Sunday, March 2, 2008

What Do Our Chromosomes Tell Us About Our Distant Relatives?

(Note: I recommend reading the previous posts 1 and 2 first)

In this post I will reveal some striking observations about our chromosomes that may come as a surprise for many, specially the religious. Basically, chromosomes are sub-cellular components of organized DNA structures that hold individual instructions. These instructions are called genes and are necessary for developing and maintaining a healthy body. In every cell in the human body there are between 20,000 to 25,000 genes located in 46 chromosomes (except for the sperm and egg cells which only have 23 chromosomes each). These 46 chromosomes appear in 23 pairs, one of each pair from the mother's egg and the other of each pair from the father's sperm. The great apes (i.e. chimpanzees, gorillas and orangutans), on the other hand, have 48 chromosomes in their cells which occur in 24 pairs.

Each chromosome has a short arm and a long arm which are separated by a specific repetitive DNA sequence called centromere. Chromosomes are protected from destruction, rearranging and fusing with each other by a specific repetitive DNA sequence at the end of each chromosome called telomere. The whole heredity information encoded in the DNA is known as the genome. An actual photograph of the chromosomes from one cell is called a karyotype. The following figure shows a normal male karyotype.

When the human genome was compared with those of great apes some interesting observations emerged. Almost half the human chromosome 5 is a reversed end to end rearrangement of the chimpanzee chromosome 4. This means that almost half of human chromosome 5 has the same linear sequence of genes as those of the chimpanzee chromosome 4, but in a reverse order[*]. There are similar segment inversions on human chromosomes 1, 4, 9, 12, 15, 16, 17, and 18 when compared to their analogous chromosomes of the chimpanzee[*].

However, more striking observations emerged when the human chromosome 2 was closely observed and compared to some chromosomes of the great apes. First, when the chimpanzee chromosomes 2p and 2q were laid end to end it created an almost identical banding structure to the human chromosome 2[*]. This can be shown in the following figure.

Second, the remains of telomere was found in the middle of the human chromosome 2. The DNA sequence of this region is exactly the DNA sequence of two head-to-head fused telomeres [*]. Third, at the place where we would expect centromere on the human chromosome 2 we find the remnants of a centromere pattern of a chimpanzee chromosome (namely chromosome 2q). Fourth, the centromere of human chromosome 2 lines up with the centromere of the chimpanzee chromosome 2q[*].

The scientific explanation for these observations is based on the theory that humans and chimpanzees shared a common ancestor who had 48 chromosomes. In the case of human chromosome 5, an inversion of a segment of chromosome 4 occurred in one of our primate ancestor. This inversion falls into well-studied class of mutations called chromosomal inversion. Incidentally, the inversion did not cause any abnormalities because the rearrangement was balanced with no extra or missing genetic information and, therefore, could have spread through the population of our ancestors. Similar mutations happened to the inverted segments of the chromosomes 1, 4, 9, 12, 15, 16, 17, and 18 during the course of human evolution.

On the other hand, the scientific explanation for human chromosome 2, as explained to me by Professor Kenneth Miller, is more complicated. A fusion between primate chromosomes 2p and 2q occurred in one of our primate ancestors. The cell in which this took place was in the germ line (the reproductive organs) or in the early embryo, so that many tissues were affected. Such a fusion, incidentally, falls into well-studied class of mutations known as Robertsonian Translocation. The individual possessing the fused chromosome would have 47 chromosomes in their reproductive tissues. In a male, this would mean that about half of his sperm would have 24 chromosome and about the other half would have 23 chromosomes. Children born by fertilization with the 23-chromosome sperm with a normal 24-chromosome egg would themselves have carried 47 chromosomes. Since the fused chromosome trait is genetically neutral, it could have spread through small population of humans or human ancestors. Eventually, two individuals with 47 chromosomes each would have met and had offspring. Half their children would carry 47 chromosomes, quarter would carry 48 chromosomes and quarter would carry 46 chromosomes that we have today. It is important to note that the same sort of change in chromosomes has also occurred in horses and (many times) in mice. The last remaining species of Asian wild horses have 66 chromosomes while domestic horses have 64 chromosomes yet they can freely interbreed.

Despite the overwhelming scientific evidence in favor of the common ancestor theory, religious people offer alternative hypotheses for explaining these chromosomal observations. One of their favorite explanation is that the fused chromosome 2 can also indicates a human ancestor who had 48 chromosome rather than a primate common ancestor. They argue that the human-chimpanzee chromosomal relationship is simply a reflection of the vast similarity in the physical characteristics between humans and chimpanzees. Put it simply, a designer (Allah) may have reused designs.

However, this explanation only focuses on the fusion issue and totally ignores other mounting evidence in favor of the common ancestor theory. Also, the assumption that Allah may have reused huge amount of the genetic material of the chimpanzees in the design of human does not fit well with the central theistic assumption that humans are very special creatures of Allah. Moreover, an omnipotent all-powerful creator can not simply reuse old designs for his final special creation and then asks us to reject all the evidence His special creation entails in favor of the common ancestor. Yet, to the contrary, He wants us to accept without evidence the creation claim. This very act can only be described as deception. Finally, by applying Occam razor to the "design re-using" hypothesis, one can simply shaves off the extra designer part thus resulting with the scientific natural explanation.

Other explanations for the above chromosomal observations are less impressive and are mainly based on fallacies. Some religious people say that whatever the evidence says, Allah is the creator of everything; human and apes are totally two distinct creatures. Clearly, this is an example of "appeal to authority" fallacy. Other religious people claim that these chromosomal observations are not correct (an "appeal to ignorance" fallacy) or fake (a "conspiracy theory"). Others fear that the scientific explanation can raise racial conflicts (an "appeal to fear" fallacy) while others say that most of the people, despite of their different religious beliefs, do not accept the theory of evolution (an "appeal to common practice" fallacy). Strangely, some people still argue that the theory of evolution is a theory, not a fact. This is a common misconception that has been thoroughly explained in a previous post.

On the other extreme of the religious explanations, theistic scientists, such as our Kenneth Miller (who is a Roman Catholic) and Francis Collins (who is the director of the National Human Genome Research Institute and also a devout Christian), provide a compromise. They accept the theory of evolution but believe that it was the way God did his creation. They, therefore, accept the that humans and apes have a common ancestor. This form of believe is usually referred to as theistic evolution. I shall not argue against this assumption in this post but would emphasize the fact that by accepting the theory of a common ancestor yet more authority is taken from God (or Allah). I shall return to argue their theistic evolution claim in later posts.

Finally, I would like to share the following clip (of Professor Kenneth Miller's distinguished presentation) with you.