The Precambrian comprises the earliest and almost seven-eighths of the history of
the Earth (4,500 to 544 million years ago).
The Precambrian is divided into three eras: the Hadean (4,500-3,800 mya) during which the earth formed and cooled; the Archaean (2800 to 2500) during which life originated and the prokaryotes evolved; and the Proterozoic (2500 to 544 mya) during which eukaryotes appeared and multicellular life evolved.
In this lecture we are concerned with the Archaean and prokaryote evolution.
The cells divide by constriction, not mitosis.
These cells lack:
This is not the case because the prokaryotes are biochemically very diverse and have played a central role in shaping the past and present environments of Earth. In fact we can say that the evolution of most of the important biochemical or metabolic pathways used by life on the planet today evolved in prokaryotes during the Precambrian.
Furthermore, it is an interesting dicotomy that prokaryotes are structurally simple but metabolically diverse, whereas eukaryotes (such as ourselves) are structurally diverse but metabolically fairly uniform. In fact, if a eukaryote wants to do something that requires innovative biochemistry (such as break down cellulose to sugar) they often co-op bacteria into a symbiotic relationship to do it for them.
Evolution of the Prokaryotes
The reconstruction of the evolutionary history of prokaryotic organisms is difficult, but not impossible if we use clues from comparative molecular biology and the fossil record.
Twenty years ago, the most primitive prokaryotes were thought to be cyanobacteria
(also called the blue-green algae). Why?
2. Cyanobacteria are photosynthetic autotrophs. That is they can produce every
resource they need by converting light energy from the sun into chemical
energy by photosynthesis. This is a fairly passive process and so was seen as
"simplier" than engulfing food and so was thought to be more primitive.
The simplist way to view metabolism in living organisms is illustrated in the following
However, as life forms multiplied, this free ATP would have been used up. At this point in time, prokaryotes that were able to create ATP through simple chemical pathways appeared and were favored by natural selection (in other words they need to also be able to do anabolism). We have such prokaryotes still living today - they are called the Archebacteria
Individual archaeans range from 0.1 to over 15 µm in diameter, and some form aggregates or filaments up to 200 µm in length. They occur in various shapes, such as spherical, rod-shaped, spiral, lobed, or rectangular.
However, their genetic transcription and translation do not show the typical bacterial features, but are extremely similar to those of eukaryotes.
For instance, archaean translation uses eukaryotic initiation and elongation factors, and their transcription involves TATA-binding proteins and TFIIB as in eukaryotes.
Several other characteristics also set the Archaea apart.
The most striking chemical differences between Archaea and other living things lie in their cell membrane. Their are four fundamental differences between the archaeal membrane and those of all other cells: (1) chirality of glycerol, (2) ether linkage, (3) isoprenoid chains, and (4) branching of side chains. These may sound like complex differences, but a little explanation will make the differences understandable:
Remember: The basic unit from which cell membranes are built is the phospholipid . This is a molecule of glycerol which has a phosphate added to one end, and two side chains attached at the other end. When the cell membrane is put together, the glycerol and phosphate end of the molecules are hydophilic hang out at the surface of the membrane, with the long, hydrophobic side chains sandwiched in the middle:
Like bacteria and eukaryotes, archaea possess glycerol based phospholipids. However, three features of the archaeal lipids are unusual:
Like Bacteria, Archaea have a cell wall. In bacteria, however, the cell wall is made up of a material called peptidoglycan, while it is made of a variety of other materials.
Archaea are usually harmless to other organisms and none are known to cause disease. Many archaeans are extremophiles (that is they live in extreme environments). Some live at very high temperatures, often above 100°C, as found in geysers and black smokers. Others are found in very cold habitats or in highly saline, acidic, or alkaline water. However, other archaeans are mesophiles, and have been found in environments like marshland, sewage, and soil. Many methanogenic archaea are found in the digestive tracts of animals such as ruminants, termites, and humans.
Important to our story of evolution are one of there - the Methanogens
1. Fermenting bacteria
In this early famine, organisms with the ability to produce their own glucose would have been favored by natural selection. Life on earth today depends primarily on photosynthesis as the process of glucose production - a solution invented more than 2 billion years ago.
The earliest photosynthetic organisms lived in an anaerobic atmosphere. They did not fix CO 2 or generate O 2 or even use chlorophyll. They used light to make ATP and absorb nutrients. We have seen this already. Archaeal photosynthesis in Halobacterium halobium uses the purple pigment bacteriorhodopsin in the external membrane and light energy to expel protons from the cell. The proton gradient used for nutrient absorption (by co-transport with protons) and for ATP production. This has a selective advantage.
Why was a purple pigment selectively advantageous to the Archaea? A purple pigment absorbs in the middle of the visible spectrum, where the sun emits light most copiously. It absorbs green light, but reflects red and blue so it looks purple. Without the ability to fix CO 2 Archaea cannot photosynthesise without external reducing agents and organic carbon. So, they use light to make energy, but need reduced carbon (food) on the menu.
2. Photosynthetic Bacteria
Trapping light energy and converting it to chemical energy is the process of photosynthesis. Organisms that do it use chlorophyll. Chlorophyll is a porphyrin molecule.
Chlorophyll probably evolved in the ancestors of the green sulfur bacteria. It occurs in the bacterial cell membrane and transfers electrons photochemically onto NAD, and the NADH so produced is used to run glycolysis backwards. How did this evolve? The respiration in early sulfur bacteria generated NADH. This must be converted back to NAD, but could not be oxidised by oxygen in an anaerobic atmosphere. Instead the electrons were passed via ferredoxin to hydrogenase. Hydrogenase combines the electrons from ferredoxin and with protons inside the cell to make hydrogen gas. This removes protons from within the cell and generates a gradient that can be used to make ATP. If this system could be run backwards , the bacteria could generate sugars by gluconeogenesis, using hydrogen as an electron donor.
All the bacteria needed was a way of generating PGA, and they could run glycolysis backwards, turning a waste-route into a way of making sugar for 'free'. The green sulfur bacteria evolved an enzyme called ATP citrate lyase, which cleaves citrate into acetyl-CoA and oxaloacetate (this is called the reductive Krebs cycle). This allowed these bacteria to run the Krebs cycle backwards and generate PGA. The enzyme was required because the Krebs cycle is essentially irreversible, so some of the 'backwards' steps needed new enzymes, in much the same way that fructose bisphosphatase acts as a work-around in gluconeogenesis for the irreversibility of the phosphofructokinase reaction in glycolysis. [Note that a number of other enzymes were required to bypass these irreversible steps, not just the lyase].
The purple sulfur bacteria chanced upon another way. All cells already contained enzymes to convert GAP to pentose phosphates (the pentose phosphate pathway) because this is needed to make nucleic acid. RuBP became the substrate for a new enzyme (Rubisco) which made it combine with CO 2 to generate PGA. Running glycolysis backwards uses hydrogen gas as a reducing agent, and this destroys the proton gradient. Modern sulfur bacteria solve this by dehydrogenating H 2 S using sulfide dehydrogenase, to produce solid sulfur, protons and electrons outside the cell. This generates the proton gradient for ATP synthesis.
(b) Cyanobacteria (blue-green algae)
A more abundant source of electrons than hydrogen sulfide, is water. But splitting
water to remove the necessary electrons requires abour 10 times more energy that it
does to split them from H2S.
Oxygen (while extremely important to our metabolism) is extremely reactive and can disrupt the delicate balance in a living cell. Even after millions of years with oxygen, living organisms can still be poisoned by high concentrations of O2: just breathing rapidly (hyperventilating) makes people dizzy.
ii. Block UV light - only biotic production of organic molecules can now occur in natural environments
To anaerobic microorganisms, the arrival of oxygen was an unmitigated disaster. Some (probably many) went extinct, some retreated into habitats without oxygen (oblgate anaerobes) - that is the only place we find many archebacteria today.
3. Facultative anaerobic bacteria
These anaerobic bacteria were the first to have evolved the ability to produce special
molecules that bind oxygen and neutralize it. These molecules, primarily porphyrins,
are still used today by microorganisms and their descendants to bind oxygen.
4. Aerobic Bacteria
In respiration, pyruvate is converted into Acetyl CoA. Acetyl CoA enters a series of reactions (known as the krebs cycle) where it is broken down to give 30 molecules of ATP, carbon dioxide, and water.
Summary (in the form of a cladogram):
Woese, C. R. 1994. There must be a prokaryote somewhere: microbiologiy's search for itself. Microbiological Reviews 58: 1-9.