Origin and first 3/4 of the History of Life

Main Points:

  1. Geological Time Scale
  2. Origin of Earth 4.5 billion years ago
  3. History of Life during the Precambrian - Prokaryotes vs. Eukaryotes
  4. Modes of life and metabolic pathways of prokaryotes

I. Geological Time Scale

Over the next few lectures, I will be discussing the history of life and its biogeochemical consequences. To put this in some sort of framework you must have some basic familiarity of the Geological Time Scale.

The Geological Time Scale consists of a hierarchical series of named time periods of various ranks. These have been established by direct study of the stratigraphic record and reflect primarily the relative order of events in time. In addition, there is a linear time scale in years, based on thousands of "absolute" radiometric dates primarily from igneous rocks.

Because this time scale is so important to your understanding of the history of life, we expect you to have a working knowledge of it.

Click here to go to the geological time scale.

II. Origin of the Earth

1. In the beginning there was a singularity.

No time or space; our present physical laws did not apply.

Then about 12-20 billion years ago or so, the Universe rapidly expanded from a tiny dot to a rapidly expanding thing we call the universe in the ...

At first there was just Hydrogen. The Hydrogen condensed into billions of local large balls of superdense Hydrogen in which fusion reactions forming Helium began and stars were born. Other elements up to the atomic weight of Iron were produced in these stars.

About 5 to 6 billion years ago. One of these stars began to run out of Hydrogen fuel. It expanded to a red giant and then collapsed on itself and exploded in a supernova. In this supernova, like billions that have occurred elsewhere in our Universe, all of the other elements were created.

2. The mass of new matter again collapsed into a disk shape mass of dust and gas (a, below left).

The center became superheated and formed a new star, our sun (b). From this disk of matter the planets began to condense (c), according to the widely supported nebular hypothesis of Immanuel Kant and Pierre Simon Laplace. The two strongest points in favor of this idea are: 1) that the disk began by rotating in one direction and the rotation of all of the planets around the sun follows the original disk; and 2) that because the disk flattened out as time progresses, all of the orbits of the planets (except Pluto) lie more or less in the same plane (d). Pluto is possibly a captured giant asteroid.

3. The earth condensed in four basic steps (1-4 at right).

  1. It began to accrete from the nebular cloud as particles smashed into each other forming so-called planetesimals. These in turn collided with each other and as their mass grew began to gather material from the nebular disk.
  2. As the mass of the Earth grew so did it's gravitational force and the Earth began to compress itself into a smaller and denser body. This happened about 4.5 billion years ago.
  3. In the third step the compression itself began to heat the interior of the Earth; also there was heat generated by radioactive decay. The interior of the earth began to melt. Because iron is the heaviest of the common elements that make up the Earth, as the Earth began to melt droplets of melted iron began to sink towards the center of the earth, where they condensed.
  4. Proceeding slowly at first it sped up to catastrophic proportions - hence it is called the iron catastrophe. Note that 3 and 4 in the figures to the right are cross sections.

It was the iron catastrophe that set up the overall structure of the Earth.

All during this time the earth was still being bombarded by asteroids and comets, a process that still occurs but at a much lower (although still significant!) rate.

At some point however, the Moon formed. Exactly how that happened is a major subject of debate. One theory claims that the Moon is a tiny planet captured by the Earth's gravity. The other is that the moon was literally splashed out of the Earth by the impact of a Mars-sized planet. The latter theory is favored now because it explains some odd but important features of the Earth's and Moon's chemical composition.

The Earth was reborn during the iron catastrophe and maybe again by the formation of the Moon. Any trace of surface structure was wiped out by the melting.

4. However, the crust finally solidified by about 3.7 billion years ago.

Gasses pouring out of volcanoes and fissures, along with lava, began to accumulate, perhaps added to by the impact of a few giant comets (which are mostly gas).

The gases that accumulated were those we still find coming out of volcanoes:

These gases combined to form:

This atmosphere would be quickly fatal to us.

5. As the crust cooled water would condense and accumulate as oceans. This happened very soon after the crust solidified.

III. Precambrian Life

1. We have already discussed what life is: Lecture 1

  1. Life is active chemical system separated from its surroundings and out of chemical equilibrium with its surroundings. Chemical equilibrium = death.
  2. Life reproduces with heredity
  3. There is variation in that heredity
  4. There is evolution of populations, adaptation, and innovation
  5. There are species that are reproductively isolated from other species and on their own evolutionary trajectories
  6. Species make societies of kin and parasitic to symbiotic relationships with other species.

2. But how did this come to be? Origin of life clouded in mystery.

  1. Creation Myths - Religion
  2. Scientific hypotheses tend to revolve around three basic questions?
  3. Where did the material basis of life come from?
  4. How did reproduction with heredity come to be?
  5. How did a source of energy come to be used?
  6. How did life become separated from the outside world?

3. Oparin-Haldane concept and Miller-Urey experiment (Interview with Stanley Miller and his view of the origin of life)

Suspect early atmosphere was reducing, that is full of substances that are electron donors.

Atmosphere was probably mostly N and CO2 with some CH4 (methane) and possibly NH4 (ammonia), HCN (hydrogen cyanide), and of course water vapor.

Material basis of life arose through abiotic synthesis of organic compounds and self-assembly in a low or no-oxygen environment - the "primordial soup". Oparin-Fox - microspheres also called coacervate droplets.

More recently it has become clear that many complex organics are present in space, on asteroids and especially comets.

Thus, we are pretty sure the early oceans were a organic "soup" the so called "primordial soup".

What is still pretty much a mystery is the origin of heredity and reproduction. Its possible that the surface of clay or some other mineral acted as a template for organization, but its very hypothetical and key experiments have yet to be performed or even rationalized.

4. Cairns-Smith - clay as a template Organic material might have used clay as substrate, reproduction with errors occur allowing evolution

5. Panspermia ( www.panspermia.org )

Life is floating around in Universe and arrives via collisions. Francis Crick and Sir Fred Hoyle are two famous proponents.

6. However it happened, we are fairly sure it happened early in Earth's history because fossil carbon in 3.5 billion year old rocks and stromatolites from about the same time have the telltale signs of life.

The major events in the biogeochemistry of life occurred during the Precambrian (4.6 - 0.54 billion years ago) 

Kona Dolomite (Michigan) 2.2 billion years old (Proterozoic age) stromatolite fossil (from World Wide Museum)

We are positive that the earliest life forms were prokaryotes, such as bacteria and cyanobacteria.

Evidently, photosynthesis must have started nearly at the beginning. At once that changed the world because of the release of free oxygen.

The photosynthetic recation and its reverse, respiration can be summarized as follows:

CO2 + H2O + e = CH2O + O2

when the reaction goes left to right it is photosynthesis; when it igoes right to left it is respiration.

The simplified phylogenetic relationships of living organisms are as follows:

It is only recently, with the advent of gene sequencing techniques, that the relationships among the prokaryotes have begun to be clarified. The names (and the spelling) of these groups have not yet stabilized. And there are still major disputes among systematists.

An excellent source of information about the phylogenetic relationships and characteristics of the of the prkaryotes can be found at the web site of the Tree of Life Project at the University of Arizona at: http://phylogeny.arizona.edu/tree/life.html

7. Up to 2 billion years ago the O2 produced by photosynthesis was used up by the oxidation of reduced iron Fe2+ to Fe3+.

Oxygen, an oxidizing agent, is an electron acceptor. Iron is soluble in compounds in the Fe2+ state but insoluble in Fe3+ compounds such as Fe2O3.

Cycles in photosynthesis thus produced cycles in O2 in the water column, which produced cycles in the oxidation and then deposition of Fe3+ compounds on the ocean floor. Likewise the draw down of CO2 in the water column would produce an increase in the pH of the water and an increase in the solubility of Si. But when photosynthesis was operating at lower levels, the pH went down and the deposition of Si would take place.bif

Thus we get what are called banded iron formations, which consist of bands of Fe2O3 alternating with Si and Fe2S.

This continues until 2 billion years ago, when we see the first appearance of red beds. These indicate that O2 is building up in the atmosphere.

About the same time, BIFs decline, an indication that reducing compounds are disappearing from the oceans.

At left: Banded Iron Formation (Michigan) (from World Wide Museum). A summary of some of the implications to the history of life of the spread of O2 and of the Proterozoic Era is at http://www.ucmp.berkeley.edu/precambrian/proterostrat.html.

8. At about 1.9 billion years ago we see the first fossils of eukaryotes. These are colonies of obligate prokaryotes with a clear division of labor.

  1. they are much larger than prokaryotes.
  2. they often lack a mucoprotein outer cell wall
  3. they have a complex architecture of internal membranes
  4. they have a nucleus surrounded by a membrane
  5. later forms have mitochondria, and if photosynthetic they have chloroplasts. Many organelles seem to have their origin as free living prokaryotes.
  6. they are much more internally homeostatic than prokaryotes and much more buffered against genetic change with the draw back that they are much more limited metabolically and slower to evolve.

Prokaryote vs eukaryote

(from Margulis and Schwartz, 1982)

At about 1.9 billion years ago we see the first fossils of eukaryotes. These are colonies of obligate prokaryotes with a clear division of labor.

Eukaryotic metabolism can get going at about 2 % present O2 levels, which is consistent with the appearance of common red beds at about the same time. Some photosynthetic eukaryotic organisms develop colonial forms and the first "sea weeds occur". This is first good evidence of eukaryotic multicellularity.

The development of sex allows for species in the same sense we know them known lineages of interbreeding individuals.

There are two main processes involved in the origin of eukaryotic cells from within Archaea among the prokaryotes.

  1. plasma membrane infolding -- endomembranes

    These greatly increase the surface are of functional structures with the cells (e.g. synthesis, processing and export of proteins; mitotic apparatus for replicating large genome, many chromosomes)

  2. endosymbiosis -- organelles like mitochondria

    eukaryotic cells are composites

    like Eubacteria in basic metabolism, cytokinesis
    like archaea in replication, ribosomes, histones

    contain functional units derived from eubacteria

    mitochondria (for aerobic metabolism)
    chloroplasts (for photosynthesis)
    possibly microtubule-based organelles

9. It is fairly certain that by 2.0 billion years ago all the major groups of procaryotes had evolved and with them all of the major metabolic pathways we see today. These are responsible for the cycling of materials within the Biosphere.

Relationships of major groups of bacteria and eukaryotes WITH THE "Six Kingdom System" of classification Superimposed. Note that the Protista is not a monophyletic group.

(modified from Raven and Johnson, 1999)

The number of seemingly different metabolic pathways is staggering and not know in great detail. However only a few basic templates of reactions are present, most based on what you already have seen in photosynthesis and aerobic respiration. However, they are only part of the picture.

The main energy capture and release mechanisms of life consists of Redox systems - Oxidation and Reduction

Oxidation is removal of electrons - releases energy (e.g. fire or rusting)

(e.g. Fe2+ -> Fe3+)

Reduction is the addition of electrons - takes energy

(e.g. Fe3+ -> Fe2+)

* For every oxidation there is a corresponding reduction

The reductant supplies the electrons and is oxidized It is the electron donor
The oxidant receives electron and is reduced.
It is the electron acceptor

Photosynthesis is a Redox reaction

When CO2 + H2O +e- -> CH2O + O2

CO2 is the electron acceptor Respiration is a Redox reaction

When CH2O + O2 -> CO2 + H2O +e-The CH2O is oxidized by the oxygen O2 is the electron acceptorThe following table summarizes the main pathways different groups of prokaryotes use for their energy metabolism. Recall also that the sources of energy for ecosystems comes from autotrophy and that photoautotrophs provide much more energy than chemoautotrophs in this world.

Note: the equations are shown here in their simplest forms and are NOT for the most part balanced. In fact the reactions are orders of magnitude more complex. If the real, even simplified, equations were shown here their similarity would be masked by details, hiding the fundamental structure.

AUTOTROPHY (CO2 fixation)
Photosynthesis
Electron- acceptor (oxidant) + electron donor (reducer = oxidizable substrate = reductant) biomass waste
CO2 + H2O + e CH2O + O2
Chemosynthesis (Chemoautotrophs)
CO2 + H + X CnHn0n + waste + energy general form (not
CO2 + H2 CnHn0n + CH4 + ATP Methanogenesis
CO2 + H2O + CO CnHn0n + CO2 + ATP Carboxydobacteria
CO2 + NH4+- CnHn0n + NO2- + ATP Nitrifying
CO2 + H2O + NO2- CnHn0n + NO3- + ATP Nitrifying
CO2 + H2S CnHn0n + SO42- + ATP Sulfur Oxidizing
CO2 + H2O + Fe2+ CnHn0n + Fe3+ + ATP Iron Bacteria
CO2 + H2 CnHn0n + H2O + ATP Hydrogen Bacteria
HETEROTROPHY (RESPIRATION)
Electron
donor
(food =
oxidizable-
substrate)
+ Electron
acceptor
(Oxidant)
waste + energy
CH20 + O2 CO2 + H2O + ATP Aerobic respiration
CnHn0n + SO42- H2S + CO2 + H2O + ATP Sulfur reduction
CnHn0n + NO3- NO2 + H2O + ATP Nitrite Respiration
CnHn0n + NO2- N2O + H2O + ATP Denitrification
Note: aerobic respiration provides more ATP than others which are anaerobic
Fermentation
Respiration without terminal electron acceptor (poor suppliers of ATP)
e.g. alcoholic fermentation: glucose ethanol
e.g. lactic acid fermentation: glucose lactic acid

Eukaryotes have much more limited array of energy metabolisms, most relying almost entirely on photosynthesis and aerobic respiration (although imbedded with are anaerobic pathways and fermentation).

Individual phylogenetic groups (clades) within the Prokaryota are characterized by different metabolic pathways and in many cases individual species specialize in individual types of molecules for energy or have extraordinarily specific environmental requirements. Some of these, such as halophiles, or thermophiles, can live in environments of extreme hostility - very high salinity or temperature, respectively. These are the so called "extremeophiles" and they greatly expand the range of conditions that life can exist.

Here are some links to extremophile and prokaryote web sites:

http://198.216.100.3/elc/learning_resource_center/monera.html

http://www.gene.com/ae/bioforum/bf03/somero/

http://www.reston.com/astro/extreme.html

http://www.nhm.ac.uk/zoology/extreme.html

It is important to realize that there are two fundamentally different ways that the evolution of major new forms of life such as eukaryotes are viewed:

Extrinsic explanations seek the origin of new forms as do to some shift in the environments making it possible for the new forms to evolve. In other words, eukaryotes did not evolve until the O2 levels had risen to about 2% of the present levels. Once that level was reached they evolved.

Intrinsic explanations seek the origin of new forms in the contingent aspects of evolution itself. In other words all the steps leading up to the new breakthrough have to have evolved before whatever key step needs to be added can be added. It took 1.7 billion years for eukaryotes to appear because the genetic systems allowing eukaryotes systems took that long to evolve because they are so complicated.

Updated January 31, 2006
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