Sunday, 29 July 2012

Pleistocene (Pt 2): Europe at the Dawn of the Ice Ages

Pachycrocuta brevirostris, a European hyena
The Pleistocene is the time of the Ice Ages, when great ice sheets rolled across much of the northern hemisphere. Nothing much lived on the ice sheets themselves, just as there is very little today in the heart of Greenland. But, as we've seen, not only were their wide bands of tundra and pine forest reaching across much of today's 'western' world (and, of course, a fair chunk of the Orient), but the ice ages weren't continuous; there were many warm gaps between them.

The mammals of the Pleistocene include what are surely the most familiar fossil mammals to most people, the ones we generally think of when we think of 'after the dinosaurs'. For this was the time of the mammoths and sabretooths. They're familiar to us because, aside from the tiny sliver of warm weather we currently live in, the Pleistocene is the most recent, and therefore the best preserved and the most easily analysed, of all the epochs of the Age of Mammals. It's been the setting for a number of films, books, and TV series - to name just two, the Ice Age cartoons, and the Earth's Children series.

Yet, when we think seriously about Pleistocene animals, there are a couple of important points to bear in mind.

Compared with Earth's history, the Pleistocene is tiny, the second shortest of all the epochs - and the present day Holocene is so absurdly short, it barely counts. If we represent the time from the creation to the Earth to the present as a single day, the Pleistocene would occupy just the last fifty seconds or so before midnight. Looked at in that way, it seems pretty short, and in the grand story of evolution, it is. We often think about how far back some fossil is from the present day in terms of how many millions of years old it is, and the Pleistocene is only two and a half million years, two ticks of the palaeontological clock.

But look at it from the opposite perspective. If you wanted to zoom quickly through human history, let's say that you consider a decade as a minute. We're familiar enough with decades; it's fairly safe bet that most people reading this blog have lived through at least one or two of them. On this scale, it takes just ten minutes to zip through the entire twentieth century, with both world wars, and then another minute or so to reach the present. The Roman Empire was a little over three hours ago. The dawn of written human history was nearly six hours before that, which goes to show how long even that has been on the time-scale of a human life.

The Holocene began, and the Pleistocene ended, about twenty hours ago, a bit under a day. That's the time since the development of agriculture. Of course, it's not as if all humans everywhere developed that at the same time, and many people continued to live a hunter-gatherer lifestyle for far longer (indeed, in some parts of the world, people still do). But it's still a benchmark.

On this scale, the Pleistocene lasted six months. When you only get nine hours to watch the whole of recorded history, six months is an immense period of time. The upshot of this is that the Pleistocene isn't a single point in time, but a vast timespan from a human perspective, and it's not surprising that mammalian faunas came and went during that time, especially given how often the climate was swinging between the ice ages proper and the warm interglacials. The animals around in the late Pleistocene were not the same as those around at its beginning.

So that's the first point I want to make about the diversity of Pleistocene life; it changed over the course of the epoch. The second point is that that's still very short in terms of how fast continents move, or in terms of large scale evolution, both of which are insanely slow processes. In terms of general geography, a map of the Pleistocene world would look pretty much the same as ours. The main difference would be that, with so much water locked up in the vast ice sheets, the sea level was lower, and many parts of the world now covered with shallow seas were then dry land. But, essentially, you do have the same continents, in broadly the same relationship to one another.

If we look around today, it's instantly obvious that the animals we see in Africa are very different from those in South America, for example. The same applies to the Pleistocene, so even if we did take a single snapshot of Pleistocene life, it's not going to be the same across the world. The Pleistocene animals of Europe were not the same as those in North America. Our natural tendency to think of mammoths and sabretooths (say) as 'Pleistocene' animals is confounded by the fact the Pleistocene fauna varied across time and across continents, and even across different environments within those continents.

Having said all of that, the Pleistocene is still the closest wholly prehistoric epoch to our own. As I've already mentioned, large-scale evolution is almost ridiculously, unimaginably, slow. Although it depends when exactly in the Pleistocene you're talking about, it's been estimated that, overall, something like 50% of the mammalian species alive then are still around now. There would have been a lot of Pleistocene animals that would have been instantly identifiable as living species - lions, for example, pre-date the ice ages.

For that matter, many of those you couldn't match to living species would have been similar enough that only a specialist would give them a second glance. The otters of the early Pleistocene for instance, would have included some species we don't have now, but they would still have been obviously otters, and the visible differences between different otter species are usually pretty subtle.

So, then, I'm going to start by looking, not at 'Pleistocene' animals in general, but at Europe and Asia in particular, and how the fauna of those continents changed over time.

The Pleistocene officially begins around 2.6 million years ago, when the Arctic Ocean first began to freeze over year round. That's a convenient marking point, since it obviously sets the stage for the ice ages. But, at this point, the ice ages proper were yet to come, and an Arctic Ocean covered with year-round ice, while a new and dramatic change at the time (it had been millions of years since it had last happened), well... that's what we have now. So Europe, at this point, had more or less the same climate it does today.

Like everywhere else on Earth, Europe was, naturally enough, a wild, untamed wilderness. Where now agriculture has taken over, even in the countryside, in those days there were vast forests, rich in wildlife. The diversity and sheer number of animals across Europe made it seem more like Africa than the continent it does today, dominated by great herds of herbivores and the animals that preyed on them.

There were deer, goats, and wild cattle, few of which are numerous today. But there were also elephants and rhinos, animals that we simply wouldn't associated with Europe today, and, for that matter, that we don't associate with cool climates at all. They were native, home-grown species, able to live on forest browse, and suited to cooler weather than their surviving relatives.

In fact, the 'elephants' weren't, strictly speaking, elephants at all: they were mammoths. Still, mammoths are members of the elephant family, and, in most respects, quite similar to the living members of that family. The European elephant of the day was Mammuthus meridionalis, an animal a couple of feet taller than the living African elephant, and presumably somewhat heavier. It had longer tusks, too, with something of a twist to them - a feature that helps distinguish mammoths from true elephants. Incidentally, most people likely think of mammoths as hairy, but most species probably weren't. Given the climate of the day, for instance, it's likely that Mammuthus meridionalis was, at best, only slightly hairier than modern elephants are.

While they might have left the mammoths alone, there were plenty of predators feeding on the other herbivores. Again, many are not animals we would associate with Europe today, even if they are familiar from elsewhere. The dense forests of early Pleistocene Europe were home to lions, cheetahs, jaguars, and hyenas, as well as more obviously 'European' animals, such as bears, wolves, foxes, and lynxes. The presence of jaguars is, perhaps, particularly surprising, and the European jaguar, Panthera gombaszoegensis, is normally considered a distinct species from the living American sort. Nonetheless, it was a close relative, and may have hunted in the dense woodland much as living jaguars do in the jungle.

Somewhat less familiar to us today would have been the sabretooth cats, already established in Europe as the Pleistocene began. The dominant species, both in Europe and in Asia, was Homotherium latidens, although it's unclear whether this was really just one species, or several. It was a large cat, about the size of a lion, although rather more slender in build, and with large, broad, canines that nonetheless fell short of those of some its more famous kin. It probably fed on gazelles, zebras, and deer, all of which seem to have been quite numerous in the fossil localities where it has been found.

Around 1.8 million years ago, the climate began to take a dramatic turn for the worse as the newly formed Arctic ice cap began to expand. The forests died back, replaced by open tundra, and many animals struggled to survive. Some, however, prospered. One of the most significant, evidently taking advantage of the expanding grasslands, is actually a vole, Allaphaiomys, which seems to have spread rapidly at this time, inhabiting steppeland from Spain to Siberia.

More noticeable to the casual viewer, perhaps, would have been the appearance of new large herbivores, adapted to colder, less forested environments than had previously been present. A dramatic example is Soergalia, a kind of musk ox, although somewhat slimmer than the modern species, and with rather odd horns that projected out and forwards. The plains also proved suitable for bison, which first appeared at this time, long before they ever appeared in America (a mere 0.3 million years ago, when the Pleistocene was nearly over). They were smaller than living bison at first, and probably closely related to the animal from which our modern cows would later be domesticated, but the grasslands evidently suited them, and they rapidly displaced the more primitive cattle of the European forests, driving them to extinction.

Goats liked the colder weather, too, and the retreat of the forests seems to have affected deer less than one might expect. The largest deer of the day had previously been Eucladoceros, almost the size of a moose, but with long, branching antlers. It survived the transition to a colder climate, and seems to have prospered across much of Asia, too, but it was soon joined by Megaloceros obscurus, a similarly sized deer with rather bizarre twisted antlers - and the ancestor of animals that would later become even more spectacular. Interesting though such dramatic animals are, it's also worth noting that fallow deer also seem to have first appeared around this time.

The carnivores seem not to have been as affected as many herbivores, presumably happy to switch their diet to slightly different hoofed animals. Wolves, Homotherium, and the lion-sized 'giant hyena' Pachycrocuta brevirostris, did particularly well, making it through the transition with just a few adjustments to their range.

Most of these creatures originated in Eurasia, although many seemed to have moved into Europe from further east at this time, if they didn't live there already. But two particularly significant animals entered the continent from Africa, travelling, perhaps, in the opposite direction to what one might expect as the world cooled. By far the larger of the two was Hippopotamus major, an enormous hippo, known to have lived across western Europe, including Great Britain.

Hippos today are the largest land-dwelling mammals after elephants and rhinos (both of which, incidentally, continued to live in Europe at this time). H. major would have looked very similar to the modern species, to which it was very closely related.  Presumably it lived in similar, albeit colder, habitats along wide rivers and marshlands. But, at an estimated weight of six tons, it was about the same weight as an African bull elephant - although, given the short legs of a hippo, it would have been far shorter.

Remarkable as this European hippo surely was, the other new African arrival belonged to a species whose descendants would, in the long run, have a far more dramatic impact on the continent, and, for that matter, the world. Europe had been devoid of apes during the earlier, temperate Pleistocene, but as the climate turned 1.8 million years ago, one arrived: Homo ergaster, very probably one of our own direct ancestors.

[Image by 'Tiberio' from the Wikimedia Commons, showing a reconstruction at the Hungarian Natural History Museum]

No comments:

Post a Comment