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7
Life Sciences
BACKGROUND
The study of life on Earth ranges from elucidating the evo-
{ution of the earliest self-replicat~ng nucleic acids to describing a
global ecology comprising over three million species, including hu-
mans. Though life has shown enormous diversity and complexity
over the last 3.5 billion years, its unifying principles are becoming
ever clearer.
Chemical and fossil evidence show that earth life as we know
it today evolved by natural selection from a few simple cells called
prokaryotes because they lacked nuclei. The earliest prokaryotes
probably already had mechanisms that allowed them to replicate
their genetic information, encoded in nucleic acids, and to express
this information by translation into various proteins. These first
cells were somehow formed during the Hadean eon that spanned
the epoch from about 4.5 to 3.5 billion years ago. Analyses of the
chemical compositions and reactions occurring on other planets, on
comets and asteroids, and in interstellar space help us reconstruct
the steps leading to the formation of the organic building blocks
of biopolymers. Understanding this prebiotic evolution is one of
60
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61
the major goals of the exobiology program biology of the early
Earth and elsewhere In the universe.
A second goad of exobiology is to understand the evolution
of the first cells with true nuclei. These cells, called eukaryotes,
were the precursors of "higher organisms: the unicellular pro-
tists, fungi, plants, and animals we know today. The important
organeDes of energy metabolis~plastids and m~tochondria-
originated 2.0 to 1.5 billion years ago by the symbiosis of prokary-
otes. In this process bacteria having one set of specialized functions
were engulfed by host cells with complementary requirements and
functions.
By the early Archean, more than 2.2 billion years ago, the
biota had used the process of photosynthesis to create an oxi-
dizing atmosphere from one previously poor in oxygen. Carbon
dioxide was also removed from the atmosphere in the form of car-
bonate precipitates. Myriad bacteria, molluscs, corals, and other
organisms contributed to vast limestone deposits and continue to
do so today. With these ~d other processes Earth's biota have
transformed a sterile planet, intermediate in character between
Venus ~d Mars, into the living planet we now enjoy.
Global biology concerns itself with the cumulative changes
wrought by the biosphere on the atmosphere, hydrosphere, and
geosphere. But, conversely, the physical and chemical constraints
of Earth have acted on the biota as wed, ~d this too falls in
the province of global biology. It is hardly a coincidence that
IJyell's Principles of Geology was so influential in the development
of Darw~n's early interest in the relationship of life add Earth. As
the impact of human activities, such as burning fossil fuels and
deforestation, increases, the predictive power of global biology will
become as important as its ability to interpret the history of Earth.
Global biology and exobiology address questions about the
nature of Earth and the origin of life that have concerned us since
even before the writing of Genesis. Yet, as these questions are now
formulated, both global biology and exobiology are young sciences.
As they have grown, they have profited from space missions and
from collaborations with earth scientists and planetary scientists.
As a result, the goals of these fields are now well defined; we can
proceed with specific missions and programs confident of valuable
results.
In contrast to global biology, exobiology, and the other space
sciences, the fields of space biology and space medicine are still
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62
formulating their basic theories and research goals. A significant
proportion of the entire scientific research effort of the United
States addresses questions closely relevant to space biology and
space medicine. Yet there have been so few space flights committed
to biological observations that the major challenge facing space
biology today is to determine how valuable a low gravitational
field (m~cr~g) laboratory might prove in addressing fundamental
research problems In biology.
Some single cells must be able to detect small changes in the
magnitude and direction of gravitational force. At a fundamental
level we need to understand the molecular mechan~srns whereby a
cell detects gravity and converts this signal to a neuronal, ionic,
or hormonal response. Plants and animate that have evolved on
Earth have done so at one g; they respond to gravity. For example,
plant roots grow down and shoots up gravitroprsm. Fertilized
eggs orient their cleavage planes relative to the gravity vector. In
larger animals ~d plants many of the responses to gravity are
additive. Hydrostatic pressure and muscle tension are good exam-
pies: the unp act of gravity is probably greater on a giraffe than
on a bacterium. Studying systemic effects produced by variations
in gravitational force will not only contribute to our theories of
physiology but test their predictive value as well.
The steering group anticipates that manned space flights ex-
tending over years will pose severe psychological and physiological
problems. The initial concern of space medicine is primarily to
identify and characterize those physiological systems that do not
adapt well to space Bight or that do not subsequently readapt to
one g. The vestibular and the cardiovascular systems, for example,
may adapt and readapt without further intervention. In contrast,
extrapolation from very Acted observations indicates that the
effects of bone remodeling win grow more severe with passing
months. Similarly, shielding cannot easily control cosmuc radia-
tion consisting of the nuclei of heavy atoms (so-called heavy ion
radiation). The unp act of one such particle can cause the death of
nondividing celIs' such as brain cells. It would be unconscienable
to launch long-term manned space flights without-a much better
understanding of these phenomena and, where indicated, devel-
opment of appropriate prophylaxis and treatment. Until these
measures have been taken, it is questionable whether such flights
should be planned or contemplated. With the number of Shuttle
flights now projected to study these phenomena, we would still
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63
have only a meager base of data by 1995. Hence, of necessity, our
projections for the period from 1995 to 2015 are tentative.
While priorities may change with tune, present observations
and extrapolations indicate clear research goals of both basic and
cImical significance. Realization of these goals demands the d~
ployment of a dedicated life sciences laboratory equipped with a
vivarium and a centrifuge of at least an Afoot radius on a space sta-
tion. Nothing less can allow us to determine whether m~crogravity
provides an important too] for the study of biology; nothing less
will give us our best chance to assure the health, performance, and
welfare of astronauts on long-term missions.
[IF E SCIENCE GOALS AND MAJOR QUESTIONS
Exobiology Goals
The goal of exobiology is to understand the origin and early
evolution of life and its cosmic distribution.
Complex anaerobic ecosystems fueled by photosynthesis ex-
isted at least 3.5 billion years ago and possibly earlier. They had
produced an oxidizing atmosphere by 2.0 billion years ago. The
oldest fossils of eukaryotic organisms are found in rocks about 1.4
billion years old. Exobiologists believe that the basic chemical
components of the first cells had to accomplish two fundamental
tasks: reproduction and energy storage. Both of these probably in-
volved the replication of polymers of KNA. These first RNAs may
have been catalytic, somehow directing polymerization of amino
acids into proteins. Selective advantage would have accrued to
those organisms that let them extract usable energy from organic
molecules, sunlight, or minerals not in their final oxidation state.
However, relatively complex, prebiotic organic reactions occurred
not only on Earth. They also occurred on other planets and in
interstellar space. The surfaces of clays or of interstellar grains
may have catalyzed these early reactions, even though these same
pathways are no longer used by existing cells. A study of this
extraterrestrial organic chemistry should provide precedents for
early reactions on Earth. It might also indicate whether condi-
tions conducive to life have ever existed on other planets in our
solar system or beyond.
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64
Major Questiom for Exobiology: 1995 to 2015
From 1995 to 2015 the steering group recommends addressing
five major questions:
1. Are there organic molecules beneath the polar caps and in
the sediments of Mars, in the atmospheres of Jupiter and Titan,
in comets and meteorites, and in interstellar space? If so, what
are they, and what ~ their distribution? More than 50 organic
compounds containing up to 11 atoms have been identified in
interstellar space; their relative concentrations differ enormously
from that predicted by equilibrium thermodynamics.
2. How can we interpret the early fossils and deposition ot
material of biological origin on Earth and relate them to materials
from other planets? Are there fossils or definitive evidence of liquid
water or former life on Mars, and, if so, what role have they played
in early biological evolution?
3. What models of prebiotic chern~cal reactions can be demon-
strated experimentally? The correspondence with reactions that
do occur or have occurred in nature should be established.
4. What models can be developed for the first replicating
system and the first true cells? A critical evaluation of such models
might outline the environmental characteristics required for the
origin and evolution of life.
5. What is the phylogeny of the archaebacteria, eubacteria,
and early eukaryote~, add what were the major events in the
evolution of the eukaryotes?
Global Biology Goals
The goal of global biology ~ to understand the evolution of the
liota and its interactions Pith Earth.
For nearly 4 billion years the biosphere has influenced, and
been influenced by, the atmosphere, the hydrosphere, and the
geosphere. The composition of our atmosphere, the abundance
of water, the temperature, and the vast carbonate and ferric iron
deposits are radically different from conditions on Mars and on
Venus. Life has evolved under the constraints of available elements
and energy fluxes of our planet. This is a dynamic system; the Bow
of sold energy to a rotating Earth, coupled with its own internal
radioactive heat source, generates numerous cycles and changes,
which act on widely varying time scales. Photosynthetic cycles, for
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65
example, have dally periods; while tune scales for plate tectonics
are In the millions of years.
Major Questions for Global Biology: 1995 to 2015
From 1995 to 2015 the steering group recommends addressing
· —
six questions:
1. What are the major features of the water cycle? Not only
is water essential to life, it drives or influences nearly aD the other
biogeochem~cal cycles as a solvent, as a source of aerosols, or as a
carrier of particulates. On a longer time scale it is the major agent
of erosion and subsequent sedimentation.
2. What are the reactions and the fluxes of the other biogenic
elements—carbon, nitrogen, sulfur, and phosphorus? During the
past century the atmospheric concentration of carbon dioxide has
increased from 200 to 300 ppm; methane, a trace gas, may provide
a significant component of the flux of carbon. The turnover of
atmospheric nitrogen is slow; we do not understand whether the
seern~ng balance between nitrogen fixation and denitrification Is
affected by the production of nitrogen dioxide by lightning. Prior
to industrialization most sulfur was introduced into the atmo-
sphere by vuIcanism. There ~ also significant biogenic production
of hydrogen sulfide and (CHESS by anaerobic organisms. Photo
phases are dissolved or borne as particulates; animal depositions
may account for most soil replenishment. It is far more difficult to
measure flux than concentration of these compounds; yet values
for flux are essential for proper modeling.
3. What are the major sedimentation and erosion processes
that the biota influence? About 99 percent of Earth's carbon
exists ~ carbonate sediments, most of them derived from biomin-
eral~zation. Rates of erosion by wind and water depend on the
extent and type of vegetation.
4. How productive are the major ecosystems? The rate of
photosynthesis is influenced by the amount of light of appropriate
wavelength reaching the Earth's surface, and, hence, is influenced
by the Earth's adbedo. Most of the energy converted by plants,
algae, and photosynthetic bacteria is expressed in the synthesis of
carbohydrates: CO2 + H2O ~ CH2O + O2. The albedo of the
Earth depends not only on its atmospheric composition, but also
on the life in its oceans and on its land manes.
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66
5. What are the impacts of major human activities? Exten-
sive use of fossil fuels and other industrial activities are increasing
atmospheric concentrations of carbon dioxide, sulfur dioxide, and
other gases. Deforestation in the tropics leads to increased ero-
sion, reduced synthesis of ozone by photosynthesis, ~d reduced
transpiration of water. The release of chIoroiluorocarbons into the
atmosphere may be reducing the ozone layer and increasing the
greenhouse effect.
6. Can the vast amounts of empirical data that will come
from satellite and other global arrays of instruments be incorpo-
rated into a predictive mode} for Earth as a whole, leading to
the formulation of new principles? To mode] Earth systems on
tune scales from days to millennia presents complexity without
precedent.
Space Biology Goals
The goal of space biology 38 to determine whether the unique
opportunity of experimentation at microgTav~ty can advance OUT
und;erstanding of basic phenomena in biology.
Throughout its entire evolution, life on Earth h" experienced
only a one-g environment. The influence of this omnipresent force
is not weD understood except that there is clearly a biological ret
spouse to gravity in the structure and functioning of living organ-
isms. Access to a microgravity (micro-g) space station laboratory
may facilitate research on the cellular and molecular mechanisms
involved in sensing an acceleration as low as lo-6 g and sum
sequently transducing this signal to a neural, ionic, or hormonal
signal. Propagating selected species of plants ~d animals through
several generations at micro In such a space laboratory would
advance our understanding of these biological responses.
Major Questions for Space Biology: 1995 to 2015
From 1995 to 2015 the steering group recommends addressing
four major questions:
I. How do plant cells detect the gravity vector and transduce
this force to hormonal Ed nonhormonal signals? This gravitropic
response utilizes growth-stimulating hormones, such as gibberellin
and indoleacetic acid, and the inhibiting hormone abscisic acid.
These same hormones, along with electric current, are involved
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67
in phototropism; the interactions between the two responses are
complex. Investigations of these processes are unport ant for the
development of a successful Controlled Ecological Life Support
System (CEL.SS).
2. Can higher plants and animals be propagated through sev-
eral generations at m~cro-g? Though many embryos orient their
cleavage planes relative to the gravity vector, we do not understand
whether gravity, per se, is essential to gametogenesis, fertilization,
implantation In animals, organogenesis, or development of normal
sensorimotor responses. Given the effects of m~cro-g on deminer-
alizatioct in bones, muscle wasting, and vestibular function, there
is some question whether vertebrates can develop normally at
micro-g.
3. What is the relative contribution of gravity to sensorunotor
functions? The otolith organs ~ the inner ear allow us to detect
linear accelerations In three orthogonal directions and distinguish
these from rotation, which is detected by the semicircular canals.
These responses, already complex, are further integrated with
visual and proprioceptive input. The ability to remove or to vary
signals from the otolith should help us understand the interactions
of these sensory systems and shed msight on the nature of motion
sickness.
4. What are the fundamental biochemistry and physics of
biom~neralization? Understanding the physical and chemical pros
cesses of biomineralization on Earth is necessary to fully under-
stand the potential effects in m~crogravity environments. This
process almost always occurs within membrane vesicles or is as
sociated with polymers of carbohydrates or of proteins. Scores
of biominerals exist, the most common being CaC03, calcite or
aragonite, and Ca~O(PO4~6(OH)2 the hydroxyapatite of bones.
Macroscopic solubility products do not readily explain the growth
of these aggregates of crystallites.
Space Medicine Goal
The goat of space medicine is to understand the human ~oi-
ology underlying the prophylaxis and therapy of maladaptations
encountered; in extended space travel and to d;evelop prophylaxis
and therapy to treat them if it is feasible.
Space flights extending for weeks or months have already
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68
caused several physiological problems that could endanger astro-
nauts during longer flights or on their return to Earth. At m~cro-
g, fluid accumulates ~ the upper body; subsequently, astronauts
must remain horizontal for hours to weeks on return to Earth.
During flight, extensor muscles atrophy, and, of greater concern,
bone calcium and phosphate are lost from weigh~bear~ng bones.
We do not know whether these effects reach a plateau. If not, they
could irreversibly compromise the health of an astronaut or even
lead to death. Numerous other effects of prolonged weightlessness,
compounded by the rigors of the confiner] environment of a sta-
tion, pose serious threats to health and performance. The heavy
ion radiation of outer space is not only mutagenic but also could
have disastrous effects on the brain. Each such particle inflicts
severe damage or even death to nondividing cells.
Major Questions for Space Medicine: 1995 to 2015
From 1995 to 2015 the steering group recommends addressing
· · —
six mayor questions:
I. Are there adverse effects of weightlessness that grow pros
gressively more debilitating as flights are extended incrementally
from months to years? We already anticipate severe problems with
bone loss, and probably with muscle atrophy =d cardiovascular
decondition~g. Others that cannot be predicted may also occur.
These could involve the immune and the endocrine systems. The
rigors of confinement will probably compound these physiologi-
cal effects and profoundly affect behavior. A series of controlled
experunents will help to sort out the effect of inevitable human
variability on responses.
2. What are the hormonal, nutritional, and mechanics cor-
relates and mechanisms of biom~neralization? Although doctors
may develop empirical procedures to alleviate the severity of de-
m~neralization, we must understand the underlying molecular and
cellular biology of bone remodeling and of calcium and phosphate
homeostasis. A similar question concerning biomineralization was
posed for space biology, rejecting the frequent and productive
interactions between clinicians and biologists.
3. What are the fundamental genetic, hormonal, and mechan-
ical factors that determine muscle development and maintenance?
Our working hypothesis is that during muscle wasting the rate
of breakdown of muscle protein exceeds the rate of synthesis. We
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must understand the factors coupling weightlessness to the controls
of those genes encoding muscle proteins as well as those genes
encoding muscle-specific proteases.
4. What are the biological effects of prolonged exposure to
heavy ion radiation? It is difficult to reproduce with particle ac-
celerators the exact spectrum of radiation encountered In outer
space where heavy iron, 56Fe, traveling at 0.9 times the speed
of light, Is the predorn~nant particle. Cultures of cells as well as
plants, rodents, and higher primates must be exposed and subse-
quently analyzed to enlarge the empirical data base. In parallel,
we should explore the basic biophysics of the heavy ion damage in
hopes of developing prophylaxis and therapies.
It would be imprudent even to plan extended missions until
these serious medFical issues are resolved.
5. How can we ensure adequate life support systems for long-
term space travel? The development of a Controlled Ecological Life
Support System (CELSS) is essential to missions of long duration.
Although this development is, ~ a sense, a technological issue' it is
so complex and requires so many advances in our understanding of
biology that we can regard it as a major scientific goal that spans
the interests of space medicine, space biology, and global biology.
Aside from the challenges of growing plants at m~cr~g, and the
utility of CELSS as a life support aid, the concept of constructing,
at least partially, an artificial ecosystem is of great interest to the
science of ecology. It may offer a research too} of considerable value
for study of the principles by which natural ecosystems function.
6. Under the conditions of space, what are the optimal inter-
actions between humans, machines, and computers? May con-
struction and observation tasks will be done under extremes of
pressure, temperature, and radiation hostile to the human organ-
ism. Human judgment and ingenuity are valuable or indispensable
to some of these jobs. This should not be seen as a choice between
man and robot, but ~ a challenge to integrate their respective
capabilities. This wall require fundamental research not only in
machine design but also in human neurophysiology.
~:COMMENDE:D PROGRAM: POST-1995
Two basic requirements dominate most of these recommen-
dations. Global biology and exobiology require ground-based
observations and experiments to calibrate and verify satellite
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observations. Space biology and medicine require one-g controls
for experiments and observations at m~cro-g.
Recommendations for Exobiology
The steering group envisions a series of observation and sam-
ple return rrlissions complemented by ground-based analyses and
experiments.
1. Determine the properties of the atmospheres of Mars, Ti-
tan, and Jupiter with greater precision. The reactions leading
to any organic compounds found there should be compared with
models for chemical synthesis on the primitive Earth.
2. Return sedimentary rock and soil samples from Mars after
a careful analysis and selection of sites. The initial compositional
analyses can be done at the sites. Complete analyses will require a
full range of laboratory procedures including electron microscopy
and microprobe analyses. This study should include searches for
organic molecules and for fossil evidence of primitive life forms, and
deterrrunation of important isotopic abundances, such as carbon,
nitrogen, oxygen, and hydrogen.
3. Return samples from asteroids and comets to Earth and
analyze them with the methods now used to analyze meteorites
and captured interplanetary dust particles. The design and execu-
tion of these missions should be closely coordinated with planetary
.
sciences.
4. Continue to explore fossils, isotope enrichment, and other
evidence of prebiotic and early life on Earth.
5. Refine laboratory experiments simulating likely prebiotic
reactions as more results from missions accumulate and as molec-
ular biology lends more insights into the fundamental structures
and reactions of cells.
Recommendations for Global Biology
The steering group projects a series of satellite flights to de-
fine the major ecosystems over the entire Earth ~d to measure
remotely their contributions to the major biogeochern~cal cycles.
Data obtained from these flights should Al be calibrates! against
measurements at selected sites to assure detection of long-term
trends.
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71
1. Determine the predominant plants for the major ecosys-
tems, and measure their chlorophyll content and rate of biomass
formation.
2. Measure the seasonal variations in carbon dioxide fixation
for these ecosystems.
3. Measure the production rates, reactions, and fluxes of sev-
eral trace gases for the major ecosystems. These measurements will
require the refinement of gas chromatography, mass spectrometers,
color imagers, laser fluoroscopes, and synthetic aperture radar.
4. Compare the unpacts of human activities, such as deforesta-
tion and industrialization, with well-established baselines. These
goals are included in the Mission to Planet Earth (described in
Chapter 2~. The two programs should proceed in close colIabora-
tion.
5. Develop correlative models to accommodate vast amounts
of diverse data. The will require that special attention be paid
to data reduction and archiving and to computational facilities.
These correlations shouic! lead to interpretative and predictive
models.
B-ecommendations for Space Biology and Space Medicine
The steering group believes that these disciplines require a
series of missions, each addressing one of the mad car research prom
lems previously discussed.
1. Construct a dedicated life sciences laboratory with a large
variable-speed centrifuge to hold plants and animals and to prm
vice one-g controls. Without adequate controls, most of the exper-
iments at micro will be of limited value. This centrifuge will also
facilitate experunents addressing the effects of reduced gravity as
found on the Moon. (As will be required for m~cr~g experiments
in the physics of solution, some experiments treating the set-
tling of amyloplasts or growth of crystab will require a frequency
analysis and isolation from accelerations exceeding loss g.)
2. Consistently employ four to eight critically selected plant
and animal species, such as the nematode Caenorhabditis elegans
and the cress Arabidopsis thaliana for as many gravity-related
experiments as possible. This focus wait extend and refine descries
tions of the physiolog~es and genetics of these species, including
techniques to clone them and extensive analyses of their genomes.
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72
3. Precede m~cro-g experiments, which will be infrequent
and expensive, with thorough simulations at one-g (e.g., extended
bed-rest for cardiovascular Reconditioning or demineralization and
clinostat experiments for gravitropism). In-flight controls at one-
g are mandatory to account for potential artifacts, such as the
acceleration of launch, vibrations, and astronaut activities.
4. Evaluate the biological effects of heavy ion radiation thor-
oughly using accelerator sources. If possible, these experiments
should employ the selected organisms already mentioned.
5. Require a prototype of CELSS to function satisfactorily on
Earth for several years before attempting to launch such a system
into space. Evaluate in highs those components whose functions
might be altered at m~cro-g.
6. Establish empirically a data base of the ranges of human
physiological and behavioral responses to m~cro-g under prolonged
isolation. All astronauts should participate in nor~invasive or mini-
mally invasive tests as well as monitors of behavior—activity levels
and general speech patterns, for example. All such tests must be
consistent with weD-established standards of personal privacy and
medical ethics and must not interfere with astronaut safety or
job performance. Many of these basic data such as pulse, blood
pressure, basic metabolic rate can be obtained from rniniatur~zed
devices worn on the skin, or from simple automated analyses of
urine and drops of blood. (Although this study does not address
the delivery of health care, it is obvious that many of the measure-
ments required for research are required for diagnosis. Economy
can be realized without compromising either function.)
7. NASA should make a major investment in robotics, both
developing new instruments and computers and optimizing the
interactions between humans and robots.
Significance of Recommendations
By 2015 we hope to have an inventory of organic compounds
found on other bodies in the solar system. By 2015 we expect
more definitive information on the past and present liquid water
on Mars, as well as the results of the preliminary search for fossils
and other evidence of past life.
The greatly enlarged data base of global biology and earth
science should permit refined predictions of atmospheric patterns
over decades. We should have much more insight into the Sequence
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of life on the general evolution of Earth. In particular, we should
understand whether major perturbations, such as glaciation or
mass extinction, reflect events originating outside the Earth or
if they are predictable transitions in the evolution of a closed
system with constant energy flux. The knowledge gained from
these measurements wiD focus our searches for minerals and fuels.
Also, with an appreciation of the ecological impacts of human
activities, rational and balanced conservation programs can be
implemented. It is quite possible that an increased respect for our
planet waif accompany an increased understanding of it.
The steering group has proposed investigations at micro-g of
several biological phenomena, such as gravitropism development,
sensorimotor integration, and bone remodeling. It has also called
for ~ Controlled Ecological Life Support System. These initial
experunents should determine whether m~cro-g offers an effective
experimental approach to some basic problems in biology. If such
experiments are performed with flexible formats and appropriate
controls, they may well reveal unanticipated phenomena of even
greater interest. Although serendipity is hardly the basis of a ret
search strategy, the steering group believes it a wise investment
to apply some of the resources of the dedicated life sciences bay
oratory to define the full range of questions best addressed at
micro-g.
By 2015, and even by 2005 if there is a dedicated life sciences
laboratory, we should have fully described the physiological and
behavioral responses of human beings subjected to m~cro-g and
to heavy ion radiation for periods of over a year. Anticipating
significant adverse effects, we should at least have defined which
of these effects might be alleviated and be wed on the way to
realizing treatments. We should have acquired a greatly refined
insight into the unique limitations and capabilities of humans and
Of robots in space flight. Most unportantly, understanding the
optimum interactions of humans and robots should permit us to
evaluate rationally their appropriate roles in commercialization,
exploration, and research in space.
This program in life sciences should be adequately funded
and properly integrated with the other space sciences and with
ground-based investigations. It will make a significant contribu-
tion to understanding the evolution of our planet and ourselves.
If pursued with reason and caution, it will help define our proper
relationship to Earth.
Representative terms from entire chapter:
space biology
