Theories are crucial to science because they provide a coherent framework for making sense out of scientific observations.
An example of such a theory is the theory of evolution. Without the theoretical framework of evolution, biologists would be limited to observing living things and noting the similarities and differences between them.
The following story illustrates the importance of theories in science.
One notable difference between organisms is seen in the chromosome numbers of the members of the family Hominidae, including humans, chimpanzees, gorillas, and orangutans.
Here is a detailed, yet incomplete, history of scientific discoveries concerning human and great ape chromosome numbers:
1958: Researchers definitively confirmed that humans have 46 chromosomes.
1960: Researchers determined that the chimpanzee chromosome number is 48.
1961: Researchers determined that the gorilla chromosome number is 48.
1961: Researchers determined that the orangutan chromosome is 48.
1975: Researchers used various techniques to compare orangutan chromosomes with the chromosomes of humans, chimpanzees, and gorillas. Their analysis confirmed the hypothesis that the four species derived from a common ancestor.
1980: Researchers noted the striking resemblance of the chromosomes of humans and chimpanzees.
1982: Researchers reported that the striking chromosomal similarities extend to orangutans and gorillas as well. They also noted that a specific pattern in the middle of human chromosome 2 bore a resemblance to the ends (telomeres) of two separate chromosomes in the great apes. Using evolutionary theory, the researchers hypothesized that human chromosome 2 resulted from the fusion of two separate chromosomes that would have been present in the common ancestor of humans and the other great apes.
1991: Researchers tested the chromosome fusion hypothesis by sequencing the previously discovered telomere region in the middle of human chromosome 2. What they found were stretches of DNA that are normally found in telomeres. This discovery confirmed the fusion hypothesis.
1992: Researchers detected evidence of a deactivated centromere sequence in human chromosome 2. Centromeres are regions normally found in the middle of chromosomes. The second centromere sequence was located right where it was expected to be found, based on the already identified corresponding chromosome in chimpanzees. This discovery also confirmed the fusion hypothesis.
2002: Researchers mapped the precise fusion point on human chromosome 2 and described its structure in detail.
2005: Researchers mapped the precise location of the deactivated centromere on human chromosome 2 and described its structure in detail.
2005: Researchers published the initial sequence of the chimpanzee genome, showing a break in alignment continuity at precisely the locations predicted by the fusion hypothesis.
1960: Researchers determined that the chimpanzee chromosome number is 48.
1961: Researchers determined that the gorilla chromosome number is 48.
1961: Researchers determined that the orangutan chromosome is 48.
1975: Researchers used various techniques to compare orangutan chromosomes with the chromosomes of humans, chimpanzees, and gorillas. Their analysis confirmed the hypothesis that the four species derived from a common ancestor.
1980: Researchers noted the striking resemblance of the chromosomes of humans and chimpanzees.
1982: Researchers reported that the striking chromosomal similarities extend to orangutans and gorillas as well. They also noted that a specific pattern in the middle of human chromosome 2 bore a resemblance to the ends (telomeres) of two separate chromosomes in the great apes. Using evolutionary theory, the researchers hypothesized that human chromosome 2 resulted from the fusion of two separate chromosomes that would have been present in the common ancestor of humans and the other great apes.
1991: Researchers tested the chromosome fusion hypothesis by sequencing the previously discovered telomere region in the middle of human chromosome 2. What they found were stretches of DNA that are normally found in telomeres. This discovery confirmed the fusion hypothesis.
1992: Researchers detected evidence of a deactivated centromere sequence in human chromosome 2. Centromeres are regions normally found in the middle of chromosomes. The second centromere sequence was located right where it was expected to be found, based on the already identified corresponding chromosome in chimpanzees. This discovery also confirmed the fusion hypothesis.
2002: Researchers mapped the precise fusion point on human chromosome 2 and described its structure in detail.
2005: Researchers mapped the precise location of the deactivated centromere on human chromosome 2 and described its structure in detail.
2005: Researchers published the initial sequence of the chimpanzee genome, showing a break in alignment continuity at precisely the locations predicted by the fusion hypothesis.
As can be seen in the list above, it literally took decades to uncover the explanation for the difference in human and great ape chromosome numbers.
It is important to note that, since they had a theoretical framework to work from, the scientists responsible for these discoveries were not limited to simply noting the similarities and differences in the chromosomes. They could actually discover the causes of those similarities and differences.
It is also important to note that scientists did not just run across this evidence and then later claim that it supported the common ancestry of humans and great apes. Instead, they used evolutionary theory to propose a testable hypothesis, to form predictions based on that hypothesis, to test those predictions by examining the chromosomes, and to ultimately confirm their hypothesis.
Theories: you can't do science without them.
For more on this topic, see the video clip below from Nova's Judgment Day: Intelligent Design on Trial.
posted by Jeremy Mohn
