What is Life on Earth Like Today?
But what is life on Earth really like today - how well do we know the
life on our own planet? The answer to
this question is probably, “Very poorly,” especially if we consider the full
complement of life on Earth. To do so, we
must take into account not only the “higher” organisms, but also the single celled,
anucleate organisms collectively referred to as prokaryotes. These organisms need to be factored into any
discussion of the nature of life on Earth, both past and present. To illustrate the way in which prokaryotes
are viewed now in comparison to just a few years ago, I offer the following
quotation from Dr. E. 0. Wilson, the noted expert on animal evolution, from his
book, The Naturalist.
If I could do it all over again and relive my vision of the 21st
century, I would be a microbial ecologist.
Ten billion bacteria live in a gram of ordinary soil, a mere pinch held
between the thumb and the forefinger.
They represent thousands of species, almost none of which are known to
science. Into that world I would go
with the aid of modern microscopy and molecular analysis.
Dr. Wilson has made immense contributions to our understanding of
macroscopic life on Earth, and in this quote expresses the opinion that it is
now time to move such thinking to the microbial level. To expand on these thoughts, I would point
out that while microbial biomass is thought to account for 50% or more of the
Earth’s biomass, we have been able to culture and characterize only a small
percentage of these prokaryotes, and thus, know almost nothing about nearly
half of our Earthly biota. This is a
sad state of affairs for a society that claims to be ready to embark on a
search for life in the Universe!
In addition to our new insights about the early appearance of Earthly
life, a number of other biological findings have changed our perceptions of
life on Earth. These new developments,
which must be factored into any search for extraterrestrial life, involve the
nature and diversity of life on Earth (e.g., the very definition of Earthly
life), as well as new insights into the toughness and tenacity of life as we
know it.
With regard to our view of the nature of life on Earth, major changes
have occurred in the past two decades.
We have moved from a peculiarly eukaryocentric view of life to one that
openly admits that the small, single-celled creatures that were once ignored
play a vitally important role in the metabolism of our planet. The view of life that most of us learned
from our biology teachers is that commonly referred to as the five kingdoms
(see below). It was derived through the
work of Linnaeus and others in the mid-1700’s.
Figure
3. The five kingdoms of life. This view represents Earthly life as composed of 5 kingdoms, 4 of
which are eukaryotic, and the 5th, the monera (what we now call prokaryotes) at
the base of the tree. This view of life
is based on structural and functional analyses of organisms, and a proposed
evolutionary line in which life moved from simple to more complex, and smaller
to larger.
This classification scheme relied upon observation of the visible
features of organisms to give each a name (e.g. Homo sapiens for
humans), and to group organisms of similar appearance together. The diagram in Figure 3 is called a
phylogenetic tree; these trees are used to illustrate the evolutionary
progression that may have occurred to result in the extant organisms (e.g., to
answer the question of which organisms preceded which in time).
Largely because of the nature of the tools available (human eye, hand
lens, and later, simple microscopes), it is not surprising that the such trees
were dominated by the macroscopic, many-celled eukaryotes such as the fungi,
plants, and animals. The tiny
eukaryotic protists (amoebae, paramecia, giardia, etc.) being visible but not
understood, were relegated to the next-to-the-bottom rung of the ladder, while
the prokaryotes (bacteria) were handily put at the bottom where they could be
acknowledged but not seriously so. This
entire approach was reasonable at the time, in the sense that structural
diversity was driving classification, and the single celled, anucleate
prokaryotes, as they are called, have little that is comparable with the
structurally and behaviorally diverse larger organisms, collectively referred
to as the eukaryotes.
Contributed by: Dr. Kenneth Nealson
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