Exploitation of gene regulation rather than the creation of new genes has been predominantly responsible for the evolutionary advances in animals and plants that are widely recognized today. Until very recently it was not possible to examine life in the absence of gravity. We can now imagine forms of life in the universe adapting to circumstances different from those found on Earth. Our own life forms would surely become different in time if they were transferred to other planets with different conditions, including much lower or higher gravity

Krikorian, Abraham D.

Markert, Clement L.

English

NASA Technical Reports Server

Life: Origin and Evolution on Earth- How Can We Escape?
 94"1921d
CLEMENT L. MARKERT
Department of Animal Science, North Carolina State University, Raleigh, NC 27695
ABRAHAM D. KRIKORIAN
Department of Biochemistry and Cell Biology, State University of New York,
Stony Brook, NY 11794 5 7- _'_/-
ABSTRACT
Exploitation of gene regulation rather than the cre-
ation of new genes has been predominantly responsible for
the evolutionary advances in animals and plants that are
widely recognized today. Until very recently it was not
possible to examine life in the absence of gravity. We can
now imagine forms of life in the universe adapting to
circumstances different from those found on Earth. Our
own life forms would surely become different in time if
they were transferred to other planets with different condi-
tions, including much lower or higher gravity.
Life arose on Earth nearly four billion years ago
as membrane-contained biochemical and biophysical
systems that were isolated from each other and pro-
tected from the environment. These systems could
enlarge, subdivide, and reproduce as individual or-
ganisms. Organic and inorganic molecules were se-
lectively absorbed and processed, and some mol-
ecules were then excreted (of. Schopf, 1983). Thus,
a form of biological replication -- crude and erratic at
first -- slowly evolved, stabilized, and began to ex-
hibit characteristics appropriate for the survival of
the fittest. It took a billion years or more for this
system to evolve beyond a bacterium-like cell stage.
The basic biochemical patterns and the mechanisms
for replicating the biochemical architecture with in-
tegrity must have evolved very slowly during an
additional two billion years. Perhaps another 700
million years passed during the evolution of multicel-
lular organisms, which takes us to the middle of the
Cambrian era of evolutionary development (cf. Avers,
1989). Then, within the next 500 million years, all of
the contemporary divisions of plant and animal phyla
developed. Mammals and higher plants have been
around for about 200 million years, since the Triassic
age of the Mesozoic era, but in the case of the mam-
mals at least, these organisms were primitive and very
small and not numerous for the next 140 million
years. Only since the dinosaurs disappeared at the
beginning of the Cretaceous era have mammals and
flowering plants undergone an explosive evolution.
The first primates go back about 60 million years.
Contemporary humans shared a common ancestor
with our close relatives, the great apes, about 10
millions years ago, and the ancestors of contemporary
human beings, Homo sapiens, have been recogniz-
able for perhaps the last two or three hundred thou-
sand years. Modern humans biologically indistin-
guishable from ourselves may go back only 40 or 50
thousand years. Thus, we are very late comers to
Earth.
The extraordinarily rapid development that we
seem to see in the formation of the most highly
evolved organisms today in terms of their size, mobil-
ity, and capacity to dominate the environment reflects
perhaps an accelerating rate of diversification of life
forms. This diversity is based upon a reassortment
and rearrangement of basically the same building
blocks (cells) -- building blocks that were created by
the billions of years of early evolution (cf. Briggs and
Crowther, 1990). Of course, human cultural, scien-
tific, and technological evolution that gave rise to
contemporary civilizations began just a few thousand
years ago, and most developments have occurred
within the last few hundred years. This recitation of
freshman biology has been made to emphasize that
the exploitation of gene regulation rather than the
creation of new genes is primarily responsible for the
evolutionary advances in metaphytes and metazoans
i.e., multicellular plants and animals. In animals
especially, but plants as well, the precise movement
and/or associations of ceils and their parts, all sensi-
tive to gravity (more precisely, in plants both meris-
tematie and non-meristematic regions may be
graviresponsive), are critical for recent evolutionary
developments.
All of this evolution, of course, proceeded with
gravity as an ever present and constant aspect of the
environment. Biologists have investigated many of
the specific mechanisms of evolution, as well as the
basic biochemical and biophysical nature of life it-
self, but we have given practically no attention to the
role that gravity has played and is playing in the
structure and functioning of contemporary organisms
(cf. Stebbins, 1982; Avers, 1989). The explanation is
35
https://ntrs.nasa.gov/search.jsp?R=19940014744 2020-06-16T17:11:25+00:00Z
MARKERT AND KRIKORIAN
Organization of Four Clusters of Homeobox Genes in The Mouse and Human Genomes
Posterior ::=:::: ......... _:=:: ............... __ Anterior
1.9 1,8 1.7 1.1 12 1,3 t.4 ].5 t.1t 1.6 Mouse
HOX-1
IJ 11 1H 1G 1A 1B 1C ID 1E 1K Human
25 2,4 23 22 2.1 2.6 23 2.8 2.g Mouse
HOX-2
2E 2D 2C 2B 2A 2F 2G 2H 21 Human
_ 3.7 3.6 3.2 3.1 33 3.4 3.? Mouse
HOX-3 _
3G 3F 3H 31 3B 3A 3C 3D 3E Human
4.8 4.7 4 6 45 4.4 4.3 42 4 1 4 g Mouse
4i 4H 4F 4D 4C 4E 4B 4A 4G Human
I I1 III IV V VI VII VIII IX X XI XII XII]
Figure 1. Distribution along chromosomes of homologous genes in mouse mad humans that contain homeoboxes.
These genes are activated in the embryo in a temporal and spatial sequence that corresponds to the linear position
of the genes along the chromosome. The homology of the homeobox sequences among many taxa has been highly
conserved throughout evolution. Diagram redrawn from Murtha et al., 1991.
simple, of course. Until very recently, it was not
possible to examine life in the absence of gravity.
Now we can do so with the space vehicles and stations
and with our ability to engage in space travel (cf.
Krikorian and Levine, 1991).
Life itself has made the Earth a very different
place from what it was before life evolved. There is
a film of living material covering virtually all of the
land surface of the Earth and, of course, also in the
water that covers most of the Earth's surface. The
gaseous atmosphere has been changed by the activity
of life, and thus has changed the environment With
consequent effects on the further evolution of organ:
isms. Life continuously creates a changing environ-
ment and then responds to the changes by further
evolution (cf. Briggs and Crowther, i990). We have
never escaped from gravity except momentarily and
then only in recent years. It is possible to imagine
many other forms of life in the universe evolving and
adapting to quite different circumstances from those
we find on Earth, and our own life forms on Earth
would surely become different if they were trans-
ferred to other planets •with different physical condi-
tions, including much lower or much higher gravity.
The current importance of understanding the role
of gravity in our own evolution and development
36
stems primarily from the fact that we plan to send
explorers to the moon and to Mars and perhaps else-
where and to take other forms of life with us (cf. e.g.,
Robbins Committee Report, 1988). The consequences
for the structure, physiology, and development of
ourselves and other organisms will surely be pro-
found, and it behooves us to understand the signifi-
cance of gravity in determining our own basic bio-
logical nature before we meet these extraterrestrial
challenges. At the most basic level of life, the indi-
vidual cell, we find a vast array of molecules, of cell
organelles, and of various elaborate structures that
collectivelymake possibie the biochemicai activities
that keep the cell alive, developing, and reproducing
(cf. Alberts et al., 1989);
Recehtly, we have come tO recognize the organi-
zation of specific groups of genes containing _se-
quences (homeoboxes) that seem to specify in time
and place the development of the organism (cf. e_g.;
Murtha et al., 1991). In mice and humans, both
species have been extensively investigated: four dif.
ferent clusters of homeoboxes have been identified,
each on a different chromosome. Within each of
these clusters, there are about ten genes distributed
along the chromosome in a precise order that is the
same for mice and humans. This physical arrange-
LIFE AND EVOLUTION WITHOUT GRAVITY
Parascaris oquorum
Normal Development
\
Germ plasm
chromosome fragmentation
and diminution
No chromosome
change
Somatic cells
Abnormal Development After Displacement of Germ Plasm by Centrifugation
Somatic cells
Germ plasm No Chcha_SOme
Gamete stem cells
Figure 2. Diagram showing results of centrifugal displacement of germinal plasm in Parascaris [Ascaris
megalocephala]. Presence of germinal plasm in a cell prevents chromosomal diminution and fragmentation of
large chromosomes into many small chromosomes. The presence of intact large chromosomes me essential for
germ cell development. Cells with reduced chromosomal content become somatic cells. After Boveri01910.
ment -- the anterior-posterior sequence -- corre-
sponds to the time of activation of these genes
during the course of early development (Figure 1).
Similar patterns of homeobox organization have
been discovered in a wide variety of organisms
stretching all the way back to the most primitive
multicellular organisms. The fact that these genes
have been extraordinarily well conserved during the
course of evolution indicates that they must have a
fundamental role in determining the structure of
organisms. All of the evidence we have so far
indicates that they do play such roles in developing
the basic morphology, very likely by affecting cell
associations and relative rates of multiplication that
could define the axial and other structures of the
developing organism. Clearly, these genes could
not be abnormally activated or inactivated without
drastic effects on the development of the organism.
Progressive slight changes, in accord with what we
might expect in evolution, could of course lead to
the variety of morphologies that we see in different
organisms. Obviously, however, the slightest per-
turbation in the activation of these genes during the
course of development would have profound con-
sequences. Thus, the role of gravity in perturbing
the program of activation of these homeobox genes
must be understood in detail if we are to safely
reproduce in space. Molecules and organelles can
be moved within the cell by centrifugation and,
therefore, must respond to gravitational forces ex-
erted on the cell; such redistributions of compo-
nents within a cell could prove fatal or at least
could change the development of embryos. The
temporary redistribution of cell contents by cen-
trifugation can frequently be overcome and the
original state of the cell restored. Many forces are
involved in molecular traffic to move and orient
molecules and organelles within the cell. Gravity
is certainly one of them (cf. Halstead et al., 1991).
How the cell manages without gravity and how it
changes in the absence of gravity are basic ques-
tions that only prolonged life on a space station
will enable us to answer (cf. Souza and Halstead,
1985; Asashima and Malacinski, 1990).
37
MARKERT AND KRIKORIAN
We know from the experience acquired so far on
space vehicles and stations, our own and those of the
Soviet astronauts (cf. Garshnek, 1988), that pro-
found deleterious physiological effects do occur and
jeopardize the functional capacity and even the sur-
vival of human beings. To counteract or circumvent
these effects, we need to know more of the basic
biochemistry and biophysics of the cell and of the
whole organism in conditions of reduced gravity.
Such knowledge is essential in order to make space
travel and residence on the moon or Mars practi-
cable. Reproduction among highly differentiated
organisms such as mammals, birds, and other verte-
brates and probably also invertebrates will obvi-
ously be seriously affected by the absence of gravity
(cf. Guyenne, 1990). Organisms with large amounts
of yolk in their eggs, such as amphibians and birds,
al., 1989). Second, we will achieve deeper insight
into the nature of our own biological structures and
activities by understanding the significance of grav-
ity in the development of morphology and physi-
ological function (cf. e.g., Oser and Battrick, 1989).
We cannot predict what the research on organisms in
the absence of gravity will produce. Otherwise, we
would not need to do the research. But that there will
be significant enlightenment seems obvious. Sur-
prises there will be, and we should be enthusiastic in
welcoming the knowledge and insights that will surely
result from biomedical research in space.
REFERENCES
Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K.,
will be seriously affected by the redistribution of and watson, J.D. 1989. Molecular Biology of the
components within the egg when gravity is greatly Cell, Second Edition. New York: Garland Publish-
reduced. Homolecithal eggs, such as those of human ing.
beings, would probably be less affected, but even in
these eggs, the molecular and organellar traffic would
surely be affected by the prolonged absence of grav-
ity. The basic hereditary organelle of the cell, the
chromosome, and the many molecules with which it
interacts to produce precise patterns ofgene activity
during development and in normal adult physiologi-
cal function would also be influenced by gravity
(Figure 2). Can such animals and plants reproduce
on a space platform and through several successive
generations? Only research on the station could
answer these questions. Moreover, we do not now
have the insights required to ask many questions that
will surely become obvious by experience in space.
Stable long term experiments through several gen-
erations are needed on a space station in order to
know our capabilities and to protect them for the
long trips to Mars or residence on the moon or Mars.
Once we develop a clear understanding of the conse-
quences of the absence of gravity on our physiologi-
cal and developmental capacities, such information
will also enlighten our response to challenges here
on Earth. In fact, such information and understand-
ing would be of great value even if we never ven-
tured into space.
In summary, there are two major reasons for
building a space station and carrying on long-term
experiments in biological and biomedical sciences
on that platform. First, we must do so if we are to
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McCormack, P.E., Swenberg, C.E., and Buecker, H.
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39

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Space Life Sciences Research: The Importance of
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This report focuses on the scientific importance of long-term space experiments for the advancement
of biological science and the benefit of humankind. It includes a collection of papers that explore the
scientific potential provided by the capability to manipulate organisms by removing Earth's gravity,
the force that has been instrumental in the evolution and development of all organisms. It discusses
the scientific justification for why the long-term space exposure that can be provided by a space station
=snecessary to conduct significant research. Early research results, limited as they have been,
show numerous scientific benefits to humankind resulting from life science research conducted in space.
14.SUBJECTTERMS
life sciences, aerospace medicine, space stations, reduced gravity, gravitational
physiology, biological effects, physiological effects, plants, animals, human beings,
cells wspace flight
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Life: Origin and evolution on Earth--How can we escape?

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