Dr. N.L. Kachhara
Evolution was briefly dealt with earlier in 4.11. The process of evolution has left behind numerous records, which reveal the history of different species. While the best known of these are the fossils, fossils are only a small part of the overall physical record of evolution. The development of genetics has allowed biologists to study the genetic record of evolution as well. Other evidence used to demonstrate evolutionary lineages include the geographical distribution of species. Scientists correlate all of the above evidence drawn from paleontology, anatomy, genetics and geography with other information about the history of the Earth.
Not much is known about the earliest development in life. However, all existing organisms share certain common traits, including cellular structure, and genetic code. Most scientists interpret this to mean all existing organism share a common ancestor, who had already developed the most fundamental cellular process, but there is no scientific consensus on the relationship of the three domains of life (Archaea, Bacteria, and Eukaryota) or the origin of life.
The emergence of oxygenic photosynthesis (around 3 billion years ago) and the subsequent emergence of oxygen rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, believed to have emerged around 2 billion years ago. In the last billion years, simple multi cellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals, the Cambrian explosion (a period of unrivaled and remarkable, but brief, organism diversity) saw the creation of all the major body plans, or phyla, of modern animals. This event is now believed to have been triggered by the development of the Hox genes. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of land ecosystems with which we are familiar.
Many critics of evolution claim that the theory robs life and the universe of any transcendental meaning. This fact is indeed one of the great strengths of the theory, many evolutionists say; it has no need for supernatural intelligence or any intelligent design. Many critics of the modern theory of evolution involve misunderstanding of the theory itself or of science in general. One of the most common misunderstandings of the theory is that one species can be "more highly evolved" than another, that evolution is necessarily progressive, or that its converse is "devolution". Evolution provides no assurance that later generations are more intelligent, complex, or morally worthy than earlier generations. Another misunderstanding is that only "microevolution" and not "macroevolution" has never been observed. "Microevolution" and "macroevolution" both refer fundamentally to the same thing, changes in gene frequencies. The difference between them is primarily one of scale, that is qualitative difference between species is the result of quantitative differences in gene frequencies. Macroevolution is microevolution over a longer period of time.
Stephan J. Gould holds that- "natural selection is not fully sufficient to explain evolutionary change for two major reasons. First, many other causes are powerful; particularly at levels of biological organization both above and below the traditional Darwinian focus on organisms and their struggles for reproductive success. Second, no matter how adequate our general theory of evolutionary change, we also yearn to document and understand the actual pathway of life's history. Theory, of course, is relevant to explaining the pathway, but the actual pathway is strongly underdetermined by our general theory of life's evolution. Life's pathway certainly including many features predictable from laws of nature, but these aspects are too broad and general to provide the "rightness" that we seek for validating evolution's particular results roses, mushrooms, people and so forth. Organisms adapt to, and are constrained by, physical principles. Laws of nature do not tell us why we have crabs and snails at all, why insects rule the multi cellular world and why vertebrates rather than persistent algal mats exist as the most complex forms of life on the earth.
The oldest rocks sufficiently unaltered to retain cellular fossils African and Australian sediments dated to 3.5 billion years old do preserve prokaryotic cells (bacteria and cyanophytes) and stromatolites. Thus, life on the earth evolved quickly and is as old as it could be. This fact alone seems to indicate inevitability, or at least predictability, for life's origin from the original chemical constituents. No one can doubt that more complex creatures arose sequentially after this prokaryotic beginning first eukaryotic cells, perhaps about two billion years ago, then multi cellular animals about 600 million years ago, while a relay of highest complexity among animals passing from invertebrates, to marine vertebrates and, finally to reptiles, mammals and humans. The most salient feature of life has been the stability of its bacterial mode from the beginning of the fossils record until today and, with little doubt, into all future time so long as the earth endures.
Our impression that life evolves towards greater complexity is probably only a bias inspired by parochial focus on ourselves, and consequent over attention to complexifying creatures, while we ignore just as many lineages adapting equally well by becoming simpler in form. Life remained almost exclusively unicellular for the first five sixths of its history from the first recorded fossil at 3.5 billion years to the first well-documented multi cellular animals less than 600 million years ago. This long period of unicellular life does include, to be sure, the vitally important transition from simple prokaryotic cells without organelles to eukaryotic cells with nuclei, mitochondria and other complexities of intracellular architecture. If complexity is such a good thing, and multi cellularity represents its initial phase in our usual view, then life certainly took its time in making this crucial step. More curiously, all major stages in organizing animal life's multi cellular architecture than occurred in a short period, beginning now known from rocks on all continents. Great diversity quickly evolved at the dawn of multi cellular animal life during the Cambrian period 530 million years ago. Although interestingly and portentous events have occurred since, from the flowering of dinosaurs to the origin of human consciousness, we do not exaggerate greatly in stating that the subsequent history of animal life amounts to little more than variations on anatomical themes established during the Cambrian explosion within five million years. Three billion years of unicellularity, followed by five million years of intense creativity and then capped by more than 500 million years of variation on set anatomical themes can scarcely be red as a predictable, in exorable or continuous trend toward progress or increasing complexity".
Sigmund Freud often remarked that great revolutions in the history of science have but one common, and ironic, feature: they knock human arrogance off one pedestal after another of our previous conviction about our own self-importance. In Freud's three examples, Copernicus moved our home from center to periphery, Darwin then relegated us to "descent from an animal world," and finally, Freud himself discovered the unconscious and exploded the myth of a fully rational mind. In this wise and crucial sense, the Darwinian revolution remains woefully incomplete because, even though thinking humanity accepts the fact of evolution, most of us are still unwilling to abandon the comforting views that evolution means progress defined to render the appearance of something like human consciousness either virtually inevitable or at least predictable. We will not smash Freud's pedestal and complete Darwin's revolution until we find, grasp and accept another way of drawing life's history. J.B.S. Haldane proclaimed nature "queerer than we can suppose," but these limits may only be socially imposed conceptual locks rather than inherent restrictions of our neurology. New icons might break the locks.
