Somewhere in East Africa, buried under a thin layer of soil for protection from curious eco-tourists, lie fossilized footprints 3.6 million years old. Soon after a long-ago volcanic eruption, two adult creatures walked across a bed of cooled and rain-moistened volcanic ash, leaving the marks of their feet. Occasionally protruding outside one of the two sets of footprints are extra toe marks, as if a large child was also part of the group, walking along and placing its feet inside the marks made by one of the adults, but occasionally missing the target by a little. Later, the ash hardened into rock and was buried by other sediment. Much later, erosion reexposed this deposit.
These footprints were made by creatures that were more than apes, yet certainly less than human, that walked the forests and grasslands of eastern Africa 3.6 million years ago and that are called australopithecines. The patterns in the ash show no trace of marks made by knuckles touching the ground, as would be expected if these beings used their arms as part of their natural locomotion the way modern-day apes tend to do. Fossil remains of other members of this group dating to the same or even earlier periods of time show ankle structures fully adapted for walking. Although not yet humans, these beings were clearly on a trajectory leading toward us.
Given how long ago this scenario happened, perhaps the most amazing fact about the subsequent history of our ancestors is how remarkably little things actually changed over an incredibly long period of time. Back 3.6 million years ago, our australopithecine predecessors were already acquiring their food from scavenging, gathering, and hunting small animals. Yet 3.59 million years later, just 12,000 years ago, our immediate (and by then fully human) ancestors were still subsisting by hunting, gathering, and scavenging, along with some fishing. A few tribes in remote areas live this way even now. Similarly, even though the use of stone tools began nearly 2.5 million years ago, thereby initiating the Stone Age, every human still lived in Stone Age cultures until the first use of metals just 6,000 to 5,500 years ago, not long before the early civilizations of Egypt constructed the pyramids. If you take this long view of our history, it is astonishing how close to the present most of the ancient ways of "making a living" survived.
From our highly accelerated modern perspective, it also seems amazing that our predecessors had so remarkably little to show by way of progress over spans of time stretching back millions of years. Today the term "glacial" is often used to describe the slowest imaginable pace, but somehow those sluggish ice sheets managed to grow and melt at least 50 times during the interval within which our ancestors were unable to move beyond a primitive style of existence. Almost everything in human history that we call "progress" has happened in just the last few thousand years, truly the blink of an eye.
Many narratives of human history go back to an asteroid some 10 km in diameter that slammed into Earth 65 million years ago. For the previous 100-200 million years, the largest organisms on Earth had been the dinosaurs, and our ancestor-mammals were small, rodentlike creatures trying to stay out from underfoot or scolding like squirrels from tree tops. Then, according to a hypothesis published by Luis and Walter Alvarez in 1980, the asteroid hit and wiped out the dinosaurs, or at least the larger forms we commonly think of under that name.
How this happened is still debated: terrific pressure waves from the force of the impact, instantaneous scorching from the heat generated, starvation soon afterward because all the vegetation had been burned, cooling over the next few years from an ejected plume of dust that dimmed the Sun's rays, the effects of highly acid rainwater over the next several decades, or global heating over later centuries because of the CO2 sent from burning vegetation into the atmosphere. Despite this uncertainty, no one doubts that a major impact occurred: it left a distinctive chemical tracer (the element iridium) that is extremely rare in Earth's crust concentrated in a thin layer of sediments worldwide. And it produced gigantic tsunami waves that drove far into the coastal regions of the continents.
Over the next several years after this "impact hypothesis" was published, critics raised objections. They pointed out that many types of dinosaurs seemed to have disappeared from the geologic record well before the impact event, and they hypothesized that climatic changes that were occurring for other reasons were part or even all of the story. But this possibility has been effectively squashed by an unusual example of participatory science. In the western United States, hundreds of nonscientific volunteers were trained to hunt outcrops for fossil remains and used as brute-force labor to sieve tons of sediment in search of smaller remains such as teeth and small bones.
The results were, in a way, predictable: the more outcrops the corps of volunteers searched, and the more soil they sieved, the more bones they found. Of course. But part of what they found was revealing in a way that had not entirely been anticipated, at least not by the critics of the impact hypothesis. For every type of dinosaur that had previously seemed to go extinct at levels well below the impact layer, the volunteers found fossil remains much closer to the impact event. The closer they looked, the more their results matched the asteroid-impact hypothesis of a single and sudden extinction. These efforts implied that an infinite number of volunteers looking at every outcrop would find the actual extinctions lying right at the level of the impact event.
Because the impact killed many life forms other than just dinosaurs, it adds another dimension to the process of evolution that Darwin had envisaged more than a century ago. For tens or hundreds of millions of years, life forms apparently do compete for the available ecological niches (and means of survival) in a more-or-less Darwinian way: they outbreed or outsurvive other, similar life forms. Success in this contest is measured by small gains and losses, as if certified public accountants from insurance companies were in control of the progression of life.
But then, every hundred million years or so, a huge chunk of rock comes flying in from outer space and completely rearranges this orderly world. Most species go extinct (70% at the impact event 65 million years ago), and suddenly many once-crowded ecological niches have elbow room for the survivors, who begin to make use of it with the slower, Darwinian diversification of their talents. This side of evolution has been called "life in the interplanetary shooting gallery." Only a few sittings ducks survive the largest assaults.
As a result of the impact event 65 million years ago, the Age of Dinosaurs became the Age of Mammals. Those small, unimpressive, rodentlike mammals soon evolved into larger, more complex life forms, including the largest creatures on Earth. One line led to small animals very much like modern lemurs, tree climbers with grasping front paws and prehensile tails that lived tens of millions of years ago. That line in turn diversified and led to a primitive kind of apes that lived 10 million years ago, after which a separate group that included chimpanzees and our own ancestors branched off near 5 million years ago. By 4.5 or 4 million years ago, those australopithecines in Africa had risen up from four legs to two and were walking upright like the ones that left those footprints in the ash. The change to an upright posture can also be detected by the position of the spine at the base of the skull: for four-legged creatures, the spine enters the back of the skull; for those walking upright (including the australopithecines), the head sits right atop the spine.
The world gradually cooled during this evolution toward our species. On Antarctica, cold-adapted vegetation gave way to small mountain glaciers, and then to larger ice sheets that repeatedly grew and melted, and finally to a thick ice sheet that has stayed in place for millions of years. Around the Arctic Ocean, temperate forests gave way to cold-adapted conifers and later to tundra. Ice caps began to appear on high mountains in the tropics.
Two causes of this gradual cooling of the Earth have been proposed. Some scientists believe that the ocean is the main reason for it. The ocean carries almost as much heat poleward as the atmosphere, and its circulation is affected by plate tectonic changes, especially when continents break apart and allow the ocean to flow through a new passage, or crunch together and choke off such flows. The most frequently cited change in these tectonic "gateways" is the separation of South America and Australia from Antarctica several tens of millions of years ago. The hypothesis is that temperate ocean water that had once been diverted by land obstacles toward the South Pole and had carried large amounts of heat to Antarctica later began to flow in an uninterrupted path around the continent, leaving it isolated and cold enough to become glaciated. The other often-cited gateway change is the final closing of the Panama Isthmus some 4 million years ago. In this case, tropical Atlantic water that had once flowed westward into the Pacific Ocean through the gap between North and South America was diverted northward toward high latitudes. The hypothesis in this case is that the extra water vapor delivered northward by the warm ocean caused ice sheets to begin growing just over a million years later.
These gateway hypotheses have their doubters, and I am one. The first gateway change in the South supposedly caused glaciation by reducing the amount of ocean heat carried poleward, while the second one in the North did so by increasing it. The use of the same line of argument but in two completely opposing directions makes me doubtful about both. I also think that these gateway changes are too scattered in time and space to provide the ongoing push needed to explain the persistent drift of climate toward colder conditions over the last 55 million years or more.
A more widely accepted explanation of global cooling is a gradual drop in the amount of the greenhouse gas carbon dioxide in the atmosphere. Although CO2 is just a tiny fraction of the total amount of gases in the air, several hundred billion tons of it are floating around up there, and the amount in the atmosphere has varied considerably through time. Think of the amount of atmospheric CO2 as water in a partly filled bathtub. This particular tub is not very tight: it has a dripping faucet and a leaky drain, so a little water is constantly coming in and a little is going back out. The water level in the tub reflects the balance between the drippy faucet and the leaky drain.
In nature, volcanoes are the dripping faucet that add CO2 to the atmosphere. Volcanoes are located mainly in areas where Earth's tectonic plates are colliding, especially around the rim of the Pacific Ocean. Every year, volcanoes explode somewhere on Earth, and when they do they add CO2 to the atmosphere, like a faucet slowly dripping tiny amounts of water into a very large tub. The leaky drain for the CO2 is right under our feet. Rainwater is slightly acidic because it contains small amounts of CO2 from the atmosphere. Rains feed groundwater that is slightly acidic for the same reason (it contains CO2), and the groundwater slowly weathers mineral grains in the soil. In the chemical reactions that occur during weathering, the CO2 in the groundwater is eventually locked up in clays in the soil. In effect, it has drained out of the tub.
To cool Earth, the amount of CO2 in the atmosphere (the level of water in the tub) has to fall. One way to do that is to reduce the amount of CO2 coming in at the faucet. Geophysicists have shown that several kinds of plate tectonic processes that create volcanoes, such as the rate of generation and destruction of new crust at deep-ocean ridges and the creation of gigantic volcanic edifices on the sea floor, have slowed during the last 100 million years. Apparently, less CO2 has gradually entered the atmosphere over time.
It also seems likely that more CO2 has been going out the drain. The normally slow process of chemical weathering that occurs in soils is accelerated if the mineral particles being attacked are ground up very fine, like preparing coffee beans for dripping water in a filter. During the last 50 million years, the slow collision of India plowing northward into Asia has created the Himalaya Mountains and the Tibetan Plateau, by far the largest feature on Earth's continents. These processes create enormous amounts of ground-up rock debris. Monsoon rains that fall against these mountain slopes attack the freshly crushed rock particles and take CO2 out of the atmosphere (down the drain) at rates much faster than normal.
The fact that both poles have cooled progressively over tens of millions of years is an argument that falling levels of atmospheric CO2 are the primary causal mechanism. Another argument for this idea is the fact that the Antarctic continent has been centered at the South Pole for over 100 million years, yet it has had significant amounts of ice for only the last half of that interval (as CO2 levels dropped). Only a much warmer atmosphere can explain a pole-centered continent with no ice on it.
By 2.5 million years ago, or not long after, the first members of our genus (Homo, meaning "man") appeared in Africa (fig. 2.1). These beings, called ho-minids, were somewhat shorter than us, and they had much smaller brains, about one-third the size of ours, and large brow ridges above their eyes. They fashioned crude tools by chipping and flaking pieces of stone of different hardness. Some of their food came from hunting small game and some from scavenging kills made by large carnivores. One use for the tools was to extract food: big hammer stones for battering and crushing bone to get at the marrow inside; smaller, sharper flakes for cutting through hides and scraping and cutting flesh to separate it from bone and sinew; and still other stones to mash raw meat to a hamburger-like consistency. To most of us, these tools look like random pieces of stone, but tools they were, at least in this primitive sense.
As the energy-demanding brains of our remote ancestors grew larger, they needed more protein in their diets, and meat obtained both from scavenging and from hunting small animals was a ready (although not dominant) source. These
Millions of Years Ago j_i_i_i_i_
Millions of Years Ago
2.1. The sequence that led to modern humans during the last 4 million years includes pre-human australopithecines that walked; early members of our genus Homo (man) that used stone tools and controlled fire; and members of our own species (Homo sapiens).
protohumans lived a wide-ranging hunting-gathering life, harvesting a wide variety of nuts and berries as they became seasonally available. Some of the shaped tools were probably used to dig out tubers and roots. Like modern chimps and some primitive people, they likely used sticks to extract termites from holes and eat them. The landscape also contained many other delicacies for those with the stomach for them: bird eggs, field mice, and other fare.
As food resources were used up in one region, these hominids moved on to another. In many regions where food resources were naturally abundant, this style of life was not harsh, and only part of the day had to be devoted to satisfying basic food requirements. But the frequent movement made it difficult to accumulate large stores of food and left these beings vulnerable to extended droughts, prolonged freezes, and other extremes of weather, especially in areas less blessed with natural resources.
By about 500,000 years ago, our genus Homo had come to look more like us, with larger, rounder skulls holding larger brains, by then about two-thirds the size of ours. These people were members of our own species, Homo sapiens ("wise man"), although not yet as intelligent as modern humans. In The Time before History, Colin Tudge comments that these creatures "would not quite pass muster in a bus queue" today. Yet they had taken a very important step by gaining control of fire, probably at first by making opportunistic use of fires started by lightning
Homo sapiens strikes, and then later by learning how to create it at will. Control of fire brought greater protection against cold weather and wild animals, the ability to render raw meat more edible, and the possibility of manipulating the landscape by intentionally burning to improve prospects for game, in effect an early form of wildlife management.
With larger brains, these beings gradually became more adept at a range of survival skills. They could begin to draw on stored knowledge of how the game had behaved in previous years and thereby anticipate the same behavior the following year and in the future. Their improving communication skills would also have made possible increasingly clever and complex group hunting strategies. And they could use stones as crude missiles to bring down small game. These people also managed to spread out of Africa and across southern Asia. Yet, considering that 2 million years had elapsed since our genus first appeared, not much progress had been made toward modernity. Humans were still in the Stone Age, still living a hunter-gatherer life, and still making crude (but now slightly more sophisticated) stone tools.
One consequence of the highly mobile life was that children had to be spaced at relatively long intervals of four years or more. Constant relocations in search of new food resources forced people to carry all their possessions from place to place, and this required choosing between carrying infants or hauling other precious cargo that was critical to basic survival. Partly as a result of spacing their children, human populations remained small. Dependence on resources that occasionally became very scarce also helped to limit populations.
Sometime between 150,000 to 100,000 years ago, nearly modern people evolved in Africa. These people were now very much like us physically: taller than their predecessors, and with much larger brains. Like their predecessors, they still made tools by chipping and flaking stone, but they did so with increasing sophistication, producing more varied, more delicate, and better-made (though still crude) tools using more clever choices of stone. We know that they buried their dead and cared for their sick: the latter conclusion is based on finds of fossil skeletons of individuals who lived through adulthood with deformities that would have required help from others. At first, like their predecessors, they continued to rely mainly on smaller game that could be killed with little or no danger to the hunter.
It is odd to think about highly intelligent human beings living such incredibly primitive lives. Their brains were capable of everything ours are now, but they lacked the base of common knowledge available today. If one of their babies were somehow brought into the modern era and raised in today's society, he or she would have just as good a chance as any of us to become an astrophysicist, a carpenter, or a billionaire manufacturer of malfunctioning computer software. This
Millions of Years Ago
2.2. Over the last 5 million years, as modern humans evolved, Earth's climate slowly cooled.
Millions of Years Ago
2.2. Over the last 5 million years, as modern humans evolved, Earth's climate slowly cooled.
juxtaposition may seem odd, but much the same thing has happened in recent decades when tribes that happened to have escaped the onrush of civilization were abruptly exposed to its complexities. People from many primitive cultures quickly adapted to modern ways.
This long story of prehuman and early human evolution played out in a world that continued to grow colder (fig. 2.2). Nearly 2.75 million years ago, the cooling reached a critical threshold, and ice sheets first appeared in high latitudes of the Northern Hemisphere. This ice was not permanent; it grew and melted as changes in Earth's orbit delivered varying amounts of sunlight to north polar regions. Through time, the glacial cycles grew larger.
Through these millions of years, our ancestors had no lasting effects on the environment or on climate. At some point they began using "fire sticks" to burn grasslands and thereby altered their local environment temporarily. But dry-season lightning strikes had been doing the same thing for the hundreds of millions of years that vegetation had been growing on land. Burning leaves the roots of the grasses untouched, while it fertilizes the soil and causes grass to grow back even more prolifically. Burning also releases CO2 to the atmosphere, but new grass reclaims it through photosynthesis during the next growing season. The people move on, and they leave no permanent trace of their presence on the landscape. Nor do they affect the level of water in the tub.
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