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Nature and Origin of Life on Planetary Bodies

Living matter (organics) has cropped up as a subject in several Sections of this Tutorial. Some readers may desire to build up a basic background in Biology, as we implemented for Geology and Meteorology, and in this Astronomy/Cosmology Section. Such an effort (in depth) is presently beyond the background of the writer (NMS). In its stead, the best Internet tutorial on this subject found by the writer seems to be An Online Biology Book by M.J. Farabee. Another extensive source is Biology-Online, also worth checking out. The writer has found this general topic so fascinating that, at the age of 77, I am slowly working my way through both online books, and then a superb text on Biology by P. Raven and P. Johnson. Hopefully, some of my readers will try to emulate this by choosing to go through one of the above Internet sources.

And here are two more Internet sites that specifically focus on paleolife topics: 1) a chapter called Paleobiology (this site is actually a part of the abovementioned tutorial on Biology, prepared by M.J. Farabee of Estrella Mountain Community College of Avondale, AZ); and 2) Looking for Extraterrestrial Life produced by the Regional Education Service Agency of Wayne County, Michigan. The writer also recommends this paper book: Life Evolving: Molecule, Mind, and Meaning, by Nobel Laureate Christian de Duve, Oxford Press. And for a thorough immersion into Earth's early life history, we recommend Andrew Knoll's Life on a Young Planet: The First Three Billion Years of Evolution on Earth, Princeton Univ. Press, Nov. 2003,

There is also a helpful Web site that treats developments in the field of Astrobiology (also called Exobiology) hosted by the Astrobilogy Branch at Ames Research Center.

The capstone of this Section on Cosmology must surely be a consideration of the most provocative and fascinating Quest in the history of human life: the attempts to determine whether life of any kind - but specifically intelligent life (Pyschozoic is a term created to generalize such a stage of life on exoplanets) - exists elsewhere in the Universe. Philosophically, many on Earth hope that we are unique - thinking beings that are the pinnacle and teleological goal of a Creator's act. Scientifically, most cosmologists, biologists, etc. are coming around to the firm conviction that life does indeed exist elsewhere - throughout the Universe. This is a logical conclusion, since a huge Universe with just one tiny inhabited body on which conscious creatures exist strikes most scientists, and a growing number of philosophers, as extremely unlikely, and, from a practical sense, even a foolish, wasteful action by any Creator (this viewpoint is touched upon again later on this page).

We remind you at the outset of the excellent 2003 book on the subject of life in the Universe, David Grinspoon's Lonely Planets: The Natural Philosophy of Alien Life, cited on page 19-2. A few comments about the history of this idea, extracted from his book, are briefly treated before we begin with the review below:

Grinspoon points out that mankind has been speculating on life beyond the Earth for more than two millenia. The Epicureans of the late period of Greek philosophy before Rome took over that part of the Mediterranean believed that living creatures with intellects lived on one or more of the planets and possibly the stars. Aristotle and Plato, however, argued against multiple worlds. Some early Christian theologians developed ideas that allowed for thinking life within the observed Cosmos (not quite a Universe as we know it today, but a realm that perhaps extended beyond the "spheres" that contained the Moon and Planets; see page 19-2). Similar speculations affected the medieval thinkers. But until Renaissance times, the vast majority held life to be unique to Earth. Copernicus, Kepler, and Galileo gave thought to the possibility of life elsewhere but never did seriously conjecture on the possiblities. Although not well-known to the public and even many scientists, the Italian Giordano Bruno (a Dominican friar fried at the stake by the Church for his radical beliefs) by 1600 had conceptualized a Cosmos filled with multiple Suns and their planets on which life was widespread.

At the dawn of the Age of Enlightenment, a treatise advocating a "plurality of worlds" was published in 1686 by Bernard le Bovier de Fontenelle. This work had a strong influence on thinkers of the day. In the first half of the 18th Century several similar and provocative books by European natural theologians followed. The German philosopher Immanuel Kant was much influenced by these writings and came up with a precursor to modern ideas for the formation of the Solar System. He espoused a much wider distribution of life within our Solar System and probably elsewhere. Laplace, who modified Kant's models, also took the view that life was established beyond Earth. The 19th Century saw similar and varied views favoring the "universality" of life. In the early 20th Century, Percival Lowell popularized this notion with his claims that "canals" existed on Mars. As the dawn of the Space Age arrived in the 1950s, many scientists and much of the general public retained the view that life was likely to be found in other parts of our solar System. The first spacecraft to fly by, orbit, and land on Mars tended to dampen this enthusiasm. But flying saucers and movies about ET and Close Encounters have pumped up the hopes of the Common Man that in time life will be found beyond the Earth.

From an anthropocentric outlook, the importance in understanding planetary formation mechanisms and history is the assumption (not yet a clearcut fact) that planets possessing certain appropriate conditions are the harbors of life. Life, it is believed by Earth dwellers who can think, may well be the most complex and advanced feature in the Universe, based on the presumption that it has evolved into a state resulting in lifeforms that perceive beyond sensing, analyze through reason, and evaluate most other aspects of known existence. Life, under this viewpoint, is the quintessential achievement in the evolution of the Universe to date. Whether life on Earth stands at the pinnacle, or somewhere below, has yet to be established - statistically, it is most likely that somewhere in the Universe even more highly developed living creatures, with superior intellects, exist today or have in the past. (The ideas just enunciated are closely associated with the modern doctrine called humanism).

The expectation that some life exists elsewhere in the Universe will depend on the nature of life itself. Life can be defined by properties that are both chemical and functional. Judging on what we know conclusively from the one sample available to earthlings - namely, life on Earth itself as the only confirmed example in the Solar System - the essential chemical incredients are carbon, the crucial element in organic molecules (of which proteins are the fundamental component) of great complexity and variety that are the basis of life, together with hydrogen, oxygen (and water which dominates the soft parts of human and many other organisms), nitrogen, phosphorus, calcium (mainly in hard parts) and sulphur, and to a lesser degree other elements as important functionaries, such as iron, magnesium, chromium, etc. The amazing thing about this assemblage of critical elements is that they all at times in the past resided in stars and much of the hydrogen itself can be traced back to the first minute of the Big Bang. You and I, as humans, are truly star people - our heritage is cosmic in that our ingredients are either primordial- or stellar-derived.

As a quick synopsis on the nature of life, here is a simple list of the "The Characteristics of Life", adapted from one put online by the Department of Zoology, Oklahoma State University.

*Organized structures that are composed of heterogenous chemicals - in units of "cells"

*Metabolism: chemical and energy transformations

*Maintain internal conditions separated from an outside environment: homeostasis

*Growth: conversion of materials from the environment into components of organism

*Reaction to select stimuli, physiologically and/or behaviorally

*Reproduction: making copies of individuals via the mechanism of genetic transfer: sections of DNA molecules that contain instructions for organization & metabolism

*Evolution: change in characteristics of individuals, resulting from mutation & natural selection - these result in adaptations; Heredity is the outcome.

Thus, the principal functional manifestations of life (based on our studies of this phenomenon on Earth - our only sample so far) are, to reiterate what was listed above: cellular-organization; reproducibility; growth cycle and dependence on nutrition; metabolism (in higher forms) respiration (in some types); (usually) movement of some kind; propensity to evolutionary modification, and, for vegetative types, utilization of photosynthesis. Intelligent life, furthermore, is marked by consciousness, reasoning, abstraction, reliance on memory, communication, and awareness of time and other essentials of existence; free will and "soul" are properties of a more metaphysical nature and harder to prove as realities.

This next diagram was taken from the Internet without any indication of its source nor any explanation of its content. I put it here without any comment, treating it as a "talking point" relating to some of the questions that are relevant to the nature and origin of life on Earth. The implications within the diagram should also pertain to life elsewhere in the Universe. Draw your own ideas and conclusions.

A speculative diagram showing possible flow paths pertaining to the origin of life on Earth.

Before entering into the commentaries below on the nature and characteristics of life, we divert long enough to state these general ideas about life. Life falls into two broad categories: prokaryotic (no cell nucleus) and eukaryotic (nucleus); single celled or multicelled; autotrophic (obtains nutrients from inorganic sources) or heterotrophic (nutrients obtained by "feeding" on other organic sources). Life is classified by a taxonomic system (first espoused by Linnaeus) of hierarchical catetories - from the highest level (broadest number of constituents) to the lowest (most limited or particular). This ranges through Kingdom; Phylum; Class; Order; Family; Genus; Species; (Subspecies). As an example, consider a honey bee: its species name is mellifera, genus is Apis, it belongs to te Family Apidae, which is part of the order Hymenoptera, a nenber if the Class Insecta, that falls within the Phylum Arthropoda, in the Kingdom Animalia. Various proposed subdivisions of life at the Kingdom level are used: a common one is the six-kingdom system proposed by Woese: Eubacteria; Archaebacteria (some use Archaea) - both being largely single celled organisms; Protista (eukaryotic; both uni- and multi- cellular), which include algae, foraminifera, radiolaria, and diatoms; (the next three are all eukaryotic and multicellular) Fungi (yeasts; mushrooms); Plantae, with nonvascular plants (mosses et al) and vascular plants (ferns, conifers, angiosperms [flowering plants]); and Animalia,, with 14 phyla, to us the most important of which is Chordata, that includes amphibians, fish, reptiles, birds, and mammals. The evolutionary "Tree of Life" has followed this generalized pattern (alternate schemes have been proposed) :

The general relation of the 6 Kingdoms in terms of evolutionary roots.

Understanding life (Biology) requires some core knowledge of Organic Chemistry and its subfield Biochemistry. Only the most rudimentary ideas can be covered on the relevant sections of this page. Most of the illustrations were taken from the Online Biology Book, cited above, prepared by M.J. Farabee. He attributes on his site most of the illustrations we will use to Purves et al.; Life: The Science of Biology; Sinauer and Assoc. Publishers and M. Freeman & Company. (PLEASE NOTE: M.J. FARABEE HAS BEEN CONTACTED FOR PERMISSION BUT HAS NOT YET RESPONDED [OUT OF TOWN FOR THE SUMMER?]. HIS ILLUSTRATIONS WILL REMAIN ON THIS PAGE TEMPORARILY BUT WILL HAVE TO BE REMOVED IF THERE IS NO RESPONSE BY THE BEGINNING OF THE COLLEGE YEAR.)

Organic molecules in their simplest form constitute the hydrocarbons which are built from Carbon atoms (which have 4 electrons in their outer shell and can accept 4 more to complete the shell. This can start from the simplest hydrocarbon (CH4, methane, and built up in chains or rings, as indicated in the diagrams below:

The simplest hydrocarbons.

From Farabee/Purves et al - see above citations

Hydrocarbons in chain and ring forms; note the double bonds (two parallel lines) between one C and one O atom.

From Farabee/Purves et al - see above citations

A wide range of more complex organic molecules can develop from adding various molecular functional groups that include N, O, P, S and other elements to open positions in the carbon shell or to hydrogens. These are the principal units:

Functional Group 1

From Farabee/Purves et al - see above citations

Functional Group 2

From Farabee/Purves et al - see above citations

Functinal Group 3.

From Farabee/Purves et al - see above citations

These molecular varieties fall into groups such as saturated hydrocarbons with single H-C bonds (Alkanes), double bond (Alkenes), and triple bonds (Alyknes); ring structures are represented by Aromatic Hydrocarbons. Isomers are organic molecules with the same chemical formula but different atomic arrangements. Among the derivative organic molecules (hydrocarbons with parts replaced by functional groups) are Alchohols (--OH functional group), Ether (--O--), Aldehyde (--CHO), Ketone (--CO--), and Carboxylic Acid (--CO3H).

Biochemistry is a vast subfield of the more general Organic Chemistry. While the subject is complex and detailed (see links above), a few gemeral ideas developed around key illustrations taken from M.J. Farabee's site (cited above), and one from Raven/Johnson are introduced here:

There are four major groups of organic molecules that also are the fundamental categories in living matter: Proteins; Lipids; Carbohydrates; Nucleic Acids (RNA; DNA)

Proteins, the most abundant constituents of organic matter, are built from linked individual amino acids. There are 100s of such acids but only 20 occur in proteins. The basic protein unit consists of a central carbon, an amino group, a carboxyl group, and a general group, labeled R, that can consists of a variety of functional groups; the general molecule looks like this:

The fundamental chemical makeup of a protein molecule.

From Farabee/Purves et al - see above citations

Five examples of individual protein types are shown below:

Amino acids: Leucine and Methionine.

From Farabee/Purves et al - see above citations

Amino acids: Cysteine, Glycine, and Proline.

From Farabee/Purves et al - see above citations

All 20 proteins are α-amino acids, meaning that the amino group is always bounded to a carbon atom. Various combinations of the 20 can link (bond) with one another (amino group to carboxyl group) to form polymers. The linkage is termed peptide bonding. Oligopeptides link only a few amino acids; chains of 100s of amino acids (polypeptides) are common:

The nature of a peptide bond.

From Farabee/Purves et al - see above citations

Protein synthesis is a major goal in biochemistry. This is proving difficult because getting the amino acids in the right order is hard, unwanted reactions among side chains is common, and peptidization gives off energy which can decompose the desired end product.Protein structure can be simple chains (primary) or helical or pleated (secondary).

Proteins are the dominant molecular types in cells. Specific proteins (composition and/or structure) can specialize into Enzymes (that promote change and formation of new organic material by catalysis (bring about reactions without themselves being changed), Hormones (perform important physiological functions), Structural proteins (hair, skin, etc.), Transport proteins (carry material across membranes); Antibodies (involved in immune systems), and Muscle proteins (actin and myosin in fibrous forms). Enzymes are particularly important since they are involved in most biochemical reactions and can be very efficient; many enzymes work by attaching to the molecule they affect, which serves as the substrate responsive to their action.

One of the most important proteins is hemoglobin, the main constituent of a red blood cell found in the circulatory systems of many mammals, including Man. As seen in the illustration below, hemoglobins consist of 4 folded units, 2 α-globin twisted chains and 2 β-globin chains, each harboring a heme (haem) group (blue disk) that contains iron. The four hemes allow loose bonding of oxygen to the iron, which is carried in the blood and released where needed to promote oxidation reactions; this globular molecule also can carry CO2 for removal during lung exhalation:

Structure of the hemaglobin molecule.

From Farabee/Purves et al - see above citations

Lipids include fats, fatty acids, certain oils, waxes, terpenes, and steroids (one example being cholesterol). They consist of polymers of CH2 and CH3. Glycerol, a typical fatty acid, has the formula: HOCH2CH(OH)CH2OH. Palmitic Acid has this structural arrangement:

Palmytic acid, a lipid that consists of repeating CH2 and H2C alternates, with C-C and H-H bonds.

From Farabee/Purves et al - see above citations

The molecular structures of some common steroids are depicted below:

Structural arrangements of 4 steroids.

From Farabee/Purves et al - see above citations

The third major group, the Carbohydrates, also known as Saccharides, includes sugars, starches, glycogens, and cellulose. The basic formulaic unit is: CH2O. One important group, the Monosaccharides, includes ribose and deoxyribose, ring structures made up of pentagonal (5 carbon) sites - these are two fundamental components of RNA and DNA. Example:

3- and  5-carbon sugars.

From Farabee/Purves et al - see above citations

Glucose is said to be the most abundant biochemical molecule within terrestrial life: Its formula is C6H12O6. Here are several views of glucose; at the top are two isomers, below is a 3-D stick and ball representation, and at the bottom is a side view:

Alpha and Beta isomers of glucose

From Farabee/Purves et al - see above citations

Stick-ball model of glucose

From Farabee/Purves et al - see above citations

Side view of glucose

From Farabee/Purves et al - see above citations

Structures comprised of two joined rings are Dissarcharides (e.g. Sucrose and Lactose). Chains of rings make up Polysaccharides" These include starches (which store food energy in plants), glycogen (energy storage in animals) and cellusose (important cell wall component in plants). A starch molecule and

Starch structure

From Farabee/Purves et al - see above citations

Many of the carbohydrates and some proteins too are important sources of energy released during metabolism and other process needed to provide the vital force to sustain life. Breakage of certain bonds are the means of releasing this energy.

The fourth group, Nucleic Acids, are of fundamental essence to life in that they contain the biochemical molecules that hold the blueprints for making and copying cells. RNA and DNA are called the information-bearing cells that determine the nature of any given organism, from simple bacteria to humans. The basic unit is called a Nucleotide, consisting as shown below of a monosaccharide 5-carbon sugar (see Ribose [RNA] amd Deoxribose {DNA] four figures above), one of 5 nitrogenous bases, and a phosphate group.

Basic makeup of a nucleotide.

From Farabee/Purves et al - see above citations

The structure and composition of the five bases (usually identified by their first letters: Adenine; Cytosine; Guanine; Thymine, and Uracil) is given by this diagram

The five nitrogenous bases found in RNA and DNA.
From Raven and Johnson, Biology, 6th Ed., McGraw-Hill, Inc

RNA occurs in single strands in which the sugars are linked by the phosphate (PO4) phosphodiester bond and the bases lie on the other side of the chain. The four bases present are A, C, G, and U. Multiple chains of nucleotides make up the nucleic acid.

Structural arrangement of RNA

From Farabee/Purves et al - see above citations

The main roles of RNA are in its intimate involvement in the production of proteins, and its intermediary action in duplicating DNA during cell division and growth.

DNA, arguably the most famous organic molecule of all, consists of two chains side by side. They contain A, C, G, and T (note that T replaces the U present in RNA_. These produce a bond between the two chains but only certain pairings are allowed: A - T and G - C. These couple by hydrogen bonds. Here is a schematic of this arrangement.

The double chain DNA pattern.

From Farabee/Purves et al - see above citations

In 1953 Francis Crick and James Watson discovered through x-ray analysis that the double-chained DNA was composed of two spiraling chains arranged in the famed appellation "Double Helix"; this discovery won them the Nobel Prize in Biology. Here is how it looks:

A segment of the DNA molecule showing the helical winding of the two chains, each bonded by photsphate unit and both linked by allowable base pairings; only a few nucleotides are shown.

From Farabee/Purves et al - see above citations

The importance of RNA and DNA reside in their roles in making new cells and in determining the nature/function of a cell by imparting correct instructions for that in the process. This subject, at the heart of concepts of genetics (genes, chromosomes, and genomes), is far too voluminous (knowledge-intensive) to cover on this page (see the Biology Tutorial Internet sites for the details). Suffice to say that the RNA and DNA strands may be very long (macromolecules consisting of 1000s of nucleotides). For DNA, the sequence of A-T amd G-C, arranged in groups of 3 (codons), can, at such lengths, encode a huge number of combinations. These lead to differences in a cell's nature (for example, different proteins are produced by RNA groupings that in turn are produced from DNA control), and when they are set in some fixed pattern of the sequenced pairings, a specific gene is determined. Genes make up the chromosomes that determine sex and heredity. The full complement of various genes in some particular pattern establishes the genome that uniquely specifies a given organism.

It is instructive to diagram one of the fundamental processes the nucleic acids are responsible for - the production of proteins:

DNA (Repl) ------------->DNA (TrScrip)-------------->m-RNA (Trlate)------------>Protein

Deceptively simple in this formulation, the process is actually complex. In the first step, Replication, involves reproduction of a cell as it divides and the DNA separates into two single strands that make complementary "copies" as short segments in which each strand undergoes A--->T, G--->C, etc. bondings; this proceeds until two identical DNA copies results. Then, Transcription, the RNA complement in one strand is synthesized by enzymes into a messenger segment, m-RNA, that is a reversed copy of the original DNA strand, but with T (thymine) replaced by U (uracil). The m-RNA serves as a template for natural protein synthesis within a ribosome (see cell diagram below. which also contains r-RNA (ribosomal RBA) This ribosome assemblage participates in the nmanufacture of a protein by Translation) using t-RNA cells (transfer-RNA) containing various amino acids that bind to the m-RNA by hydrogen bonding. The process continues as the polypeptide protein builds until addition of a 'chain termination' group signals the particular protein with its diagnostic codification is completed. The specific protein composition depends on the sequences involved in the synthesis.

During replication mistakes in reproducing a gene pairing constitute the biochemical explanation for the mutations that Charles Darwin and his compatriot Alfred Wallace cited as the basic cause for natural selection. That is the cornerstone of the Theory of Evolution - this holds that individuals in a species that have the best adapted means for survival in their environment(s) will live longer and therefore have the better likelihood of passing their specific gene makeup to offspring in the gene pool ("Survival of the fittest"). Occasional mutations over thousands of generations in an organism's reproductive history lead to gradual changes until differences are great enough to warrant designation as new species. More fundamental and long-ranging changes lead to generic, familial, and higher level variants that become new types of animals or plants.

A brief comment about how living organisms derive energy needed to function, usually through metabolism, which describes several possible chemical reactions. A general way is any process that releases energy when bonds are broken. In plants, photosynthesis is an endothermic reaction involving interaction of sunlight with chloroplasts. In animals, respiration and fermentation release energy when appropriate organic molecules are catabolized (degraded or broken down). Aerobic organisms use oxygen to break down glucose into carbon dioxide and water, releasing energy. About half that energy is stored in a group of molecules, chief of which is ATP (Adenosine TriPhosphate) synthesized naturally from ribose sugar, an adenine base, and phosphate molecules. Here is its structural formula:

The ATP molecule, a major energy storage component of many organisms, from which that energy can be retrieved by metabolic processes such as reaction with water.

From Farabee/Purves et al - see above citations

Mentioned in passing are viruses. These are strands or segments of nucleic acids encased in a lipid and/or protein coating. They are not living matter although they can reproduce in cells. Although some viruses are beneficial, most disrupt chromosomes, and can insert themselves into DNA or RNA, causing infections. The best known virus today is HIV, which through AIDS destroys the body's ability to overcome disease and cancer. These belong to the class of organic materials known as antigens. In animals, the antigens that threaten their well-being are attacked by antibodies, generated by immune response systems (e.g., lymphocytes)

The HIV virus.

This is probably a good place on this page to talk briefly about cells. Two types exist: Prokaryotic (no nucleus) and Eukaryotic (nucleus). Both single-celled and multi-celled organisms exist among each type. Both types contain DNA and ribosomes. Prokaryotic cells typically are 1 to 10µm in size; Eukaryotic cells can be 100 µm wide; larger cells are knownThis figure depicts the two cell types:

Schematic showing the main components of Prokaryote and Eukaryote cells.

A Eukaryote cell is much more complex, as suggested by this generalized diagram showing its makeup:

A Eukaryote cell.

Each of these components will be concisely defined:

The Cell Wall (called Cell Membrane in animals), in bacteria is usually made of peptilogycan and in eukaryotes cellulose or chitin. Its functions are to enclose the cell interior, protect the cell, and aid in transfer of material in and out.

The Nucleus, commonly spherical and enclosed by a double membrane, contains the chromosomes (gene assemblages of DNA).

The Nucleolus is host to genes for rRNA synthesis.

Cytoplasm refers to the semi-fluid (mainly water; some ions) medium (shown in wine-color above) within which "float" all the organelles (every body in the interior other than the nucleus)

Ribosomes are protein-RNA complexe that are sites of protein synthesis.

Vacuoles are open sacs available for digestion or storage of waste products; may contain degraded protein, can become water-filled.

Gogli Apparatus consists of stacks of vesicles (openings) in which proteins made in the cell are prepared for export from the cell

Lysosomes are vesicles, derived from Gogli A., containing digestive enzymes that attack defunct organelles and other cell debris.

Centrioles are specialized organelles that produce microtubules that influence cell shape, move chromosomes during division, and aid in developing cilia and flagella.

Peroxisome use enzymes to remove superfluous electrons and hydrogen atoms; hydrogen peroxide is a by-product.

Mitochondria are double-membrane organelles that provide "power" from the cell by oxidative metabolism.

Endoplasmic Reticulum serve as networked membranes that aid in making vesibles; also assist in synthesizing proteins and lipids.

Not shown in diagram: Chloroplasts, which control photosynthesis in plants and Chromosomes, which are long DNA threads that host hereditary information.

We have elected to omit any direct discussion of genes, chromosomes, and genetics (heredity) but will point you to three websites that provide some insight into this topic: Chromosome 1; Chromosome 2; Genetics. We will make one comment here that answers one topic of confusion: In the human gene assemblage there are 46 (23 times 2) chromosomes (long strands of DNA containing all the genetic information [the number varies among species]). Two of these are the sex chromosomes; the remainder are "autosomes" in which each chromosome has its own individual assemblage of genes that controls some aspect of the genetic makeup (other than the sex) of the organism (22 on the haploid strand of a human).

Now, with this background we move on to facts and speculations about the origin snd development of life on Earth (and by inference, elsewhere in the Universe). There is general consensus that the first organic ingredients that became plentiful in oceans ond other energy-rich environments associated with water were amino acids and later nucleic acids. RNA probably organized before DNA, and was needed to synthesize the proteins. Both are important in the production of nucleic acids. These constituents had to develop before they could organize into symple prokaryotic cells. The first organisms - bacteria - were single-cells that containing mostly water enclosed in a membrane, with RNA strands and ribosomes as the principal internal organelles.

Evidence is abuilding about the Earth's atmospheric history and its relevance to the appearance of life. The most primitive atmosphere containing nitrogen, free hydrogen, some carbon dioxide, and no oxygen. In the first two billion years or so, the Sun's energy output was 70-80% of today's radiant release. To keep the early ocean's from freezing, some mechanism was needed to maintain proper temperatures. Carbon dioxide - the greenhouse gas - could perform part of that function, but its quantity was probably too low (based on absence of FeCO3 or Siderite in the geologic record of early times; likewise, calcium and magnesium carbonates [limestones] were noticeably rare). Researchers now think that carbon reacted with hydrogen atoms to form methane (CH4) which was more efficient than CO2 in absorbing outgoing thermal radiation. A class of living microorganisms - methanogens - could have arisen and flourished for millions of years. As these proliferated, they expelled methane in the atmosphere of the time until that gas reacted with hydrogen and other constituents to form a "smog" (similar to that on Saturn's Titan) that built up. This in turn would absorb incoming solar irradiation and lead to a reversal of temperatures to the extent that cooling brought about an Ice Age ("Snowball Earth") some 2.3 billion years ago. Thereafter, methanogens never regained their importance as oxygen slowly built up in the changing atmosphere.

There are still questions about how life actually began. The key components - proteins, RNA and DNA - had to precede living cells. Speculation still continues over the mechanisms and circumstances by which the components were first produced. Although not definitively accepted as the actual scenario, an experiment in 1953 by a graduate student, Stanley L. Miller at the University of Chicago, under the tutelage of Nobel Prize winner Harold Urey, is regarded as one of the classic scientific efforts ever in the field of biology. Here is a diagram that depicts the experimental set-up:

Miller and Urey produced a primeval ocean in one chamber. Heating it expelled water vapor into a second chamber containing the gases they thought might have existed after the molten Earth cooled to a crust and primitive ocean/atmosphere at a time much hotter than the present. Into the top flask, the water vapor-gases mix was subjected to frequent electrical sparks (to simulate lightning). As days went on, and the condensed mix was sampled and analyzed, sequences of organic molecules were synthesized, as shown here:

The organic molecules produced by the Miller-Urey experiment.
From Raven & Johnson, Biology, 6th Ed., McGraw-Hill Higher Education.

Variations of this famed experiment have produced still other organic molecules. The key conclusion it points to is that a reducing, hot atmosphere with compositions similar to the one they used could have generated some of the basic ingredients that later organized into life. Other sources of energy have been proposed. The conditions that prevailed then were probably like those we assign the word "extremophile" to. One plausible alternative are the hot waters around the deep-sea "black smokers" found around oceanic spreading ridges; carbon escaping from a primitive earth's mantle would react with other subsurface constituents, or those in the water, to yield building-block molecules (life today is found around the smokers - apparently produced there - but today's water contains more oxygen than in primitive earth environments [in fact, oxygen tends to destroy these simpler molecules]). Another view holds that at least some organic molecules were added to Earth after its general melting and bombardment by asteroids/comet. These extraterrestrial bodies are known to contain various amino acids. An experiment at Lawrence Livermore Laboratory in which a group of amino acids were held in a material subject to high speed impact (from a gun) yielded the surprising result that these acids formed peptide chains, the building blocks of proteins. Thus, life on Earth could have begun internally and/or externally.

On Earth, as stated above the first life was unicellular (microbial, including an abundance of bacteria), followed much later by unicellular plant life which eventually acquired the ability to photosynthesize carbon compounds using solar energy into monosaccharide carbohydrates, releasing oxygen as a by-product (a build-up of oxygen leads to formation of upper atmosphere ozone which, in turn, protects life below from destructive UV radiation). Sometime in the last billion years, multicellular eukaryotic plants and then primitive animal forms (e.g., protozoa) that have evolved into today's varieties (metazoa). Energy sources that favor life are solar radiation, terrestrial heat, and change of state heat (nuclear decay which supplies much of Earth's heat from the interior may also provide radiation that could synthesize certain organic molecules under the right conditions). (A fourth possibility is gravitational [tidal] energy which might produce life-developing conditions; future exploration of Europa will test this mechanism by seeking life beneath its icy crust). The presence of water and a suitable atmosphere (life on Earth began in a reducing atmosphere but with photosynthesis, oxygen has increased, allowing the process of metabolism (oxidation) to provide internal energy for living organisms) also are determinants.

Thus, the frame of reference of any investigations of life elsewhere in the Universe continues to reside in the extensive studies of organic chemistry and biology of organisms dominating the only known place where life's existence is confirmed: our planet. Life on Earth began at least 3.5-3.8 billion years ago. Since then the history of life has been increasing complexity and diversity and adaptation of ever more (and changing) environments. This chart summarizes this history:

The history of life.
From Raven & Johnson, Biology, 6th Ed., McGraw-Hill Higher Education.

So, as described above, almost certainly the first living bodies were microscopic in size, being single-celled. Bacteria were the dominant, perhaps the only, major life forms. Below are two modern day examples:

A simple modern Eubacterial microbe.

An Archaebacterica; coccus form.

A more advanced modern unicellular animal is the Protist Paramecium, with dual nuclei and numerous tiny "whips" (cilia) for locomotion:

A Paramecium, with stained nuclei and cilia.

Familiar to many is the amoeba. This protist moves by extending parts of its cell as "pseudopods" in certain directions and then pulls the remainder of the cell towards one or more of these protuberances. This photomicrograph shows the Proteus species of Amoeba:

Amoeba proteus.

Claims of planktonic microbial life (animals and plants living at or near water surface; free-floating; largely microscopic; utilize photosynthesis in autotrophic or heterotrophic assimilation of foodstuff; in the oceans and lakes planktons are at the base of the food chain) as old as 3.8 b.y. in rocks from Greenland have been made. Here is a modern phytoplankton (microscopic plant)

Modern-day phytoplankton.

Generally accepted evidence of bacterial life from the 3.5 b.y. Apex Formation of Australia has been published by J. Wm. Schopf (UCLA) and others. Here are two photo illustrations of what has been discovered:

Color photos of bacterial life in the Apex Formation of Australia.

Additional photos of bacteria in the Apex Formation, with sketches of their appearance.

Microbial life has now been found in the rocks from the Barberton Formation in South Africa, of 3.5 billion year age. The example shown here is in a rock that was emplaced as a glassy lava before crystallizing. It is postulated that this life form actually "fed" on the rock material itself (now recrystallized into a basaltic type).

Presumed microbes in the Barberton Formation of South Africa.

One of the prevalent life forms (perhaps as far back as 3+ billion years) falls in the general category of cyanobacteria (also known as blue-green algae). Fossil examples from two different ages are shown here:

Cyanobacteria from rocks about 2 billion years old.

A type of cyanobacterium present in the Bitter Springs Formation of Australia, dated at 850 million years.

Cyanobacteria were dominant for at least 2 billion years and some forms still exist today. They produce large amounts of oxygen by photosynthesis (using sunlight to convert CO2 and H2O to simple sugar and free oxygen. They played a key role in the transition of the Earth's atmosphere from reducing to a gradual buildup of oxygen. One of the sedimentary rock types supposedly influenced by bacteria is the Banded Iron Formation (BIF) which occurs worldwide; it forms rich iron ore in Minnesota and Michigan. For an extended period, iron in water environments grabbed the free oxygen, slowing the buildup of that gas but in time the iron was depleted and oxygen then accumulated more rapidly. Here is an example of this BIF rock in which the red is rich in hematite:

Banded Iron Formation.

Another famous locality containing a variety of ancient life forms is the Gunflint Formation (1.9 b.y. in age) in Minnesota and southern Canada. These are examples of microfossils found in rocks made up of chert from this unit:

Gunflint Formation microfossils; A, B, and C are blue-green algae, D is an algal spore; F is a bacterium; attributed to work by Elso Barghoorn and Stanley Tyler.

Other life forms were fungi and algae. The oldest and most famous of the larger fossils are the stromatolites of Western Australia. Stromatolites are mounds of prokaryotic algae and cyanobacteria. Modern stromatolites occur today along the Australian coast.

Modern-day stromatolites in Australia.

These compare well with excavated ancient stromatolites found around Marble Bar in Western Australia dated at 3.45 billion years:

Ancient stromatolites

In cross-section these stromatolites have a conspicuous curved layering.

Cross-section through these ancient stromatolites.

Stromatolites are also found in the Gunflint Formation, described above. Here is an outcrop on Lake Superior:

Strommatolitic layers in the Gunflint Formation.

The first eukaryotic life forms may be as old as 2 billion years ago. Grypania spiralis has been found in ancient rocks in Michigan. This fossil is preserved because it formed formed simple shells:

Grypania spiralis.

Life continued in this primitive state until about a billion and a half years ago when photosynthesis became a common process that helped plant life flourish and released oxygen.

The greatest explosion of life in earth history took place about 600 to 500 million years (late Proterozoic into Cambrian) ago with the appearance of distinctive and diverse animal forms. One significant marker was the first appearance of animals with bilateral symmentry about 580-600 million years ago. These animals were very small, generally microscopic, and usually found in shales; the samples had to be thin-sectioned by guesswork to find the tiny objects which were preserved soft parts. Representative of these is Vernanimalcula, found in China:

Vernanimlacula, soft parts; the creature possesses bilateral symmetry

A more general view of the life in the time frame from about 600+ to 542 million years ago (end of Proterozoic and Precambrian into the oldest Cambrian), known as the Ediacaran or Vendian, is found at this New Zealand site; it mentions Australian and other geographic localities where the assemblages have been found. The fossil life represents entirely creatures with soft parts only and suggestions that these may be ancestral to later phylla observed at the beginning of the Paleozoic. Below is an artist's sketch of some of these creatures:

Some typical Ediacaran fossils.

Around 535 m.y. ago, in the late Vendian, a soft parts assemblage of fossil forms has been found in the Chengjiang Formation in Yunnan Province of southern China. Here is one typical life form;

Life form of indeterminate taxonomic placement found in the Chengjiang Formation  of China.

By the opening if the Cambrian, many forms of invertebrate life had developed (mainly for protection) external carapaces or coverings that, after death, survive as fossil shells. A survey of Cambrian stratigraphy indicates a dramatic increase in diverse marine life forms, so much so that this abundance of living creatures has been referred to as "The Cambrian Explosion".

The life forms with hard parts make their first appearance in the Middle Cambrian. By far the best locality where both hard and soft parts are well preserved and displayed is in the Burgess black shale of British Columbia (Prof. Charles Walcott is famed for his pioneering studies of this assemblage) deposited about 520 m.y. ago . This sudden burst of evolution may have been tied to oxygen reaching threshholds near the present day levels. A good review of the types of fossils found there (MacKenzie Mountains) is given by the Peabody Museum at Yale University. Here is an outcrop of this black shale near Mt. Burgess.

The Burgess shale.

Here is an artist's conception of typical animal and plant life in the shallow sea in which the Burgess shale was deposited:

Artist's rendition of some of the life forms present in the Burgess shale sea.

Some of the typical life forms in the Burgess shale, each fascinating in its own right, are shown below:

Two animals similar to Trilobites.

Another fossil of the Arthropod type.

A segmented worm.


Vauxia, an elongate sponge.

Jellyfish imprint in the Burgess shale.

An excellent review of early life on Earth is available at this web site maintained by the University of Munster (unfortunately, many of the links no longer are active).

A second peak time in the abundance of shell-surviving life forms was in the Upper Ordovician (by this time also, the first larger vertebrates, fossil fish, had appeared). Below are two illustrations: the first, an artist' conception of marine invertebrate life in the late Ordovician; the second, a typical slab of Ordovician limestone (from Indiana) containing the fossil types listed in its caption:

A painting depicting a typical Ordovician seafloor, with crinoids, bryozoa, cephalopods, trilobites, and brachiopods.

Slab of Ordovician limestone with a mix of brachiopod, bryozoa, gastropod, and trilobite parts.

The oldest known vertebrate life may be a tiny fish (8 cm in length) called Anatoleptis, of very late Cambrian age (510 million years; younger specimens (470 m/y.) have been found in Scandanavian rocks. The photo below is a microscope view of scales from this fish, whose remains have been found in Wyoming and other parts of North America:

Primitive fish scales associated with other fossil remains of Anatoleptis, possibly the oldest vertebrate found to date; specimen from Wyoming.

A rather fanciful panorama of life forms (Kingdoms, Phylla, Families, etc.) from the Late Precambrian to the Present is shown in this mural that is found on the Humboldt State University campus. On the left, pre-Paleozoic animals with soft and hard parts give way to the Invertebrates of the Early Paleozoic, the first fish (Ordovician; sharks in the Devonian), then Amphibians that appeared at about the same time (Mississipian) that land plants took root, with the first reptiles and dinosaurs near the end of the Paleozoic (about the time the first extensive forests spread in the Pennsylvanian), reptiles and small mammals in the Mesozoic, along with the first birds, and finally a dominance of mammals, flowering plants, and widespread forests in the Cenozoic.

The panorama of life found as macrofossils, from about 700 million years ago to the present.
To see entire panel, scroll bottom bar to the right

At the far right of this panel would be the story of the hominids, which includes today's mankind. This subject is far too involved for any extended treatment in this Tutorial. A good, quick overview that carries back to the beginnings of life but gives some emphasis to the appearance of humans is found in the Wikipedia Timeline of Evolution webpage. For this page, these three diagrams are helpful. The first is one of several variants that include hominids in the Great Ape family.

Phylogeny of the Gorilla/Ape/Man group of animals.

In this version, there are two major divergences over the last 30 million years. One line includes old world monkeys, the other contains several branches of which one is the hominids. While the closest tie between Man and other similar animals is said by many to be with the Gorilla, the Chimpanzees are also related (97% of the thousands of DNA genes in the genomes of Man and Chimp are present and located in common).

We repeat here a diagram similar to the one shown on page 19-2a that lists the main genera and species that tie in modern humans in the last million years to other related hominids.

Ancestry of Man in the naturalistic model developed from the concept of Evolution.

Part of this can be displayed in terms of an evolutionary pattern that depends on the principal evidence - interpretation of skeletal remains, in this case reconstructed heads.

Another phylogenic classification of the principal hominids.

We need to expand our thoughts on how life originates in space. This page is continued as page 20-12a, reached through the Next button.

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Primary Author: Nicholas M. Short, Sr. email: [email protected]