1.6What Is Natural Science?
Life science began to develop in the 17th century because of the spread of scientific ideas and the realization that life could be understood within the framework of scientific thought. What then was the original method of scientifically promoting ideas (scientific way of thinking)? Several definitions were offered by numerous people, but finally, science was generally defined by the following characteristics:
(1) The objective of observation is clearly defined.
(2) Discussion is based on data obtained by experiment and observation.
(3) The method is objective and can be repeated by other researchers.
(4) Hypotheses must be tested.
(5) Incorrect hypotheses can be replaced with correct ones.
Science is a rational method of determining truths about nature. First, nature is observed, and then a hypothesis is formed. The hypothesis must then be tested. If the hypothesis is rejected as a result of testing, it must be discarded. Thus, only good hypotheses survive.
Now let us look at items (1) - (5). (1) is obvious. For example, UFOs are not seen by everyone. (2) is also important. There should be discussion about what can be concluded about data obtained from experiments, and correct conclusions cannot be obtained from traditional ideas or the researcher's own assumptions. (3) is obvious, but retesting is particularly important. Retesting often is not performed because the researcher doing the retesting does not receive as much recognition as the original discoverer. However, when a scientist assumes that he alone is correct and neglects retesting (by not including data that can be retested in his research paper), he might have to face problems such as suspicion of fraud. In this sense, research papers must be written such that the experiment can be repeated. Consequently, theories that have been established after being retested many times are the closest to scientific truth.
(4) has been strongly asserted by Karl Popper; we will therefore discuss it below. Popper claimed that verifiability is the most important characteristic of science, and that an evolutionary theory that cannot be verified (and falsified) is not science. He also claimed that absolute truth does not exist. He claimed that what is called a scientific truth is nothing more than a hypothesis that has not been found to be in error and would someday be replaced. Such types of debates exist, and there is also an opinion that what scientists call "faith in evidence" is itself probably just a kind of fundamentalist faith. Regarding life science, however, Richard Dawkins' opinion that we should distinguish hypotheses to be tested from hypotheses that need not be tested may be appropriate (see Column in Section 7 of Chapter 1).
(5) is also important. Science does not progress in a society in which the opinions of one individual are considered to be the last word. It is important for a society to avoid assuming that preconceived ideas are true, but to constantly replace ideas with new ones that have been verified to be true, and to permit this process. Furthermore, life science has been criticized for its probable application to only life on earth instead of explaining the entire universe as physics or chemistry do. It has also been said that life is too complex and diverse to be explained by reductionist theories. However, the DNA of many organisms has been sequenced, and it has become clear that DNA determines how life is formed. It has also become possible to understand human behavior, consciousness, and the like using modern techniques of molecular biology. These methodologies are believed to be sufficiently applicable to alien life if it is ever discovered.
Evolution and Bitter-taste Receptors
One well-known human genetic trait is sensitivity to phenylthiocarbamide (PTC, Column Fig. 1-2). The artificial substance PTC is perceived as bitter by some people but not by others, and this trait is hereditary. PTC has been found to bind to the T2R38 receptor, which is expressed in taste buds, and PTC sensitivity is determined by receptor polymorphisms (see Chapter 3). These polymorphisms are caused by differences in only 3 out of 333 amino acids. It has been discovered that people whose 49th, 262nd, and 296th amino acids from the N-terminus of T2R38 are proline, alanine, and valine are highly sensitive to bitterness (these people are called "PAV-type" based on the first letter of each amino acid). People who have alanine, valine, and isoleucine at these amino acid positions do not taste bitterness (these people are called "AVI-type"). It has also been found that heterozygous people (see Chapter 3) with one each of PAV and AVI, have significantly lower PTC sensitivity than homozygous people with two PAVs (i.e., they taste slight bitterness).
Chimpanzees that do and do not taste PTC have been discovered. Unlike in humans, however, the difference in their sensitivity is not caused by PAV or AVI types. Chimpanzees that do not taste PTC have a mutation in the start codon (see Chapter 3) of the T2R38 gene. The 2nd T of the start codon of this gene has mutated into G, and the 97th amino acid further downstream into methionine. That is, in PTC-insensitive chimpanzees, the T2R38 protein is smaller than normal, and the function of this small protein has deteriorated. An interesting question is why there are both humans and chimpanzees that do and do not taste bitterness. A simple answer is that diversification is necessary for species survival, but what is the real answer? Organisms that taste bitterness might be more sensitive to poisons in the environment and have probably survived because of their ability to identify poisons. Organisms that cannot recognize poisons may benefit from the ability to eat bitter substances that enhance health (the anti-cancer effects of Brassicaceae species such as broccoli). The answer to this question remains unknown.
Column Fig. 1-2 Bitterness and Genotype
(A) The structure of phenylthiocarbamide (PTC). PTC exists in large quantities in Brassicaceae species. Upon licking PTC, some people taste bitterness and others do not.
(B) PTC binds to the T2R38 receptor. People differ in their sensitivity to bitterness depending on the type of receptor they have.