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s
The Evolution of
Cellular and Mullicellular Life
INTRODUCTION
Our perspective on biological evolution is that it is a cosmic phenome-
non, born of galactic and solar-system processes and influencing the further
development of planetary surfaces where it occurs. Although traditional
studies of terrestrial evolution have considered biology to be a system apart
from and, in many respects, independent of the physical Earth on which it
resides, increasing evidence compels us to reject this view in favor of a
concept of life as intimately linked with the crust, sediments, oceans, and
atmosphere, through an interacting series of biogeochemical cycles. In-
deed, life is an outgrowth of solar-system and planetary evolution.
The characteristic feature of the evolutionary process is its dependence
on context. The unfolding of terrestrial life must be understood as contin-
gent on the particular course of this planet's development. Both the origins
of life on Earth and its subsequent evolution have been influenced strongly
by events in the evolution of the physical Earth and by extraterrestrial
phenomena (such as halide bombardment) that have impinged the Earth
throughout its history. To understand the evolution of terrestrial life, a
much more integrated understanding of Earth's biological and physical his-
tory must be developed. Such an understanding is requisite to determining
the extent to which the course of biological evolution on Earth can be
regarded as a general feature of life and, thereby, likely to be representative
of life throughout the universe.
The earlier phases of evolution are most likely to share common charac-
teristics on different planets. As biological systems on Earth became more
complex, they came to have a greater influence on their own evolutionary
development and that of the planetary surface as well. The major determi-
91
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THE SEARCH FOR LIFE'S ORIGINS
nants of early evolutionary change on Earth, however, were changes in the
physical environment. Hence, although the course of evolution of more
complex organisms elsewhere in the universe is difficult to predict using
Earth as a model, the major transition from physical to biological evolution
of organic matter, similar to that thought to have occurred on Earth, is
expected to characterize all planets whose early physical evolution is com-
parable to that of our own.
In recognition of the need for an integrated approach to evolutionary
problems, another report of this committee (SSB, 1981) recommended the
following: "It is essential that data obtained using molecular methods with
live organisms be evaluated in the context of the sedimentary rock record."
Since the preparation of that report, methodological progress has been so
substantial that it has become possible, indeed necessary, to expand and
refine the recommendations of that report.
Integration of the Earth's biological and physical history now seems to
be attainable: molecular approaches permit the inference of evolutionary
relationships for all extant life. This advance is complemented by advances
in electron microscopy, making it possible to define ultrastructural pheno-
types and trace their development in microorganisms. Renewed exploration
of diversity in prokaryotic metabolism, spurred by the recognition of the
archaebacteria as a distinct prokaryotic kingdom, has demonstrated the ex-
istence of evolutionarily important bacterial metabolisms that were unknown
a decade ago. Knowledge of the early Earth has expanded in concert with
this biological progress and, for the first time, evidence has accumulated
that forces us to consider the role of extraterrestrial factors in determining
patterns of terrestrial evolution.
With this in mind, the committee has articulated four goals for future
research on the evolution of cellular and multicellular life. These four
goals, which are components of a larger primary goal of understanding the
interrelationships between physical and biological evolution on planetary
surfaces, seem appropriate for NASA sponsorship and coordination. These
goals and objectives have not been prioritized because all are necessary for
the integrated understanding to which we aspire. Balance, rather than pri-
oritization, is the key to a successful research program in cellular and mul-
ticellular evolution.
In delineating the specific objectives of these goals and the recommended
research, it is important to note the critical role of NASA in coordinating
and catalyzing the interdisciplinary study that will be necessary. No other
agency is capable of providing the conceptual or intellectual umbrella for
the evolutionary research advocated in this chapter.
The goals defining a strategy for research on cellular and multicellular
life, together with their component objectives, are described below.
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THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE
93
GOAL 1: To develop a universal understanding of the temporal se-
quence and evolutionary relationships of life on Earth.
Recent molecular data support the view that all extant (living) organisms
on Earth descend from a common ancestor. At this time, there are three
principal lines of evolutionary descent from this common ancestor, namely,
the three major phylogenetic groups: the archaebacteria, the eubacteria, and
the eukaryotes. Evolutionary relationships within and among these groups
can be examined by a variety of biological, paleobiological, and geological
means. Traditionally, evolutionary relatedness has been assessed by com-
parison of phenotypic characters. This system of inquiry has worked rea-
sonably well for plants and animals, but it has been of only limited value in
defining relationships among fungal, protistan, and prokaryotic microorgan-
isms because of their simple morphology (phenotype) and lack of fossil
preservation. The committee suggests that a different combination of in-
sights (items 1 to 3 below) will provide a markedly improved understanding
of archaebacterial, eubacterial, and eukaryotic evolution:
1. Molecular phylogeny: All living organisms contain an extensive
record of their own phylogenetic history. Nucleic acid sequencing technol-
ogy now provides ready access to much of this biologically incorporated
history. From sequence comparisons (for homologous functions), quantita-
tive evolutionary relationships can be inferred, and these serve as concep-
tual frameworks within which to relate phenotypes and their temporal evo-
lution, as well as the ecological and geological conditions surrounding the
evolutionary process.
2. The characterization of phenotypes: The characterization of pheno-
types provides an independent set of biological data that can indicate path-
ways and possible causes of morphological and/or biochemical evolution.
Among prokaryotes (archaebacteria and eubacteria), many of the most im-
portant characters currently used in phenotypic characterization are meta-
bolic, whereas in eukaryotic protists they are predominantly ultrastructural.
In plants, animals, and fungi, these are largely anatomical and morphologi-
cal. Consideration of phenotypes and ecology in the context of molecularly
derived phylogenetic relationships permits the generation of hypotheses
concerning the conditions under which evolutionary innovations arose.
3. Paleontological and geological record: The paleontological and
geological record provides a testing ground for these hypotheses and can
further illuminate the causes of environmental changes or occurrences asso-
ciated with significant evolutionary events. Paleontology traces the actual
course of terrestrial evolution, indicating the sequence of appearances and
disappearances of preservable phenotypes and placing constraints on the
timing of evolutionary origins. Equally important, the analysis of sedimen-
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THE SEARCH FOR LIFE'S ORIGINS
TABLE 5.1 Information Content of a Precambrian Fossiliferous Rock
1. Physical environment of deposition
Bedding and sedimentary structures
2. Chemical environments of deposition and diagenesis
Petrology and geochemistry
3. Biota living in, or transported to, the site of deposition (incomplete sampling)
Micropaleontology
Chemical indicators of metabolism (e.g., carbon isotopes of carbonate and
organic matter; sulfur isotopes of pyrite and sulfate; molecular fossils)
Traces of microbial activity (stromatolites, microphytolites, oncolites)
4. Chemical indices of global tectonic and biogeochemical systems
Strontium isotopic ratios
Carbon isotopic ratios
Sulfur isotopic ratios
Rare earth element abundances
Oxidation state of weathered surfaces
tary rock sequences in which fossils are found provides important clues to
the environmental evolution of the Earth's surface. A given sedimentary
sample contains information on the sedimentary, tectonic, and geochemical
environments in which organisms lived and/or were buried. It also records
many features of its subsequent diagenetic history and may contain isotopic
indices to the pulse of biochemical cycles and tectonic activity (see Table
5.11. From the examination of samples along environmental gradients in a
single time plane come paleoenvironmental and paleogeographic reconstruc-
tions, as well as the determination of paleoecological distributions of coex-
isting organisms. Analyses of time sequences of samples (normalized for
environment) provide the geological evidence for both biological and physi-
cal evolution and the means of relating the two.
OBJECTIVE: To study a wide variety of organisms by using the tech-
niques of molecular phylogeny and biochemical and morphological charac-
terization.
A decade ago biologists regarded our understanding of eukaryotic phylo-
geny as fairly complete, whereas evolutionary relationships among bacteria
were considered unknown quantities. Today bacterial relationships at higher
taxonomic levels are regarded as well known, whereas increasing data have
exposed our ignorance about eukaryotic phylogeny. The committee be-
lieves that the time is ripe for concerted effort on fundamental questions of
eukaryotic cell evolution.
Speculation has long focused on certain bacterial characters of the major
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THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE
95
organelles the mitochondria and chloroplasts. It is now abundantly clear,
from molecular phylogenetic comparisons particularly of rRNAs that
mitochondria and chloroplasts are derived from specific eubacterial groups.
However, there are many morphologically and biochemically distinct ver-
sions of these organelles, only a few of which have been inspected in terms
of molecular phylogeny. Questions therefore arise as to whether mitochon-
dria and chloroplasts each arose only once or many times. Moreover, it is
now clear that substantial genetic exchange has occurred between mito-
chondria and chloroplasts and their host eukaryote nuclei. The mechanisms
and rationale for the genome mixing are not understood, but the occurrence
has important implications for the evolution of eukaryotic cells. The con-
tinuing molecular investigations of eukaryote and prokaryote diversity should
include analysis of the organelles as well, so that their origins and coevolu-
tion with host cells can be evaluated.
Nucleated organisms have been thought to be descendants of forms re-
lated to present-day prokaryotes and, hence, viewed as more recently evolved
taxa. However, the emerging phylogenetic framework inferred from com-
parisons of small-subunit (~16S) RNAs shows that the sequence diversity
of eukaryotic RNAs eclipses that of archaebacteria or eubacteria. Such a
finding is consistent with the hypothesis that the eukaryotic nuclear line of
descent represents an extremely old superkingdom, derived directly from
the "progenote" rather than from prokaryotes belonging to the other two
superkingdoms. The eubacterial line of descent, however, contributed the
two major organelles, the mitochondria and the chloroplasts. The breadth
of genotypic and biochemical diversity of the eukaryotes observed thus far
is represented by members of the Protista; the microsporidians, euglenoids,
and trypanosomatids have been identified as particularly early branches on
the eukaryotic tree. Other eukaryotic divisions especially plants, animals,
and fungi appear to have arisen nearly simultaneously during a relatively
recent radiation.
Despite these revelations, the understanding of eukaryotic evolution is
limited by the small number of taxa examined to date and by the inability to
bring genotypic phylogenies into juxtaposition with relationships inferred
from comparisons of phenotypes and the fossil record. Several other protis-
tan groups, including the dinoflagellates and oxymonads, may represent
additional early branches in the eukaryotic line of descent, but comparative
morphology cannot be solely relied upon for inferring branching order. The
uncorroborated assignment of primitive status to particular characters is
frequently difficult to determine. Recent progress in ultrastructural research,
coupled with molecular phylogeny, promises to clarify these issues. No-
table successes include the new phylogeny of green algae, which differs
appreciably from traditional phylogenies, and which greatly clarifies the
evolutionary history of this group and its descendants, the land plants.
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TlIE SEARCH FOR LIFE'S ORIGINS
Metabolic Diversity
Molecular data show that the main phylogenetic diversity of life on Earth
is in the microbial world, both prokaryote and eukaryote. Traditional micro-
biological investigations have relied on laboratory cultivation for the char-
acterization of organisms. Yet, it is known that many, perhaps most, organ-
isms in natural microbial populations have not been, and perhaps cannot be,
maintained as pure laboratory cultures. This is attested to by the continued
discovery of novel organisms, the abundant occurrence of symbiotic asso-
ciations, and the recognition of large, so far uncultivated populations of
organisms. An outstanding example of the latter is the marine planktonic
microbiota, which was long considered sparse on the basis of laboratory
cultivation. Over the past decade, however, direct microscopic investiga-
tions have revealed an abundance of marine "picoplankton" (organisms less
than 2 sum in diameter), including eukaryotes, prokaryotes, phototrophs, and
heterotrophs. Cultivation attempts have repeatedly failed to retrieve repre-
sentative forms; thus, a potentially major influence on the biosphere re-
mains uncharacterized. Methods for analyzing phylogenetic and quantita-
tive aspects of natural microbial populations, without laboratory cultivation,
are now available. These methods use RNA gene cloning and sequencing
approaches. The expansion of such approaches to many populations and
environments should be encouraged so that a full representation of phylo-
genetic diversity may be achieved.
It is of critical importance to the understanding of eukaryotic evolution
that the sequence sampling be integrated with expanded studies of the ul-
trastructure, ultrastructural development, and biochemistry of sequenced taxa.
A fuller understanding of eukaryotic evolution is necessary if data from this
superkingdom are to be pooled with those from eubacteria and archaebacteria
to make informed inferences about the nature of the "progenote."
Just as electron microscopic studies have revealed a hitherto unappreci-
ated diversity of eukaryotic phenotypes in terms of ultrastructure, so too
have recent biochemical studies of prokaryotes revealed a metabolic diversity
significantly broader than previously thought. The isolation and
characterization of microorganisms having previously unknown metabolic
capabilities (especially among the archaebacteria, cyanobacteria, and sulfur-
reducing prokaryotes) have markedly revised some of the traditional "meta-
bolic dogma." The discovery of these metabolic potentials has allowed the
expansion of our understanding of modern ecosystems, both aerobic and
anaerobic, and thus has enlarged the cast of characters available for modeling
ancient ecosystems. The phylogenetic characterization of these new organ-
isms will provide further insights into the evolution of metabolism and may,
in conjunction with geological data, allow scientists to constrain the time of
origin of certain metabolic phenotypes.
Through such coupled phenotypic, molecular, and geological studies it
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THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE
97
may be possible to resolve more satisfactorily the succession of ecosystems
that have characterized our planet through history. Limiting steps in such
ecosystem reconstruction are the isolation and characterization of organ-
isms whose unique metabolic capabilities allow them to persist in extreme
environments.
For example, although seemingly unusual in the context of today's envi-
ronments, recently discovered thermophilic, anaerobic, sulfur-dependent
archaebacteria may provide important clues to the nature of the early bio-
sphere.
The expanding repertoire of prokaryotic metabolisms can be used to guide
further research. For example, among presently known archaebacteria,
many—but not all—of the complementary metabolisms needed to complete
biogeochemical cycles are known to exist. The question of whether
archaebacteria could maintain element cycles in the absence of eubacteria is
especially important for an understanding of biological diversification on
the early Earth.
GOAL 2: To determine the properties of the universal ancestor of
extant organisms.
Understanding the universal ancestor of life on Earth is critical to under-
standing evolution on this planet. Because of the general evolutionary
principle of descent with modification, the nature of the earliest biological
entities constrained subsequent evolution and, in the attempt to link biologi-
cal to prebiological chemical evolution, also constrains our views of how
life arose.
The earliest life forms on Earth must have been far simpler than any
organism now alive. Although these primordial organisms are extinct, clues
to their genetic organization, metabolic capabilities, and other phenotypic
characteristics are retained in the biological traits of unicellular organisms
that represent early branchings in each of the primary lines of descent. By
comparing features common to these evolutionary lineages, it may be pos-
sible to infer the phenotypes of the earliest organisms; it is probable that
features common to early branching groups will reflect features possessed
by their common ancestors. As mentioned above, studies of molecular
phylogeny have brought the universal phylogenetic "tree" within reach. In
addition to providing the starting point for inquiries into the course of
biological evolution within the three primary kingdoms, this set of phylo-
genetic relationships provides a framework for asking about life's earliest
history before segregation of the three extant lineages.
OBJECTIVE: To root the universal phylogenetic tree.
Our understanding of the ancestor of all extant life rests heavily upon
knowing the root of the universal "tree" of phylogenetic relationships. One
needs to know, for example, whether the archaebacteria and eubacteria are
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THE SEARCH FOR LIFE'S ORIGINS
specifically related to one another to the exclusion of the eukaryotes. One
also needs to know whether the archaebacteria are more primitive in pheno-
type than the other two major types. Although the position of the root of
the universal tree must be known to answer both questions, a partial answer
to the second can be given in the absence of this knowledge.
Although phylogenetic relationships are customarily rooted by invoking
known outgroup species, that is not an option for the tree that encompasses
all life. However, this does not mean that the root of the universal tree is
unknowable. Another way to determine the rooting is through comparisons
among sequences of genes that have doubled and functionally separated
while still in the universal ancestry state, provided that their sequences have
retained a sufficient degree of similarity. Thus, the search for gene families
overlaps the problem of rooting the universal tree.
The universal ancestor, the so-called progenote, is considered to have
been a distinct entity ancestral to, but qualitatively different from and more
rudimentary than, any of its daughter lineages. It was sufficiently primitive
that its capacity to transmit information from genotype to phenotype would
have been more limited than that of its descendants; that is, translation at
this stage was presumably more inaccurate (prone to errors) than in later
organisms (see also Chapter 4~.
Four salient characteristics follow from such translational inaccuracies:
(1) proteins at this time would not have been typical of proteins seen in
cells today (e.g., they were probably smaller than modern proteins if they
were accurate translations of genetic messages); (2) the genes the organism
carried would have been fewer in number than prokaryotes now carry; (3)
genes at this stage were perhaps not arranged in large linear arrays (genomes);
rather, they may have been physically separate entities, probably composed
of RNA, not DNA; and (4) in all respects the level of biological specificity
for the progenote was presumably lower than now exists.
As living systems evolved from this state to those represented by the
three major kingdoms, they would have evolved a more varied set of genes
coding for proteins of ever-increasing variety and specificity. Traces of
this evolution are preserved in gene families of living organisms. For this
reason, therefore, the committee recommends as a principal objective of
research the recognition and evolutionary evaluation of gene families.
GOAL 3: To understand what factors drive the biosphere.
As emphasized in an earlier report of this committee (SSB, 1981), bio-
logical evolution has not proceeded independently of planetary evolution,
nor has it been immune to influences from Earth's cosmic environment. The
fossil record documents numerous episodes of evolutionary radiation; mo-
lecular phylogeny also suggests that evolution occurred in bursts. Why
does evolutionary history appear to have this pattern? Traditional explana-
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THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE
99
lions have stressed the evolution of new phenotypic characters (innova-
tions) that confer the ability to utilize an underexploited resource or to
compete for ecologic success, and some radiations may indeed represent
such phenomena. However, many of life's major radiations appear to be
related to environmental changes that presented new opportunities or re-
moved long-standing constraints. Mammals, for example, radiated early in
the Tertiary period not so much because of any innovation on their part as
because the previous ecological dominants in many terrestrial niches, the
dinosaurs, became extinct at the close of the Cretaceous period. Among
eubacteria, evidence begins to suggest that oxygen utilization arose at about
the same time in different lineages, probably during the late Archean to
Early Proterozoic, which geochemical markers indicate was a time of rising
atmospheric oxygen tensions.
An interesting example highlights these close interrelationships among
biological, tectonic, and environmental changes. Approximately 600 mil-
lion years ago, after more than 3 billion years of microbial evolution, macro-
scopic animals radiated over the face of the Earth. Why did this evolution-
ary burst occur 600 million years ago rather than 1 billion years ago or
some other time? Biologists have long stressed that animal life requires
certain minimum oxygen tensions to support exercise metabolism, to ensure
the diffusion of oxygen to internal cells in organisms having multiple cell
layers, and to complete certain biochemical reactions. Recent geochemical
evidence suggests that just prior to the observed radiation of the Ediacaran
fauna, significant amounts of oxygen may have accumulated in the atmo-
sphere a consequence of abnormally high rates of organic carbon burial.
The anomalous carbon burial, in turn, correlates well with sedimentary and
geochemical evidence for continental breakup and the opening of Late Pre-
cambrian ocean basins and suggests a relationship similar to that docu-
mented for the Permian through Early Cretaceous periods, when the breakup
of Pangaea promoted the accumulation of economically important concen-
trations of organic matter. The model that is emerging involves a tectonic
event, its biogeochemical consequences, and attendant changes in the com-
position of the atmosphere that remove an environmental constraint to the
evolution of tissue-grade multicellularity. This model and others like it
must be tested and refined in light of new geological and geochemical data,
but they underscore the point that biological evolution cannot be understood
outside of the context of physical Earth history, and vice versa.
OBJECTIVE 1: To integrate the biological accounting of the Earth's
historical development with that obtained from studies of the geological
record.
Substantial progress in unraveling the environmental evolution of our
planet will require detailed sedimentological, stratigraphic, paleontological,
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THE SEARCH FOR LIFE'S ORIGINS
and geochemical analyses of well-preserved sedimentary basins, particu-
larly those of Archean and Proterozoic age. Global syntheses are only as
useful as the data that go into them, and at present the base of carefully
collected and analyzed data is insufficient to tie the geological record to the
biological record in the way the committee believes is possible.
OBJECTIVE 2: To determine the influence of Earth's cosmic environ-
ment on evolution.
In 1985, NASA published an important workshop report: The Evolution
of Complex and Higher Organisms (ECHO). The object of the ECHO
report was to explore the last 600 million years of biological evolution on
Earth in the context of the Earth's cosmic environment. The report con-
cluded with the recommendation that NASA initiate a new study program
designed to link existing programs in planetary biology and thereby to in-
clude in NASA's overall research effort the important evolutionary events
that took place in the interval between the appearance of multicellular life
and the evolution of man.
During the last 600 million years of Earth history (the Phanerozoic eon),
animals, plants, and fungi have diversified on this planet, initially in the
oceans and then on land. Complex social behavior has arisen in several
phyla and technology in one. In short, the modern biota has taken shape
during the last 15 percent of the Earth's development. This evolution was
once seen as an orderly progression from simple to complex, with more
complex organisms being selected in favor of their more primitive ances-
tors. The seeming inevitability of this progression was used as a model for
the evolution of life in any planetary system. The evolution of life on Earth
was also seen as operating in a closed system not significantly influenced
by events and processes in space.
The general view of the evolution of advanced life now appears to be
grossly oversimplified, to the point of being essentially wrong. The fossils
preserved in the Phanerozoic record show evolutionary change (sometimes
gradual, sometimes spasmodic), but they do not conform to predictions of
linear models of progressive evolution. There is no evidence, for example,
that the extinct trilobites of the Paleozoic era were simpler or less special-
ized than their modern counterparts. Tropical reefs have been in existence
and have flourished throughout most of the Phanerozoic, yet the framework
builders of these reefs have varied markedly through time; the coral reef of
modern seas is just the current version of a recurrent ecosystem. There is
no reason, from first principles, to argue that mammals should have ap-
peared when and how they did. Humanoid intelligence evolved only once,
but there is no reason it could not have evolved several times in separate
lineages or not at all.
It is becoming increasingly clear that the Earth's cosmic environment has
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101
important influences on biological evolution. The history of life on Earth
can no longer be seen as operating in a closed system. During the Earth's
history, the Sun's luminosity has increased about 30 percent, day length has
increased, ocean tides have decreased, the planet has been bombarded by
comets and asteroids, and the solar system has undoubtedly been influenced
by the gravitational effects of randomly passing field stars and by occa-
sional supernovae. Phanerozoic time has included three galactic years, and
the Earth has passed through the plane of the galaxy perhaps 20 times. The
ECHO report considered these and other effects while focusing on two
questions. (1) Do events and processes in our cosmic environment leave
recognizable signatures in rock or fossil records? (2) How have these
events and processes influenced the evolution of advanced life? On the one
hand, innovative geochemical analyses will be necessary to constrain better
the history of atmospheric oxygen or ocean chemistry. On the other hand,
better constraints on temperature history, solar radiation, and the like may
require information from improved models of solar-system evolution based
on comparative planetology and observations of the Sun.
There have recently been a number of striking research successes in
relating past global biology to solar-system~or galactic influences. Work
with the marine micropaleontology of the past 700,000 years has shown
rather decisively that cycles of climatic change, including the several pulses
of continental glaciation, can be tied directly to Milankovich cycles of or-
bital change in the Earth-Moon-Sun system. This work is being extended to
recognition of Milankovich cycles deeper in the geologic past. It also
shows promise of having important implications for problems of global
climate and predictions of future climatic change.
Another success is the discovery of geochemical and geophysical evi-
dence for a major comet or asteroid impact 65 million years ago, at the time
of the terminal Cretaceous mass extinction. Although there is still consid-
erable controversy over the role of this impact in the mass extinction of
dinosaurs and other organisms, the work has dramatically increased the
attention being paid to large-body impacts as influences on past life.
At the very least, research on the possible effects of large-body impacts
has sensitized the scientific community to think more in terms of cosmic
influences on Earth systems. Current estimates of comet and asteroid im-
pact rates call for about 12 impacts of objects 10 km or larger, and up to
3600 impacts of objects 1 km or larger, during the Phanerozoic. Although
the environmental consequences of these impacts are still poorly known,
there is an intriguing possibility that the large number of smaller impacts
has been responsible for the lesser regional extinctions that punctuate the
history of life.
It seems likely that research to date has barely scratched the surface of a
new and exciting field of science. Regular and irregular events in space
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THE SEARCH FOR LIFE'S ORIGINS
may be crucial elements in our evolutionary system. In fact, arguments can
be made that in the absence of this sort of physical disturbance, biological
evolution would have reached steady state hundreds of millions of years
ago, thereby preventing the evolution of advanced life as we know it. The
most likely scenario is one in which externally induced environmental shocks
eliminated dominant organisms at certain times, thus accommodating the
innovations so important to long-term evolution.
Research on the evolution of advanced life may also have direct benefits
in other aspects of space science. For example, if solar-system and galactic
history can be documented from the geologic and fossil records, astronomy
will have, for the first time, a means of empirical verification of time-
dependent processes that can otherwise be treated only theoretically.
The foregoing discussion must remain open ended because of the fledg-
ling nature of the field. Emphasis is given to Milankovich cycles and large-
body impacts because these are areas in which there has been some prelimi-
nary success; however, totally different aspects of the cosmic and planetary
environment may prove to be important to the global biology of the past,
present, and future. To improve techniques for the quantitative understand-
ing of environmental conditions on the early Earth, NASA should continue
to take the intellectual lead in fostering interdisciplinary research and com-
munication among scientists having disparate specialties. It is particularly
important that NASA encourage improved communication among molecu-
lar or evolutionary biologists, paleontologists, Precambrian geologists, and
planetary modelers by sponsoring workshops, symposia, and innovative inter-
disciplinary research projects.
GOAL 4: To generalize our understanding of environmental and early
cellular evolution on Earth by comparative studies of Mars.
Because no other planet in the solar system appears to harbor living sys-
tems, most scientists have assumed that any comparative study of biologi-
cally active planets will necessarily involve other solar systems at great
distance from the Earth. This may be true if only present planetary surfaces
are considered, but if we look at the geological records of ancient planetary
conditions, this assumption may prove to be wrong. The case for exobiol-
ogical input into Mars sample return missions has been made by the SSB
Task Groups on Planetary and Lunar Exploration (SSB, 1988b, pp. 99-106)
and Life Sciences (SSB, 1988a, pp. 47-51~. This committee simply under-
scores the importance of exobiological research in any and all future Mars
. .
missions.
OBJECTIVE: To investigate the sedimentary record of Mars which,
because of similarities to Earth in its early stages of planetary develop-
ment, offers a unique opportunity to expand and generalize our understand-
ing of environmental and early cellular evolution.
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103
According to the logic developed at the beginning of this chapter, if
biological processes arise from physical ones under a given set of physical
conditions, and if early stages of evolution on different planets appear, in
principle, to be more likely comparable than later stages, then there are
compelling intellectual reasons to conduct detailed investigations of Mars
for possible evidence of prebiotic and early biological evolution. For these
purposes, studies on Mars should concentrate on early supracrustal succes-
sions and include well-designed site and sample selection strategies that
maximize the potential for evaluating the environmental and possible early
cellular evidence of evolution on Mars.
PRIORITY CONSIDERATIONS
This chapter has stressed the goal of developing an integrated under-
standing of the evolution of life on this planet, as well as the exciting
prospect of testing the universality of early events in this history through
the nonchemical and naleontolo~icn1 examination of ~nri~.nt ~~,nr~r~r'~t~1
rocks from Mars. The research advocated here is highly interdisciplinary,
with the consequence that program balance must take precedence over strict
prioritization. Nevertheless, the committee can highlight three principal
components of any successful research program aimed at understanding the
course of evolution on our planet:
1. development of robust phylogenies relating living microorganisms,
through the comparison of sequences in informational macromolecules,
especially small subunit ribosomal RNAs;
2. elucidation of the biochemical and ultrastructural characters of micro-
organisms in order to relate patterns of phenotypic diversity to phylogeny;
and
3. development of improved data on the biological and physical devel-
opment of the Earth through careful sedimentological, geochemical, and
paleontological analysis of ancient sedimentary basins. Geological research
should be aimed not only at the elucidation of environmental evolution, but
also at understanding the cosmic influences on terrestrial environments and
evolution.
As discussed in the final chapter of this report, much of the planning for
Mars sample return missions will be spearheaded by groups outside of the
exobiology research community; however, the committee views the partici-
pation of exobiologists in mission planning and execution as essential. It is
difficult to imagine more exciting and fundamental questions that can be
addressed by such a mission than those concerning the early surficial envi-
ronment and the possibility of chemical or even biological evolution on the
early surface of our neighboring planet.
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104
THE SEARCH FOR LIFE'S ORIGINS
Some areas of exobiological research are supported by other agencies in
addition to NASA, especially NSF. NASA's continuing support is critical,
however, because only it can provide the programmatic integration that
promotes the necessary cross-fertilization of the various disciplines relevant
to exobiology. Given the structure of NSF, the search for interstellar mole-
cules, Archean geochemistry, and microbial metabolism are necessarily
viewed as unrelated topics. Only under NASA's aegis are they integrated
as components of a single research effort. This fact cannot be overempha-
sized.
Representative terms from entire chapter:
multicellular life
