* Astronomy

Scientists have identified a major climate crisis that struck Africa about 70,000 years ago and which may have changed the course of human history.


Tanganyika

The evidence comes from sediments drilled up from the beds of Lake Malawi and Tanganyika in East Africa, and from Lake Bosumtwi in Ghana.
It shows equatorial Africa experienced a prolonged period of drought.
It is possible, this was the reason some of the first humans left Africa to populate the globe.
Certainly, those who remained on the continent at that time would have had to be extremely resilient to make it through such hard times.

"This was a profound impact on the landscape. So it must have had a major impact, not just on humans but on all species in equatorial Africa at this time" - Christopher Scholz, Syracuse University, US.

Dr Scholz presented data from the drilling project here at the Fall Meeting of the American Geophysical Union.
The cores reveal that prior to 75,000 years ago, Lake Malawi, which is currently an inland sea some 550km long and 700m deep, was reduced to a couple of pools no more than 10km across and 200m deep.
Lake Bosumtwi, currently a 10km-wide lake that fills an old space impact crater, lost all of its water.

The 1.07 million year old Bosumtwi impact structure is surrounded by a circular depression and an outer ridge of diameter 20 km, and was caused by a 10^8 ton iron meteorite.

Only a prolonged continent-wide drought could have had this effect.

What makes the timing so fascinating is that it ties in with the "Eve hypothesis" of human evolution.
Genetic studies suggest modern humans are descended from a very small group, no bigger than 10,000 individuals who lived in East Africa at the time of this crisis.
Immediately after the drought ended, human populations started to expand rapidly - and many of our ancestors began moving out of Africa and into the Middle East, Asia and Europe.
Scientists are increasingly convinced that tragedies in the deep past have shaped human evolution.
The intriguing thought is that we owe our existence to a small band of survivors who clung on to life during a crisis of epic proportions or who simply decided they had to move to find water.

"We think there may be a connection between this climatic release - that is the rise in lake levels following this major desiccation event - and the order of magnitude increase in early modern humans. And, also, there may be a connection with the exodus of early modern humans out of Africa and this climatic release. There's been recognition that speciation of hominids is controlled by environmental factors - whether that's long-term changes in aridfication in Africa or perhaps the dramatic increase in variability in environmental conditions, such as in precipitation, temperature, and so forth" - Christopher Scholz.

So what caused the climate change?
A likely culprit is the Toba supervolcano in Indonesia that exploded 74,000 years ago.
Volcanic ash can affect the global climate. The fine ash and sulphur dioxide blasted into the stratosphere reflects solar radiation back into space and stops sunlight reaching the planet. This has a cooling effect on the Earth.

By comparing the amount of ash ejected by past volcanoes with their effect on the Earth's temperature, we can estimate the impact of the Toba eruption on the global climate .

"We can see this kind of plot predicts that the Toba eruption was so large that the temperature change after Toba in degrees Celsius would have been about a 5 degree global temperature drop, very significant, very severe global cooling. Five degrees globally would translate into 15 degrees or so of summer cooling in the temperate to high latitudes. The effects on agriculture, on the growth of plants, on life in the oceans would be catastrophic."

Lynn Jorde and Henry Harpending are scientists specialising in human genetics. By looking at mitochondrial DNA they noticed that there should be a wider range of genetic variation.

"Mutations in the mitochondria take place with clock like regularly, so the number of mutations give us a clock essentially that we can use to approximately date the major event. In the case of a population bottleneck we think that this would have occurred roughly 70-80,000 years ago, give or take some number of thousands of years. So then the real question is: what could have caused such a reduction, an extreme reduction, in the human population down to as few as 5 or 10,000 individuals?"

However, it should be noted that temperatures at other times during the last ice age fell far lower and did not come close to wiping out humankind; and the exact age of the bottleneck is poorly constrained so that linking it to the Toba eruption is speculative.