THE ORIGIN OF LIFE AND EVOLUTION OF EARLY LIFE

By Edward L. Crisp, Ph.D.

Professor of Geology

Hadean Eon: 4.6 to 4.0 Billion Years Ago
Archean Eon: 4.0 to 2.5 Billion Years Ago
Proterozoic Eon: 2.5 Billion to 543 Million Years Ago
Phanerozoic Eon: 543 Million Years Ago to Present

CONDITIONS ON EARTH DURING LATE HADEAN TIME (CHECK OUT THIS LINK: Geol 2C Hadean lecture at http://people.hofstra.edu/faculty/j_b_bennington/2cnotes/hadean.html

Earth barren, hot, with thin dark igneous crust (but may have been completely molten soon after formation).
Lots of primordial heat from gravitational collapse of formation and radioactive decay (Earth much more radioactive than at present and additional short lived radioisotopes present).
The atmosphere consisted primarily of CO₂, H₂O vapor, N₂, H₂, CH₄, NH₃, and CO, with hardly any molecular oxygen (O₂) and no ozone (O₃) layer (so no filtering of ultraviolet radiation).
Earth rotated on its axis in about 10 hours. The moon was much closer and caused a huge tidal effect.
Earth eventually cooled down, water condensed and began to accumulate in the ocean basins.
Meteorite and comet bombardment slowed - very slow by 4 billion years ago.
By 3.8 billion years ago there were small areas of continental crust.
Life had definetly evolved by 3.5 billion years ago (probably by as early as 3.8 billion years ago, maybe prior to that in the late Hadean Eon prior to 4 billion years ago).

INTRODUCTION

The precambrian (4.6 billion to 543 million years ago) represents about 87% of geologic time, with the Phanerozoic Eon representing only about 13% of geologic time.
Formal subdivision of the Precambrian rocks is difficult; complexly deformed and metamorphized in many cases, much as non-stratified rocks (difficult to use principle of superposition); contains few fossils; so must relie on radiometric dates for age and correlation.
1982 - North American Commission on Stratigraphic Nomenclature - named two eons: Archean (3.8 to 2.5 bya, but has recently been revised to 4.0 to 2.5 bya) and Proterozoic (2.5 bya to begining of Cambrian Period, now recognized to be 543 mya). Note: Some refer to the Pre-Archean (4.6 to 4.0 bya) as the Hadean Eon (but only a few rocks on Earth have been dated in that time range: 3.96 bya for some rocks in the Canadian Shield and 4.2 bya for some detrital zircons in metamorphic rocks of Australia).
Alternate Classification Scheme - U.S.G.S.-1972
Hetertrophic organisms and chemosynthetic autotrophic organsims may have evolved by about 4 billion years ago on Earth.
So-called chemical fossils occur in significant concentrations in rocks that are about 3.8 billion years old. These chemical fossils are organic molecules called pristine and phytane that are breakdown products of the organic substance chlorophyll (which is itself involved in the photosynthesis process in some organisms). These may indicate that life had evolved on Earth by 3.8 billion years ago and were already photosynthesizing organisms (thus autotrophic) .

THE EVOLUTION OF THE ATMOSPHERE AND OCEANS

The first atmosphere on Earth consisted of H and He. These gases boiled off and were swept to the outer solar system by the solar wind.
Soon afterwards, a primeveal atmosphere formed via volcanic outgassing from the Earth's interior.
Modern volcanoes outgas water vapor, carbon dioxide, sulfur dioxide, caron monoxide, chlorine gas (Cl₂), molecular nitrogen (N₂), and moleular hydrogen (H₂). Volcanoes probably emitted these gases during Pre-Archean and Archean time also.
From chemical reactions, the gases methane (CH₄) and ammonia (NH₃) probably fomed during Archean time and were fairly abundant in the atmosphere.
No molecular oxygen (O₂) at the beginning of Archean time, however, by late Archean photochemical dissociation of water vapor (by incoming ultraviolet radiation) had generated some molecular oxygen (this may have eventually supplied up to 2% of our present atmospheric oxygen; at 2% moleuclar oxygen ozone forms to cut-off photochemical dissociation.
By the end of Archean time (about 2.5 bya) maybe about 1% of our present atmospheric oxygen.
The other major souce of atmospheric oxygen is photosynthesis, which probably did not happen in Archean time (maybe a little by late Archean???).
Oceans started accumulating in Archean time but their extent is unknown. The oceans started forming due to condensation of water vapor from volcanic outgassing.

ARCHEAN LIFE (INORGANIC TO ORGANIC EVOLUTION)

Life originated on Earth by at least 3.5 bya (probably earlier). How did life originate? What is life???
The minimum requirements for living organisms is 1) reproduction and 2) metabolism.
Viruses (which contain DNA or RNA in a Protein Capsule) reproduce and metabolize in host cells, but outside of host cell do neither. Are viruses living organisms? Note: Some suggest that viruses probably evolved after life had evolved. Probably evolved from bacteria-like cells - lost protoplasm and became parasites.
From: Examples of Viruses at http://www.accessexcellence.org/AB/GG/examples_of_viruses.html
Micorspheres: organic molecules that form spontaneously and have more organization than inorganic objects (such as minerals). They can even grow and divide, but randomly and do not always pass on the same molecular structure. They are not considered living, but evolution of life may have passed through this stage. In 1924 Russian biochemist A. I. Oparin postulated that life originated without free oxygen (molecular oxygen destroys organic molecules).
A. I. Oparin
(image courtesy of Encylopædia Britannica)
The absence of an ozone layer during Archean time allowed large amounts of ultraviolet radiation to penetrate to the Earth's surface, perhaps this was the energy source for putting organic molecules together to form life (an energy source is needed, remember the Frankenstein movie and the lightening).
For life to orginate needed 1) a source of elements for organic molecules to form and 2) a source of energy for chemical reactions to form organic molecules.
All organisms consist mostly of carbon, hydrogen, oxygen, and nitrogen.
All these elements were present in the Earth's early atmosphere.
It has been postulated by biochemists that the elements carbon, hydrogen, oxygen, and nitrogen combined to form simple organic molecules called monomeres. Some typical monomeres are amino acids (the building blocks of proteins).
The energy source for these monomeres was ultraviolet radiation and lightening.
In the 1950s, Stanely Miller synthesized several amino acids by circulating gases like the ones in the Earth's early atmosphere through an apparatus that used electrical sparks for energy. He formed several amino acids typical of living organisms. (CLICK THE RIGHT PICTURE BELOW TO READ ABOUT THE STANLEY MILLER EXPERIMENT IN 1953 AT THE UNIVERSITY OF CHICAOGO, WHERE HE WAS A GRAD STUDENT)
Stanley Miller apparatus (Left diagram from: http://www.uky.edu/ArtsSciences/Geology/webdogs/time/hadean/hade2.jpg)
(Right diagram from: EXOBIOLOGY: An Interview with Stanley L. Miller at http://www.accessexcellence.com/WN/NM/miller.html)
More recently, all 20 life essential amino acids have been synthesized in the laboratory.
From: Amino Acids found in Proteins - Part 1 at http://www.accessexcellence.org/AB/GG/aminoAcids1.html
Molecules of organisms are polymers (linked monomeres), such as proteins and nucleic acids.
How did polymerization occur? Aqueous solutions usually cause depolymerization.
Sidney Fox at the University of Florida has synthesized small molecules (protenoids) consisting of 200 amino acid units. Heat seems to cause polymerization.
In Fox's experiments protenoids form microspheres, which are bounded by cell-like membranes. They can grow and divide much like bacteria. These microspheres are called protobionts, but are somewhere between life and inorganic chemical compounds.
It is thought that monomers were abundant in early Archean seas. Maybe they washed onto the shore where evaporation concentrated them. They were then heated by solar radiation and polymerization took place. They then washed back out to sea for more reactions to take place. But a membrane had to form around them or they would be destroyed.
How did reproduction take place? RNA or DNA is needed for reproduction. RNA molecules may have evolved (without enzymes) that could reproduce.

THE EARLIEST ORGANISMS (PROKARYOTIC HETEROTRPHS AND AUTOTROPHS)

The earliest organisms (which probably evolved prior to 3.5 billion years ago) were probably anaerobic, heterotrophic prokaryotes.
The oldest known fossils are 3.3 to 3.5 billion year old Cyanobacteria from Australia. These are photosynthesizing (thus autotrophic) anaerobic prokaryotic bacteria.
Cyanobacteria form layered mound-like structures called stromatolites. The oldest stromatolites are in 3 billion year old rocks of South Africa. Many Precambrian sedimentary and metasedimentary rocks have abundant stromatolites.

The earliest atmosphere on Earth was without oxygen. The primeval atmosphere evolved from the gases expelled during volcanism, a process called volcanic outgassing.

The earliest organisms were one celled organisms that were anaerobic (did not metabolize with oxygen) and were most likely heterotrophs. The first cells evolved prior to 3.5 billion years ago. However, the remains of the oldest one celled organisms that have been found in the rock record are 3.5 billion year old autotrophs (organisms that make their own food). About 3.5 billion years ago, anaerobic autotrophic organisms evolved. These one celled organisms are called cyanobacteria (Figure 1). Autotrophs make their own food by photosynthesis:

PHOTOSYNTHESIS:

Carbon Dioxide + water + chlorophyll + energy from Sun ----> Carbohydrates + molecular oxygen

Figure 1. Filamentous procaryotic microfossils from 3.5 Billion year old black cherts of the Archean Warrawoona Group, Pilbara shield of western Australia (Originally courtesy of J. W. Schopf and B. M. Packer to: Levin, Harold L.; 1991; The Earth Through Time (4th ed.); p. 265, Figure 7-18; Saunders College Publishing; 651 p.

Prokaryotic and Eukaryotic Cells

The oldest one celled organisms were PROKARYOTIC cells. Prokaryotic cells are small, have no nucleus or cell partitions. Bacteria are examples of organisms that possess prokaryotic cells.

About 1.7 billion years ago (perhaps by about 2 bya) more advanced cells evolved. These EUKARYOTIC cells are larger and more complex. Eukaryotic cells have an nucleus and organelles that perform certain cell functions.

a) Prokaryotic Cell Eukaryotic Cell

Figure 2. a) Comparison of prokaryotic and eukaryotic cells. From: Introduction to Microbiology
(http://www.eng.rpi.edu/dept/chem-eng/Biotech-Environ/FUNDAMNT/streem/ese.htm)
b) Schematic diagrams of a prokaryotic cell (below left) and a eukaryotic cell (below right). From: Sepkoski, John, Jr., 2001, Foundations Life in the Oceans, in Gould, Stephen J. (ed.) , 2001, The Book of Life: W. W. Nortan & Company, New York and London, 256 p.

The evolution of the eukaryotic cell is probably the most important event in the history of life.

ENDOSYMBIOTIC THEORY for the origin of eukaryotic cells. Certain prokaryotes ingested other prokaryotes. The ingested prokaryotes remained alive inside their host and adopted special functions as organelles for a more complex cell. Both the host cell and the ingested cells found a mutual benefit from the association.

There is much supporting evidence for the endosymbiotic theory for the origin of eukaryotic cells. The DNA and RNA of certain organelles is like that of prokaryotic cells and different from the nucleus of eukaryotic cells. Certain organelles have separate cell membranes and organelles have separate reproductive mechanisms.

Figure 3. One theory for the evolution of eukaryotes from prokaryotes, the Endosymbiotic Theory.
(From: Levin, Harold L.; 1991; The Earth Through Time (4th ed.); p. 264, Commentary; Saunders College Publishing; 651 p.

Two other evolutionary steps had to occur before life as we know it could have evolved. 1) The origin of sexual reproduction and 2) the evolution of multicelled organisms. Sexual reproduction allows for the reshuffling of genetic material to get different combinations of traits (than the parents had). Whereas, asexual reproduction allows for only very slow variation due to minute mutations that occur over long periods of time. Sexual reproduction allows recombination of DNA in the offspring, thus tremendous variety can occur in a short period of time (geologically speaking).

MULTICELLED ORGANISMS

Multicelled organisms had evolved by about 1.2 billion years ago (ProterozoicEon). The first multicelled organisms were types of algae. Multicelled animals had evolved by 670 million years ago (Late Proterozoic Eon). Remains of these soft bodied multicelled organisms were first discovered in Australia. They have now been discovered several places around the world. These remains of Late Proterozoic soft bodied organisms are referred to as the Ediacaraian Fauna.
Spriggina, a soft-bodied multicelled organism (perhaps related to arthropods) from Precambrian rocks in Australia. Note the segmented, bilaterially symmetrical body plan. From: http://www.ucmp.berkeley.edu/vendian/critters.html Vendian Animals
Go to this site and read about the Ediacaraian Fauna.

Most multicelled organisms reproduce by sexual reproduction (although some can reproduce via asexual reproduction). Sexual reproduction allows for the reshuffling of traits and more variation.

CHEMICAL CODING - FROM GENOTYPE TO PHENOTYPE

DNA - THE BLUEPRINT OF LIFE

The similarity of DNA (deoxyribonucleic acid), blood proteins, and other organic molecules among organisms is strong evidence that organisms share a common ancestor. DNA carries the code that is the blueprint to build each organism.
The DNA molecule consists of chains of nucleotides. Nucleotides consist of a sugar (deoxyribose in DNA and ribose in RNA), joined to a phosphate group on one end of the sugar and a nitrogenous base on the other end of the sugar. The nucleotides are linked together by phosphate bonds to form two strands. Complimentary nitrogenous nucleotide bases are linked between the two strands by hydrogen bonds to form two complementary strands that wind in a helical pattern around each other (see figure below). In DNA the nucleotide bases are adenine, guanine, cytosine, and thymine (thymine is replaced by uracil in RNA). Guanine and adenine are referred to as purines, whereas cytosine and thymine (and also uracil) are pyrimidines (see below). Adenine always links with thymine between the two strands of DNA via two hydrogen bonds, while Guanine always links with cytosine between the two strands via three hydrogen bonds. Each double-stranded molecule is wrapped in protein to form a chromosome. During replication of DNA, the two strands unravel (with the aid of an enzyme) and a new complementary strand forms with each of the original strands, with complementary pairing always between adenine and thymine and between guanine and cytosine. The replication process always moves from the 5 ' (5 prime) end of the single new strand towards the 3' (3 prime) end of the single new strand. By this replication process and the complimentary base pairing of the nucleotide bases, an exact copy of the DNA code can be made.
From: DNA Molecule - Two Views at http://www.accessexcellence.org/AB/GG/dna_molecule.html
The four nucleotide bases in DNA.
From: BIOL 1400 -- Lecture Outline 21 at http://www.accessexcellence.org/AB/GG/dna_molecule.html
The similarity of DNA among organisms is considered by many as the strongest line of evidence in favor of evolution.

Diagram illustrating the sugar-phosphate backbone of the two DNA strands and the hydrogen bonding between the nitrogenous bases (viewed as if uncoiled). The phosphate groups form bonds that hold the sugar bases together; these are called phosphodiester bonds.

DNA Molecule:

RNA (RIBONUCLEIC ACID)

DNA is the template that carries the genetic code. Messenger RNA (mRNA) reads the code. By complimentary bases that can attach to a DNA strand (with uracil substituting for thymine in RNA), the mRNA forms a sequence of nucleotide bases that are complimentary to the DNA strand that is being read: RNA to DNA such that uracil links to andenine, adenine to thymine, cytosine to guanine, and guanine to cytosine
With the assistance of transfer RNA (tRNA) and ribosomal RNA (rRNA), mRNA translates the coded information from the DNA to form proteins (linked amino acids). Each set of three mRNA nucleotides forms a codon that is specific for the synthesis of a particular amino acid. For example, GGC (guanine-guanine-cytosine) would code for the amino acid glycine. The three letter codes are called codons. Translation starts at one end of the mRNA and proceeds, one codon read sequentially after the other, to the other end of the mRNA strand. Thus the DNA transcribes a message to mRNA, which then translates the message to form proteins (chains of linked amino acids).
DNA-----------TRANSCRIPTION----------RNA--------TRANSLATION--------PROTEINS
(see Figures 4.10 and 4.11 in Kardong (2005) for a summary of this process.
Of course, some proteins that are formed by the above process are enzymes that promote chemical pathways for the formation of fats (fatty acids) and carbohydrates (sugars). Proteins, fats, and carbohydrates are the basic building blocks of living cells.

METABOLIC PATHWAYS, CARBON FIXATION, AND PHOTOSYNTHESIS - See your text (Kardong, 2005) and read this material.