PALEOBIOLOGY: THE PRECAMBRIAN: LIFE'S GENESISAND SPREAD

Table of Contents

Origin of the Earth and Life | Is There Life on Mars, Venus, Anywhere Else?? | The Origins ofMulticellularity | The Precambrian

Learning Objectives | Terms | Review Questions | Links | References

Image of the precambrian reduced in size fromhttp://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_11/ArcheanLandscape.jpg.

One way to represent geological time. Note the break during the precambrian. If the vertical scale was trulyto scale the precambrian would account for 7/8 of the graphic. This image is fromhttp://www.clearlight.com/~mhieb/WVFossils/GeolTimeScale.html.

Origin of the Earth and Life | Back to Top

Scientific estimates place the origin of the Universe at between 10 and 20 billion years ago. Thetheory currently with the most acceptance is the Big Bang Theory, the idea that all matter in theUniverse existed in a cosmic egg (smaller than the size of a modern hydrogen atom) thatexploded, forming the Universe. Recent discoveries from the Space Telescope and other devicessuggest 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 theyemit is shifted to the red side of the visible-light spectrum. Those moving towards usare shifted to the violet side. This shift is an example of the Doppler effect. Similareffects are observed when listening to a train whistle-- it will sound higher (shorterwavelengths) approaching and lower (longer wavelengths) as it moves away.Likewise red wavelengths are longer than violet ones. Most galaxies appear to bemoving away from ours.

2) Background radiation: two Bell Labs scientists discovered that in interstellarspace there is a slight background radiation, thought to be the residual afterblastremnant of the Big Bang.

Soon 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 weunderstand them did not exist, but rather they formed soon after the bang. After 10 million to 1billion years the universe became clumpy, with matter beginning to accumulate into solarsystems. 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 somedistance from the star, their increasing gravitational fields sweeping stray debris into larger andlarger planetesimals that eventually formed planets.

The processes of radioactive decay and heat generated by the impact of planetesimals heated theEarth, which then began to differentiate into a "cooled" outer cooled crust (of silicon, oxygenand other relatively light elements) and increasingly hotter inner areas (composed of the heavierand denser elements such as iron and nickel). Impact (asteroid, comet, planetismals) and thebeginnings of volcanism released water vapor, carbon dioxide, methane, ammonia and othergases into a developing atmosphere. Sometime "soon" after this, life on earth began.

Where did life originate and how?

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 analyzedand found to contain amino acids, the building blocks of proteins, one of the four organicmolecule groups basic to all life. The idea of panspermia hypothesized that life originated out inspace and came to earth inside a meteorite. Recently, this idea has been revived as CosmicAncestry. The amino acids recovered from meteorites are in a group known as exotics: they donot occur in the chemical systems of living things. The ET theory is now not considered bymost scientists to be correct, although the August 1996 discovery of the Martian meteorite andits 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 theoryis outside science (at least our current understanding of science). Science classes deal withscience, and this idea is in the category of not-science.

Organic Chemical Evolution: Until the mid-1800's scientists thought organic chemicals (thosewith a C-C skeleton) could only form by the actions of living things. A French scientist heatedcrystals 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 A.I. Oparin, in1922, hypothesized that cellular life was preceeded by a period of chemical evolution. Thesechemicals, he argued, must have arisen spontaneously under conditions exisitng billions of yearsago (and quite unlike current conditions).

Ingredients used in Miller's experiments, simple molecules thought at the time to have existed

on the Earth billions of years ago. Image from Purves et al., Life: The Science of Biology, 4th Edition,by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

In 1950, then-graduate student Stanley Miller designed an experimental test for Oparin'shypothesis. Oparin's original hypothesis called for : 1) little or no free oxygen (oxygen notbonded to other elements); and 2) C H O and N in abundance. Studies of modern volcaniceruptions support inference of the existence of such an atmosphere. Miller discharged an electricspark into a mixture thought to resemble the primordial composition of the atmosphere. Fromthe water receptacle, designed to model an ancient ocean, Miller recovered amino acids.Subsequent modifications of the atmosphere have produced representatives or precursors of allfour organic macromolecular classes.

A diagrammatic representation of Miller's experimental apparatus. Image from Purves et al., Life:The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman(www.whfreeman.com), used with permission.

The primordial Earth was a very different place than today, with greater amounts of energy,stronger storms, etc. The oceans were a "soup" of organic compounds that formed by inorganicprocesses (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 haveexisted over 3 billion years ago were such that the spontaneous (inorganic) formation of organicmacromolecules could have taken place. The simple inorganic molecules that Miller placed intohis apparatus, produced a variety of complex molecules:

Molecules recovered from Miller's and similar experiments. Image from Purves et al., Life: TheScience of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman(www.whfreeman.com), used with permission.

The interactions of these molecules would have increased as their concentrations increased.Reactions would have led to the building of larger, more complex molecules. A pre-cellular lifewould have began with the formation of nucleic acids. Chemicals made by these nucleic acidswould have remained in proximity to the nucleic acids. Eventually the pre-cells would havebeen 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.

1. The capacity for replication from one generation to another. Most organisms today useDNA as the hereditary material, although recent evidence (ribozymes) suggests that RNAmay have been the first nucleic acid system to have formed. Nobel laureate Walter Gilbertrefers to this as the RNA world.

2. The presence of enzymes and other complex molecules essential to the processes neededby living systems. Miller's experiment showed how these could possibly form.

3. A membrane that separates the internal chemicals from the external chemical environment.This also delimits the cell from not-cell areas. The work of Sidney W. Fox has producedproteinoid spheres, which while not cells, suggest a possible route from chemical tocellular life.

Fossil evidence supports the origins of life on earth earlier than 3.5 billion years ago. The NorthPole microfossils from Australia (the Apex Chert) are complex enough that more primitive cellsmust have existed earlier. From rocks of the Ishua Super Group in Greenland come possibly theearliest cells, as much as 3.8 billion years old. The oldest known rocks on Earth are 3.96 billionyears old and are from Arctic Canada. Thus, life appears to have begun soon after the cooling ofthe Earth and formation of the atmosphere and oceans.

These ancient fossils occur in marine rocks, such as limestones and sandstones, that formed inancient oceans. The organisms living today that are most similar to ancient life forms are thearchaebacteria. This group is today restricted to marginal environments. Recent discoveries ofbacteria at mid-ocean ridges add yet another possible origin for life: at these mid-oceangeological structures where heat and molten rock rise to the Earth's surface.

Is there life on Mars, Venus, anywhere else?? | Back to Top

The proximity of the earth to the sun, the make-up of the earth's crust (silicate mixtures,presence of water, etc.) and the size of the earth suggest we may be unique in our own solarsystem, at least. Mars is smaller, farther from the sun, has a lower gravitational field (whichwould keep the atmosphere from escaping into space) and does show evidence of running watersometime in its past. If life did start on Mars, however, there appears to be no life (as we knowit) today. Venus, the second planet, is closer to the sun, and appears similar to earth in manyrespects. Carbon dioxide build-up has resulted in a "greenhouse planet" with strong storms andstrongly acidic rain. Of all planets in the solar system, Venus is most likely to have some formof C-based life. The outer planets are as yet too poorly understood, although it seems unlikelythat 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!"

Mars: In August 1996, evidence of life on Mars (or at least the chemistry of life), wasannounced. Click here to view that article and related ones. The results of years of study areinconclusive 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, manyscientists consider that the chemistry of life appears to have been established on Mars. Searchfor Martian life (or its remains) continues.

The Origins of Multicellularity | Back to Top

The oldest accepted prokaryote fossils date to 3.5 billion years; Eukaryotic fossils to between750 million years and possibly as old as 1.2-1.5 billion years. Multicellular fossils, purportedlyof animals, have been recovered from 750Ma rocks in various parts of the world. The firsteukaryotes were undoubtedly Protistans, a group that is thought to have given rise to the othereukaryotic kingdoms. Multicellularity allows specialization of function, for example musclefibers are specialized for contraction, neuron cells for transmission of nerve messages.

The Precambrian | Back to Top

The Archean Eon encompasses the time from the formation of the earth until 2.5 billion yearsago. The rocks formed during this eon are the most ancient rocks known, up to 3.96 billionyears old. The nature of this rock inducates that there were/are even older rocks that, if they stillexist, have yet to be located and dated. Perhaps the biggest development during the Archean wasthe first appearance of life. The earliest forms of life were simple prokaryotic cells, in a fewcases remarkably similar to living prokaryotic forms (at least in terms of observable cellstructure and size). Fossil evidence supports the origins of life on earth earlier than 3.5 billionyears ago. Specimens from the North Pole region of Western Australia are of such diversity andapparent complexity that even more primitive cells must have existed earlier. Rocks of the IshuaSuper Group in Greenland yield possibly the fossil remains of the earliest cells, 3.8 billion yearsold. Life appears to have begun soon after the cooling of the Earth and formation of itsatmosphere and oceans.

These ancient fossils occur in marine rocks, such as limestones and sandstones, that formed inancient oceans. The organisms living today that are most similar to ancient life forms are thearchaebacteria (the archaea in modern usage). This group is today restricted to marginalenvironments. Recent discoveries of life at mid-ocean ridges add yet another possible place oforigin: at these mid-ocean ridges where heat and molten rock rise to the earth's surface.

Archaea and Eubacteria are similar in size and shape. When we do recover fossilized bacteriathose are the two features we will usually see: size and shape. How can we distinguish betweenthe two groups: the use of molecular fossils that will point to either (but not both) groups. Sucha chemical fossil has been found and its presence in the Ishua rocks of Greenland (3.8 billionyears 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 fromhttp://www.astrobiology.ucla.edu/ESS116/L15/1515%20Apex%20Chert.jpg.

Many of the ancient phototrophs and heterotrophic bacteria lived in colonial associations knownas stromatolites. Cyanobacteria occur on the outer surface, with other photosynthetic bacteria(anoxic, which do not produce oxygen from their photosynthesis process)) below them. Belowthese phototrophs are layers of heterotrophic bacteria. The layers in the stromatolites arealternating biogenic and sedimentologic in origin. Stromatolites become more common in theProterozoic and decline during the Cambrian. Modern stromatolites are found in marineenvironments 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 fromhttp://www.dme.wa.gov.au/ancientfossils/sharkbay2.jpg.

Diagram of a stromatolite and its structure as a series of alternating layers of algae andsediments. Image fromhttp://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_10/stromatolites.htm.

A fossil stromatolite from the North Pole deposits in Western Australia. These deposits areapproximately 3.5 billion years old. Image fromhttp://www.carleton.ca/~tpatters/teaching/intro/precambrian/precambrian7.html.

The Proterozoic Eon covers 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 simpleeukaryotes approximately 1.5-1.2 billion years ago. With the appearance of eukaryotes comesthe development of sexual reproduction, which greatly increased the variation that naturalselection could operate on. A major enbvironmental change, initiated by living things, was thedevelopment of oxygenic photosynthesis. This led to increasing oxygen levels during laterProterozoic. Geologists refer to the "great iron crisis" when the rising levels of oxygen in theworld's oceans caused the formation or iron oxide (Fe2O3), often preserved as the banded ironformation (an important commercial source of iron).

Banded iron formation, illustrating the alternating layers of magnetite and hematite (the red iron)and chert. Image from http://www.agso.gov.au/education/factsheet/ironform.html.

The first protist (eukaryotic) fossils have commonly been thought to be in rocks approximately1.2-1.4 billion years old (Proterozoic) from the Bitter Springs Formation in Australia. The BitterSprings deposits also yield a variety of bacteria and cyanobacterial types. Recent study of theBitter Springs eukaryote fossils suggests they may in fact be cyanobactria. A group ofundoubted 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.

Multicellular protists appeared in the fossil record more than 600 million years ago near thevery end of the precambrian. This time is referred to as the Vendian Period (650 to 544 millionyears ago), and is characterized by the appearance of soft-bodied animal fossils. Multicellularanimal fossils and burrows (presumably made by unknown multicellular animals) first appear700 million years ago, during the late precambrian time. All known Proterozoic animal fossilshad soft body parts: no shells or hard (and hence preservable as fossils) parts. There are somepaleontologists 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, manyanimals in the Vendian assemblages are quite unlike anything living today, while others can betraced to extant phyla.

Dickinsonia sp. a Vendian animal fossil thought related to the annelid worms. Image is fromhttp://www.ucmp.berkeley.edu/vendian/dickinsonia.jpg.

Spriggina sp. an enigmatic fossil from the Ediacara Hills in Australia. This fossil has beenclassified with the annelid worms as well as recently an unknown group of arthropods. Imagefrom http://www.ucmp.berkeley.edu/vendian/spriggina.gif.

Some problemmatic fossils, thought by some paleobotanists to be multicellular algae, have beenfound in rocks approximately one billion years old. However, the multicellular algae are usuallyclassified 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 http://www.carleton.ca/~tpatters/teaching/intro/cambrian/cambrianex8.html.

Orientation of continents in Rodinia supercontinent at the close of the precambrian. Image fromhttp://astro.sau.edu/~earth/html/md-tectonics.html.

Learning Objectives | Back to TopDescribe 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 availableon?What did Miller's experiment prove? What did it NOT prove? How does this experiment fit with eachof 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 biologicaldevelopment paved the way for the modern world?

Terms | Back to Top

amino acids Archean Eon archaebacteria autotroph Big Bang Theoryfossil fossil record nucleic acids photosynthesis prokaryotes proteinoidProterozoic Eon Protistans ribozymes

Review Questions | Back to Top1. Which of these is not a type of cell? a) bacterium; b) amoeba; c) sperm; d) virus2. The Earth's early atmosphere apparently lacked ___. a) oxygen; b) carbon dioxide; c) water vapor; d)

ammonia3. The oldest fossil forms of life are most similar to _____. a) animals; b) modern bacteria; c)

archaebacteria; d) fungi

Links | Back to TopFossils, 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 twelvestandard hours, from midnight to noon.Geologic Ages of Earth History This site by Jeff Poling give a much more detailed geologic time scalethan is commonly encountered in introductory textbooks. Rest assured, as you get deeper into anyfield of science, that things will most assuredly get more detailed!The Major Fossil Groups This U.S. Geological Survey page provides additional details about selectedgroups of fossil-producing organisms.The Origin and History of Life (U Texas) A methodical outline of the major events in the origin oflife.The RNA World (IMB Jena, Germany) Links to WWW RNA sites and resources. Lots of very coolimages.Life on Mars? American Association for the Advancement of Science (AAAS) Symposium on amonumental 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 summarizingthe 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 andbiology.Cyanobacteria Morphology and Introduction to the Archaea: Life's extremists. . . These University ofCalifornia Berkeley sites from their Museum of Paleontology (UCMP) offer some pertinentinformation 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...theArchaea 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 onsuch topics as panspermia, Oparin, and exobiology.Ediacara Biota: Ancestors of Modern Life or Evolutionary Dead End? This page from the MillerMuseum, presents a series of images and text discussing the Vendian-aged Ediacara animal fossils andtheir possible roles in the evolution of m more modern forms.INTERNATIONAL ORGANISATION OF PALAEOBOTANY (IOP) This wonderful site houses thePlant Fossil record database among other superb resources for the specialist and interested generalpublic alike.Evolution and the Fossil Record This primer from the American Geological Institute is available byfollowing links from this page to an ADOBE PDF file that can be viewed and printed from yourcomputer.

References | Back to Top

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 AnnSynge. Oliver & Boyd, 207p.

Oparin, A. I., 1968 (translated from a book published in Russian in 1966), "Genesis andEvolutionary Development of Life," Academic Press, New York, 203 p.

Schopf, J. W. 1999 Cradle of Life: The Discovery of earth's Earliest Fossils. PrincetonUniversity Press, 367 p.

All text contents © 2001, by M.J. Farabee, all rights reserved. Use for educational purposes is encouraged.

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Email: mj.farabee@emcmail.maricopa.edu

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