If the term glacial era means an era of repeated ice ages with glaciers advancing and retreating at the start and end of each ice age, then major glacial eras occurred as follows.
| Time (mya) | Duration (m years) | Period(s) | Comments |
|---|---|---|---|
| 800-600 | 200 | late Proterozoic | |
| 460-430 | 30 | parts of Ordovician and Silurian | somewhat less extensive |
| 350-250 | 100 | Carboniferous (Pennsylvanian) and Permian | |
| 2-0 | 2+ | Pleistocene | maybe extends back to 4 mya |
Over 20 ice ages have occurred during the glacial era commencing two million years ago. We are still in that glacial era today. The present-day climate represents a warm period between ice ages.
The last ice age is thought to have ended 15,000 years ago, resumed 13,000 years ago, and finally ended 11,500 years ago. It had started about 74,000 years ago, the preceding interglacial having commenced about 130,000 years ago. Prior to that, ice ages had happened about every 100,000 years, but varying between 80,000 and 120,000 years.
According to BBCi – Walking with Cavemen – episode 1 science:
Many experts believe that the key to the creation of the broken mixed environment of trees, shrubs and savanna grasslands in Africa in which afarensis evolved, is the monsoon. Many millions of years ago, India collided with the continent of Asia, buckling the surface of the earth to create the huge mountain range that we call the Himalaya.
This geological event created a monsoon that released vast quantities of rain, drying out the air that flowed through it. This same air flows across East Africa, and caused rainfall to drop sharply, drying the rain-forests and replacing them with broken scrub and woodland. It is no coincidence that the monsoon intensified 6-8 million years ago – the time at which the common ancestor lived.
Another take on this was given in Mountains that Changed the World, BBC Radio 4, 28th May 2003. The basic thesis here is that the elevation of the Himalayas, starting 50 mya, increased the monsoons which, through (1) weathering of calcareous rocks and (2) vegetable matter being carried down by swollen rivers and buried in the seabed, depleted carbon dioxide from the atmosphere, resulting in an inverse greenhouse effect and cooling the earth's climate.
In addition, a 20,000-year climatic cycle has resulted in alternating wet and dry conditions in equatorial Africa. This cycle is caused by a wobble in the earth's orbit around the sun driven by planetary interactions.
The effect of these two phenomena has been numerous near-extinction events among our ape and hominin ancestors in a varied landscape, driving evolution through trial and error towards an adaptable form – ourselves.
According to BBCi – Walking with Cavemen – 150 kya:
Professor David Goldstein, a molecular biologist at UCL in London, has uncovered evidence of a very ancient population bottleneck. A bottleneck is an event that reduces the genetic difference, or diversity, in a population of animals.
One way this can occur is through a catastrophe that wipes out a large proportion of a population. If we compare the genes of modern people from all over the world, they are remarkably similar, suggesting that the ancestors of all living people expanded from a small population that survived a bottleneck. The ancient bottleneck proposed by Professor Goldstein must have occurred in Africa, where modern humans evolved.
"Our data suggests there was a bottleneck that was not that recent," says Goldstein. The genetic data puts the likely date for this event at just before 100,000 years ago.
It's not known what caused this bottleneck. But a plausible candidate is emerging. By measuring the ratios of different oxygen isotopes in ice cores, scientists can reconstruct climatic changes over time. Oxygen isotope data suggests that between 190,000 and 130,000 years ago – a period known as 'oxygen isotope stage 06' – Africa was drained of moisture and became a parched wasteland, with little to sustain populations of modern humans.
"I'm not in a position to say what caused the bottleneck, but it certainly could be a something like that (drought). That scale of climatic change could be responsible for what we see in the genetic data," says Goldstein.
There may also have been other bottlenecks that contributed to the small amount of genetic diversity we see in modern humans. Professor Stanley Ambrose of the University of Illinois at Urbana-Champaign believes that the eruption of the volcano Toba in Sumatra roughly 70,000 years ago was responsible for a volcanic winter that caused an instant ice age.
The large amount of sulphur thrown up into the atmosphere by the eruption reflected sunlight away causing temperatures around the world to plummet. Temperatures in Africa may have fallen by as much as 9°C, creating a freeze that lasted 1,400 years.
"It was a long time, it was unrelentingly cold," says Ambrose. But it didn't just get cold, a temperature change of this magnitude would almost certainly have caused another terrible drought. "Lakes dried up, the earth turned to sand. Every year of drought was geometrically worse than the year before," adds Ambrose.
Copyright © 2004 Alan J. White; all rights reserved. Last updated January 2004.