While history should be explained in terms of society's internal dynamic and never purely in terms of external factors, the environment does impose constraints on what can happen. People cannot live on ice sheets, for example, and difficult terrain, such as forest or desert, means low scale and low development.
To understand how people colonised the planet, we therefore need to take account of the environmental context, i.e. the changes in climate, sea level and ice cover over the last 40,000 years.
The methods for reconstructing ancient climates are described by a number of books. The one I used was Global Environments Through the Quaternary by David Anderson, Andrew Goudie and Adrian Parker. The book's Amazon page links to some other titles covering the same area. Another one worth mentioning is Earth's Climate: Past and Future by William F Ruddiman.
The Quaternary Ice Age
Viewed on geological timescales, the 40 thousand years of the human story have occurred during an unusually cold phase of our planet's history. This is the Quaternary Period, which began some 2 million years ago and is regarded by geologists as an ice age.
Geological time is divided into various periods, which can be recognised by differences in the type of rock laid down (e.g. rocks from different periods contain different kinds of fossil). These periods are often named for the regions where the associated rocks were first noticed (e.g. the Jurassic takes its name from the Jura), although some get their names in other ways (e.g. the Cretaceous takes its name from the Latin for chalk, since that is what the rocks consist of).
The Quaternary is the most recent period. Its name comes from a former scheme that divided earth history into Primary, Secondary, Tertiary and Quaternary periods. Of these, only the Tertiary and Quaternary are still recognised (though even this terminology is now coming into question). Conventionally, the Quaternary began 1.8 million years ago, but many geologists now put this back to 2.6 million years. The controversy is irrelevant to us, since we are only interested in the last 40 thousand years - roughly the last 2 percent of the Quaternary - the time for which modern humans have been in existence.
What distinguishes the Quaternary is that during this period the earth has been in its 'ice house' mode, whereby there are ice caps at the north and south poles. It is not usual for the earth to have such ice caps. There were none during the great age of the dinosaurs, 100 million years ago, for example. However, from time to time, the earth's climate goes through a noticeably cooler phase. No one knows why this occurs, even though there are many theories. Whatever the reason, over the last 3 billion years, there have been some half dozen such glacial episodes, each lasting up to 100 million years. In the Quaternary, we are currently in the early stages (if past durations are anything to go by) of the latest of these ice ages.
The current cooling of the earth's climate began as long as 65 million years ago, at the end of the Cretaceous (when the dinosaurs died out). The cooling trend continued throughout the Tertiary, the period immediately before our own. In fact, ice sheets were already appearing in the late Tertiary. It is when these became persistent, and temperatures reached their current low, that is taken to mark the start of the Quaternary.
While the Quaternary as a whole has been cold, temperatures have by no means remained constant during this time. In fact, not only have temperatures fluctuated but there have been smaller fluctuations within larger fluctuations. The more evidence climatologists gather, and the closer they look, the more ups and downs become apparent. Within the Quaternary, there have been warmer and colder millennia; within a given millennium, there have been warmer and colder centuries; within a given century, there have been warmer and colder decades; within a given decade, there have been warmer and colder years. Climate fluctuates on all scales.
The fact that climate fluctuates on all scales suggests it is a chaotic system. External inputs--such as variations in solar activity, continental drift, the passage of the solar system through interplanetary clouds, or gases released by volcanoes and living organisms--energise the system. They do not directly drive change, except possibly large, long-term change, but rather prevent the atmosphere from reaching equilibrium. The system remains unstable in itself, and is bound to change in an essentially patternless, unpredictable manner, on account of complex, multi-level feedbacks between climatic variables.
One of the earliest pieces of work in chaos theory was, in fact, the research of Edward Lorenz on computer models of the atmosphere. He found his simulated weather systems wandered all over the place in a way that had no simple relationship to the input conditions. Given that the weather seemed to vary continuously and unpredictably, he questioned whether it even makes sense to speak of the earth's 'normal' climate.
The diagram below shows some of the differing climatic regimes of the Quaternary, which will be explained in the following sections. Red represents warmer phases, and blue colder ones.
Glacials and interglacials
The grossest temperature fluctuations within the Quaternary are the glacials (or glaciations), when ice sheets grew massively around the world, and interglacials, when the ice sheets retreated and in some cases disappeared.
The most recent glacial is known as the Würm glacial in Alpine Europe or as the Wisconsin in North America (and has other names in other regions). This began around 75,000 years ago, was at a point of maximum coldness around 20,000 years ago (the Last Glacial Maximum or LGM), and came to an end around 11,500 years ago. We are currently in an interglacial.
Pleistocene and Holocene
The relatively warm period that began 11,500 years ago is considered to be a distinct sub-division (or epoch) of the Quaternary, called the Holocene. All the rest of the Quaternary, before this, is called the Pleistocence. Pleistocene means 'very recent'. Holocene means 'completely recent'.
Some geologists dispute the notion of the Holocene as a special epoch. They have a point, as it is only the last of a series of interglacials. The idea that it marks a new epoch seems to be exaggerating the importance of a relatively minor change simply because of our closeness to the event. That said, the Holocene provides a useful label for the current warm phase, whatever its geological status.
Marine isotope stages
The broad pattern of glacials and interglacials was first identified in the nineteenth century by the characteristic valleys and deposits of debris left by ancient glaciers. However, a much more detailed record of our planet's changing ice cover is now available from studies of ocean sediments.
Seawater molecules (H2O) contain two isotopes of oxygen, 16O, the lighter of the two, and 18O, the heavier. When water is evaporated from the oceans, to fall as rain or snow over the continents, the lighter water molecules, containing 16O, evaporate a little more easily. If the earth is going through a cold spell, and ice sheets are growing, these lighter water molecules become locked up in the ice, leaving the oceans with a higher concentration of the heavier molecules. Later, when the earth warms and the ice sheets melt, the lighter water molecules return to the oceans, reducing the concentration of heavier molecules there. This means that the concentration of 18O in seawater reflects the size of the earth's ice sheets, with higher concentrations of 18O when the ice volume is larger.
We can reconstruct past variation in 18O concentrations because the shells of tiny animals that live in the oceans reflect the chemical composition of the water that surrounds them. If 18O is more concentrated, these shells also contain a higher concentration of 18O. When the animals die, their shells fall to the bottom of the oceans and build up as layers of sediment, creating a record of the changing concentration of 18O and hence of the changing size of the earth's ice sheets.
Although this 'marine isotope' record shows fluctuations of all sizes and durations, geologists recognise in it an overarching pattern of swings between warmer (less ice) and colder (more ice) stages. The current warm swing is designated marine isotope stage (MIS) 1. The previous cold swing is MIS 2. The warm swing before that is MIS 3, and so on. Odd numbers correspond to warmer stages and even numbers to colder stages.
The marine isotope record does not tie up exactly with the more traditional division into glacials and interglacials, but rather reveals the complexity of climatic fluctuations. The current warming (MIS 1) had its beginnings in the last few millennia of the Würm glacial. The previous cold phase (MIS 2) was an exceptionally cold part of the Würm glacial, spanning the LGM. Before that, MIS 3 was a less cold phase, but still within the Würm glacial and not as warm as today.
The ending of the last glacial
The overlap between the nominal end of the last glacial and the beginning of MIS 1 reflects the fact that the colder climate did not terminate in a once-and-for-all manner. Within the warming, there were setbacks, involving a temporary return to colder conditions. First there was a warming lasting a little under 2000 years (the Bølling), then a short cold snap of about 3 centuries (the Older Dryas), then another warming of a little under 1000 years (the Allerød), then a longer cold snap of nearly 1500 years (the Younger Dryas). The end of the Younger Dryas marks the end of the glacial.
It should be apparent from all this that identifying the termination of the glacial is somewhat arbitrary, and requires a degree of hindsight we do not currently possess. Whether the present warming should be seen as part of a longer-term warm phase or just as a warmer interval in a longer-term cold phase depends on what happens in the future.
Climatic variation in the Holocene
During the Holocene, climates have continued to fluctuate. In Europe, the first 1500 years (the PreBoreal) were relatively cool. There then followed 7500 years of relative warmth, ending 2500 years ago (i.e. around 500 BC). The beginning and end of this phase (the Boreal and SubBoreal) were both warm and dry, while the middle part (the Atlantic), lasting about 3000 years, was warm and wet. Finally, the last 2500 years (the SubAtlantic) have been relatively cool again.
Adapted from: D Anderson et al. Global environments through the Quaternary (Oxford 2007) p. 11.
Again, within the current (SubAtlantic) phase, there have been shorter term fluctuations. The heyday of the Roman Empire was relatively warm. The end of the Empire and the early medieval period (the 'Dark Ages') was colder. The high middle ages, the time of the monasteries and crusades, was warm, with grapes being grown in Britain, and the Vikings settling Greenland. The early modern period, the time of Shakespeare, Elizabeth I and Philip II, up to the Victorian period was colder (the 'Little Ice Age').
The last hundred years or so have been relatively warm, but still by no means uniformly so. The first half of the twentieth century was warm, and scientists spoke of global warming as a boon to humanity, bringing not just better weather but better growing conditions for crops. The late 1940s to 1970s were cooler, leading to talk of a renewed ice age, with soaring energy costs for heating, and the threat of famine; in the 1970s, British harvests were on average 11 days later than they had been in the mid-twentieth century. The 1980s and especially the 1990s were warm again, so that talk was once more of global warming, though now as a source of concern and even fear. Finally, temperatures in the first decade of the twenty-first century have shown little trend either way.
To repeat, climatic fluctuations occur on all scales, and the closer one looks, the more variation one sees. This is variation not just in time but also in space. Episodes like the medieval warm period and subsequent little ice age do not appear to have occurred in other regions the same way they occurred in Europe, and temperature changes in the southern hemisphere seem sometimes to have been in the opposite direction to those in the northern hemisphere.
Sea level changes
Changes in global ice cover cause corresponding changes in the global sea level. More ice means less water in the oceans and larger areas of dry land.
This would have affected people's ability to get from A to B, and is important for how they migrated around the world.
The chart at right (source: Wikipedia) shows changes in sea level through the late Quaternary. The light and dark shaded bands indicate the marine isotope stages (note that MIS 5 is subdivided into 5a, 5b etc.).
For most of human existence, sea level has been lower than today, reaching a minimum at the LGM, when it was more than 100 metres below the present level. Around 5000 years ago, however, sea level was some 10 metres higher than it is today.
We can translate the above chart into maps of how the continents would have looked at different times, courtesy of an applet developed by Sebastien Merkel at the University of Lille. You enter a given sea level (metres above or below the present) and the applet draws the land as it would then appear. (There are actually several applets, for the world as a whole and for different regions.)
I have used Sebastien's applet to create a slideshow of changing sea level, spaced at 5000-year intervals, from 40,000 years ago to the present.
Here is a Youtube version:
I have also created a set of Google Earth layers showing the ancient coastlines. (This does not include a layer for the present, since you can get that from Google Earth itself.)
Below, is an animated version, which requires the Google Earth plug-in to see it. Move the slider to change the date. (If you do not want to install the plug-in, but have a standalone Google Earth browser, you can download the animated coastlines here.)
Lower sea levels meant that the world's land surface was more connected in the past than it is today. The British Isles were joined to continental Europe. There was a land bridge, known as Beringia, between Asia and North America. The islands of modern Indonesia were mostly joined to each other and to the mainland. Australia was the only separate continent, cut off by a sea crossing of about 100 miles, but was joined to New Guinea. The entrance to the Black Sea was dry land, so the Black Sea was then a lake. However, the Gibraltar Strait remained submerged, so there was a short sea crossing between Africa and Spain, while the Mediterranean still opened into the Atlantic Ocean.
Early human migrants would have followed the coasts in spreading around the continents, and followed the rivers into the interior. The early settlement of Australia shows they could also cross the sea. Evidently, they had boats, which would have served them for both fishing and transport. They could thus have crossed between Africa and Spain, and reached offshore islands, such as those of the Mediterranean, Caribbean and South China Sea.
The rise in sea levels means that most of the sites occupied by human migrants 40-50,000 years ago are now beneath the waves. Future advances in underwater archaeology can be expected to reveal much more about this time, and give a clearer picture of the colonisation of our planet.
The pattern of coastlines and land connections represents only part of the information we need for thinking about how early humans moved around the planet. We also need to know the type of terrain that confronted them.
For example, while the low sea levels of the LGM produced the Beringia land bridge between Asia and North America, the heavy glaciation of that time meant that Beringia was blocked from the rest of the North American continent by an ice sheet. Given the traditional belief that humans only reached the Americas after the LGM, the moment at which the ice sheet had retreated enough to leave an ice-free corridor from Alaska to the Great Plains provides an important constraint on the timing of their arrival. (For an animation of the retreat of the North American ice sheet, see this site.)
My view is that the earth, including America, was colonised in essentially one great movement, at the start of the Upper Paleolithic, i.e. 20-30,000 years before the LGM. Beringia existed at that time, while the North American ice sheet extended only a little way beyond Hudson's Bay, and did not block movement via the west coast. That said, we should not discount the possibility that humans arrived in America via the Atlantic or Pacific. It may be a long voyage, but the Americas present a huge target.
I have prepared a set of maps, in Google Earth, of global climate/environment at 10,000 year intervals, from 40,000 years ago to the present. They are derived from those produced by the Quaternary Environments Network plus a certain amount of guesswork (the QEN maps are quite patchy in their chronological and regional coverage, and I have had to fill in the gaps to create a consistent set of maps at regular intervals).
Terrain is classified into eight types, using the following colour scheme. In a nutshell, the lighter the green, the drier and more open the terrain (plus yellow for desert, including polar desert, and white for ice).
You can see these maps either with the Google Earth plug-in below, or by accessing the Google Earth files directly here. Move the slider to change the date.
Note that these maps take into account the different coastlines at different periods, which is why they may show grassland etc. in what is now sea. The map for the present shows potential vegetation (i.e. as it would be in the absence of human influence). Actual vegetation can be very different due to the effects of industry and agriculture, the main thing being the widespread clearance of forests.
For most of human existence, the climate has been colder and drier than today, resulting in more desert and less woodland. However, around 10,000 years ago, climate was generally moister than today, and the Sahara desert was converted to grassland and steppe. That said, the global environment did not vary as one. Climatic change could, for example, mean a shift in wind patterns, carrying moisture away from one region and towards another, so that the first region became drier and the second one wetter.
This post has provided a narrative of climatic variation over the period of human existence, plus some relevant resources in terms of maps of changing coastlines and terrestrial environments. It has not reached any particular conclusions but is intended to provide the high-level background for subsequent discussion of humans' discovery and conquest of their world.