June 2004 - Issue 6
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The Burning tips of Chromosomes
Some twenty thousand years ago we jumped out of trees to spread across the land into a highly evolved species-Homo sapiens. As palaeontologists shown that the species come and go, the questions like will we persist for many millions of years to come, or are we headed for an evolutionary makeover or even extinction are being widely discussed now a days.
For the over 100 years, scientists, have grappled with the cause of “background” extinction. Mass extinction events, like the wiping out of dinosaurs 65 million years ago, are impressive and dramatic, but account for only around 4 percent of now extinct species. The majority slip away quietly and without any fanfare. Over 99 percent of all species that ever lived on Earth have already gone extinct!
Most species seem to have long stable periods followed by a burst of change: not the slow, steady process predicted by the theory of natural selection proposed by Charles Darwin. In recent years more detailed studies have backed up this idea.
Then what determines and controls such events? Twenty years ago late Stephen Jay Gould suggested that internal genetic mechanisms could regulate these quiet evolutionary periods but until now no one could explain how it would work. According to Mr. Reinhard Stindl, of the Institute of Medical Biology in Vienna, the answer to the above question could lie at the tips of our chromosomes called telomeres. His theory suggests that all eukaryotic species have an evolutionary “clock” that ticks through generations, counting down to an eventual extinction date.
Chromosomes are the genetic elements, which determine the genetic formulation of the organism through genes- the hottest elements in recent trend of science research. Like the plastic tips on the end of shoelaces, all eukaryotic species have telomeres on the end of their chromosomes to prevent instability of the chromosomes. However, cells seem to struggle to copy telomeres properly when they divide, and very gradually the telomeres become shorter. This leads to a tiny loss of telomere length between each generation, mirroring the individual ageing process. Once a telomere becomes critically short, it causes diseases related to chromosomal instability, or limited tissue regeneration, such as cancer and immunodeficiency, finally a population crash. This can explain the disappearance of a seemingly successful species, like Neanderthal man, our older ancestor, with no need for external factors such as climate change.
Telomere erosion is a compelling theory, helping to explain some of the more mysterious patterns in evolution and extinction. There are few data-partly because telomeres are tiny and difficult to measure- but new DNA sequencing techniques could soon change that. Studies have already shown a huge variation in the telomere length between different species.
Current estimates suggest that telomeres shorten only a tiny amount between each generation, taking thousands of generations to erode to a critical level. Many species can remain stable for tens to hundreds of thousands of years, creating long flat periods in evolution, when nothing much seems to happen.
What after extinction?
After a population crash there are likely to be isolated groups remaining and inbreeding within these groups could “reset” the species clock, elongating telomeres and potentially starting a new species. Studies on mice provide strong evidence to support this. Established strains of lab mice, inbred intensively from a small population had exceptionally long telomeres compared to those in wild mice, their ancestors.
How far the telomere theory can be believed? The correlation between extinction risk and the telomere length is widely accepted. The palaeontologists are finding the telomere hypothesis interesting but they strongly demand the results of tests against factors like geographic extent, or population size and variability, that have already been proven effective in predicting extinction risk. The idea that all endangered species have short telomeres is not completely acceptable since there are other extinction mechanisms resulting form human threats to ecosystems. More experiments are needed to signify this idea.
What can be done now?
This would be the greatest question before the present generation of evolutionary biologists. Telomerease – the enzyme needed for the elongation of telomeres during cell division, can be induced in its activity in embryos, so that the newborns possess elongated telomeres. Although inbreeding seems to have been the traditional way of lengthening telomeres, there could be a less drastic alternative.
The greatest opportunity before us, now, is to save the endangered species by elongating their telomeres. The embryonic stem cell research is the best way to target the Telomerease activity and we may even be able to save ourselves when our own telomeres become critically short, to take hold of destiny and prevent our own extinction.
Acknowledgements: Kate Ravillious