The New
Frontier
As a child I grew up seeing the standard Geologic Chart,
and as a child, not really understanding it. But I did get a sense of the great
stretch of time the Earth had existed through, as well as a sense of the layers
of geologic periods, stacked on top of one another like books in a pile - each
containing a story about life.
And now, in
the year of Darwin's 200th anniversary, I begin to clarify and again to learn.
As with all
science, everything is in flux; everything is open to change, and the Geologic
Chart that I grew up with is gone. And a new chart is coming into being....
When
Charles Darwin wrote On the Origin of Species, he and most
paleontologists believed that the oldest animal fossils were the trilobites and
brachiopods of the Cambrian Period, now known to be about 540 million years
old.
Many
paleontologists believed that simpler forms of life must have existed before
this but that they left no fossils. A few believed that the Cambrian fossils
represented the moment of God's creation of animals, or the first deposits laid
down by the biblical Flood.
Darwin
wrote, "the difficulty of assigning any good reason for the absence of
vast piles of strata rich in fossils beneath the Cambrian system is very
great," yet he expressed hope that such fossils would be found, noting
that "only a small portion of the world is known with accuracy."
That was
200 years ago.
Since
Darwin's time, the fossil history of life on Earth has been pushed back to 3.5
billion years before the present.
In The Precambrian
Eon
MYA |
Periods |
Major Events |
|
543 |
Neoproterozoic |
Oldest animal fossils |
|
900 |
Mesoproterozoic |
|
|
1600 |
Paleoproterozoic |
Approximate origin of
Eukaryotes
Transition to Oxygen Atmosphere |
|
2500 |
Archaean |
Oldest known fossils
Oldest known rocks |
|
3800 |
Hadean |
|
|
4500 |
|
|
|
|
Most of the
fossils found there are microscopic bacteria and algae. However, in the latest
Proterozoic -- in a time period now called the Vendian (see below) -- macroscopic
fossils of soft-bodied organisms can be found in localities around the world,
and now confirm Darwin's expectations.
So, let's
look at this new frontier of knowledge, the expanding Proterozoic and how its
new Vendian Period fits in.
The
Proterozoic Era
Many of the
most exciting events in the history of the Earth and of life occurred during
the Proterozoic -- stable continents first appeared and began to grow (accrete)
in a long process that took about a billion years.
Also coming
from this time are the first abundant fossils of living organisms,
- Bacteria
- Archaeans
- Eukaryotic cells
Here's a
few words about each of these groups.
Bacteria
Bacteria
are often maligned as the causes of human and animal disease ( like the one
shown below: Often airborne in hospitals, schools, and other public places, Streptococcus bacteria are responsible for infections such as strep throat, scarlet fever,
and some types of pneumonia ).
However,
certain bacteria, produce antibiotics such as streptomycin and nocardicin,
while others live symbiotically in the guts of animals (including humans) or
elsewhere in their bodies, or on the roots of certain plants, converting
nitrogen into a usable form.
For
example, Bacteria put the tang in yogurt, the sour in sourdough bread, and they
help to break down dead organic matter; bacteria make up the base of the food
web in many environments.
Bacteria
are of such immense importance because of their extreme flexibility, capacity
for rapid growth and reproduction, and great age - the oldest fossils known,
nearly 3.5 billion years old, are fossils of bacteria-like organisms.
Fossilized
Bacteria
Archaea
Life's extremists. . .
The Domain Archaea wasn't recognized as a major domain of
life until quite recently.
Until the 20th century, most biologists considered all
living things to be classifiable as either a plant or an animal. But in the
1950s and 1960s, most biologists came to the realization that this system
failed to accommodate the fungi, protists, and bacteria.
By the 1970s, a system of Five Kingdoms had come to be
accepted as the model by which all living things could be classified. At a more
fundamental level, a distinction was made between the prokaryotic bacteria and
the four eukaryotic kingdoms (plants, animals, fungi, & protists).
The distinction recognizes the common traits that eukaryotic
organisms share, such as nuclei, cytoskeletons, and internal membranes.
However, the scientific community was shocked in the late
1970s by the discovery of an entirely new group of organisms -- the Archaea.
 Dr. Carl Woese and his colleagues at the University of
Illinois were studying relationships among the prokaryotes using DNA sequences,
and found that there were two distinctly different groups.
Those "bacteria" that lived at high
temperatures or produced methane clustered together as a group well away from
the usual bacteria and the eukaryotes.
Because of this vast difference in genetic makeup, Woese
proposed that life be divided into three domains:
- Eukaryota,
- Bacteria, and
- Archaea.
The three domains are shown in the illustration to the
right, which illustrates also that each group is very different from the
others.
Further work has revealed additional surprises. Most
archaeans don't look that different from bacteria under the microscope.
However, biochemically and genetically, they are as different
from bacteria as you are.
 Finding Archaea : The hot springs of
Yellowstone National Park, USA, were among the first places Archaea were
discovered. At left is Octopus Spring, and at right is Obsidian Pool. Each pool
has slightly different mineral content, temperature, salinity, etc., so
different pools may contain different communities of archaeans and other
microbes. The biologists pictured above are immersing microscope slides in the
boiling pool onto which some archaeans might be captured for study.
Archaeans inhabit some of the most extreme environments
on the planet. Some live near rift vents in the deep sea at temperatures well
over 100 degrees Centigrade. Others live in hot springs (such as the ones
pictured above), or in extremely alkaline or acid waters.
They have been found thriving inside the digestive tracts
of cows, termites, and marine life where they produce methane. They live in the
anoxic muds of marshes and at the bottom of the ocean, and even thrive in
petroleum deposits deep underground.
Some archaeans can survive the effects of extremely salty
water.
However, archaeans are not restricted to extreme
environments; new research is showing that archaeans are also quite abundant in
the plankton of the open sea. Much is still to be learned about these microbes,
but it is clear that the Archaea is a remarkably diverse and successful group
of organisms.
Eukaryota
The
Eukaryota include the organisms that most people are most familiar with
- Animals,
- Plants,
- Fungi, and
- Protists ( Amoeba,
Paramecium, protozoa, algae, slime and water molds, etc. ).
Eukaryota
also include the vast majority of the organisms that paleontologists work with.
Although
they show unbelievable diversity in form, they share fundamental
characteristics of cellular organization, biochemistry, and molecular biology.
Besides these groups of life, with the beginning of the
Middle Proterozoic comes the first evidence of oxygen build-up in the
atmosphere. This turned out to be good for us much later on, but at the time
quite deathly for some of the earlier forms of life.
For example, this global catastrophe spelled doom for
many bacterial groups, but made possible the explosion of eukaryotic forms.
These include multicellular algae, and toward the end of the Proterozoic, the
first animals.
So, let's look at the newest period on the Geologic Chart,
the Vendian Period. |
