Humans Nearly Went Extinct 70,000 Years Ago

Josh Fink brings us a CNN story discussing evidence found by researchers which indicates that humans came close to extinction roughly 70,000 years ago. A similar study by Stanford scientists suggests that droughts reduced the population to as few as 2,000 humans, who were scattered in small, isolated groups. Quoting: "'This study illustrates the extraordinary power of genetics to reveal insights into some of the key events in our species' history,' said Spencer Wells, National Geographic Society explorer in residence. 'Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world. Truly an epic drama, written in our DNA.'"

Comments

The concept of races

[PIPBoy3000]

This event probably ended up establishing the concept of "races", meaning small groups of geographically isolated humans ended up having a lot of genetically distinct features. As their populations grew, they seemed very foreign to each other and only in modern times those barriers to gene flow seem to be falling.

I look forward to the day when people stop saying "I'm X race" and instead say "I carry the genetic markers for A, B, and C." Well, perhaps it's unlikely, but an ex-biologist can dream, can't he?

Re:The concept of races

[jd]

You are correct to be suspicious. The other event I mentioned was much stronger - there was a definite genetic bottleneck, there was a geologically determinable drought, there was a reduction in human activity, and humans were still more-or-less in one region and thus much more likely to be affected by a drought. Numbers can be calculated directly from evidence of remains, but also by looking at what would have existed in the way of food and water, then calculating the maximum supportable population. You can do that with a single cluster.

This newer claim must be treated with caution, because it involves humans that have spread out (less likely to find remains, less likely the humans would have been affected catastrophically) and it's much harder to calculate numbers, because it's much harder to determine what would have been available to whom and what level of trade would have existed when levels of critical resources differed between human-inhabited areas.

DNA is also a dangerous thing to go by. We know there was a mitochondrial Eve, and we know a date but not whether it was the date of the event horizon (the point at which all surviving humans were descendents of Eve, within a timeframe in which differences in mtDNA would not yet be significant in the only regions we have really mapped for such purposes) or the point of singularity itself (when Eve lived). We also don't know why homogenious mtDNA occured - unless it conveyed such dramatic advantage as to be always selected (mtDNA handles energy conversion in cells), there's nothing that makes it obviously preferential, so all mtDNA lines should have survived on a completely random distribution.

Only twelve descendent lines exist in the whole of Europe and Asia. Another eight pretty much covers the rest of the planet. I say "only", but remember at least one actual catastrophic drought and this supposed one happened much later than mtDNA Eve. If a uniform, homogenous strain was preferential, we should not be getting such divergence now. It's not a simple picture.

(Also, dating an event by mutations is dangerous, since mutations can revert, not all markers mutate at the same rate, and all kinds of other factors make such calculations extremely messy. On the DNA mailing list, people often point out that the margins of error on last common ancestor calculations are so broad as to make the calculation worthless.)

It's a Douglas Adams kind of situation: even if we knew for certain, we wouldn't really know what it was we were certain about, or indeed that we were even certain about it.

Re:The way things are going

[clonan]

The problem with global warming is three fold....

#1 it is unequally balanced..the temp changes more at the poles where the ecosystem is more sensitive to temperature. Therefore a small global change will mean dramatic changes in isolated areas.

#2 if you look through history, the average GLOBAL temperature over a one year period has typically hovered around 0 deg C for most of history. I hear that is an important temperature for something..... Anytime the temperature strays from freezing dramatic changes happen to the global environment.

#3 Consistency. So much of our modern society is based an the extremly mild conditions the earth has experienced over the last 20,000 years. Most of Europe is inhabitable ONLY because of the gulf stream and atlantic currents. Agriculture is ONLY possible because the temperature has been consistant year to year. We are in a sweet spot environmentally that is very unusual in earths history. screwing with the temperature is not going to help.

Re:The way things are going

[Reality Master 101]

Good old fashion starvation and disease. For reference, see the current food prices and how these are liked in the developing world. Biofuel mania has something to do with it, but increased consumption by people and animals people eat is the major problem.

Yes, it's entirely possible to get crop failures leading to starvation. But how many deaths? 1M? 10M? Not even a small dent in human population.

The flaw in your thinking is very common -- it assumes a static world that does not adjust. If people are dying by the millions, then things will adjust. Hunger is by far a distribution problem, not a food production problem.

Re:The way things are going

[Reality Master 101]

Volcanism. With global warming, the melting of the polar ice will result in a major redistribution of mass. The planet will want to conserve angular momentum. Something will have to give.

Huh? I suggest going to look up the mass of the earth, compared to the mass of all the water. The mass of ALL the water is proportionally tiny, much less the mass of just the ice. Then try and remember that the world goes through periodic ice ages that redistribute water mass all the time.

Seems a bit shaky to me

[NewsWatcher]

This whole article seems to rest on the premise that humans left Africa en masse about 60,000 years ago. This is likely, but still a hotly contested theory. A rival theory contends that modern humans (Homo sapiens sapiens) originated about the same time from Homo erectus, whose bones have been found in Asia and Africa (the multiregional theory).

It stands to reason that the tests on mitochondrial DNA of a group in Africa is only useful if you assume everyone left Africa sometime after 60,000 years ago.

Given there are numerous sites in Australia that claim to have artefacts stretching back at least that far (and possibly 176,000 years ago) it is very likely there were pockets of humans in other parts of the world much earlier than 60,000 years.

This research actually only shows that there is evidence of a population crash in Africa. Not that homo sapiens across the world had a population crash.

Re:The flood!

[QuantumG]

Floods typically do follow droughts.. so I wouldn't be surprised if an oral tradition was formed around how the global flood (which is a legend in most every culture) was passed on for 70,000 years or so.

70,000 is co-incidental with another event...

[puppetman]

the explosion of the Toba volcano, in Indonesia, that was believed to take humans to the brink of extinction:

Across the world the last eruption of a super volcano was the Toba volcano in Indonesia. This erupted around 75,000 years ago spewing out tremendous quantities of rock and ash and is thought to have reduced global temperatures by up to 21 degrees centigrade.

Re:The way things are going

[Knara]

AFAIK things like ice cores can give us indirect (but very usable) evidence of temperatures for much longer time periods. Of course, with all the ice shelves/glaciers melting, that particular method might not be all that useful for much longer. However, I imagine that other geological methods can also give us indirect, usable evidence of climate over longer periods than, say, just using tree rings or the like.

Re:It bothers me

[MetalPhalanx]

I'm giving up modding to point this out, but perhaps you might want to consider that many systems in nature tend to be a kind of check-and-balance. There are effects in the system which dampen the issue, things which remove carbon from the air and bind it. If we continue to increase the CO2 levels, we will overwhelm those checks and then all hell will break loose.*

The other thing I'd like to mention is that there really are more things to consider than just CO2 levels in terms of global warming. I don't think that human carbon dioxide emissions will be the end of us, but it could trigger the chain of events that leaves our planet much less hospitable to us. Have you heard of the methane hydrates in the cold sea bed?** It's possible that a small shift caused by our increasing carbon dioxide emissions - even if they have to increase by another 30% or maybe more - will push the temperature over a critical threshold and trigger a cascade which will again cause all hell to break loose.

So in a way, you are right. Except in climates which are around a sensitive temperature (e.g. Those areas where the temperature hovers near 0 degrees C) there is very little change right now. That could be that CO2 emissions are having a very minimal effect on the temperature, or more likely IMO, that's just that we haven't quite overwhelmed the checks that are in place. /rant

* (IANA Environmental Scientist, so there may be a margin of error in the direness of my predictions)
** http://dsc.discovery.com/news/2008/02/26/methane-global-warming.html

Re:It bothers me

[mosb1000]

What I'm saying is: if we're already half way there, where are the effects we should be seeing today? Where are the droughts and famines and floods that everyone is talking about? Is there some reason to believe that there's a threshold value, and once we cross it the problems will begin. It seems to me that if the CO2 if trapping heat, we should see the temperature rise with CO2. That would mean that we can expect another 1/2 degree rise at the most in the next 50 years.

Re:So...the Neanderthals could have wiped us out

[HomoErectusDied4U]

Precisely. The journalist who wrote this article does not understand the difference between population census (gross size) & effective population size. 70,000 years ago, the scope of genetic variation of humans - who have living descendants today - was contained in approximately 2,000 individuals. It's a more sophisticated idea, but it's also a far cry from the more sensationalist 'only 2,000 people survived'. To put this idea into a modern perspective, there are over 6,500,000,000 people alive on the planet today, but our species' effective population size is only about 10,000. If human populations 70,000 years ago had the same amount of diversity as we do today, then there were about 1,300,000,000 people alive 70,000 years ago. Obviously this is an absurdly high figure; we know from historic records that there were not more than a billion people alive as recently as 1800. What it does imply, however, is that our species, over the course of the last 70,000 years, has become more genetically homogeneous. This can only be explained by gene flow & natural selection. Recent work by Greg Cochran & John Hawks has shown that adaptive evolution has been accelerating rapidly over the last 40,000 years; our comparatively low Fst value (a measure of population differences) indicates gene flow between regions has also been increasing.

Re:The way things are going

[Hal_Porter]

It is possible. Freeman Dyson wrote a paper on spraying particulates into the atmosphere. So did Edward Teller. Recently people have proposed a plan to stabilise the population in the Arctic

http://www.rollingstone.com/news/story/12343892/can_dr_evil_save_the_world/print

A real-life experiment in the Arctic was, of course, out of the question. But after some discussion, Caldeira and Wood decided to run some computer modeling to see if shooting particles into the stratosphere over the North Pole could help stabilize the region. How much sunlight, they wondered, would you have to reflect to stop the ice from melting? What effect would it have on the rest of the Earth's climate?

Scientists routinely use such computer models to test the effects of various climate-related scenarios, from rising CO2 levels to the impact of deforestation on global warming. After several weeks of running a climate simulation on Stanford's superfast computer network, Caldeira concluded that shading the sunlight directly over the polar ice cap by less than twenty-five percent would maintain the "natural" level of ice in the Arctic, even with a doubling of atmospheric CO2 levels. Push the shading up to fifty percent, and the ice grows. Even better, the restoration happens fast: Within five years, the temperature would drop by almost two degrees. 2 degrees Centigrade is a lot in global warming terms. Wikipedia says "The average global air temperature near the Earth's surface increased 0.74 ± 0.18 degrees C (1.33 ± 0.32 degrees F) during the hundred years ending in 2005".

The modeling results interested Wood. He calculated that it would take roughly 300,000 metric tons of particles each year to shade the sunlight in the Arctic by twenty-five percent -- a tiny amount, on a planetary scale. As for how to get those particles up there, Wood thinks that a half-dozen 747s could do the job. Even better, you could build a Kevlar tube fifteen miles long, with a diameter slightly larger than a garden hose. The bottom of the hose would be connected to a combustor that created the aerosols, while the top would be held in place by high-tech kites or a high-altitude airship that the Defense Department is developing. "It's nothing more than a fancy blimp," Wood says.

In Wood's view, this was a no-brainer. You could stabilize the ice, save the polar bears and demonstrate the virtues of planetary engineering for less money than it takes to feed and clothe the soldiers in Iraq for a year. Because the aerosols are launched only over the Arctic, there is little danger of directly impacting humans. And best of all, you can try it for a few years and see if it works. If something goes wrong, you can quit, and within a year or so, all the particles will have dissipated, returning the region to its "natural" state. I like this quote too.

"Human beings are like cockroaches," Wood says with typical black humor. "It's fairly easy to kill the first ten percent of the population. And if you try really hard, you might even get the next ten percent. But no matter what you do, you'll never get that last ten percent. We will find a way to survive." That's the spirit.

Re:The way things are going

[Guppy]

History constitutes less than 2000 years. Thats the farthest back for which there are any usable records. Chinese records go back a bit further than that. While the oldest writings formally intended to serve as historiography are 2.1k years old, there are about 3k years of actually readable materials recorded by contemporaries of that time.

In addition, there are several thousand years worth of recorded events before that, but by historians living long after (although still ancient by our perspective) they supposedly occured. Many such cases can be considered the "historical" myths of their time, although in other cases historians mention the titles of prior works now unknown to us (thus indicating that these were written, rather than oral, legends).

Have you ever actually talked to a geologist?

[snowwrestler]

I have. I talked to a lot of them when I was getting my geology degree in college. You're right that the climate is constantly changing. You're wrong if you think that implies that humans cannot change the climate.

You're also wrong if you think that recorded human history is the only record of past climate that we can reference. There are numerous natural records of past climate that go back much further into the past. And by the way, the best estimate for an average global surface temp is actually about 14 degrees C, not 0. I have no idea where the grandparent got that number. Maybe they mistook temp anamoly for absolute temp.

Finally, it may surprise you to learn that many researchers of past and current climate do in fact hold geology degrees.