|A scene from northern Spain|
Of course, "large, land-dwelling" is something of an arbitrary qualification, and one more rooted in the natural prejudices of our own species than in an actual reflection of Earth's biodiversity. The most numerous animals throughout the era, and, for that matter, through the Age of Reptiles that preceded it, would have been insects. But, unless you're standing in the middle of a swarm of midges, most people don't notice insects in the same way they would notice, say, a herd of antelope, or a prowling tiger. Mammals aren't even the most numerous vertebrates today, and, by sheer species count, it's the fish that are dominant, and some of those are pretty big.
Even on land, in terms of number of species, mammals are the least numerous of the four vertebrate classes - birds come in first, and reptiles still hold on to second place, followed by amphibians. Of course, most of those reptiles are small lizards, and birds are also generally quite small. Even so, there are ostriches, crocodiles, and anacondas, among others, and, in fairness, most mammal species are mouse-sized. So there's a reasonable case that what this should really be is the Age of Birds. But I, for one, am going to stick with the standard term.
Even in geology, eras are pretty large chunks of time, and so they are subdivided into 'periods'. The Age of Mammals used to be divided into two such periods, the Tertiary and Quaternary. When geologists first described these periods, back in the eighteenth century, they had no idea of the real age of the Earth, and knew nothing of evolution - the Tertiary was originally assumed to represent sediments laid down in Noah's flood, and the Quaternary to be everything since then. Even when it became clear that that wasn't true, the periods kept their names, for lack of any good reason not to do so.
It soon became clear, however, that the Quaternary was ridiculously short in comparison with the Tertiary, or indeed, any other period of geological history. It covered just the last two million years, whereas the Tertiary covered the 63 million year timespan before that. In 1989, the International Union of Geological Sciences decided that it had enough, and ditched the scheme, creating two entirely new periods, the Paleogene and Neogene. The Neogene, which was the most recent, and therefore included the present day, was still the shorter of the two, at 23 million years in length, but they are were at least more balanced than they had been.
That, however, annoyed the International Union for Quaternary Research, presumably at least partially on the grounds that they didn't want to sound as if they were researching something that didn't exist. (It's also likely the case that, when you spend your life studying one thing in particular, it's more apparent to you how it differs from everything else than it might be to someone who's less of a specialist). After quite a lot of arguing, a compromise was finally reached in 2009, and the Quaternary was restored, while the Tertiary wasn't. So, as we stand today, the Age of Mammals is officially divided into three periods: the Paleogene, the Neogene (now a little shorter than it originally was), and the Quaternary.
As an aside, this also changed the name of the geologic boundary at the base of the Age of Mammals; the point in time where the non-avian dinosaurs went extinct. Long called the "K/T extinction", where the T stood for "Tertiary", this is now officially the "K/Pg extinction". It's worth noting that a lot of people, including professional scientists, don't take any notice of this, and still widely use the older terms.
Part of the reason that they can get away with ignoring the correct terminology is that, frankly, periods aren't terribly useful chunks of time for dividing up the Age of Mammals. It's far more useful to use the finer divisions of periods, which are called epochs. In total, there are seven such epochs in the Age of Mammals, and, while there have certainly been plenty of arguments as to where the lines between them should be drawn, the basic scheme has been in place for a long time without truly drastic alteration.
For true specialists, even epochs are too long, and so are subdivided into smaller spans of time. Unfortunately, there are at least three different ways to do this, and some of those aren't necessarily consistent worldwide. For instance, people whose main interest is mammalian fossils use divisions based on the kind of fossils they find at different ages. Which is all very well, but you're not going to find fossils of the same animals in North America as you are in Africa, which means that there has to be a different scheme for each continent. Now, if you only ever work in one continent, that's great, but it can be a little confusing for somebody coming in from the outside.
Ignoring that, then, let's look at the Quaternary specifically. It's by far the shortest of all geologic periods, and is divided into two epochs: the Pleistocene and the Holocene. The Holocene is almost absurdly short; it's the epoch we're in today, and started just 0.012 million years ago, which is to say, around 10,000 BC. That's roughly when humans first developed agriculture, and the Holocene is therefore the epoch dominated by us, and by our ability to change the world around us on a large scale.
The Pleistocene, therefore, occupies the entire remainder of the Quaternary. The compromise that restored the period also added a further 782,000 years onto its beginning, and it is now considered to have lasted for about 2.6 million years. That's very short for a geological epoch, but it's still an immense period of time on a human scale - hundreds of times longer than the entire span of recorded history.
The Pleistocene is dominated by the Ice Ages; times during which the world cooled and the polar ice caps expanded so far that huge ice sheets rolled across much of the northern continents and the tip of South America. But how many Ice Ages were there?
It really depends on how closely you look. There were four or five main ones, with the last ending at the start of the Holocene, 12,000 years ago. But none of these were truly continuous; there were always breaks within them, times when the world warmed to, or beyond, its present-day temperature, before sliding back into the ice again.
In the last 700,000 years, traditionally thought to include the latest two or three of the main Ice Ages, more recent evidence has shown at least four. By extension, therefore, there were also four occasions when the world was much warmer, in between the glaciations. Deep-sea cores, which are able to measure worldwide temperature, show that there may have been as many as nineteen warm phases during that time, which further reduces our picture of the Pleistocene as a time of everlasting cold.
Having said that, the individual Ice Ages were certainly long enough on any human time scale. Looked at from this perspective, there's also no reason to suppose they are over yet. The current warm period began 12,000 years ago, but the one before that lasted about 16,000 years, and may well have been one of the shorter ones. Since the climate then was just as stable as it is now, if not more so, and the world was actually a degree or two warmer than it is today, there's no reason to assume that the ice won't be back in a few thousand years. If we were to try and define epochs by climate on this sort of time scale, then we'd have to admit that the Pleistocene hasn't really finished yet.
For all that the Ice Ages may have lasted only a few tens of thousands of years each, which is nothing on a geological time scale, the world was a dramatically different place when they were at their height. If we could have looked at the world from space back then, the glaciers themselves would have been the most obvious change from today. Northern Europe, Canada, Siberia, and Patagonia were under vast thick ice sheets, as Greenland and Antarctica are today. So were several major mountain ranges, such as the Alps and southern Andes. In these environments, nothing much can have lived; there was just a vast wilderness of ice and snow.
Most of the world, of course, wasn't covered in ice, but it was still a lot colder than today. Pine forests stretched across much of the present day USA, reaching as far south as Louisiana. Inland, where it was too dry for forests, the Great Plains were covered in chilly steppe-land, stretching down into west Texas. France, and much of central and eastern Europe, were permafrost coated tundra, too cold for woodland, with pine forests in Italy, Spain, and Greece.
Further south, the jungles retreated. They never went away entirely, and the equatorial regions of the world would still have been fairly warm, but they were much smaller and more fragmented than they are now, replaced largely by open savannah.
Granted, this picture is of the Ice Ages shows them at their worst. As we've seen, most of the time, it wasn't so dramatic, and at times, the world was positively sweltering, with coastlines flooded by melting ice caps. But such swings in climate, if not so sudden as today, were at least as dramatic. Animals had to move, evolve, or die out as the environment around them changed. Some sheltered in refugia, places of milder climate where they could still survive, following shifting vegetation to maintain their diet. Others evolved, fitting themselves to eat whatever food had replaced that of their ancestors.
In part 2, I'll look at some of the specifics; the mammals that prospered during the Pleistocene.
[Picture by Mauricio Antón. Released under the Creative Commons Attribution License.]