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a r t i s t s ’ a r t i c l e
The Tree, the Spiral and the
Web of Life: A Visual Exploration
of Biological Evolution for
Public Murals
a b s t r a c t
The authors created the Spiral
of Life as a new, accessible
symbol for the evolution of life.
This novel visual interpreta-
tion of evolution challenges
traditional tenets of the field in
light of emerging new themes
in research. The Spiral brings
recent principles to the general
public and also provides scien-
tists with a new visual concept
to support further discussion.
The Spiral emerged from the
combination of the analysis of
the latest scientific research
with an artistic process to
create new images and icons. A
resulting complementary series
of artworks was installed in five
cultural institutions and muse-
ums in Pittsburgh, Pensilvania.
Joana Ricou and
John Archie Pollock
Our science outreach work constantly reveals
the growing disconnect between the current state of science
and the general public’s understanding of fundamental prin-
ciples of science. In keeping with what C.P. Snow observed in
Two Cultures [1] and a recent National Academy of Sciences
report highlighted [2], this disconnect has had a profound,
pervasive and negative impact on science education, público
policy and health in the United States.
Evolution struck us as an interesting topic within which to
address this divide. The coincidence of Charles Darwin’s 200th
birthday and the 150th anniversary of the publication of On
the Origin of Species in 2009 was the impetus for a worldwide
movement that promoted awareness of evolution and spurred
our own efforts. The dialogue surrounding this topic is obfus-
cated by the cultural debate that still rages over the validity of
the science in this area and the lack of understanding of the
relevance that evolution has to everyday life. Communicating
the principle of common descent and evolutionary relation-
ship of all living things was also interesting to us because the
subject is currently actively researched by scientists, who still
see defining over-arching principles as a significant challenge.
Visualization is a part of evolutionary research. Different
diagrams are created depending on the specific point of view,
intellectual framework, data sets and other constraints. Mientras
scientific visualizations must not allow for “artistic license,"
they always have an aesthetic component [3]. Since the cur-
rent research into evolution still has many questions to ad-
vestido, and the way that the data is visualized is important to
data interpretation, we find that image explorations can con-
tribute both as support for visualization and as propositions.
These characteristics make evolution promising territory for
a fruitful art-and-science collaboration.
The traditional split between the roles of the scientist and
the artist is harder to define in our collaboration than is often
Joana Ricou (artist), 631 Grand Street, Floor 4, Brooklyn, Nueva York 11211, U.S.A. Correo electrónico:
John Archie Pollock (scientist, educator), Department of Biological Sciences, 222 Mellon
Sala, Duquesne University, pittsburgh, Pensilvania 15282, U.S.A. Correo electrónico:
Ver
issue.
article Frontispiece. Spiral of Life III: Animal Evolution, digital art on
vinyl, 18 × 10 ft, 2009, Pittsburgh Zoo & PPG aquarium. this piece
focuses on the animal kingdom. (© Joana ricou and John Pollock)
the case, because Ricou is an artist
trained in cellular biology and ge-
netics and Pollock is a physicist/
biology researcher with a second-
ary career in science visualization
and education. Both artistic and
scientific opinions contributed to
the making of this project, con un
concern for the general accessibility
of the science. The decision-making
process was guided by the tug-of-
war between the scientific point of view that resists generaliza-
tion and the artistic point of view that is traditionally more
concerned with the big picture. As Goodsell and Johnson sug-
gest [4], artistic license allows selective disclosure, purposeful
distortion of scale or perspective and simplification, entre
other tools, to serve the goal of clarifying a bigger picture and
exciting interest. The artist is willing to incorporate what is
known and create a visual hypothesis of what is not certain,
while the scientist in both of us reached for exacting content
based on the modern molecular genetic data in the published
scientific literature.
Given the pace of research on evolution, we are aware that
any images committed to paper today will become outdated
in a few short years. Sin embargo, there is a need for a sym-
bol that captures and clarifies the principles currently under
discussion. We needed a symbol that would be accessible to
the general public and have the potential to serve as a men-
tal framework for researchers to model, anticipate and hypo-
thesize with.
The first step in our creative process was to review the
current scientific literature, news articles, imagery and web
resources. In this process we considered over 84 different pri-
mary sources, texts and web projects [5]. The primary research
data we collected had unsettled, changing and/or contradic-
tory aspects. We resolved individual conflicts by consulting a
panel of experts (listed in this paper’s acknowledgments). Para
ejemplo, opinions on the branching pattern and the dates of
the branching in bacteria are not settled. Different evidence
points lead to conflicting answers. We based our project on
the dates provided in Madigan and Martinko’s 11th edition
of Brock Microbiology of Microorganisms [6], with guidance from
John Stolz of Duquesne University. We collected data simulta-
neously with spreadsheets and diagrams and subjected both
to feedback from our scientific advisors.
©2012 ISAST
LEONARDO, volumen. 45, No. 1, páginas. 18–25, 2012 19
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Higo. 1. (digitally enhanced) (a, izquierda) First evolutionary tree drawn by charles Darwin in his
Notebook b in 1837 (public domain). For Darwin, the tree described the relationships
between groups of organisms, revealing that all species are related through common ances-
try and that they change over time. (b, bien) Evolution of Man—Pedigree of Man Plate XV by
Haeckel, 1879 (public domain). Within a decade of Darwin’s publication of The Origin of Spe-
cíes, ernst Haeckel had drafted multiple views of evolutionary trees, which typically placed
humans as the most evolved species.
The Tree
While Charles Darwin was not the first to
use a tree structure to organize living or-
ganisms, the “tree of life” he drew in 1837
became an icon and cognitive model for
evolution (Higo. 1a) and is recognizable
in modern cladograms. His model es-
tablished that all species are related by
common descent (common root) y
that they diversified (ramification). El
length of the branches can signify time
or evolutionary difference. As decades
passed by, the popularized interpretation
of this revolutionary image accumulated
misconceptions: Por ejemplo, that evolu-
tion is a linear, vertical process and that
all of life descends from a single common
ancestor and culminated in the human
species (as shown in Fig. 1b, a Haeckel
illustration [7]). Another is that evolu-
tion has direction or is a progression
[8], which has ended in modern times.
While within the scientific community
there is consensus regarding the funda-
mental principle of evolution, emerging
research wrestles with the constraints
of this iconic framework, En particular
regarding the role of horizontal gene
transfer processes, which we discuss
más tarde. Most modern imagery and visual
herramientas, such as the Tree of Life web proj-
etc. [9], continue to use the tree icon as a
estructura.
The CirCle
Históricamente, a circular design has also
been used to represent the organization
of the natural world and has inspired
more recent evolutionary diagrams that
are radial or circular [10] (Higo. 2). El
radial diagrams include the branching
pattern but place the origin of the dia-
gram at the center, unequivocally con-
noting a common origin. An example of
this type is the interactive Tree of Life
[11]. Sin embargo, this type of geometry is
the least represented in sites to which the
general public has ready access, como
in museums (less than 10% in a sample
de 112) [12].
. . . and The Spiral
The most important message that we
seek to impart with the Spiral is that all
life is connected by evolution and all life
is evolutionarily equal. Para tal fin, nosotros
used the circle theme, with time as the
radial axis, labeled explicitly like rings in
a tree. In this way, all branches of organ-
isms living today have the same radial
length, stretching from the origin to the
“today” ring, to emphasize the equality
of evolutionary paths. An organism at a
different scale represents each group of
organisms; this “purposeful distortion”
[13] made microscopic and macroscopic
20 Ricou and Pollock, The Tree, the Spiral and the Web of Life
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organisms appear similar in size, to fur-
ther communicate their evolutionary
equality. A “future” ring tantalizes the
imagination and serves as a reminder
that evolution is ongoing. Through an
iterative process, we combined a logical
organization of the visual space with the
constraints of multiple discrete branches
and enormous time (see “Collapsing
Time” below). In this way, a spiral shape
emerged at the center of our image, em-
phasizing the connection between the
evolution of all major taxa. This shape
became the heart of our reconceptual-
ization of evolution as the Spiral of Life
(Color Plate A).
From Spiral To Web
The idea that many natural and human
processes are best represented as a col-
lection of linked entities or network has
achieved significant prominence in the
cultural and scientific zeitgeist in recent
años. It should come as no surprise that
evolutionary biology is also participating
in this conceptual revolution. Scientists
now recognize that “horizontal transfer,"
processes that violate the strictly tree-like
flow of genetic information proposed by
Darwin, have been contributing to evolu-
tion over the millennia [14] (Higo. 3).
Traditional cladograms represent
“vertical” processes and refer to the pas-
sage of genetic material from a parent
generation to an offspring generation.
We found that there is another type of
proceso, “horizontal,” that plays an im-
portant role in evolution. Horizontal
transfer occurs when genetic informa-
tion gets passed from one individual to
another not through reproduction but
by a virus, transposable elements [15]
or other means, and this information
is passed on to progeny. The existence
of horizontal processes means that evo-
lution can occur during the lifetime of
an organism, which is an idea closer
to Lamarck’s theories than Darwin’s.
There is a consensus that these pro-
cesses, while rare, have happened and
continue to happen and are crucial in
evolution [16–19]. Sin embargo, horizontal
gene transfer (HGT) is not included in
most representations of the tree of life
in museums [20]. The added concen-
tric links between branches signify the
“horizontal” molecular and cellular pro-
cesses that transfer genetic material be-
tween species. The idea of the evolution
of life as a net was originally suggested
en 1993 [21], and a visualization of per-
vasive HGT was published in 2005 [22]
(Higo. 3).
The fusion of bacteria and archaea
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Higo. 2. David M.. Hillis, Derrick Zwickl and robin Gutell of the Uni-
versity of texas created this circular diagram based on DNa sequence
analysis of small unit ribosomal rNa sampled from about 3,000 species
[43]. Originally intending a long banner, Hillis et al. found that a circle
allowed them to pack in more species [44]. (© David Hillis)
Higo. 3. barth smets proposes this image to capture the role of
horizontal gene transfer in evolution [45]. the image also rep-
resents the origin of life as a complex web instead of a single
line. reprinted by permission from Macmillan Publishers ltd:
Naturaleza (doi:10.1038/nrmicro1253), © 2005.
Higo. 4. the white circle indicates the evolution of eukaryotes. this step resulted from the
fusion of the archaea and bacteria branches and a third background branch. Horizontal
connections (depicted above, outside the circle) represent later endosymbiosis events: el
engulfment of cyanobacteria that led to the modern chloroplast and the subsequent endo-
symbiosis of the latter. (© Joana ricou and John Pollock) (recuadro) the “ring of life” as pro-
posed by rivera and lake [46]. in this paper, the authors provide evidence for the fusion of
bacterial and archaea genomes in the evolution of the eukaryotic genome and suggest that
the tree is really a ring at this stage. reprinted by permission from Macmillan Publishers ltd:
Naturaleza (doi:10.1038/nature02848), © 2004.
that gave rise to the eukaryotic cell,
called eukaryogenesis (see Fig. 3), is a
well-established horizontal step [23]. En
the Spiral, this fusion is represented by
the literal fusion of branches from both
grupos, arriving at an image similar to
the “ring of life” suggested by Rivera
and Lake [24] (Higo. 4, recuadro). Nosotros elegimos
to include a third branch in the fusion
step stemming directly from the origin.
This third branch establishes the likely
contributions of genetic material from
unknown sources like plasmids or viruses
[25,26]. In our original art, the branches
are colored according to their domain
and outlined to distinguish their start
and end points. Fusions are indicated
with knots to clarify what branches are
involved. The third stem disappears after
eukaryogenesis, indicating that the dif-
ferent successful combinations of build-
ing blocks that make up the archaea,
bacteria and eukarya domains increased
in stability and eventually out-competed
all others. Those that disappeared are
forgotten to the modern world, excepto
for the ghosts left behind, hidden as fos-
sils in the DNA record [27].
Other accepted “horizontal” examples
are the initial and subsequent endosym-
biosis of cyanobacteria and particular
eukaryotes that evolved into the modern
chloroplast—the foundation of all green
plants. We represented these processes
also as concentric lines (Higo. 4). El
perpendicular branches were nudged
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Ricou and Pollock, The Tree, the Spiral and the Web of Life 21
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Higo. 5. Horizontal processes (solid concentric lines) are known
to be common across unicellular life. emerging research suggests
that they may also be more common than previously thought in
multicellular organisms (dashed concentric lines). adding these
concepts converts the spiral into a web. (© Joana ricou and
John Pollock)
Higo. 6. Spiral of Life diagram—linear time, digital art, 2010. este
diagram’s linear timescale shows how multicellular life (negro
branches) emerged significantly later than unicellular life
(grey branches), alrededor 1 billion years ago. Major evolutionary
events for multicellular organisms have occurred, entonces, de 1
bya to today (range indicated by shaded ring). (© Joana ricou
and John Pollock)
Higo. 7. Spiral of Life II: Plant and Animal Co-Evolution, digital art on canvas, 30 × 40 en, 2009. this piece was installed at Phipps conservatory &
botanical Gardens and identifies major steps in the evolution of plants and their co-evolution with vertebrates and arthropods. in this piece,
and in Spirals I and III, we made use of perspective to distort the spiral shape and focus on the most relevant set of branches. en este caso, nosotros
focus on the plant branches and show key evolutionary milestones such as the appearance of vessels, seeds and flowers. (© Joana ricou and
John Pollock)
22 Ricou and Pollock, The Tree, the Spiral and the Web of Life
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parent 50% that we depict. In this way
we realize that a net-like pattern is the
regla, not the exception. Emerging re-
search in hybridization [31], the role
of viruses [32] and transposons [33,34],
or the particular case of documented
HGT between bacteria and fungi [35],
or hybridization in human evolution
[36] supports this line of investigation,
suggesting that the spiral (or radial)
shape is likely a first step towards the
understanding of the evolution of life
as a web (Higo. 5). With this addition, nosotros
postulate that, in the future, evolution
will be modeled as a web (not a partial
web).
Common rooT and
branCheS
The existence of a Last Universal Com-
mon Ancestor was predicted by Darwin
Higo. 8. Spiral of Life III, detail. evolutionary milestones in the evolution of animals were
explicitly labeled and illustrated. (© Joana ricou and John Pollock)
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Higo. 9. Spiral of Life V: Bird Evolution, digital art on cintra, 6 × 5 ft, 2009, National aviary,
Pittsburgh Pa. this piece contextualizes the evolution of modern birds. the main spiral is
depicted in the lower center and the “dinosaur” branch is exploded to show the demise of
the dinosaurs and the relatively recent evolution of birds and their diversification. (© Joana
ricou and John Pollock)
up and down as needed to clarify events
that happened close in time but in a spe-
cific order.
It is also accepted that within and
between the bacteria and archaea do-
mains HGT continues to be a ubiqui-
tous evolutionary process [28,29], cual
is likely common across most unicellular
vida. Adding an artistic interpretation
of HGT to the unicellular branches of
the spiral created solid concentric lines
interspersed across half the spiral, sug-
gesting that the pattern of evolution is
a combination of vertical and horizontal
procesos (Higo. 5). With this addition, el
Spiral becomes a “partial web” that in-
cludes vertical and horizontal processes,
as the horizontal processes are only in
the unicellular branches.
diverSiTy
As Lynn Margulis put it, “[para] mayoría
people today, life is readily divided into
three categories: plants . . . animals . . .
and germs (to be vanquished)" [30]. Nosotros
wanted the Spiral to give a broader view
of the variety of organisms on our planet,
in particular to address the microscopic
majority of the biosphere. The Spiral
shows the top-level organization of life
as the three domains Archaea, Bacteria
and Eukarya. Sin embargo, to depict intra-
domain diversity accurately, the Spiral
should have many more branches for
Bacteria than the other two domains
combined. Since Eukarya includes ani-
mals, plants and fungi, which are organ-
isms our audience would more easily
recognize, we chose to show each do-
main at different levels of detail. Bacteria
is represented with a subselection of its
kingdoms, while Eukarya is represented
down to the deeper level of phylum.
De este modo, multicellular life is represented
at a higher level of detail (and appears
to have relatively more branches) than
unicellular life. Working with the visual
and semantic contents of the branches
highlighted for us the utility but also the
limitations of the taxa. Taxonomy is a
classification system based on evolution-
ary relationships and, tal como, is equally
in flux. So, in the Spiral series, we decided
to obfuscate lower taxa distinctions in
order to clearly identify meaningful
grupos.
Returning to the web visualization,
we noted that horizontal transmission
of genetic material in multicellular
life is considered the exception, si arriba-
posed to its prevalence in unicellular
vida. Multicellular life is part of Eukarya
and so should appear to constitute close
a 10% of the biosphere and not the ap-
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Ricou and Pollock, The Tree, the Spiral and the Web of Life 23
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Higo. 10. Spiral of Life VI, aproximadamente 8 ft diameter, soft sculpture, 2010, children’s
Museum of Pittsburgh. Joana ricou performs the Story of Evolution, reading the sculpture like
a book with children, exploring the vertical and horizontal processes as physical structures.
the soft sculpture doubled as a storytelling stage and a tactile playground. (© Joana ricou
and John Pollock. Photo © Josh Gates.)
and supported by several investigations,
including a recent 2010 Nature study
[37]. De este modo, the origin of life is repre-
sented as a single point or single line in
many diagrams. In the Spiral, sin embargo, nosotros
chose to represent the origin as a large
and knotted root (Higo. 4) to suggest the
origin of life to be a tumultuous jumble
of building blocks and not a single en-
tity (as the term Last Universal Common
Ancestor would imply)—arriving at an
image similar to Fig. 3. This depiction
implies the possibility that the origin of
the three domains may be more primitive
or more varied than previously thought
[38], reinforcing the possible role of
other genetic elements such as plasmids
and viruses [39,40], and the likelihood
that horizontal processes may have been
the primary mechanism for evolution at
this early stage [41].
The branches of a traditional diagram
are also simple lines, but in the Spiral the
branches were drawn as organic shapes
to suggest that within a population each
lineage waxes and wanes, and evolution
does not imply increasing numbers or
complejidad (Color Plate A).
CollapSing Time
The distribution of taxa represented
in the Spiral is biased toward multicel-
lular organisms, as mentioned above.
Multicellular life emerged around 1
billion years ago (bya), which is signifi-
cantly later than unicellular life (ver
Higo. 6). To allow the image to show
more detail in the evolution of multi-
cellular life, we collapsed the timeline
with a combination of both non-linear
and linear representations along the
radius. A logarithmic scale is used be-
entre 4 billion and 900 million years
atrás, and a linear scale is used between
900 million years ago to today. El
multiplicity of installations of the Spiral
series at different venues allowed us to
counterbalance these compromises by
making different decisions at each host
institución.
The SerieS
We created a series of six pieces based
on the Spiral symbol, each to inhabit a
different cultural institution in Pitts-
burgh (Article Frontispiece and Figs
7–10) [42]. We wanted the Spiral to
become a recurrent theme that capital-
ized on the strengths of distinct cultural
institutions. The identity, mission and
audience of each host influenced the
individual pieces. Thus each gained a
unique style and story. By complement-
ing the content of each collection and
24 Ricou and Pollock, The Tree, the Spiral and the Web of Life
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the educational and experiential goals
of each museum, we created a signifi-
cant point of access and communication
between the science of evolution and
the public perception. Patrons who visit
more than one cultural institution begin
to recognize the common elements of
the Spiral of Life and therefore build a
stronger understanding and acceptance
of the science that is revealed with these
images.
This objective of creating a core sym-
bol that is explored by a series of inter-
pretative pieces led us to create an image
that is significantly richer in information
than any of its instances yet is flexible
enough for visitors to see it from differ-
ent points of view and at different lev-
els of detail. This process also kept the
work scientifically honest, a particular
concern in creating work that intends to
teach and that we hope may contribute
not just to the appreciation of art but also
to the understanding of science. Estos
are goals that we feel resonate with the
challenges articulated by C.P. Snow over
50 years ago.
The development of the Spiral allowed
us to reveal evolution as a pattern of verti-
cal and horizontal processes, but much
room remains for continued exploration
in the re-conceptualization of the 21st-
century understanding of evolution.
Future work will include the rhythms of
evolution: punctuated equilibrium, el
role of viruses, the redefinition of sexual
reproduction in evolution and much
más.
acknowledgments
Scientific advisors: Brady Porter, Associate Professor
of Biological Sciences; John F. Stolz, Director, Center
for Environmental Research and Education, Profes-
sor of Biological Sciences; David Lampe, Associate
Professor of Biological Sciences; Michael Seaman,
Associate Professor of Biological Sciences, de
Duquesne University; and Matthew C. Lamanna,
Assistant Curator, Section of Vertebrate Paleontol-
ogia, Carnegie Museum of Natural History. A Science
Education Partnership Award from the National
Center for Research Resources, a component of
the National Institutes of Health (R25 RR020403)
provided principal funding to Pollock. “The Story
of Life: Reading a Sculpture,” a performance and
public interaction at the Children’s Museum from
the F.I.N.E. Artist Residency Series, was partially
funded by the Fine Foundation. Research assistants:
Brinley Kantorski and Allison Pogue. Additional art
by Robert Hoggard and Laura Gonzalez. Thank you
to Angela Seals, Lois Winslow and Penny Lodge at
the Children’s Museum of Pittsburgh; Erin Estell,
Todd Kazner, Steve Sarro and Caitlin Stone at the
National Aviary; Dave Mintz, Connie George, Tracy
Gray, Jennifer Hicks, Mark Reardon, Margie Marks
and Kevin O’Connell at the Pittsburgh Zoo & PPG
Aquarium; Laura Micco, Sarah Presogna, Mar-
gie Radebaugh and Kelliann Walsh at the Phipps
Conservatory; Dennis Bateman, Azur Cherin,
Mike Hennessy and Brad Peroney at the Carnegie
Science Center.
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references and Notes
Unedited references as provided by the authors.
1. Charles P. Snow, The Two Cultures (Cambridge,
REINO UNIDO.: Prensa de la Universidad de Cambridge, 1960).
2. Norman R. Augustine (Committee Chair), “Ris-
ing Above the Gathering Storm, Revisited: Rapidly
Approaching Category 5,” National Academy of Science
Prensa (2010).
ings of the National Academy of Sciences for the United
States of America. volumen. 103, No. 28, páginas. 10696–10701
(11 Julio 2006).
19. Sarah Schaack, Clément Gilbert, Cédric Fe-
schotte, “Promiscuous DNA: Horizontal transfer of
transposable elements and why it matters for eukary-
otic evolution,” Trends in Ecology and Evolution. volumen.
25, Issue 9, páginas. 537–546 (2010 Septiembre).
39. Koonin et al. [25].
40. Claverie [26].
41. Woese [17].
42. For pictures of each Spiral and related learning
extensions, please see
20. macdonald [8].
43. Pennisi [10].
3. James Elkins, “Art history and images that are not
arte,” Art Bulletin. volumen. 77, No. 4, páginas. 553–571. College
Art Association (1995).
21. mi. Hilario, Peter Gogarten, “Horizontal transfer
of ATPase genes—the tree of life becomes a net of
vida,” Biosystems, volumen. 31 (2–3), páginas. 111–119 (1993).
44. David Hillis, personal communication, 2010.
45. Smets and Barkay [14].
46. Rivera and Lake [24].
4. David S. Goodsell, Graham T. Johnson, “Filling
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cladogram—a diagram used to show ancestral rela-
tions between organisms.
evolution—the principle that organisms change over
time due to natural selection.
horizontal processes, horizontal gene transfer—pro-
cesses where different species of the same generation
pass genetic information to one another. Horizontal
gene transfer (HGT) refers specifically to transfers of
genetic material. Bacterial conjugation is a horizon-
tal process, where a bacterium can pass a copy of an
extra-chromosomal plasmid to another bacterium.
Endosymbiosis is the process where two organisms
fuse and give rise to a new kind of organism.
vertical processes—mechanisms that allow an indi-
vidual organism or population to pass on genetic
information to the next generation. Reproduction
is a vertical process.
Manuscrito recibido 30 Junio 2010.
Joana Ricou is an artist who explores conten-
tious and/or complex biological concepts in
a variety of 2D media, from oil painting to
animation, videogames and the Web
Partnership in Education. Ricou has a Bach-
elor’s in Science and Arts from Carnegie Mel-
lon University and a Master’s in Multimedia
Arts from Duquesne University.
John Pollock, Doctor. (biophysics), is an As-
sociate Professor of Biological Sciences at
Duquesne University with primary research
in the molecular biology of the developing
nervous system
of Partnership in Education, which uses art
and multimedia to reveal fundamental prin-
cipals of science
cipal support comes from a Science Education
Partnership Award and a Darwin Evolution/
Revolution Award, both from the National
Center for Research Resources (NIH).
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Ricou and Pollock, The Tree, the Spiral and the Web of Life 25
