Historical Evolution of Plants | Biosphere | Geography

In this article we will discuss about historical evolution of the plants that can be traced through the geological periods.

(1) Evolution of organisms on the earth’s sur­face begins from the pre-cambrian period when prob­ably the first life in the form of single celled or unicellular microscopic organisms resembling the present-day cocoid or spheroid bacteria, was evolved in aquatic environment. These early organisms were anaerobic heterotrophs (these organisms did not use oxygen because at that time there was no oxygen in the environment).

It may be pointed, out that there were no autotrophs in the beginning due to absence of photo­synthesis. Thus, the initial organisms were heterotophs which depended for their food on non-biologically produced organic food. The early organisms used to live in water and derived their food in the form of dilute nutrients in liquid form.

As regards the origin of non- biologically produced organic food, it is believed that a few organic compounds might have been synthesized from the mixture of some simple substances like meth­ane, ammonia, water vapour etc. by ultraviolet radia­tion (it may be remembered that in the beginning of the evolution of organisms there was no ozone layer be­cause of the absence of atmospheric oxygen and hence most of ultraviolet radiation reached the earth’s sur­face) or electrical discharges.

(2) It is believed that the initial heterotrophic organisms might have given birth to autotrophic plants which could have become able to manufacture their food themselves. The initial autotrophic organisms included purple bacteria, blue-green algae and green algae. It is also important to note that like initial heterotrophic organisms initial autotrophic plants were also originated in water areas.

These aquatic plants are supposed to have formed oxygen through biological processes. The oxygen, thus formed, was gradually diffused in the atmosphere and hence new aerobic organisms (organisms which use oxygen) were evolved. The gradual concentration of oxygen in the atmos­phere resulted into the formation of ozone gas (O₃ = O₂ + O) which controlled the temperature of the lower atmosphere and the earth’s surface by absorbing ultra­violet solar radiation.

This resulted in the evolution and development of several aerobic complex multi­cellular plants and animals. It is believed that about 600 million years ago the atmospheric oxygen was about 3 percent of its present level in the atmosphere. Even this meagre amount of initial oxygen enabled a few multi­cellular organisms like sponges, corals, worms, shell­fish and the ancestors of vertebrates to evolve and develop on the earth’s surface.

(3) It is believed that the origin and colonization of plants on the land surface began in late devonian period when the first terrestrial green plants were evolved from the aquatic red and brown algae.

During this period the landmasses were characterized by dry environment and the first green land plants were such that they could adjust with the dry environment. Rhynia and cooksonia are supposed to be the first plants to evolve on the land surface. The early land plants were characterized by vascular system, xylem, cuticle, stem-like green axes, stomata and terminal sporangia.

(4) During the beginning of Devonian period several new species of flora were developed and the first representative plants of the modern club-mosses and the ancestor of horsetails were exclusively evolved during early Devonian period. The first tree-like plants were evolved during middle and late Devonian period (e.g., large arborescent lycopods, tall horsetails etc.).

Lycopods and horsetails became extinct in their origi­nal form but they could persist in transformed form like herbaceous plants. Ferns were evolved from the spores hidden in the earlier plants. By the end of Devonian period the floras attained a uniform character because of the development of vegetation in uniform climatic conditions and the vegetation communities were trans­formed into forests.

(5) All the landmasses continued to be united in the form of pangaea upto carboniferous period. The disruption of Pangaea and drifting of its broken land- masses had a great influence on the development of world flora. Though the breaking of Pangaea and continental drift have been validated on the basis of plate tectonics (which is based on two convincing evidences of palaeomagnetism and sea-floor spread­ing) but there is no unanimity about the time of disrup­tion. Most of the scientists believe that Pangaea was disrupted during jurassic period while some consider the disruption of Pangaea in the carboniferous period.

It is believed that Pangaea consisted of two big landamasses e.g., Laurasia (including Asia, Europe and North America) and Gondwana land (including S. America, Africa, India, Australia and Antarctica). The northern segment of Pangaea (Laurasia or Angaraland) was characterized by the spread of dense luxuriant forests of tall arborescent plants during Carboniferous period.

The vegetation included several genera of giant club-mosses (Lepidodendrum and Sigillaria), cone- bearing plants having the height of more than 30m, the giant horsetail plants, ancestral forms of modern conif­erous trees etc. which developed within swampy litto­ral habitats. These vegetations were buried under sediments and compressed and ultimately were con­verted into peat and coals. Most of the coal formations of North America and Western Europe belong to the Carboniferous period.

There was a global climatic change during Car­boniferous period when most of the Gondwanaland was covered with ice sheet and thus pre-Carboniferous Ice Age vegetation of Gondwanaland, which was simi­lar to the vegetation of Laurasia or Angaraland, was drastically changed.

The deglaciation of ice sheets resulted into the origin of new sets of climatic condi­tions over Gondwanaland e.g., cold and humid temper­ate climates. During this period the glossopteris flora covered very extensive area of Gondwanaland. The remains of glossopteris are still found over the mem­bers of the Gondwanaland e.g., South America, Africa, India, Australia and Antarctica.

There was substantial change in the vegetation of Laurasia (Angaraland) during the carboniferous and permian periods and consequently three main types of vegetation were developed e.g.:

(i) Eumerican flora in the western part of Laurasia (present north­eastern North America and Western Europe);

(ii) Angara flora in modern Siberia, and

(iii) Cathysian flora in S.E. Angaraland (modern China and S.E. Asia).

(6) There was a large-scale change in the cli­mate, plant and animal life during mesozoic era (which includes jurassic, triassic and cretaceous) mainly due to disruption of Pangaea and large-scale drifting of continents and ocean basins. First, Pangaea was bro­ken into two big landmasses known as Laurasia or Angaraland and Gondwanaland. These were separated by Tethys Sea. These two landmasses were further disrupted into many parts which drifted in different directions to acquire their present position.

Dry climatic conditions prevailed over major areas of the northern landmasses during triassic period with the result most of the vegetation developed during carboniferous and permian periods was replaced by sparse vegetation of desert climate. Many species of dense forests of carboniferous and permian periods disappeared and became extinct but plant-eating dianasaurs could survive because of the availability of their food from cycads, conifers, ferns and horsetails which also could survive because they become suc­cessful in adapting themselves with new environmen­tal conditions.

(7) Jurassic and cretaceous periods provided ideal climatic conditions for the development of a variety of plants and animals. Sub-tropical humid climate developed over major areas became most fa­vourable for evergreen forests which included coni­fers, ginkgos, and cycads during Jurassic period.

(8) The world flora underwent a tremendous transformation because of the emergence of flowering plants (phanerogams) during cretaceous period. The evolution of flowering plants (both angiosperms and gymnosperms) was so rapid and abrupt under episodic events that they soon changed into dense forests.

This sudden spread of dense forests of flowering plants resulted into marked decrease in the number of earlier species and their populations, because flowering plants became fittest and most dominant of all earlier plants. These plants included oak, poplar, sycamore, maple, ash and species of modern tropical trees e.g., breadfruit, fig, palm etc.

These forests were characterized by dense undergrowth of shrubs and small trees which included laurel, dogwood, holly etc., and herbaceous plants. Thus, the vegetation community was well or­ganized by the end of cretaceous period and the strati­fication was well developed e.g., top layer (stratum) of tall flowering plants, second layer of shrubs, third layer of herbaceous plants and fourth or ground layer of mosses and lichens.

The following reasons have been suggested by the palaeobotanists to account for the abrupt evolution of flowering plants and their domi­nance over other (earlier) plants:

(i) The Jurassic period was characterized by relative stability of environmental conditions but the Cretaceous period was punctuated by several events of earth movements (e.g., formation of mountain ranges, volcanic eruption and massive lava flows, faulting etc.), sea level changes resulting into transgression of seas on the continental margins and climatic changes.

All these events resulted into the formation of new habitats having varying environmental conditions. The flowering plants possessed enormous capacity for their adaptation and adjustment to a variety of environmen­tal conditions. The flowering plants possessed enor­mous capacity for their adaptation and adjustment to a variety of environmental conditions.

Consequently, the flowering plants fully exploited the new physical conditions of newly created habitats as they were quickly adjusted and adapted to new hut varied envi­ronmental conditions. This resulted into rapid coloni­zation of the earth’s surface by flowering plants.

(ii) Flowering plants and insects were evolved together (side by side) through the process of mutual­ism. Thus, the insects helped in the cross-pollination among the flowering plants which resulted in the genetic and ecological diversification which ultimately gave birth to the evolution of new species of flowering plants.

(iii) The capability of flowering plants to adapt and adjust with varying physical conditions enabled them to develop a great variety of life forms and habit which allowed a great variety of species of flowering plants to share and exploit the same habitat. This process resulted into the formation of different vertical strata or layers of plants communities.

Though there is a lot of controversy about the exact place of the origin of flowering plants but the western part of Gondwanaland is generally considered as the birth place of flowering plants. From this part of Gondawanland flowering plants were dispersed to other places. Large-scale global orogeny during terti­ary period (origin of alpine mountains like the Rockies, the Andes, the Alps, the Himalayas etc.) and related climatic changes, glaciation of major parts of North America and Eurasia (pleistocene glaciation) during quarter-nary period, frequent changes in climate and sea level because of sub-glacial and interglacial peri­ods etc. have largely affected the life-form and patterns of vegetation at global and local scales.