Our knowledge of the tree of life—a phylogenetic tree summarizing the evolutionary relationships among
all life on Earth—is expanding rapidly. “Mega-trees” with millions of tips (species) are expected to appear
imminently (for example, see http://www.opentree.wikispaces.com). Unfortunately, there has so far been no
intuitive way to explore even the much smaller trees with thousands of
tips that are now being routinely produced. Without a way to view megatrees, these wondrous objects,
representing the culmination of decades of scientific effort, cannot be fully appreciated. The field
really needs a solution to this problem to enable scientists to communicate important evolutionary
concepts and data effectively, both to each other and to the general public.
Just like Google Earth changed the way people look at geography, a sophisticated tree of life
browser could really change the way we look at the life around us . . . Our advances in understanding
evolution are moving really fast now, but the tools for looking at these big trees are lagging behind.
(Westneat in , February 2009)
Displaying large trees is a hard problem that has so far resisted solution. We are still waiting for
the equivalent of a Google Maps. (Page in , June 2012)
In this manuscript, we introduce a new approach that solves the problem. Trees with millions of tips,
richly embellished with additional data, can now be easily explored within the web browser of any modern
hardware with a zooming user interface similar to that used in Google Maps.
Escaping the Paper Paradigm
Much of the difficulty with phylogenetic tree visualization (and with data visualization more generally)
is that we all too often constrain ourselves to the “paper paradigm”—the practice of displaying data in
ways that are optimized for printing on paper. Many applications fail to take full advantage of the freedom
that a computerdisplay gives us over printed sheets; we read and write documents and
browse web pages that are constrained to be optimal for printing, but fail to realize that such documents
are unlikely to be optimal for visualization on a digital device. Of course, the paper paradigm of
visualization seems most natural to us, but is this only because we are too familiar with the paper format?
Society is undergoing a rapid transition in terms of its use of computing devices. Computers have enabled
us to generate and store large amounts of data that would not have been possible using paper. We now need
to take the next step with a transition to data visualization that is optimized for interactive displays
rather than printed paper.
Recent methods of phylogenetic tree visualization attempt to buy extra space in the paper paradigm—for
example, by using walls consisting of multiple displays (see Figure 7a in ). This approach is costly and does not give tree visualization capabilities to
the masses, which is what is really needed. Furthermore, expensive display technology does not really
solve the problem—according to our estimates, even the most advanced technology, such as NASA's
“Hyperwall2” of 128 LCD displays , would not be large enough to clearly display 5,000 tip trees using conventional
techniques. Other currently available methods make exploration of phylogenetic trees interactive,
enabling the user to expand or magnify parts of the tree  that may be too small to see in detail at the scale of the screen.
Hyperbolic tree browsers ,  are a good example of this and they can display large trees, but users do
not find them intuitive  and we don't see the inclusion of rich metadata as being realistically
achievable. An optimal tree viewer should be able to 1) handle large megatrees; 2) be explored in an
intuitive way; 3) incorporate significant amounts of metadata; and 4) be visually appealing and
immersive, especially if public users are expected. We have yet to find an existing viewer that we feel
convincingly meets these requirements.
Zooming in on a Potential Solution
We introduce a new phylogenetic tree viewer that allows interactive display of large trees. The key
concept of our solution is that all the data is on one page so that all the user has to do is
zoom to reveal it—hence the name OneZoom (http://www.onezoom.org). Our interface is analogous to Google Earth, where one can
smoothly zoom into any local landmark from a start page showing the whole globe, recognizing familiar
landmarks at different scales along the way (e.g., continents, countries, regions, and towns).
Equivalently, OneZoom can zoom smoothly to one tip of the tree of life—say, human beings—passing the
familiar clades of animals, vertebrates, mammals, and primates at different scales along the way (see
1, which used data from
). Trees with millions of tips may require a page of paper larger than the observable universe to be
printed: they break the paper paradigm, but on screen, users can still easily zoom in through the tree to
any point of interest (see Figure
2, which used data from ).
Figure 2.An example IFIG of a 408,135-tip phylogeny of small-subunit RNAs
using data from SILVA [
In order to place the phylogenetic data on a scalable two-dimensional map, OneZoom uses concepts
inspired by fractal geometry. Fractals are objects that look similar at different scales and have a
dimension that is not a whole number; they often appear in the natural world , . For example, an effective lung requires a large surface area to be packed into
a small a volume—its surface is therefore so convoluted and labyrinthine that at some scales it can be
regarded mathematically as having a dimension greater than two (a surface) but less than three (a solid
object). Fractal geometry can be used to produce stunning and apparently complex images—including many
that resemble trees and other plants—based on remarkably simple sets of underlying growth rules that are
applied repeatedly (“L-systems” ). Our algorithms (Text S1) build on this to produce a zoomable object, inspired by fractals, but
embellished with useful data (such as images, graphs, and text) at every scale—an interactive
fractal-inspired graph (IFIG). We give the branches of the tree a width so that they appear as filled
shapes rather than narrow lines and thus can contain rich metadata within them that is easily accessible
through zooming. Ultimately, these methods should make it possible to usably view the complete tree of
life together with all its metadata on any modern computing device (Box 1 and Software S1, S2).
Box 1. Features of OneZoom
By varying the angles between the branches and the successive ratios of branch lengths and widths,
many different fractal structures are possible for an IFIG of a phylogenetic tree (Figure
3). By allowing these to be defined uniquely at each split according to the tree's properties, the
balance can be seen at a glance for every part of the tree.
Time scales and dated trees
Many phylogenetic trees also include branch lengths, which provide information about the time scale
of evolution. Our software plots the phylogeny in a shape that can reflect the topology and balance,
whilst the timescales are expressed in ways that are independent of shape including color, text, and
animation. In future work it should be possible to express timings with additional new fractal
Phylogenetic trees frequently include polytomies, which are nodes with more than two descendent
branches. These generally represent phylogenetic uncertainty in the relationships among clades. Our
software can highlight the nodes that make up polytomies in one of a number of ways, or, alternatively,
it can not draw branches of length 0 so that polytomies appear as physical breaks. In the future, it
should be possible to display polytomies as a number of branches joining at a single point.
In a tree of the proportions OneZoom can visualize, it is necessary to have an efficient search
function so that users can find leaves or clades easily. OneZoom's search function ranks clades
according to the total number of search hits and the proportion of tips in a clade that represent a
hit. Search results can be highlighted on the tree or shown instantly. OneZoom also allows users to fly
through the tree to search hits with an animation feature.
There is no theoretical limit to the amount of metadata that can be put on a leaf or branch of the
tree because these data can be displayed at any size and zoomed in on. Allowing branches to have a
thickness assists the concept of scale when zooming and allows data such as graphs, maps, paragraphs of
text, and images to be embedded inside branches and leaves. There are many further possibilities for
using the colors and shapes of branches and leaves to reflect different metadata (Figure
Figure 4.A mammal tree using data from  incorporating IUCN Red List metadata and common names
Zooming to reveal further details as implemented in OneZoom feels intuitive because it is akin to the
way we explore the real world by moving closer to objects of interest to see them in greater detail. Data
is always displayed in both intuitive and raw formats. For example, in the “natural” view of the tree of
life (see Figure
3), the balance at each node (indicative of the ratio of species richness in each descendent branch)
can be taken in at a glance from the thickness and angles of the branches, but the raw numbers can also be
found by zooming in on the nodes. In OneZoom, we can be wasteful of space because there is an infinite
quantity of space available. For example, structures that do not form a strict hierarchy, such as food
webs, could be expressed by repetition of the same elements in multiple places. OneZoom is effective
without fast processors, large high-resolution screens, stereoscopic vision, or multi touch inputs;
however, these technologies, where available, could be straightforwardly exploited to provide an enhanced
Visions for the Future
A significant issue with all tree visualization is that the ordering of descendent branches from any
node and the positions in space of any node can be changed whilst still describing exactly the same tree
. This means that there are many ways to view a tree, each of which might
emphasize different properties. We included three forms in the first release of OneZoom (see Figure
3), but all these have the common property that evolutionarily distinct species and clades appear
larger. The natural view emphasizes tree balance over rates of diversification. We envisage the
development of further forms for OneZoom in the future. For example, nodes could be aligned based on
their dates using a non-linear timescale, with branches becoming progressively thinner towards the
present day. This would complement the other forms by placing emphasis on diversification rates,
timescales, and the species richness of clades; the tree would also then appear more like a familiar
The concept of deep zooming for data presentation is not novel (see for example http://www.prezi.com), but we believe the idea has been under-utilized. We suggest
that this may be because suitable methods for automatically laying out information on an infinite space
are lacking; we hope that the OneZoom concept of using fractals will help resolve the problem. The
fractal forms used within OneZoom need not look like trees, and further alternatives may be better and
allow OneZoom to have applications outside of biology. For example, we speculate that an IFIG of the
global financial markets could give an intuitive overview of the relative performance of each sector and
subsector whilst allowing the more minute details to be revealed by zooming. Scientists exploring a
scatter plot or color map could zoom in on any point or pixel to reveal further graphs, text, and
information associated with just that data point. The files on a computer, the Internet, news, mind
maps, genealogies, online stores, complex software structures, and industrial plants are all further
examples of large and complex data sets that we imagine could be explored with OneZoom, even on a
We most look forward to seeing OneZoom bring to life the remarkable and powerful phylogenetic datasets
that many evolutionary biologists have strived to collect over recent years. We hope, together with a range
of collaborators, to use OneZoom in the near future as a way to tackle the challenge of public education
about evolution. For example, we envisage putting “microdots” on the branches of the tree, that when zoomed
into, show fossil images and other evidence backing up the hypothesized evolutionary path of that branch. A
richly annotated IFIG may help make the evidence, logic, and beauty of evolution easy to explore and
understand in a way that is compelling and fun. The phylogenetic tree is also the most logical structure
within which to explore the breadth of biodiversity on Earth, and so OneZoom could potentially be used to
browse existing ecological databases of species such as the Encyclopedia of Life (http://eol.org/). These databases do face difficulties besides visualization, for
example lack of available data and inconsistent naming of species. Nevertheless, we are confident that
these problems will eventually be resolved simply by making gradual improvements to the many databases
over a long period of time.
Our dream for the more distant future is an easily accessible web page presenting all that we know about
life on Earth in one place. The logical way to do this is to build around the tree of life visualized using
OneZoom; we may yet see the Google Maps equivalent for all life on earth.
Citation: Rosindell J, Harmon LJ (2012) OneZoom: A Fractal Explorer for the Tree of
Life. PLoS Biol 10(10): e1001406. doi:10.1371/journal.pbio.1001406
Funding: James Rosindell was funded by the Natural Environment Research Council (grant
NE/I021179). The funders had no role in study design, data collection and analysis, decision to publish, or
preparation of the manuscript.
Competing interests: The authors have declared that no competing interests
October 16th 2012: The OneZoom manuscript is published in PLoS Biology, the first in
the journal's new "Cool Tools" series. The OneZoom website, videos and software all go live.
Joel Cracraft on OneZoom: "This will revolutionize how we teach and understand the Tree of Life. It is an
invaluable tool for communicating the grand scope of life's history."
David Hillis on OneZoom: "The problem of visualizing an evolutionary tree for millions of species is a
challenging one. OneZoom provides a creative solution that will be of great utility, especially for
web-based representations of the Tree of Life."