Changes that affected humanity were evolutionary, not revolutionary. Although Darwin saw selection operating on individuals as the primary mode of evolution, biologists today have widened our view to include changes among whole populations on regional, and sometimes global, scales. That evolution still occurs competitively by means of the usual adaptations to changing environments, but broadened, cultural opportunities for living—and thinking—within the past million years quickened the pace of evolution. And it hasn’t slowed since.

Environmental changes act as the motor of evolution, allowing some life forms to adapt successfully while forcing others to extinction. There were winners and losers, and no amount of political correctness can alter that assertion. As noted earlier in this CULTURAL EPOCH, some of the most dramatic environmental change on Earth, excepting asteroid impacts, is caused by the climate. Not least, glacial cycling drove biological evolution on our planet for eons and cultural evolution more recently. Harsh climate ~30,000 years ago, in fact, may have been the main reason that the Cro-Magnons replaced the Neandertals; the former had invented the technical skills needed to survive at the height of the most recent ice age.

Apart from seasonal effects occurring over monthly durations and continental drifts spanning millions of years, planet Earth experiences intermediate-scale changes in global climate over the course of thousands of years (Figure 7.15). Surprisingly detailed climatic records dating back nearly 1 million years have been derived by a variety of methods, including analysis of trapped gas and dust in core samples taken from the Greenland and Antarctic icecaps, of pollen in sandy sediments extracted from the seafloor of the North Atlantic, and of land-based geological data exposing freeze-thaw cycles. To give but one example, some snails coil to the left in cold water and to the right in warm water; the proportion of each type in their fossil remains yields a profile of ocean temperature over hundreds of millennia. Since plants and animals are acutely sensitive to changes in climate, their fossils extend our knowledge of climate, albeit less reliably, hundreds of millions of years into the past.

FIGURE 7.15 — The average surface temperature of Earth has changed during the past 0.5-million years. Recall that 0o C equals the freezing point of water. Earth's current globally averaged temperature is nearly 15o C.

Ice Ages Geochemists have accumulated strong evidence that during the most recent 1 million years our planet has cycled through ~10 (including 4 major) episodes of cool, dry climate—intermittent periods commonly known as ice ages. Hence, there was no single Ice Age per se, but several of them in recent years, geologically speaking. Though some of the data are incomplete, each cold ice age, as well as its opposite warm interglacial period, apparently lasted several tens of thousands of years. We now reside in an interglacial period—a temporary thaw of sorts before heading back into the deep freeze, though probably not for another ~20,000 years. In fact, all of human history—including the rise of agriculture, nation-states, and technology—has occurred within the current, ~10,000-year-long, interglacial warming trend.

What causes these cycles of heating and cooling on our planet? Some geologists contend that glaciation increases during periods of global volcanic activity when ejected dust reduces the amount of sunlight penetrating Earth’s atmosphere. Others maintain that periodic reversals of Earth’s magnetic field have caused the protective Van Allen belts to collapse, thereby sporadically allowing unusually high doses of solar radiation to heat the ground and thus decrease glaciation. Still other researchers note that ice ages could have been induced on our planet by variations in the output of the Sun itself, or passage of Earth through an interstellar dust cloud, or altered circulation of deep water in Earth’s oceans, or reduction of greenhouse gases in the atmosphere, or any one of a long list of other proposals.

Recently, oceanographers have found convincing evidence to support yet another theory, dubbed the Milankovitch effect after the Serbian mathematician who championed it in the mid-20^th century. According to this idea, subtle though regular changes in Earth’s attitude toward the Sun trigger the ice ages as variable amounts of sunlight hit our planet. These changes are the combined result of 3 astronomical effects, each in turn cyclical and caused by the normal gravitational torques (or twisting forces) exerted on Earth by the Sun, Moon, and other planets in the Solar System: First, change in the shape or “eccentricity” of Earth’s orbit about the Sun. Second, precession or “obliquity” of Earth’s spin axis. Third, change in the tilt or “wobble” of Earth’s spin.

Specifically, Earth’s elliptical orbit alters its shape every ~100,000 years, becoming more circular, then more oval, and so on in a regular periodic way. When the orbit is most elliptical, the Earth receives ~30% more radiation when it’s closest to the Sun than when it’s farthest away. Also, Earth slowly and smoothly precesses like a spinning top, returning to its starting point every ~26,000 years, thereby changing the presentation of Earth’s hemispheres toward the Sun and hence the amount of sunlight received from our star. Finally, over the course of ~40,000 years, the tilt of Earth’s axis (currently 23.5^o relative to its orbital plane) wobbles by a few arc degrees (from 22.1^o to 24.5^o), which is enough to change further the temperature contrast between winter and summer.

The first effect would produce warmer summers and colder winters during periods of high orbital eccentricity. The second also alters seasonal differences, driving extremes in climate when tilt combines with eccentricity unfavorably. And the third likely causes milder winters and cooler summers with reduced melting when Earth’s axial tilt is low. The net result of these 3 effects—for all 3 operate simultaneously yet over different timescales—sometimes causes abnormal solar heating such as we are now experiencing; yet at other times, that heating is distinctly reduced, producing a decline in global temperature and widespread glaciation.

This theory of astronomically induced ice ages is currently favored among the majority of working scientists mostly because samples of seafloor sediments show that, during the past 0.5-million years, tiny sea plankton have thrived at certain times, while barely surviving at others. Studies of the abundance of fossilized plankton known to prefer warm or cold water (a little like the coiling of shellfish noted above) provide estimates of the prevailing water temperature during their lives. This inferred sea temperature correlates well with the expected heating and cooling of Earth by means of the 3 combined astronomical cycles.

Apparently, then, slight changes in Earth’s axial tilt and orbital geometry are mainly responsible for triggering the ice ages. Whether they are the only trigger remains to be proved by further research. At least to some extent, the Milankovitch model reinforces yet again a robust astrobiology connection at work in Nature, for the cosmic stirring of the ice ages must have had profound impacts on the evolution of life on Earth.

The most recent major glaciation began nearly 100,000 years ago, after which the climate pretty much returned to the way we now know it by ~10,000 years ago. At the height of this ice age, ~30,000 years ago, as depicted by Figure 7.16, an ice sheet nearly 2 km (about a mile) thick extended from the Arctic far enough south to cover much of North America as well as a good deal of northern and central Eurasia. Earth’s overall surface temperature then averaged ~5^o C (or ~9^o Fahrenheit) lower than it does today, while the sea, with much water locked up in ice bergs, was ~100 m (or ~330 feet) below current levels.

Figure 7.16 — Glacial ice covered large segments of the Earth at the peak of the last major ice age, ~30,000 years ago. (Lola Chaisson)

Snowball Earth Even thicker, more extensive ice probably covered most, and perhaps even all, of Earth’s surface long ago—suggesting that mass extinctions of life could have been caused by cold air and glacial ice, not merely by the fire and brimstone of asteroid hits or the rising tides of oceanic waters. Some geologists have recently argued on the basis of glacial debris and biological tracers in ancient rock that massive glaciers might have completely entombed the entire globe ~600 million years ago, just prior to the Cambrian outburst of multicellular organisms. Reaching even into the tropics, 1-km-thick sea ice might have encapsulated all the oceans, sealing them off from the atmosphere and potentially cutting off life from its usual source of energy, the Sun. How such a “snowball Earth” got itself into such a deep freeze is an unsolved puzzle, but how it got out is an even bigger conundrum. The only reasonable exit would seem to have been volcanoes that belched out enough CO₂ to create an enhanced greenhouse warming of the planet—which, in turn and for a while at least, probably caused a brutal episode of heating sufficient not only to melt the ice but also bake the planet. How life survived the rigors of such severe climate reversals is another problem, unless it did so exclusively on geothermal internal energy without recourse to any outside solar energy.

An alternative, milder model (called “slushball Earth”) contends that, in the tropics at least, the snow didn’t freeze solid. If some open waters stayed unfrozen in equatorial refuges, the danger of life’s extinction would have been lessened. Scientists are currently troubled about the catastrophic harm potentially done to the biosphere during this multi-million-year-long cold spell that once (and maybe often) wracked our planet’s surface—but coming as it did just prior to the Cambrian period, perhaps the wave of heat that followed actually stimulated the evolution of diverse animal life more than harming it. The jury is still very much still out on this historical puzzle.

The snowball-Earth scenario of ~600 million years ago brings to mind the so-called faint-Sun paradox of ~4 billion years ago. The issue here, noted earlier midway through the STELLAR EPOCH, is the slow rise in the Sun’s brightness over the course of time; all stars experience this small change as hydrogen in their cores increasingly converts to helium. Both the theory of stellar evolution and the measured amount of energy reaching Earth today imply that our Sun is currently brightening by ~1% every 100 million years. Extrapolating back 3-4 billion years and the faint young Sun was probably ~¹/₃ less bright than today. Water on Earth should then have been frozen solid for its first ~2 billion years or so. Hence the paradox: How did primitive life survive, or even get started, if the solar energy reaching Earth billions of years ago was insufficient to melt ice? Liquid water does seem to be a vitally important ingredient for life as we know it. The possible answer, much as for the snowball-Earth scenario more recently, holds that Earth’s early biosphere must have been well warmed above freezing by greenhouse gases (not only carbon dioxide but also ammonia and methane) released from surface volcanoes and undersea vents. Otherwise, the geological record, whose ancient sedimentary rocks show clear evidence that Earth’s climate has always kept oceans in the liquid state, cannot be reconciled.

Such global challenges to life may well have been regular, ongoing episodes in the natural history of our planet—much as both natural and anthropogenic events, locally and globally, will likely continue to threaten future life on Earth.

Gaia Hypothesis The controversial concept of Gaia is also relevant here—the idea that living organisms can alter their environments and not just the other way around. Modern-day Gaians—disciples of the Gaia hypothesis and cultural descendants of worshippers of the terrestrial Greek goddess—go even further, claiming that planet Earth is a single, vast superorganism, indeed that Earth itself is alive (“strong Gaia”). Reality or metaphor, the notion of Gaia argues for living creatures that affect the composition of Earth’s biosphere, where both environment and life act in a coupled way so as to regulate the former for the benefit of the latter (“weak Gaia”).

Microorganisms surely excrete metabolic waste products that modify their environmental conditions, a property that enhances their own species’ survival, which impressively extends over billions of years. Accordingly, living systems have seemingly prevented drastic climatic changes throughout much of Earth’s history, as evolution has endowed organisms with improved ability to keep surface conditions favorable for themselves. For example, as our Sun ages and therefore sends more heat toward Earth, life responds by changing Earth’s atmosphere and surface geology to keep the climate fairly constant—a kind of geophysical thermostat that basically adjusts CO₂ levels down by preferentially trapping it in calcium carbonate (CaCO₃, or calcite) under high temperatures, thereby cooling the atmosphere. If those temperatures drop too low, the reaction to form calcite decreases and the carbon dioxide and the air temperature both rise, the whole cycle acting again in feedback fashion to keep water liquid and Earth habitable.

This regulating effect of life on climate will not occur indefinitely; Earth’s thermostat will eventually fail. In roughly 1 billion years—well before the Sun terminates in ~5 billion years—rising solar energy will outpace the offsetting, cooling effects at Earth. Most water will escape and the CO₂, no longer absorbed in the oceans, will accumulate rapidly. Our planet will then likely experience runaway, Venus-like greenhouse heating too great to sustain life. Any far-future descendants will presumably have to depart our home planet long before the Sun itself actually dies.

At any rate, ecologists are certain that the last 10,000 years have seen the glaciers retreat, the coastal plains flood, the vegetation bloom, and the ocean and atmosphere warm. The change in climate from the peak of the most recent ice age to its present state occurred quickly, by geological standards. And although global weather has remained anomalously benign during these past 10 millennia when civilization developed, ongoing changes in Earth’s regional environments, such as those produced by widespread oceanic (especially North Atlantic) circulation and atmospheric (El Nino) oscillation or by local droughts and floods, have surely helped foster the many advances made by our human ancestors during this period of rich innovation. Some of those natural changes, no doubt aided and abetted by anthropogenic actions like internal strife and tribal warfare, may have led to the demise of whole peoples and civilizations, such as possibly the Mesopotamians and later the Mayans—just as innumerable climatic warmings and coolings pushed unfit mammals of old to extinction, all the while spurring the evolution of new, better adapted species. Humans were forced to adapt—biologically, culturally, and rapidly—to changes throughout the air, land, and sea. The motor of evolution had indeed quickened.

Early Americans In one notable, if provincial, example of climatic change, the ice ages themselves may have accelerated the migration to and colonization of North and South America. Anthropologists know that humans didn’t evolve in the Americas since fossil evidence has never been found there for ape-like creatures from which they could have ascended. Current consensus has it that the so-called Clovis hunters arrived in the New World of the Americas <30,000 years ago, and possibly as recently as ~12,000 years ago. Their artifacts (notably Clovis arrowheads, first found near the present-day New Mexico town of that name) are strewn across parts of North America, but human remains are few. Kennewick Man who lived in what is now the state of Washington ~9000 years ago is the most celebrated case of a full skeleton of an early American. Precisely how they transitioned from the Old World is unknown, but during one of the more recent glaciations, enough water would have been wrapped up in ice to have lowered the sea level by ~30 m (~100 feet), allowing humans to walk dry-shod across the Bering Strait between what is now Siberia and Alaska.

This, then, is the prevailing view: Native North, Central, and South Americans are descendants of Asians who chased and hunted mammoths into the Great Plains hardly more than a hundred centuries ago. Once settled in the Americas, these migrants further developed arts, languages, tools, and many other cultural amenities. Whether civilizations of the Americas experienced cultural evolution independent of those in Eurasia, or whether they had some as-yet undiscovered contact with them, is another of those contentious controversies in modern anthropology.