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Evolution of Micro-organisms and Plants


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Pre-Cambrian Era, 4570 - 541 MYA
Cambrian Period, 541 - 485.4 MYA
Ordovician Period, 485.4 - 443.4 MYA
Silurian Period, 443.4 - 419.2 MYA
Devonian Period, 419.2 - 358.9 MYA
Carboniferous Period, 358.9 - 298.9 MYA
Permian Period, 298.9 - 252.2 MYA
Triassic Period, 252.2 - 201.3 MYA
Jurassic Period, 201.3 - 145.0 MYA
Cretaceous Period, 145.0 - 66.0 MYA
Tertiary Period, 66.0 - 2.588 MYA
Quaternary Period, 2.588 MYA - present

The Geologic time scale is adopted from the 30 December 2011 version by the International Union of Geological Sciences (IUGS). MYA = million years ago, FA = first appearance.


Pre-Cambrian Era, 4570 - 541 MYA

Pre-Cambrian Family Tree At the time of the appearance of the first organisms about 3.8 billion years ago (see Figure 11-04a), there was no free oxygen, as there is now, but rather a "reducing atmosphere" composed of methane, carbon dioxide, ammonia, and hydrogen (see Figure 11-02a). The microorganisms of this period utilized methane or hydrogen rather than oxygen in their metabolism - they are therefore referred to as "anaerobic" (non-oxygen-using). Fermentation is modern example of anaerobic metabolism. This type of metabolism is 30 to 50 times less effective than oxygen-based ("aerobic") metabolism, or respiration.

Figure 01 Pre-Cambrian Era
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Figure 02 Plants Family Tree [view large image]


Mitochondria Chloroplasts Up to 700 MYA, life remained fairly primitive, the distinctions between plants and animals were not very clear-cut. For example, the bacteria (monera) show a variety of forms. Some take in chemicals and depend on atmospheric CO2, like plants (autotrophs - from Greek autos = self, trophe = nourishment). Others take in organic material for food, like animals (heterotrophs - from Greek heteros = other). Some autotrophic bacteria, called chemosynthetic, do not need light;

Figure 03 Mitochondria


Figure 04 Chloroplast [view large image]



they obtain their energy from mineral chemical reactions, such as the conversion of sulfur to sulfate or the production of CH4 from CO2 and H2.

Ecocycle Among the multitude of micro-organisms, two bacteria destined to become the major components for the further development of the living world. The mitochondria (see Figure 03) are the sites of the principal oxidation reaction linked to the assembly of ATP, which supplies energy to most of the organisms today. Their closest relative among present-day bacteria have been identified to be the alpha-proteobacteria. The chloroplasts (see Figure 04) are the agents of photosynthesis in unicellular algae and plants. They store the solar energy in the form of glucose (sugar), which becomes food for the "heterotrophs". Their present-day relative are the cyanobacteria (formerly known as blue-green algae), which is believed to be responsible for the first generation of atmospheric oxygen. As shown in Figure 05 the chloroplasts supply food and oxygen for the heterotropic organims, which in turn produced CO2 for photosynthesis. This ecocycle generates a lot of atmospheric oxygen as shown in Figure 11-02 today. The evolutionary steps for the micro-organisms are shown in Figure 01, which shows that the first organisms were anaerobic (anoxygenic) autotrophy.

Figure 05 Ecocycle

Blue-green Algae Another hypothesis proposes that the first organisms were "heterotrophs"; they derived their food from other organisms or organic matter which they were able to consume. Very soon all the available organic matter was exhausted, and life would have cannibalized itself to extinction, were it not for the appearance of a new type of organisms, capable of manufacturing their own food. They are the very early green plants, which were actually an extremely primitive form of algae, similar to the modern blue-green algae, which assume many forms from one-celled to colonial or filamentous (loose association of cells, Figure 06a). But the generation of oxygen in the atmosphere started a crisis for the anaerobic organisms, which became either extinct or have to adopt to the new environment. Since then most of the organisms acquire their energy from aerobic metabolism. The organisms evolved into animals captured mitochondria for this purpose. While the plant cells incorporated both mitochondria and chloroplasts to survive. They still retain copy of their own DNA within. The DNAs are in a circular form reflecting their more primitive origin (from the bacteria).

Figure 06a Blue-green Alage [view large image]


Sulfur Spring Photosynthesis Fossil evidences have been accumulated over the years that anoxygenic photosynthesis were used by bacteria 3.4 billion years ago in a sulfuric environment such as the hot spring shown in Figure 06b. It is also known that oxygen-producing form of photosynthesis emerged

Figure 06b Sulfur Spring [view large image]


Figure 06c Photosynthesis Evolution [view large image]


about one billion years later. The question is : why it takes so long ?
    The following evolutionary sequence is a probable scenario to explain the train of events that leads to the present day photosynthesis.

  1. Green Sulfur Bacteria - Hydrogen sulfide H2S is a close relative of water H2O. They belong to the same group in the periodic table except that the electron configuration of H2S is further away from the nucleus. Such structure makes H2S easier to break into S + 2H+ + 2e+. The conventional view is for the photosynthesising bacteria first exploited the easy-to-oxidise molecules as shown in Figure 06c (1) in the type-II reaction similar to the photosystem II in Figure 04 with H2O replaced by H2S (see examples of oxidation in Redox Gradients).

  2. Proto-Cyanobacteria (H2S) - Early in their history the green sulfur bacteria experienced some kind of genetic glitch which duplicated the entire set of genes for the type-II reaction. This spare copy was free to modify the codes (without harming its host). Eventually, it evolved an ability to recycle electrons but can only make the ATP molecule to store energy, it could also produce oxygen (Figure 06c (2)). This is called type-I reaction similar to the photosystem I in Figure 04 with H2O replaced by H2S. Thus, this kind of proto-cyanobacteria contained both the type-II and type-I reaction centers. However, they were not linked, only one type could be turned on at a time. The type-I reaction would be switched on when the H2S concentration ran low. This theory will be vindicated if such "missing link" bacteria with no coupling between the two types have ever been found.

  3. Proto-Cyanobacteria (Mn) - It happened on occasions that the proto-cyanobacteria got swept into shallow ponds with no H2S but rich in manganese (Mn). They could duly switch on the type-I reaction to get the electron supply from Mn. However, it didn't help because the manganese produced too many electrons choking up the organisms (Figure 06c (3)).

  4. Cyanobacteria - Until one day the type-I and type-II centers were turned on and coupled, then the excessive electrons could go over to the type-II reaction to relieve the blockage. The modern version of photosynthesis were in place with the manganese getting the electrons from H2O (Figure 06c (4)). Since this kind of modification involving two coincidences (the use of Mn and the link between the two types) is highly improbable, therefore it took a long time (about 1 billion years) to institute.
There is a contentious scenario in which the type-II center evolved first. It was acquired by another group and gradually being modified to the type-I center including the manganese cluster which generates oxygen. These two types merged later via gene transfer. According to this theory, the "missing link" is the bacteria with oxygen production capacity type-I center only (nicknamed indigo bacteria). The debate between these two hypotheses can be settled by the discovery of living representatives of either the proto-cyanobacteria or the indigo bacteria. Whatever the true sequence of photosynthesis evolution, the outcome is hugely important to the world we are inherited.

Stromatolites Not all of the single-celled organisms of this time were solitary. Beginning around 2.4 billion years ago blue-green algae (cyanobacteria) would often grow into large mats, called Stromatolites (see Figure 07). Modern-day Stromatolites can still be found in a sheltered bay (Shark Bay) in West Australia, where the water is so salty that creatures that would otherwise eat them are not able to exist.

Figure 07 Stromatolites [view large image]


Green Algae Plant Cell Then evolution took another step toward complexity about 1.5 billion years ago. The bacteria acquired a nucleus and advanced to eukaryotic cells (protista). According to one theory, the ancestral eukaryote is envisioned to be the fruit of an early fusion between an anaerobic, thermophilic, and wall-less bacteria, and a motile eubacterium (bacteria with a rigid cell wall). The product of this union inherited from its progenitors the rudiments of what later became the hallmarks of true eukaryotic cells: the genetic core, histones and actin precursors from the archaeon (thermophile), metabolism and propulsion from the eubacterium.

Figure 08a Green Algae
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Figure 08b Plant Cell
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Subsequent fusion with mitochondria generated cells ancestral to those of animals and fungi. Acquisition by cyanobacteria in yet another round of fusion
launched the photosynthetic protists and plants. It was against this background the green algae emerged as the ancestral to the first plants (see Figure 02). There are single-cells, colonial, filamentous, and multicellular green algae. Algae are grouped according to their pigmentation (green, golden brown, brown, and red) and biochemical differences, such as the chemistry of the cell wall and the chemical compound used to store excess food. Figure 08a shows the structure of a motile green alga - the Chlamydomonas. Figure 08b depicts the structure of a modern plant cell. Comparison shows that all the major organelles in the plant cell are already in place in the green algae. The green algae also have some of the same characteristics as plants. All types of plant life cycles (see Figure 10-06) are seen in this diverse group. For example, the multicellular Ulva (sea lettuce) shows the alternation of generations life cycle in the same way as terrestrial plants. Since the diplontic life cycle involves the fusion of two gametes, the green algae must have learnt about sex already. Actually sexual reproduction took a long time to evolve, it started about 1.2 billion years ago and there have been many intermediate stages in its working out. Protists and some plants to this day maintain the alternative binary fission and sexual reproduction. Incidentally, sex is not equal to reproduction - there can be sex without reproduction such as in abortion or there can be reproduction without sex such as cloning. The two processes are connected only in sexual reproduction.

Symbiotic relationship between organisms is much alive today. A modern example of bacteria incorporating into vertebrate cells is provided by the 2010 study of spotted salamander embryo containing algae. The bright green color from the embryos themselves, as well as from the enclosing capsule betrays the presence of the algae as shown in Figure 09. It seems that the nitrogen-rich waste produced by the embryo is useful to the algae, and the algae in turn supply the oxygen for the respiring embryos. Such a close co-existence with a photosynthetic organism has previously been found in invertebrates, but never in a vertebrate. Because vertebrate cells have the adaptive immune system, which destroys
Salamander Embryos biological material not considered "self". But in this case, the salamander cells have either turned off their internal immune system, or the algae have somehow bypassed it. Experiment reveals that the algae gain entry into the embryo when its nervous systems begin to form. The presence of algae in the oviducts of adult female spotted salamanders raises the possibility that symbiotic algae are passed from mother to the offspring's jelly sacs during reproduction. Another intriguing possibility is for the genome of the alga incorporated into the salamander germ cells.

Figure 09 Salamander Embryos Co-existing with Algae [view large image]

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