Biology: PALEOBIOLOGY: THE PRECAMBRIAN: LIFE'S GENESIS AND SPREAD
Table of Contents
Origin of the Earth and Life | Is There Life on Mars, Venus, Anywhere
Else?? | The Origins of Multicellularity | The Precambrian
Learning Objectives | Terms | Review Questions |
Image of the precambrian reduced in size from
One way to represent geological time. Note the break during the precambrian.
If the vertical scale was truly to scale the precambrian would account
for 7/8 of the graphic. This image is from
Origin of the Earth and Life
and 20 billion years ago. The theory currently with the most acceptance is the
(smaller than the size of a modern hydrogen atom) that exploded, forming
the Universe. Recent discoveries from the Space Telescope and other
devices suggest this theory smay need some modification. Evidence for
the Big Bang includes:
1) The Red Shift: when stars/galaxies are moving away from us the energy they emit
is shifted to the red side of the visible-light spectrum. Those moving
towards us are shifted to the violet side. This shift is an example
of the Doppler effect. Similar effects are observed when listening to
a train whistle-- it will sound higher (shorter wavelengths) approaching
and lower (longer wavelengths) as it moves away. Likewise red wavelengths
are longer than violet ones. Most galaxies appear to be moving away
from ours.
is a slight background radiation, thought to be the residual afterblast
remnant of the Big Bang.
after the Big Bang the major forces (such as gravity, weak nuclear force,
strong nuclear force, etc.) differentiated. While in the cosmic egg,
scientists think that matter and energy as we understand them did not
exist, but rather they formed soon after the bang. After 10 million
to 1 billion years the universe became clumpy, with matter beginning
to accumulate into solar systems. One of those solar systems, ours,
began to form approximately 5 billion years ago, with a large "protostar"
(that became our sun) in the center. The planets were in orbits some
distance from the star, their increasing gravitational fields sweeping
stray debris into larger and larger planetesimals that eventually formed
planets.
processes of radioactive decay
heat generated by the impact of planetesimals heated the Earth, which
then began to differentiate into a "cooled" outer cooled crust
(of silicon, oxygen and other relatively light elements) and increasingly
hotter inner areas (composed of the heavier and denser elements such
as iron and nickel). Impact (asteroid, comet, planetismals) and the
beginnings of volcanism released water vapor, carbon dioxide, methane,
ammonia and other gases into a developing atmosphere. Sometime "soon"
after this, life on earth began.
Where did life originate and how?
text-decoration: none;">Extra-terrestrial: In 1969, a meteorite (left-over bits from the origin of the solar
system) landed near Allende, Mexico. The Allende Meteorite (and others
of its sort) have been analyzed and found to contain
of panspermia hypothesized that life originated out in space and came
to earth inside a meteorite. Recently, this idea has been revived as
Cosmic Ancestry. The amino acids recovered from meteorites are in a
group known as exotics: they do not occur in the chemical systems of
living things. The ET theory is now not considered by most scientists
to be correct, although the August 1996 discovery of the Martian meteorite
and its possible fossils have revived thought of life elsewhere in the
Solar System.
Supernatural
Since science is an attempt to measure and study the natural world,
this theory is outside science (at least our current understanding of
science). Science classes deal with science, and this idea is in the
category of not-science.
text-decoration: none;">Organic Chemical Evolution: Until the mid-1800's scientists thought organic chemicals (those
with a C-C skeleton) could only form by the actions of living things.
A French scientist heated crystals of a mineral (a mineral is by definition
inorganic), and discovered that they formed urea (an organic chemical)
when they cooled. Russian scientist and academecian
of chemical evolution. These chemicals, he argued, must have arisen
spontaneously under conditions exisitng billions of years ago (and quite
unlike current conditions).
have existed on the Earth billions of years ago. Image from Purves
et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
1950, then-graduate student Stanley Miller designed an
: 1) little or no free oxygen (oxygen not bonded to other elements);
and 2) C H O and N in abundance. Studies of modern volcanic eruptions
support inference of the existence of such an atmosphere. Miller discharged
an electric spark into a mixture thought to resemble the primordial
composition of the atmosphere. From the water receptacle, designed to
model an ancient ocean, Miller recovered
or precursors of all four organic macromolecular classes.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
amounts of energy, stronger storms, etc. The oceans were a "soup"
of organic compounds that formed by inorganic processes (although this
soup would not taste umm ummm good). Miller's (and subsequent) experiments have not proven life originated in this way,
only that conditions thought to have existed over 3 billion years ago
were such that the spontaneous (inorganic) formation of organic
placed into his apparatus, produced a variety of complex molecules:
Molecules recovered from Miller's and similar experiments. Image from Purves et al.,
(http://www.sinauer.com/) and WH Freeman (
used with permission.
concentrations increased. Reactions would have led to the building of
larger, more complex molecules. A pre-cellular life would have began
with the formation of
to the nucleic acids. Eventually the pre-cells would have been enclosed
in a lipid-protein membrane, which would have resulted in the first
cells.
Biochemically, living systems are separated
from other chemical systems by three things.
- The capacity for replication from one generation
to another. Most organisms today use DNA as the hereditary material, although recent evidence
(ribozymes) suggests that RNA may have been the first nucleic acid system to
have formed. Nobel laureate Walter Gilbert refers to this as the RNA world. - The presence of enzymes and other complex molecules
essential to the processes needed by living systems. Miller's experiment
showed how these could possibly form. - A membrane that separates the internal chemic
chemical environment. This also delimits the cell from not-cell areas.
The work of Sidney W. Fox has produced
life.
years ago. The North Pole microfossils from Australia (the Apex Chert)
are complex enough that more primitive cells must have existed earlier.
From rocks of the Ishua Super Group in Greenland come possibly the earliest
cells, as much as 3.8 billion years old. The oldest known rocks on Earth
are 3.96 billion years old and are from Arctic Canada. Thus, life appears
to have begun soon after the cooling of the Earth and formation of the
atmosphere and oceans.
ancient fossils occur in marine rocks, such as limestones and sandstones,
that formed in ancient oceans. The organisms living today that are most
similar to ancient life forms are the
discoveries of bacteria at mid-ocean ridges add yet another possible
origin for life: at these mid-ocean geological structures where heat
and molten rock rise to the Earth's surface.
Is there life on Mars, Venus, anywhere else??
crust (silicate mixtures, presence of water, etc.) and the size of the
earth suggest we may be unique in our own solar system, at least. Mars
is smaller, farther from the sun, has a lower gravitational field (which
would keep the atmosphere from escaping into space) and does show evidence
of running water sometime in its past. If life did start on Mars, however,
there appears to be no life (as we know it) today. Venus, the second
planet, is closer to the sun, and appears similar to earth in many respects.
Carbon dioxide build-up has resulted in a "greenhouse planet"
with strong storms and strongly acidic rain. Of all planets in the solar
system, Venus is most likely to have some form of C-based life. The
outer planets are as yet too poorly understood, although it seems unlikely
that Jupiter or Saturn harbor life as we know it. Like Goldilocks would
say "Venus is too hot, Mars is too cold, the Earth is just right!"
In August 1996, evidence of life on Mars (or at least the chemistry
of life), was announced.
The results of years of study are inconclusive at best. The purported
bacteria are much smaller than any known bacteria on earth, were not
hollow, and most could possibly have been mineral in origin. However,
many scientists consider that the chemistry of life appears to have
been established on Mars. Search for Martian life (or its remains) continues.
The Origins of Multicellularity
Eukaryotic fossils to between 750 million years and possibly as old
as 1.2-1.5 billion years. Multicellular fossils, purportedly of animals,
have been recovered from 750Ma rocks in various parts of the world.
The first eukaryotes were undoubtedly
kingdoms. Multicellularity allows specialization of function, for example
The Precambrian
years ago. The rocks formed during this eon are the most ancient rocks
known, up to 3.96 billion years old. The nature of this rock inducates
that there were/are even older rocks that, if they still exist, have
yet to be located and dated. Perhaps the biggest development during
the Archean was the first appearance of life. The earliest forms of
life were simple prokaryotic cells, in a few cases remarkably similar
to living prokaryotic forms (at least in terms of observable cell structure
and size). Fossil evidence supports the origins of life on earth earlier
than 3.5 billion years ago. Specimens from the North Pole region of
Western Australia are of such diversity and apparent complexity that
even more primitive cells must have existed earlier. Rocks of the Ishua
Super Group in Greenland yield possibly the fossil remains of the earliest
cells, 3.8 billion years old. Life appears to have begun soon after
the cooling of the Earth and formation of its atmosphere and oceans.
ancient fossils occur in marine rocks, such as limestones and sandstones,
that formed in ancient oceans. The organisms living today that are most
similar to ancient life forms are the
marginal environments. Recent discoveries of life at mid-ocean ridges
add yet another possible place of origin: at these mid-ocean ridges
where heat and molten rock rise to the earth's surface.
and Eubacteria are similar in size and shape. When we do recover fossilized
bacteria those are the two features we will usually see: size and shape.
How can we distinguish between the two groups: the use of molecular
fossils that will point to either (but not both) groups. Such a chemical
fossil has been found and its presence in the Ishua rocks of Greenland
(3.8 billion years old) suggests that the archeans were present at that
time.
Microfossils from the Apex Chert, North Pole,
Australia. These organisms are Archean in age, approximately 3.465 billion
years old, and resemble filamentous cyanobacteria. Image from
which do not produce oxygen from their photosynthesis process)) below
them. Below these phototrophs are layers of heterotrophic bacteria.
The layers in the stromatolites are alternating biogenic and sedimentologic
in origin. Stromatolites become more common in the Proterozoic and decline
during the Cambrian. Modern stromatolites are found in marine environments
where the presence of herbivorous :grazers" is limited.
Image of Sharks Bay, Australia extant stromatolites,
a cross section of one of these structures, and a closeup of the cyanobacteria
that make up the bulk of the feature. Image from
Diagram of a stromatolite and its structure
as a series of alternating layers of algae and sediments. Image from
A fossil stromatolite from the North Pole deposits in Western Australia. These deposits are approximately
3.5 billion years old. Image from
the time span from 2.5 billion to 544 million years ago. Simple, prokaryotic
cells still dominated the world's environments until the evolution of
simple eukaryotes approximately 1.5-1.2 billion years ago. With the
appearance of eukaryotes comes the development of sexual reproduction,
which greatly increased the variation that natural selection could operate
on. A major enbvironmental change, initiated by living things, was the
development of oxygenic photosynthesis. This led to increasing oxygen
levels during later Proterozoic. Geologists refer to the "great
iron crisis" when the rising levels of oxygen in the world's oceans
caused the formation or iron oxide (Fe2O3), often
preserved as the banded iron formation (an important commercial source
of iron).
Banded iron formation, illustrating the alternating
layers of magnetite and hematite (the red iron) and chert. Image from
to be in rocks approximately 1.2-1.4 billion years old (Proterozoic) from the Bitter Springs
Formation in Australia. The Bitter Springs deposits also yield a variety
of bacteria and cyanobacterial types. Recent study of the Bitter Springs
eukaryote fossils suggests they may in fact be cyanobactria. A group
of undoubted eukaryote fossils is the "acritarchs". This term
applies to resting cycts of single-celled algae. Acrtitarchs have been
recovered from sediments that are as old as 1.8 billion years.
This time is referred to as the Vendian Period (650 to 544 million years
ago), and is characterized by the appearance of soft-bodied animal fossils.
Multicellular animal fossils and burrows (presumably made by unknown
multicellular animals) first appear 700 million years ago, during the
late precambrian time. All known Proterozoic animal fossils had soft
body parts: no shells or hard (and hence preservable as fossils) parts.
There are some paleontologists who suspect that the Vendian faunas were
reduced by an extinction event, possibly related to massive glaciation,
at the close of the vendian time. In any event, many animals in the
Vendian assemblages are quite unlike anything living today, while others
can be traced to extant phyla.
Dickinsonia
a Vendian animal fossil thought related to the annelid worms.
Image is from http://www.ucmp.berkeley.edu/vendian/dickinsonia.jpg.
Spriggina sp. an enigmatic fossil from the Ediacara Hills in Australia. This
fossil has been classified with the annelid worms as well as recently
an unknown group of arthropods. Image from
algae, have been found in rocks approximately one billion years old.
the multicellular algae are usually classified based on their pigments,
which commonly are not preserved in the fossils.
Reconstruction of the sea floor during the
Vendian times when the Ediacaran organisms thrived. Image from
Orientation of continents in
Learning Objectives
- Describe the major scientific ideas on the origin of life and the
evidence supporting each one. - List the basic physical and biological requirements for life. What
planet(s) would these be available on? - What did Miller's experiment prove? What did it NOT prove? How does
this experiment fit with each of the hyoptheses of the origin of life
discussed here? - What was the world of the Archean like? How about that of the Proterozoic?
What major biological development paved the way for the modern world?
Terms
Review Questions
- Which of these is not a type of cell? a) bacterium; b) amoeba; c)
sperm; d) virus - The Earth's early atmosphere apparently lacked ___. a) oxygen; b)
carbon dioxide; c) water vapor; d) ammonia - The oldest fossil forms of life are most similar to _____. a)
animals; b) modern bacteria; c) archaebacteria; d) fungi
Links
- Fossils, Rocks, and Time This online version of
a U.S. Geological Survey general interest publication, by Lucy E. Edwards
and John Pojeta, Jr., gives additional insight to application of the
study of fossils. - Life Through Geologic Time: A Pictorial History of Life on Earth
This online geologic time scale allows you to click on any period of
time you wish, and see what life was like way back when! - A Geologic Timeline This site gives the majopr events
in Earth hostory presented against twelve standard hours, from midnight
to noon. - Geologic Ages of Earth History This site by Jeff
Poling give a much more detailed geologic time scale than is commonly
encountered in introductory textbooks. Rest assured, as you get deeper
into any field of science, that things will most assuredly get more
detailed! - The Major Fossil Groups This U.S. Geological Survey
page provides additional details about selected groups of fossil-producing
organisms. - The Origin and History of Life (U Texas) A methodical
outline of the major events in the origin of life. - The RNA World (IMB Jena, Germany) Links to WWW RNA
sites and resources. Lots of very cool images. - Life on Mars? American Association for the Advancement
of Science (AAAS) Symposium on a monumental discovery...or was it? "Surrender
earthling!" - Comet's Gift: Hints of How Earth Came to Life Excerpted
from the New York Times, article by Wm. J. Broad, web version by Warren
Huff, U. of Cinn. - COSMIC ANCESTRY Life comes from space because life comes from life
A website summarizing the salient points of panspermia and a new idea
of how and where life originated. - Volcanoes and the Origin of Life by Blake Edgar.
A short article about the connection of geology and biology. - Cyanobacteria Morphology and
to the Archaea: Life's extremists. . . These University
of California Berkeley sites from their Museum of Paleontology (UCMP)
offer some pertinent information about a group of organisms that resemble
some of the earliest known fossils. - Exobiology and the Origin of Life - here - and out there
Learn more about life on the edges...the Archaea on Earth...and possibly
life elsewhere? - From Primordial Soup to the Prebiotic Beach: An interview with exobiology
pioneer, Dr. Stanley L. Miller This online interview
by Sean Henahan of Access Excellence offers a different perspective
on such topics as panspermia, Oparin, and exobiology. - Ediacara Biota: Ancestors of Modern Life or Evolutionary Dead End?
This page from the Miller Museum, presents a series of images and text
discussing the Vendian-aged Ediacara animal fossils and their possible
roles in the evolution of m more modern forms. - INTERNATIONAL ORGANISATION OF PALAEOBOTANY (IOP) This wonderful site houses the
Plant Fossil record database among other superb resources for the specialist
and interested general public alike. - Evolution and the Fossil Record This primer from the American Geological
Institute is available by following links from this page to an ADOBE
PDF file that can be viewed and printed from your computer.
References
Miller S. L. 1953 A production of amino acids
under possible primitive Earth conditions, Science; 117: 528-529.
Oparin, A. I. 1961 Life: its nature, origin and development.
translated from Russian by Ann Synge. Oliver & Boyd, 207p.
Oparin, A. I., 1968 (translated from a book published in Russian in
1966), "Genesis and Evolutionary Development of
Life," Academic Press, New York, 203 p.
Schopf, J. W. 1999 Cradle of Life: The Discovery of earth's
Earliest Fossils. Princeton University Press, 367 p.
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