a "Volkswagen
syndrome" the tendency for outward aDnearano~f~c. to remain the come
_. 1_ 1 ~1 1 -
r r ~ -^ -~ ~
wane internal mechanisms are undergoing substantial change. The
ancient prokaryotes developed all of the basic biochemical machinery
on which later life clepends. Ant! in doing so they transformed the
earth's environment from one hostile to life to one in which advanced
organisms could prosper.
Unraveling the threads of Precambrian evolution has been a difficult
process and is far from over, says Schopf. It is an interdisciplinary
undertaking' drawing on fields from throughout biology and beyond.
`'To address the problems of three anti a half billion year old pond
scum, one has to worry about such things as geology and mineralogy,
microbiology and paleontology, organic chemistry, biochemistry, at-
mospheric evolution' a little bit of comparative climatology, and the
history of science." Progress in the field has sometimes been slowed,
Schopf says' by the tendency of scientists to focus on particular fields
an(l ignore their interdisciplinary connections. But C`nature is not com-
partmentalized''' Schopf observes. Witherers a great imperfection in our
science' and ~ think it's a function of the way we educate our students.
There's something to be said for an interdisciplinary education, and
perhaps my remarks in some small way illustrate that point.
Human societies are now reaching a point where the interconnect
98 SHAPING THE FUTURE
OCR for page 99
edness of the biological and the nonbiological can no longer be ignored.
By burning fossil fuels and destroying vegetation, we are increasing the
amount of carbon dioxide in the earthts atmosphere. Carbon dioxide
.
acts as a greenhouse gas, trapping infrared radiation and raising the
temperature of the globe. If computer models of atmospheric processes
are accurate, global temperatures will rise several degrees over the next
century, shifting agricultural and ecological zones and possibly raising
sea levels.
Humans are also modifying one of the influences that made the
modern ecosystem possible. industrial chemicals known as chlorofluo-
rocarbons have been breaking down ozone molecules in the upper
atmosphere, allowing more ultraviolet radiation to reach the ground.
Besides increasing skin cancer rates, this increased ultraviolet radiation
could eventually harm terrestrial and marine plants and animals, with
untold ecological consequences.
By studying the geologic, atmospheric, oceanographic, and biologic
processes that have shaped our modern world, biologists hope to learn
more about how these forces will continue to interact. Lithe past de-
termines the present," says Schopf, Land in the same sense the present
determines the future."
EVOEUTION AND THE BIOSPHERE 99
Representative terms from entire chapter:
amino acid
