Tree Of
Life
The
diversity of life on Earth is incredibly widespread. From microscopic organisms thriving in
conditions similar to boiling battery acid to creatures as delicate as
butterflies, there seems to be no limits to nature’s imagination. But with this diversity comes the challenging
task of categorizing a number of organisms on the planet that is virtually too
large to count.
Prior to
the 1970s, people categorized Earth’s inhabitants according to a natural
taxonomy that relied only on fossil records and morphology – the form and
structure of an organism. The first
system of classification split all the Earth’s creatures into one of two
categories, plants or animals. Following
Charles Darwin’s publication of The
Origin of Species in 1859, which articulated the now well-accepted model of
natural selection, scholars sought to establish a more detailed and accurate
depiction of how organisms are related on Earth. In 1866, Ernst Haeckel formally challenged
the pre-existing “tree of life”, which at that time consisted of only two
branches: plant and animal. It was
during this time that protists – single-celled microorganisms – were first
identified by scientists. These
organisms did not fit well into either of the two categories of life. Based on this observation, Haeckel proclaimed
that the tree of life should contain three branches, not two.
Although
Haeckel, and eventually other scientists, continued to add branches to and
therefore diversify the tree of life, the primary distinctions made between the
branches was based on physical traits and characteristics of organisms, not on
actual genetic relationships. By the
1970s the tree of life had grown to include 5 main branches, called kingdoms: Animalia, Plantae, Fungi, Protista, and
Monera. Although this five kingdom
designation gained rapid popularity and is still widely taught today,
microbiologists have been articulating the need for a different classification
based on molecular and cytological understanding of cells. For over 100 years, microbiologists have
argued that the primary division of life should lie between bacteria and
eukaryotes followed by the division between plants and animals. As a result, a two kingdom classification
system is also commonly accepted: Prokaryotes
and Eukaryotes.
Two recent
discoveries have significantly changed how we define the modern tree of
life. In the 1960s, Linus Pauling and
Emile Zuckerkandl introduced a new method of examining organisms, molecular
phylogeny, that examined the molecular building blocks of life (such as genes
and proteins) to determine actual genetic relationships amongst organisms. As a result of this type of research, in 1977
Carl Woese and George Fox announced their discovery of archaebacteria,
single-celled organisms that resemble bacterial microorganisms no more than
they resemble eukaryotes. This
revelation upset the two kingdom classification system of Prokaryotes versus
Eukaryotes. On the cellular level,
archaeabacteria (now commonly known as archaea) resemble other prokaryotes as
they lack membrane-bound organelles and possess circular rather than linear
DNA. However, on a genetic level,
archaea are no more similar to other prokaryotes as they are to
eukaryotes. In 1990, Carl Woese and
colleagues formally proposed a restructured tree of life. In this modern tree, there are three domains
that supercede all pre-existing kingdoms:
Archaea, Bacteria, and Eukarya.
Just as the
tree of life has changed over the past several hundred years, we can surely expect
it to change as our understanding of true genetic relationships between
organisms is expanded. Scientists are
currently sequencing entire genomes of organisms and determining relationships
never before anticipated. What began as
an endeavor to illustrate the relationships of living creatures on Earth as
depicted by
What is
complicating our search for a last common ancestor to life on Earth? According to our basic knowledge of
inheritance, a particular individual possesses genetic material which it then
passes on to its offspring. Sometimes
during this process, an individual will develop mutations in its genetic
material which can, in turn, be passed on.
Regardless, the main idea is that genetic material is passed from parent
to offspring in a top-down, linear manner.
If this were the only mechanism for gene transfer, then it would be
logical to assume that the tree of life would have a clean set of traceable
limbs and branches, allowing us to easily determine a common ancestor for life
on Earth. However, this is not the only
mechanism. A different process, lateral
or horizontal gene transfer, has profoundly influenced the development and
evolution of life. This process entails
the exchange of genetic material between
species, not from parent to offspring.
If one were to draw a picture of how this would look in an actual tree,
one could imagine two branches from distant limbs on a tree to suddenly have a
branch growing between them, linking them together. Horizontal gene transfer has significantly
increased the complexity of creating a universal tree of life. But in our efforts to establish this tree of
life, we will learn more and more about the origins and evolution of life on
our planet and, hopefully, extend that knowledge to other planets or moons.
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