Molecular Clock

However, it now appears that such premises may have been wrong. For the past few years researchers have been suggesting that paternal mitochondrial DNA may go through recombination since it can survive for a few hours inside the human egg. In fact while studies of rats and other organism have actually shown recombination, no evidence has been found regarding the recombination of human mitochondrial DNA.

But Erika Hagelberg of Cambridge University has found clear evidence showing mitochondrial DNA recombination on the tiny island of Ngura, Vanuatu, in the Pacific.

Hagelberg and her group were studying the history of human migrations through mitochondrial DNA appraisal. Through such study, it became clear that within the area surrounding Nguna were three main population groups. But limited to Nguna, all three groups showed occurrence of a distinctive mutation of mitochondrial DNA that were previously known only from northern Europe.

If mitochondrial DNA is inherited only through the maternal line, there should be no recombination between each groups, which lead us to assume that a very rare type of mutation had occurred separately and accidentally among the three mitochondrial groups in such a tiny isolated island.

But since such an occurrence is highly improbable, it is more reasonable to assume that DNA recombination had occurred.

Furthermore, a group of researchers from Sussex University who had been studying "homeoplasies--common mutations in mitochondrial DNA occurring among different groups", announced that it is probable that genetic recombination had been occurring. Previously, homeoplasies were thought to be the result of mutations occurring coincidently among different groups on sites where mutation are thought to occur at a high frequency. But their research found no evidence of such sites.

If paternal mitochondrial DNA would go through recombination, even rarely, the previous premises of the molecular clock will completely fall into pieces.

Additionally, the constancy of the mutation of mitochondrial DNA itself may be faulty.

Consider why the destruction of the ozone layer has caused the problem of ultraviolet rays shining on the earth surface? It is because ultraviolet rays can cause mutations that may lead to cancer. No one thinks ultraviolet rays directly affect the mitochondrial DNA inside the reproductive cells. But botanical studies revealed that an increase of ultraviolet rays affect plants beyond generations. It is now feared that the destruction of the ozone layer would have a bad effect on plants. Apart from the ultraviolet rays, various radiation is pouring onto the earth. Nobody can declare that such radiation would not affect the mutation of mitochondrial DNA.

Furthermore, through the long history of human beings, solar activity was not always stable. The 11-year ycle is quite well-known, but solar activity is constantly changing in much longer cycles.

Such facts signify that the amount of radiation pouring onto the earth was not always the same. Of course, it also varies according to the areas.

Therefore, it is also probable that DNA mutation may not occur constantly.

If we look at in much longer units, such as in millions or in tens of million years, such periodic changes may level off and therefore such study may function as the molecular clock. But in units below hundred thousand years, the unit in which the origin of modern population can be considered, the changes are too biased to rely on as the molecular clock.

Humans experienced a long period where the land was covered with glaciers. A drop in solar activity is thought to be one of the causes of the ice age. If the radiation falling on the earth dropped severely during the ice age, the rate of mutation that may occur in DNA should also have dropped. If this could be true, modern men may had separated at much earlier stage then it is currently thought.

Before the common ancestor of the human being evolved into modern men, Peking man and Java man, those representing the Homo erectus, had spread from Africa to throughout the world. This is now considered to be a definite fact. The common ancestor of Africa that many DNA studies have ended up reaching could be the Homo erectus.

These ideas suggest that it is far from reasonable to declare, based on DNA studies, that every modern person are the descendants of modern Homo sapiens that appeared in Africa some 150,000 years ago.

Translated by Rie Ishida