In this article we will discuss about the evolutionary relationships of bacteria.
The early investigators considered bacteria as animalcules and place them with the Protozoa. This viewpoint was held up to the middle of the last (nineteenth) century. Some assigned them a position of the border line between plants and animals.
They place this group of structurally simple organisms, which cannot readily be classified as either plants or animals, in the Protista. Even today the affinities of bacteria are uncertain.
However, it has now been established beyond doubt that bacteria are members of the plant world because in most of their structural features particularly in their mode of nutrition and presence of a rigid cell wall they resemble plants more than animals. The nearest relatives of bacteria in the kingdom Plantae are the fungi and algae. Many workers hold that the bacteria are related to fungi.
In the chitinous nature of cell wall, the absence of chlorophyll and mode of nutrition as parasities or saprophytes, the bacteria show a nearer approach to the fungi. In their unicellular plant-body they resemble the yeasts.
But here the resemblance ends. Bacteria as a rule reproduce by fission. Because of this feature and the above-mentioned resemblances with the fungi, the supporters of this viewpoint referred to bacteria as fission fungi and placed them in the class Schizomycetes coordinate with the other classes of true fungi.
The opponents of this thesis point out that bacteria differ from the fungi in their small size, unicellular nature, lack of a definite nucleus, mitochondria and endoplasmic reticulum in the cell and lastly reproduction, as a rule, by fission. Furthermore, there is no group of fungi with which the bacteria share any relationships.
The relationship of bacteria with other classes of plants is totally obscure. The modem trend, however, is to club them with the Cyanopyceae (blue-green algae).
In the opinion of the supporters of this hypothesis, the two groups are closely related.
This assumption is based on the following common features which the two share between themselves:
1. Both groups include members which are one-celled.
2. Obscure nuclear structure and lack of mitochondria in the cells of both.
3. Cell wall structure and cell division similar in both.
4. Frequent development of gelatinous sheaths in both.
5. Formation of resting cells or spores in both.
6. Presence of similar cell forms, such as cylindrical, spherical and spiral in both.
7. Ability of some members of both the groups to withstand extreme desiccation and endure temperatures approaching boiling point of water.
8. Ability of few blue-green algae to live on dead organic matter like saprophytic bacteria when light is not available for photosynthesis.
9. Ability of certain Cyanophyceae to fix free atmospheric nitrogen like the nitrogen-fixing bacteria.
10. Multiplication in both by fission.
11. Lack of motile asexual spores in both.
12. Entire absence of sexual reproduction in both. Recent experiments, however, indicate that some bacteria are capable of a primitive type of sexual reproduction.
The sharing of above-mentioned common features particularly the striking similarity of the nuclear apparatus in the two groups tempted some students of blue-green algae to place bacteria and blue-green algae in a single phylum “Schizophyta”.
Some even go to the length of placing them in a separate kingdom, the Monera which includes the prokaryotic organisms (lacking distinct nuclei).
It is subdivide into the following two phyla:
1. Phylum Schizophyta which includes the bacteria.
2. Phylum Cyanophyta which comprises the blue-green algae.
The International Code of Botanical Nomenclature, however, does not recognise, as yet, more than two Kingdom systems.
The evolutionary relationships of the two phyla with each other is still not clear. One view is that the bacteria and the blue-greens arose from the common prokaryotic ancestor.
This hypothesis is supported by the possession of so many features in common between them.
This evolutionary scheme can be represented as follows:
The other hypothesis is that the two groups do not share a common ancestry. They have evolved independently of each other. The proponents of this hypothesis point out the great differences between the two groups which preclude any close affinities between them.
These are:
1. Blue-greens are aerobic whereas many bacteria are anaerobic.
2. Many bacteria are flagellate whereas blue-greens are aflagellate.
3. All blue-greens are photosynthetic and use chlorophyll a to capture light energy. Only a few bacteria are photosynthetic but lack chlorophyll.
4. In photosynthesis the blue-greens use hydroxzyl ions derived from water as electric donors and release oxygen as a by-product. In bacterial photosynthesis hydrogen sulphide is the source of hydrogen and the by-product is sulphur and not oxygen.
The supporters of the second hypothesis thus advance the following evolutionary scheme:
There is a third suggestion which advocates that the bacteria have evolved from the blue- green algae by the loss of chlorophyll and adaptation to a saprophytic or parasitic mode of existence. The fossil record, at present, is of little help in deciding the issue.
The fact that some bacteria possess flagella suggests the possibility of their origin from simple flagellated progenitors which also gave rise to the green algae. There is also a possibility.
It different groups of bacteria have evolved independently from different ancestors. However, our knowledge of the evolutionary relationships of bacteria is unsatisfactory.
The modem bacteriologists, however, agree that the simple flagellar structure of bacteria which differs from 9+2 pattern flagellar structure of all other organisms alone is a sufficient feature to indicate isolated position of bacteria in the plant world.
In this text they have been assigned to the division Schizomycophyta or Schizomycota coordinate with the true fungi.