Global cooling: from 1.7 million years ago

Global cooling: from 1.7 million years ago

It is about a million years ago that our ancestors, in the form of Homo erectus, first move out of Africa. At that time the planet is undergoing a series of slow but fairly drastic temperature changes, in a long sequence of glacial periods (also known as ice ages) interspersed with warmer spells. There have been fluctuations of this sort in the earth's climate since about 1.7 million years ago and they are still continuing.
We are at present some 10,000 years from the end of the last glacial period, and perhaps a little more than 20,000 years from the beginning of the next.
Each glacial period provides stimulating challenges for early humans. Islands become accessible as new territories, in some places because deep channels freeze and in others because the general drop in the level of the ocean (from water piling up on high ground as ice) results in a new land bridge.
Changes in vegetation, caused by the advancing or retreating ice caps, create new environments in which some species face extinction and others find improved opportunities.
Almost the entire span on earth of Homo erectus falls within this period of intermittent ice ages. His ability to adapt to the changing conditions must have been a large part of his success in spreading throughout the world. That adaptability is in part the result of greater thinking power. Over a span of a million years, from early African fossil skulls to those in China and Java, the braincase of Homo erectus shows on average a 25% increase in size. (Both Peking man and Java man date from about 500,000 years ago.)
This increase in intelligence no doubt leads to intermittent but important improvements in the way humans carry out the main everyday tasks on which life depends - hunting animals and gathering edible plants.