Lecture 8:  Early Life: Archean and Proterozoic Eons  

Focus Question:  How did bacteria change the early Earth?

1.        Review from last lecture-Sun, rocky inner planets (Mercury, Venus, Earth, Mars) with heavier elements, and outer planets composed of lighter elements (Jupiter, Saturn, Uranus, Neptune)

2.       Precambrian timescale is divided into two Eons for which we have rocks on Earth: the Archean Eon (dating from about 3.8 billion years ago to about 2.5 billion years ago) and the Proterozoic Eon (dating from about 2.5 billion years ago to about 0.5 billion years ago).  The Hadean Eon is represented only by those Jack Hills zircons, which are 4.2 to 4.4 billion years old, having eroded out of ancient rocks now long gone.

3.       Is there fossil evidence of life in these old rocks?  If so, what kind of life was it?  There is evidence, and it is of two types:  fossilized bacteria and archea (collectively, microbes) and chemical traces of their activity.  From these two lines of evidence, we know the first sign of life in the fossil record is of

a.        Prokaryotes-single celled bacteria and archea. 

b.      Prokaryotes have no nucleus, and carry the DNA in a single simple chain loose in the cell.  The cell wall is complex, and contains folded membranes.

c.       These prokaryotes made their living through fermentation and photosynthesis.  The photosynthetic ones were cyanobacteria (known as blue-green algae).  They are still around today.

4.        The first prokaryotes appear around 3.8 to 3.5 billion years ago, in the early Archean Eon.  The cyanobacteria (blue-green algae) were in this group. 

1.        The energy to fuel these cells and all living organisms today is provided by the molecule ATP.  In prokaryotes, this is produced  in the outer bacterial membrane, and in eukaryotes, ATP is produced in mitochondria, an important group of organelles in eukaryotes. 

2.       Of all the metabolic methods used by early prokaryotes, the one we are especially interested in is photosynthesis.  Remember that in photosynthesis, higher plants, algae, and photosynthetic bacteria take up Carbon dioxide, and in the presence of sunlight and water, can produce glucose and also the byproduct, oxygen.  Photosynthesis can generate a lot of ATP-more than other methods like fermentation.  And it also produces oxygen.

3.       Bacteria (and plants, too) are able to photosynthesize by catching light and using it to drive the photosynthetic reaction.  Pigments in the membranes of the outer wall of the bacterial cell absorb certain wavelengths of light. 

4.       The oxygen produced by photosynthetic cyanobacteria built up over time.  We can trace this in the rock record.  Before about 3.5 billion years ago, so little free oxygen was available that minerals were not oxidized.  Beginning about 3.5 billion years ago, and continuing to about 1.5 billion years ago, the oxygen levels started to rise, fluctuating up and down.  They stabilized about 1.5 billion years ago at about 10% of modern values.  We know this because of an important recorder-the Banded Iron Formations.

5.       Banded Iron Formations (aka “BIFs”) are the great Precambrian iron ore deposits.  They were originally ocean sediments accumulating in ocean basins, and they had a lot of iron in them.  The iron is enclosed in chert (silica), and enormous open pit iron mines are mined for these deposits.  The iron is the basis of the steel industry.   There are big deposits in Minnesota, the Michigan Upper Peninsula,  and Wisconsin.  Ore ships still travel frequently across Lake Erie, bringing the ore from these BIF mines into Ohio, and then on trains to Pittsburgh and other Pennsylvanian steel towns.     

6.       As the cyanobacteria in the ancient Precambrian oceans started photosynthesizing, they added oxygen to the ocean water and the atmosphere.  While it was fluctuating, the iron settling out of the ocean and forming those ancient sediments was oxidized (red color) when oxygen was abundant, and unoxidized (black/grey) when there wasn’t any oxygen around.  Finally, enough oxygen was produced to oxidize surface minerals, and the “stripes” of black/red stopped forming, and the iron was red (oxidized) after about 1.5 billion years ago.   We can summarize all this activity in a diagram (below) showing

The rise of oxygen (O2), the range of time the BIFs were forming, and the time scale in billions of years:

                                                                                                                                Global Glaciation Events

So we can answer the focus question here:  The bacteria strongly affected the early Earth by changing the composition of the atmosphere-oxygen is a byproduct of photosynthesis, and the cyanobacteria in Archean and early Proterozoic time produced that oxygen.  This also changed the surface ocean chemistry, and drove the precipitation of these big metal rich deposits, such as the BIFs and, among others, the Kalahari Manganese deposit, and other major economic metal deposits. 

7.  Eukaryotes did not appear in the fossil record until about 1.2  billion years ago, more than 2 billion years after the first appearance of Prokaryotes .

                      a.        Prokaryotes-single celled organisms, no nucleus, complex cell wall, include the Bacteria and Archaea

                      b.      Eukaryotes-single celled (Protistans) and multicelled (everything else-fungi, plants, animals), nucleus in cell, organelles in cell (e.g. mitochondria and chloroplasts).

8.    The first clues that these two kinds of cells, Prokaryotes and Eukaryotes, were related appeared in the structure of the cells.

                      a.        The mitochondria of Eukaryotes looked like bacteria

                      b.      The  chloroplasts looked like cyanobacteria

9.       This similarity led to the development of a hypothesis that eukaryotes developed from prokaryotes that had engulfed other prokaryotes, did not digest them, and became dependent on them.   This hypothesis, following numerous tests, became known as “Endosymbiont Theory”.  Summary of the evidence:

                      a.        Mitochondria and chloroplasts have their own DNA

                      b.      They store some of that DNA in the cell nucleus, so they can’t survive outside the cell

                      c.       They produce ATP, just like the bacteria

                      d.      The membranes in the organelles look like, and function like, those in bacteria

10.        First Metazoans (multi-celled eukaryotes) were the Ediacaran Fauna (aka Vendian Fauna), which ranged from about 0.67 to 0.54 billion years ago. 

                    a.        Soft bodied-no shells

                    b.      Just imprints of these organisms found

                    c.       Strange symmetries

                   d.      Found in coastal or shallow marine environments

Next Lecture:  The Cambrian Explosion and Early Paleozoic Life