Origin And Evolution Of Life
Life had a beginning. Since when life appeared on Earth? What is the mechanism involved in the origin of life ?Such questions are very difficult to answer because we cannot go back in time and observe life’s beginning. Since its appearance, life on Earth has changed through time. Hence, the history of life actually comprises two events -first, the origin of life and, second, the mechanism involved in the changes of living organisms through time or evolution of life.
ORIGIN OF LIFE:
The oldest surviving terrestrial rocks, about 4.3 billion years old, contain no definite trace of life. At least not recognizable as yet. Some rocks, about 3.9 billion years old: contain carbonates. Geologists interpret that these carbonates have resulted from life processes. Therefore, life was present on Earth about 3.9 billion years ago. However. the oldest micro fossils discovered so far are that of photosynthetic cyanobacteria that appeared 3.3 to 3.5 billion years ago. Given this background, we can only speculate how life has originated and when.
Theories on the Orlgin of Life:
Many theories have been advanced to explain the origin of life. Most of the ideas on the origin of life fall into one of our categories.
(i) Special Creation: This idea embodies that life on Earth is a special or divine creation of one or more superior, intelligent, and all-powerful being, the God. It also attributes the origin on Life to a supernatural or vitalistic event, at a particular time in the past. It upholds that life is immutable and has not changed ever since its. origin.
(ii) Spontaneous Generation: According to this theory, life originates spontaneously from lifeless matter abeogenesis for example, frogs could arise from moist soil. However, this idea had been put to rest when Louis Pasteur (1862) provided proof that micro-organisms come only from other micro-organisms. Since then, it is being widely accepted that life originates from pre-existing life- Biogenesis.
(iii) Extraterrestrial or Cosmic Origin: This idea holds that life is coeternal with matter without any beginning. This notion advocates that life could have originated once or several times in various parts of the Galaxy in the Universe. The alternative name of this cosmozoan theory is panspermia. which holds that ’spores’ or ’see s (sperm) having extraterrestrial origin might have infected the barren Earth at the time of its origin or afterwards. Likelihood of this notion is remote because the hazards known to exist in the interplanetary space are not conducive to life. These non-conducive conditions are extremely low temperatures. Lack of atmosphere, utter dryness and very high flux of cosmic and ultraviolet radiation from the sun.
(iv) Terrestrial orAbiogenic Origin :This idea holds that life arose by a series of sequential chemical reactions. There is a general consensus of scientific opinions that life originated on Earth from collections of organic molecules that were produced early in the history of Earth.