Posted on 04/14/2004 6:15:04 AM PDT by Momaw Nadon
Every species seems to come and go. Some last longer than others, but nothing lasts forever. Humans are a relatively recent phenomenon, jumping out of trees and striding across the land around 200 000 years ago. Will we persist for many millions of years to come, or are we headed for an evolutionary makeover, or even extinction?
According to Reinhard Stindl, of the Institute of Medical Biology in Vienna, the answer to this question could lie at the tips of our chromosomes. In a controversial new theory he suggests that all eukaryotic species (everything except bacteria and algae) have an evolutionary "clock" that ticks through generations, counting down to an eventual extinction date. This clock might help to explain some of the more puzzling aspects of evolution, but it also overturns current thinking and even questions the orthodoxy of Darwin's natural selection.
For over 100 years, scientists have grappled with the cause of "background" extinction. Mass extinction events, like the wiping out of dinosaurs 65m years ago, are impressive and dramatic, but account for only around 4% of now extinct species. The majority slip away quietly and without any fanfare. Over 99% of all the species that ever lived on Earth have already passed on, so what happened to the species that weren't annihilated during mass extinction events?
Charles Darwin proposed that evolution is controlled by "survival of the fittest". Current natural selection models imply that evolution is a slow and steady process, with continuous genetic mutations leading to new species that find a niche to live in, or die. But digging through the layers of rock, palaeontologists have found that evolution seems to go in fits and starts. Most species seem to have long stable periods followed by a burst of change: not the slow, steady process predicted by natural selection. Originally scientists attributed this jagged pattern to the imperfections of the fossil record. But in recent years more detailed studies have backed up the idea that evolution proceeds in fits and starts.
The quiet periods in the fossil record where evolution seems to stagnate are a big problem for natural selection: evolution can't just switch on and off. Over 20 years ago the late Stephen Jay Gould suggested internal genetic mechanisms could regulate these quiet evolutionary periods but until now no-one could explain how it would work.
Stindl argues that the protective caps on the end of chromosomes, called telomeres, provide the answer. Like plastic tips on the end of shoelaces, all eukaryotic species have telomeres on the end of their chromosomes to prevent instability. However, cells seem to struggle to copy telomeres properly when they divide, and very gradually the telomeres become shorter.
Stindl's idea is that there is also a tiny loss of telomere length between each generations, 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. "The shortening of telomeres between generations means that eventually the telomeres become critically short for a particular species, causing outbreaks of disease and finally a population crash," says Stindl. "It could explain the disappearance of a seemingly successful species, like Neanderthal man, with no need for external factors such as climate change."
After a population crash there are likely to be isolated groups remaining. Stindl postulates that 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 have exceptionally long telomeres compared to those in wild mice, their ancestors," says Stindl. "Those strains of lab mice were inbred intensively from a small population."
Current estimates suggest 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.
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 telomere length between different species.
Other scientists are going to take some convincing. David Jablonski, a palaeontologist from the University of Chicago, says: "The telomere hypothesis is interesting, but must be tested against factors like geographic extent, or population size and variability, that have already been proven effective in predicting extinction risk."
Stindl accepts that more experiments need to be done to test his ideas. "We need to compare average telomere lengths between endangered species and current successful species," he says. "I don't expect all endangered species to have short telomeres, since there are clearly other extinction mechanisms resulting from human threats to ecosystems, but I would expect some correlation between extinction risk and telomere length."
If Stindl is correct it will have interesting implications for mankind. Although inbreeding seems to have been the traditional way of lengthening telomeres, there could be a less drastic alternative. Stindl believes that it may be possible to elongate telomeres by increasing the activity of the enzyme telomerase in the embryo. So humans could perhaps boost biodiversity and save endangered species simply by elongating their telomeres. We may even be able to save ourselves when our own telomeres become critically short, making humans the first species to take hold of destiny and prevent their own extinction.
Indicators for human extinction Human telomeres are already relatively short. Are we likely to become extinct soon?
Cancer: Cancer incidence does seem to have increased, but it is hard to say whether this is due to longer lifespans, more pollution, or telomere erosion. The shortest telomere in humans occurs on the short arm of chromosome 17; most human cancers are affected by the loss of a tumour suppressor gene on this chromosome.
Immunodeficiency: Symptoms of an impaired immune system (like those seen in the Aids patients or the elderly) are related to telomere erosion through immune cells being unable to regenerate. Young people starting to suffer more from diseases caused by an impaired immune system might be a result of telomere shortening between generations.
Heart attacks and strokes: Vascular disease could be caused by cells lining blood vessels being unable to replace themselves - a potential symptom of telomere erosion.
Sperm counts: Reduction in male sperm count (the jury is still out on whether this is the case) may indicate severe telomere erosion, but other causes are possible.
Overstating the case for Neanderthals. Neanderthals were bigger, stronger, and probably smarter then we are. But their abundance of stress factors, and such of their dietary predilictions and difficulties as we can recover, suggest that they were fierce opportunistic predators. We are not. We are pack hunters who learned to cache, conserve, routinize and strategize the hunt. The Neanderthals are gone because they were tasty.
But, to give rightwhale justice, atoms imply you. You don't imply atoms. --You could infer atoms, God willin' and the H-Band don't rise.
This isn't the first rap on telemore damage as a control mechanism - it's been suspected in human somatic cell control as well in this study and there have been a number of allegations that free-radical controls in human cells work through minimizing this damage. And as the scientists quoted in the article know full well, they have a bit of a difficulty in their test population inasmuch as there are different telomere signalling pathways between mice and humans.
Interesting stuff. If I could manage to minimize telomere damage to the point where my body would last to age 300 I might yet see the Mariners win a World Series...
Can't be well dressed if your DNA suffers from split ends.
Codswallop. First of all, having the largest possible genetic diversity is an advantage to any species when disaster strikes. The more directions in which a genome spreads, the more likely it is that some obscure corner of it will contain something to help it live through the trial. Today's "maladaption" is tomorrow's key to survival.
Second, it's ridiculous to believe that because wolves and bears and snakes no longer have us on the menu, we are therefore under no genetic selection pressure. It's just that the pressures have changed. In the past century, the Tasmanians were wiped out, segments of the Bantu, Hottentot and Siamese populations were dealt serious blows from the AIDS epidemic (along with any genes responsible for male homosexuality in the West), and white Caucasians decided to stop replacing their numbers through breeding. I'm sure you can come up with many more examples, besides.
But seriously, only when all our eggs are in the same basket do we risk extinction. It will happen eventually unless we can colonize another world.
Unless, a' la Aldous Huxley, you plan to re-design the human program to be more in keeping with that of the bumblebee or the ant, this dog won't hunt.
And if you do think that's a good option--than my sypathies lie with the volutary human extinction program. Depending on genocide to keep the human genome humming along is sort of outside my palatability zone.
Yeah, but you would have to help pay for twenty-three new stadiums in the process.
Short-sighted solution. The sun will only last as a blast furnace about 4 1/2 billion more years, and will become quite unpleasant for us long before that. Besides, the sun could get clobbered by another sun most any millenia now. It's always problems with the furnace when it comes to real estate.
To really kill everything ? Supervolcanoes and asteroids are your best friends.
You cannot prove the proposition empirically. And if I disagree with the proposition, can you tell me with any certainty that I ought to agree with it, and that even if I ought to, how I could change my beliefs on the subject?
Cordially,
Two more propositions here. See my response to RightWhale in #92.
Cordially,
So you think humans, as they are today, will still exist after the great judgement?
I think we'll have new "bodies" and will be essentially a whole new "species".
Let's see, at the rate we're going, maybe next Thursday.
True, but I was referring to potential energy, rather than specific lethality.
I was wondering if there was anything that might theoretically someday be done about it. The magma reservoir is about eight miles long, and under such pressure that it is full of dissolved gasses. When any lessening of pressure takes place, the dissolved gasses will suddenly expand back into their gaseous state, and that's when the whole mess goes up. That will be one hell of a bang- you'll be able to hear it from just about anywhere on the planet.
Once atoms begin to organize they do not obey pure statistical laws anymore. Argument must take the form of discourse and the law of the excluded middle is no longer valid. Something in the nature of atoms must allow the development of consciousness, otherwise how would our material beings have consciousness?
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