Frozen Earth: The Once and Future Story of Ice Ages (33 page)

Figure 24.
There seem to be climate cycles at almost every scale that can be examined.
This graph shows temperature fluctuations as recorded in a central Greenland ice core between fifty and twenty thousand years ago, a time of rapid evolution of
Homo sapiens.

It would be foolish, of course, to try to squeeze every aspect of human evolution into a theory that singled out the Pleistocene Ice Age
as the only cause.
There have been multiple influences at work, as well as a strong element of chance.
We know that by the height of the most recent glacial advance some 20,000 years ago,
Homo sapiens
had already spread to most continents, and our species had learned to live in the harsh climate of Europe, to make clothes and to build shelters against the cold.
But in spite of our dispersal around the globe, the human population remained small until quite recently.
In such an environment, a series of abrupt climate changes of the kind exemplified by the Younger Dryas must have led to repeated fragmentation and sometimes demise of individual populations.
In the end, though,
Homo sapiens
emerged from this trial-by-climate with enough innovations that, eventually, climate no longer mattered in the big picture of species survival, although locally it could (and can still, even today) have devastating effects.

If ice age climate has played a significant role in human evolution, a crucial question is, Why are we, among all the closely related primates, the only ones to have developed a large brain, complex language, and all the other traits of the modern human race?
Why didn’t the plunge into an ice age affect the evolution of other apes similarly?
At the very simplest level, it may be that bipedalism—Stanley’s terrestrial imperative—was the key.
Chimpanzees and others remained arboreal.
Their populations probably declined as the rain forests of Africa shrank in size, but they were still able to travel through the forest canopy and find enough food there to survive.
Australopithecus,
on the other hand, relied on the trees of open woodlands for safety, and as dryness increasingly fragmented these woodlands, their populations became fragmented too, and they eventually died out, giving rise to the fully land-living
Homo erectus.
The terrestrial environment had potential advantages but also dangers for this new genus.
According to Calvin, Stanley, and some others, surviving in this new ecological niche required new skills, necessitating more neurons in the brain, conditions that shaped human evolution.
And the continuing rapid variations of ice age climate cycles forced and sped up the process, leading in a very short time to
Homo sapiens.

Although for us human evolution is perhaps the most interesting example of the effects of ice ages on evolution, it is far from the only one.
Recent research, especially research using genetic techniques, shows that many plant and animal populations that lived during the Pleistocene Ice Age were greatly affected.
Particularly at the higher latitudes of Europe and North America, the present-day distribution of many species is the result of normal biological diversity strongly influenced by climate changes and geography.
DNA analysis has been especially valuable for working this out, because it allows genetic diversity within a species to be examined and specific lineages to be tracked.
As might be expected, the ranges of individual species expanded and contracted during the warm-cold cycles of the Pleistocene Ice Age, but an interesting discovery is that the expansion phases were particularly important for establishing the present distributions.
As the climate quickly warmed during the “rapid terminations” that characterize the end of glacial periods, small groups of “colonizers” from a shrunken original population would establish themselves and quickly expand into areas of newly suitable habitat.
Sometimes they would take over very large areas with little or no competition.
Later “invaders” from the original population had little chance of making a significant imprint on the genetic makeup of the exponentially expanding population of colonizers.
Repeated rapid climatic cycles accompanied by these kinds of population changes led to significant genetic differences within species, and sometimes to entirely new species.
Such histories are now well documented for many plants, animals, and insects, and they are relevant to our discussion of human evolution, because they are very much akin to the boom-and-bust scenario championed by William Calvin.

A concrete example comes from studies of the common grasshopper.
Today, in terms of their genetic makeup, there are about five different populations of this insect in Europe.
But just one of these dominates a very large area, all of northern Europe and south into the Balkans.
During the last glacial maximum, 20,000 years ago, the vegetation on which grasshoppers feed was restricted to a few regions of southern
Europe in the Balkans, Spain, and Italy (this can be deduced from analysis of pollen in lake sediments).
The implication of the current population distribution is that Balkan grasshoppers were the colonizers as the ice retreated and suitable habitat opened up rapidly to the north.
They expanded very quickly into this new ecological niche, with no significant barriers to their mushrooming population.
Groups of grasshoppers that had been restricted to Spain and Italy during glaciation, however, were limited by geography as the climate warmed—the Pyrenees and the ice-capped Alps, respectively, prevented them from dispersing northward as rapidly as their Balkan cousins.
The genetic makeup of many other species in Europe and North America, from hedgehogs to fish to bears, also shows distinct geographic groupings that can be linked directly to ice age climate variability.
Both in simulations and in real life, the colonizing populations that expand rapidly to fill new environmental niches have a relatively homogeneous genetic architecture, while the parent population retains greater diversity.
This is paralleled in modern humans by the observation that there is greater genetic diversity among the African population than is present, for example, in modern Europeans.

It seems clear that the Pleistocene Ice Age has played an important role in the evolution of humans and other life of Earth.
Before closing this chapter, it is worthwhile to ask the question, Did earlier ice ages also influence evolution?
It is tempting to assume that they
must
have had evolutionary consequences—after all, some, like the Snowball Earth episodes, were much more severe than the current ice age.
But the great difficulty in investigating this question is the sparseness of the fossil record.
While ever more sophisticated analytical techniques allow us to document environmental changes in the distant past, it is difficult to tie the comings and goings of an individual species or genus, or even a family, with specific changes in climate.
There is nothing remotely resembling the detailed, continuous ice-core and deep-sea sediment records that are available for the entire duration of the Pleistocene Ice Age.
Instead, we have to be content to examine sedimentary rock sections
from scattered locations around the world, a few hundred meters here, another few hundred there, and patch them together into a coherent and, one hopes, more or less continuous record.
The problem of cause and effect becomes much more acute than it is for the Pleistocene Ice Age.
And while there is a suggestion that the rate of extinction—usually measured in the fossil record as the fraction of existing species that became extinct during a given time period—increased during the Late Paleozoic Ice Age that occurred around 300 million years ago, the evidence for the earlier ice ages is even more nebulous.

However, some of those working on the Snowball Earth glacial episodes point to changes in the fossil record that followed these intervals as an indication that they did indeed influence the course of life on Earth.
The problem is that the Snowball Earth ice ages occurred over a long period of time, between about 800 and 600 million years ago, and the appearance of new life forms in the fossil record occurred millions of years later.
Whether or not they can be linked is still an open question.

Before the Snowball Earth ice ages began, the only living things that inhabited our planet, as far as we know, were one-celled organisms—bacteria and algae—and the slightly more advanced eukaryotes, which had cellular nuclei and more complex internal structures.
The fact that eukaryotes survived the Snowball Earth interval, with the oceans frozen from pole to pole, has been touted by critics of the idea as evidence that no such episode occurred.
How could life survive under such condition?
This might be a powerful argument if every square centimeter of the planet had been frozen solid.
However, even the proponents of a “completely frozen” ocean during Snowball Earth point out that there would have been active volcanic regions analogous to Iceland or Hawaii that might have sporadically or even continuously provided small regions of warmer, open water where eukaryotes could endure.
In addition, open cracks and leads in the sea ice would likely have been present in the tropics, as they are in the present day Arctic Ocean in summer.
Even in the
coldest winter months there are fairly large patches of open water that occur regularly in the arctic, always in the same localities, presumably due to winds and currents.
They are called polynyas, and they attract an abundance of bird and animal life.
Thus it doesn’t seem too difficult to imagine eukaryotes (and bacteria and algae too) surviving even severe global glaciation.
But, like
Australopithecus
or the hedgehogs or grasshoppers of the current ice age, their habitats would have been restricted.
They would have retreated to
refugia
, the term that paleontologists and ecologists use to describe places that provide islands of habitable environment during times of crisis.
The sparse fossil evidence does seem to corroborate this view; it shows a decrease in diversity among the eukaryotes at about the time of the Snowball Earth glaciations.
The climatic shocks of extreme cold during these periods, possibly followed by extreme warmth during short “super greenhouse” intervals, would presumably have led to the same kinds of expansion, contraction, and speciation among the eukaryotes as is seen in plants and animals during the current ice age.

Shortly after the end of the Late Proterozoic glacial episodes, a completely new type of fossil appears in sedimentary rocks, and it is these that have caused most speculation about the effects of Snowball Earth on evolution.
The new organisms are called the
Ediacarans,
after the locality in Australia where they were first identified.
They were soft-bodied, large (at least compared to the single-celled creatures that had dominated the oceans for billions of years before them), and displayed a range of body shapes.
They appear quite abruptly in the fossil record, diversified rapidly, and then disappeared almost as quickly.
Not long afterward, about 540 million years ago, the famous “Cambrian explosion” occurred, documented by the sudden appearance in sedimentary rocks of a wide range of creatures that had begun to make shells, carapaces, and skeletons—and are thus much better preserved as fossils than their soft-bodied predecessors.

The connection between the appearance of these new organisms and the Late Proterozoic Snowball Earth episodes is tenuous, but because ice ages are rare in the Earth’s history and the bursts of evolution followed a series of these rare events, it is just possible that the cold intervals played a part in shaping the evolutionary pathways.
But much more evidence will be required before that link can be made with any certainty.

CHAPTER ELEVEN

The Last Millennium

We are now about twenty thousand years past the peak of the most recent glacial advance of the Pleistocene Ice Age and are in the midst of the maximum warmth of an interglacial period.
Record high temperatures have been the norm in recent years; newspapers report the melting of permafrost in Alaska and the possibility of an ice-free Arctic Ocean at the North Pole.
And yet in the winter of 2000–2001, Scots were delighted when a spell of very cold weather allowed them to hold curling tournaments on lakes that had not been frozen for decades.
In 2003, there was snow in New York in early April, a rare event by historical standards.
Several years ago, a cartoon in a U.S.
newspaper poked fun at the concern over global warming.
It showed a bundled-up man with mittens and a scarf reading a sign, posted on a closed door, that read “Lecture on global warming cancelled due to cold weather.”
The same cartoon could easily have been recycled during the arctic cold spell that gripped the northeastern United States early in 2004.
These things remind us that there is no such thing as average weather.
Weather is what we experience daily, and it can be misleading, because we are impressed most by the extremes.
Only over a longer timescale, by observing trends in temperature, precipitation, and windiness, can we really determine the direction in which the climate is moving.
It is the
ability to provide us with a long-term record of climate that makes cores from glaciers, lake and ocean sediments, and even trees so very valuable.
We have already seen that they portray a Pleistocene Ice Age consisting of a seemingly endless series of cycles, cold succeeding warmth and dry periods succeeding wetter ones.
Change, often very rapid change, is the dominant theme.
It seems to be most frequent and most severe during transitions between glacial and interglacial periods, but it is omnipresent.
Quite often the change is regular, following the astronomical cycles, but sometimes—especially at shorter timescales—it is chaotic.
Given our uncertain understanding of the causes of climate change, this makes short-term predictions of the future somewhat hazardous.