Time of origin of life on this planet, various organisms were evolved and dominated this planet during various periods of geological time chart. This has been found by the evidence obtained from the discovery and study of fossils which allows biologists to place organisms in a time sequence. As geological time passes and new layers of sediments are laid down, the older organisms should be in deeper layer, provided the sequence of the layers has not been disturbed.
In addition it is possible to date/age rocks by comparing the amounts of certain radioactive isotopes they contain. The older sediment layers have less of these specific radioactive isotopes than the younger layers. A comparison of the layers gives an indication of the relative age of the fossils found in the rocks. Therefore, the fossils found in the same layer must have been alive during the same geological period.
You can have an idea about the temporal distribution of various forms of life both plants and
animals in the various geological periods ( fig.1.3)
Phyletic Lineage Of Time
When we look at the biodiversity (the number and variety of species in a place), we ind that there are nearly 2,500,000 species of organisms, currently known to science. More than half of these are insects (53.1%) and another 17.6 % are vascular plants. Animals other than insects are 19.9 % species) and 9.4 % are fungi, algae, protozoa, and various prokaryotes.
This list is far from being complete. Various careful estimates put the total number of species between 5 and 30 millions. Out of these only 2.5 million species have been identified so far.
The life today has come into existence through Phyletic lineages or evolving populations of the organisms living in the remote past. Evolutionary change often produces new species and then increases biodiversity. A phyletic lineage is an unbroken series of species arranged in ancestor to descendant sequence with each later species having evolved from one that immediately proceeded it. If we had a complete record of the history of life on this planet, every lineage would extend back in time to the common origin of all early life. We lack that record because many soft bodied organisms of the past had not left their preserved record as fossils.
Science is a systematized knowledge. Like other sciences, biological sciences also have a set methodology. It is based on experimental inquiry. It always begins with chance observation. Observations are made with ive senses viz, vision, hearing, smell, taste and touch, depending upon their functional ability. Observations can both be qualitative and quantitative. Quantitative observations have accuracy over qualitative as in the former variables are measurable and are recorded in terms of numbers. An observer organizes observations into data form and gives a statement as per experience and background knowledge of the event. This statement is the hypothesis, which is tentative explanation of observations.
At this stage you should look at the ways of devising hypothesis. There are two ways of formulating
hypothesis. A hypothesis can be the result of deductive reasoning or it can be the consequence of
Deductive reasoning moves from the general to the species. It involves drawing species conclusion from some general principle/assumptions. Deductive logic of “if ……. then” is frequently used to frame testable hypothesis. For example, if we accept that all birds have wings (premise #1), and that sparrows are birds (premise # 2), then we conclude that sparrows have wings. If all green plants require sunlight for photosynthesis, then any green plant when placed in dark would not synthesize glucose, the end product of photosynthesis. The other way of reasoning used in the formulation of hypothesis is inductive reasoning which is reasoning from the species to the general. It begins with specific observations, and leads to the formation of general principle. For instance, if we know that sparrows have wings and are birds, and we know that eagle, parrot, hawk, crow are birds, then we induce (draw conclusion) that all birds have wings. The science also, therefore, uses inductive methods to generalize from specific events.
In fact sometimes scientists also use other ways to form a hypothesis, which may include
- Intuition or imagination
- Aesthetic preference
- Religious or philosophical ideas
- Comparison and analogy with other processes
- Discovery of one thing while looking for some other thing.
These ways can also sometimes form basis for scientific hypothesis. Hypotheses as you already know, are subjected to rigorous testing.
Repeated exposure of a hypothesis to possible falsification increases scientist’s confidence in the
hypothesis when it is not falsified. Any hypothesis that is tested again and again without ever being
falsified is considered well supported and is generally accepted. This may be used as the basis for
formulating further hypothesis. So there is soon a series of hypotheses supported by the results of
many tests which is then called a theory. A good theory is predictive and has explanatory power.
One of the most important features of a good theory is that it may suggest new and diferent
hypotheses. A theory of this kind is called productive.
However even in the case of productive theory the testing goes on. In fact many scientists take it as a challenge and exert even greater efforts to disprove the theory. If a theory survives this skeptical approach and continues to be supported by experimental evidence, it becomes a scientific law. A scientific law is a uniform or constant fact of nature, it is virtually an irrefutable theory. Biology is short in-laws because of the elusive nature of life.
Examples of biological laws are Hardy-Weinberg law and Mendel’s laws of inheritance. You will learn about them in later chapters. You can see that laws are even more general than theories and afford answers to even more complex questions, therefore there are relatively a few laws in biology.