The climates of the past 5 million years:
5.3 to 1.8 mio years ago: climate cooler and dryer but otherwise fairly similar to today
1.8 mio to 10,000 years ago; unstable climate with many glacial cycles (the “ice age”)
Started ca. 10,,000 years ago (some begin the period at 12,000 or 11,000 years ago): increasingly warm and wet.
The Holocene is “our” era even if 10,000 years is much too short a time to be sure that our pleasant Holocene climate together with global warming concerns will last. The Holocene may well turn out to be a brief climatic blip in an ongoing ice age – as so many warm intervals in the depth of the ice age have been (see chart below). So do not throw away your warm socks yet.
Since the industrial revolution less than 300 years ago the number of Homo sapiens on earth has broken all records – and seems set to increase further. But as the saying goes “what goes up must come down” or even “the higher up they go, the lower they fall”. Looking at the graph, these are not comfortable conventional wisdoms.
The colder the climate, the more water is locked up in ice formations (mostly at the poles) and consequently the more the average sea level falls, i.e. the more coastal shelve areas, land bridges and other shallow waters turn to dry land. It is noticable that with one brief exception around 125,000 years ago, sea levels have never been higher than they are now – and they are still rising.
For the possible effect of the Toba volcanic eruption around 73,000 years ago click the link.
Climate exercised a tremendous influence on our ancestors – and it is still a major influence on us today. The geological era we live in now is the Holocene, a period that has lasted for only 10,000 years since the end of the Pleistocene ice age. Our pleasant Holocene climate (along with warm-up fears) might well turn out to be a brief climatic blip in an ongoing ice age.
The ice shield in the northern hemisphere during the last glacial period (see chart above) reached its maximum extent just before the end of that glacial. At that stage around 10,000 years ago it covered the area shown in the map below. At that stage, temperatures had already started to rise and the ice has retreated, more or less steadily, ever since.
Skin colour is a function of latitude: the stronger the solar radiation, the darker the skin as this chart of measured skin colours clearly shows.
Skin colour is a genetic trait that protects against strong solar radiation. When the pre-humans lost their furry coats in Africa hundreds of thousands of years ago their skin must have been black already or they became black-skinned while they shed their fur. Skin colour is a genetic trait (not to be confused with the individual colouring by the sun’s rays!) that can change rather rapidly. “Rapidly” here means in as little as 1,000 generations or ca. 18,000 years if we allow 18 years per generation from birth to first parenthood. Since the start of the Great Migration out of Africa 100,000 years ago, ca. 5,500 generations have passed which the map below shows was clearly enough time for newly-arriving populations to adjust their skin colour. Note that these are not degrees of “sunburn” but genetically-set (inherited) skin colours.
African skin colours of long-established populations conform well with long-term solar radiation intensity despite a unexplained moderate skin lightening around the Gulf of Guinea.
Australia was settled by Homo sapiens: around 50,000 years ago yet he skin colour shows up in well-defined stripes that mirror the intensity of the sun. The stripes indicate that this was enough time to adjust the genetic skin colour of a migrant population.We donot, of course, know whow the dark the skin of the Australians was on arrival. It is possible that they have lightened their colour (except for the inexplicably very dark-skinned Tasmanians (see the section on genetics in the Tasmanian section).
America was settled later than Australia but not later than 15,000 years ago. Late or not, the genetic skin-colour stripes are clearly visible.
Europe and Asia (settled by modern humans for not much less than 100,000 years) show a clear and largely undisturbed stripe pattern.