How likely is human extinction?

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.

Some interesting information I found on Toba, doing some quick research after reading the post in this thread on Toba and which was the first I recall of ever hearing about this ancient event:

The eruption of 2,800 cubic km of magma at Toba caldera 75,000 years ago was the largest eruption in the last 2 million years. The eruption may have release as much as 10E12 kg of sulfuric acid, an order of magnitude more than Laki in 1783 and Tambora in 1815, two of the greatest Holocene eruptions. The Toba eruption may have caused about 3 to 4 degree C cooling at the surface but this impact is hard to detect because of concurrent glacial conditions (Sigurdsson, 1990).

Formed by a stupendous prehistoric volcanic explosion, the 100 km long lake is the largest is Southeast Asia and one of the deepest and highest in the world. The drama of that cataclysmic birth persist in 500 meter cliffs dropping into the blue-green waters, Surrounded by steep, pine covered sloped, the climate is fresh and pleasant, with just enough rain to support the lush vegetation.

Smoke Over Lake Toba, Indonesia(NASA Visible Earth)

The super-eruption of Toba, did it cause a human bottleneck? FJ Gathorne-Hardy1, WEH Harcourt-Smith2, 1 Archaeology department, West Street, Sheffield University S1 4ET, 2 Palaeontology department, AMNH, Central Park West 79th Street, NYC, NY10024, U.S.A: It has been claimed that the super-eruption of Toba, Indonesia, about 73.5 Ka, caused a global “volcanic winter” which was responsible for a bottleneck in human populations. We show that the eruption’s radius of direct impact was probably less than 350 km and that the global cooling associated with Toba was not unusual. We argue that at 73.5 Ka humans were adaptable and wide-ranging omnivores, so unlikely to suffer greatly from global cooling. No plant or animal extinction is attributed to the effects of the eruption. Genetic evidence appears not to support the hypothesis of a Toba-induced population bottleneck. We conclude that the Toba super-eruption is unlikely to have caused a bottleneck in human populations.

COMMENT: Above argues against effect of Toba on humans. However ...

URI oceanographers investigate link between last Ice Age and Indonesian volcanic eruptions
January 27, 2004
Approximately 75,000 years ago, a massive explosive eruption from a volcano in western Indonesia (Toba caldera) coincided with the onset of the Earth's last Ice Age.
In the current issue of Geology, University of Rhode Island geological oceanographers Meng-Yang Lee and Steven Carey; Chang-Hwa Chen and Yoshiyuki Iizuka of the Academia Sinica in Taiwan; and Kuo-Yen Wei of National Taiwan University describe their investigation into the possibility that eruptions from the Toba caldera on the island of Sumatra caused a severe "volcanic winter" and the initiation of a glacial period.
The magnitude of the oldest Toba eruptions had not previously been documented due to the difficulty in recognizing their widespread erupted products in marine sediments. Lee and the team of scientists present new data on the distribution of volcanic ash from the oldest Toba eruption in Ocean Drilling Program cores and piston cores in the South China Sea and Indian Ocean. By using high-resolution litho-, magneto-, and oxygen isotope stratigraphic records, the geologists were able to clarify the correlation between distribution patterns in the cores, refine the age of the layers, and reestimate the eruptive volume of the early eruption of Toba.
The results of their analysis indicate that the glass shards from the first Toba eruption 788,000 years ago were dispersed more than 2000 miles from the source. Fallout from the eruption was deposited from clouds that drifted over both the Indian Ocean and the South China Sea, producing an extensive ash blanket that may have been comparable in size to that of the last Toba eruption 75,000 years ago.
Comparing material from the oldest Toba eruption to that of the youngest, or last, Toba eruption, the scientists were able to draw surprising conclusions. The youngest Toba eruption (75,000 years ago) has been proposed as a triggering mechanism for the onset of large-scale glaciation, which brought the last interglacial stage to its end. The coincidence of the oldest Toba eruption with the transition from a glacial stage to an interglacial stage, however, appears to be an opposite effect.
Although the estimated volume of the oldest Toba eruption is not as large as the youngest Toba, they are both enormous eruptions involving discharges of tremendous amounts of magma. However, the warming trend following the oldest supereruption of the Toba appears to suggest that factors other than volcanism have played more influencing roles in governing glacial to interglacial transitions over the last 3 million years.
University of Rhode Island

Genomics, evolution, history, and geography
By polarbearcub , Section Biology
Posted on Mon Apr 22nd, 2002 at 04:04:46 PM PST
Dinosaurs were terminated during KT boundary, which is long long long time ago (65 million years ago). With very limited fossil record and the abrupt extinction of dinosaur, it is difficult and meaningless to study the "evolution" of dinosaur. More interesting is what trigger this event, which is a big unknown, some said asteroid, some said volcano, some said disease.
Human evolution is different, it is recent, there is fossil evidence and genetic evidence and we are not extinct yet =). Genetic evidence (mtDNA, HLA, Y chromosome, Alu sequence) provides us lots of insight about evolution. For example, just by estimate coalescence time and diversity about genetic systems, genetic evidence can tell us about time of human origin, which was around 2 million years ago, and location, which was Africa.
One interesting fact is that human evolution went through a bottleneck event around 75 thousand years ago. And there is evidence that during this bottleneck event, human population went from 100,000 to 10,000. With this short population expansion from a small population after such a recent bottleneck, no wonder we are so similar genetically. So what was going on during this bottleneck? It turns out there was a giant eruption from volcano Toba, in Indonesia. This event was 10,000 more powerful than St. Helen and decreased global temperature by as much as 15 degree Celsius!

COMMENTS: Haven’t found any gene link yet. Found arguments back and forth on the real impact of Toba on human evolution or decrease in population.

Opps! Left of some information. Also, I don't think the science folks at this site wanted me to post this, but :

massive volcanic eruption changed the course of human history by severely reducing the human populations (called a 'bottleneck In population genetics and evolutionary biology, a population bottleneck (or genetic bottleneck) is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing, and the population is reduced by 50% or more, often by several orders of magnitude. A graph of this change resembles the neck of a bottle, from wide to narrow; hence the name. ..... Click the link for more information. '): Around 75,000 years ago the Toba Lake Toba is a large lake, 100km long and 30km wide, in the middle of the northern part of the Indonesian island of Sumatra. Geology In 1949 the Dutch geologist Rein van Bemmelen reported that Lake Toba was surrounded by a layer of ignimbrite rocks, and was a large volcanic caldera. Later researchers found rhyolite ash similar to that in the ignimbrite around Toba in Malaysia and India, 3000km away. Oceanographers discovered Toba ash on the floor of the eastern Indian Ocean and the Bay of Bengal. ..... Click the link for more information. supervolcano The term supervolcano has no specifically defined scientific meaning. It was coined by the producers of a BBC Popular Science programme in 2000 to refer to volcanoes that have generated Earth's largest volcanic eruptions. As such, a supervolcano would be one that has produced an exceedingly large, catastrophic explosive eruption and a giant caldera. Because there is no well-defined minimum ..... Click the link for more information. in Indonesia The Republic of Indonesia is a large archipelago located between the South East Asian peninsula and Australia, between the Indian and Pacific Oceans. Indonesia borders Malaysia on the island of Borneo, Papua New Guinea on the island of New Guinea and East Timor on the island of Timor. Republik Indonesia (In Detail) (Full size) National motto: Bhinneka Tunggal Ika (Old Javanese: Unity in Diversity) ..... Click the link for more information. erupted with a force three thousand times more powerful than Mount St. Helens Mount St. Helens (MSH) is a volcano in Skamania County, Washington state, in the Pacific Northwest region of the United States. It forms part of the Cascade Range. MSH erupted catastrophically at 08:32 on Sunday, May 18, 1980. Before the eruption, the summit of Mount St. Helens was 9,677 feet (2,950 meters). The eruption reduced its peak to 8,364 feet (2,550 m) in elevation and replaced it with a one-mile-wide (1.5 km) horseshoe-shaped crater. ..... Click the link for more information. .

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