|
Million Years Ago Stages of Evolution |
|
|
700 |
early animals |
|
620 |
first animal brains |
|
580 |
shells and skeletons |
|
500 |
vertebrates |
|
450 |
plants come ashore |
|
400 |
amphibians and insects come ashore |
|
350 |
seed ferns |
|
300 |
fungi |
|
250 |
reptiles |
|
225 |
conifers, dinosaurs |
|
200 |
mammals |
|
150 |
birds |
|
125 |
flowering plants |
|
70 |
extinction of dinosaurs |
|
65 |
early primates |
|
35 |
monkeys |
|
20 |
apes |
|
10 |
great apes |
|
4 |
upright walking “Southern apes” |
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ways of life. There are about 750,000 known species of insects today, three times as many as all other animal species together.
During the 150 million years after they left the sea, the amphibians evolved into reptiles, endowed with several strong selective advantages—powerful jaws, drought-resistant skin, and most important, a new kind of eggs. As the mammals would in their wombs later on, the reptiles encapsulated the former marine environment in large eggs, in which their offspring could prepare themselves fully for spending their entire life cycles on land. With these innovations reptiles rapidly conquered the land and evolved into numerous varieties. The many types of lizards that still exist today, including the limbless snakes, are descendants of those ancient reptiles.
While the first lineage of fish crawled out of the water and turned into amphibians, shrubs and trees were already thriving on land, and when the amphibians evolved into reptiles they lived in lush tropical forests. At the same time, a third type of multicellular organism, the fungi, had come ashore. Fungi are plantlike and yet so different from plants that they are classified as a separate kingdom, which displays a variety of fascinating properties. 46 They lack the green chlorophyll for photosynthesis and do not eat and digest, but absorb their nutrients directly as chemicals. Unlike plants, fungi do not have vascular systems for forming roots, stems, and leaves. They have very distinctive cells, which may contain several nuclei and are separated by thin walls through which the cell fluid can flow easily.
Fungi emerged more than 300 million years ago and expanded in close coevolution with plants. Virtually all plants that grow in the soil rely on a tiny fungus in their roots for the absorption of nitrogen. In a forest the roots of all the trees are interconnected by an extensive fungal network, which occasionally comes up through the earth as mushrooms. Without fungi the primeval tropical forests could not have existed.
Thirty million years after the appearance of the first reptiles, one of their lineages evolved into dinosaurs (a Greek term meaning “terrible lizards”), which seem to hold endless fascination for humans of all ages. They came in a great variety of sizes and
254
THE WEB OF LIFE
shapes. Some had body armors with horny beaks, like modern turtles, or horns. Some were herbivores, others were carnivores. Like the other reptiles, dinosaurs were egg-laying animals. Many built nests, and some even developed wings and eventually, around 150 million years ago, evolved into birds.
At the time of the dinosaurs the expansion of reptiles was in full swing. The land and waters were populated by snakes, lizards, and sea turtles, as well as by sea serpents and several species of dinosaurs. Around 70 million years ago the dinosaurs and many other species suddenly disappeared, most likely because of the impact of a giant meteorite measuring seven miles across. The catastrophic explosion generated an enormous cloud of dust, blocking out sunlight for a prolonged period and drastically changing worldwide weather patterns, which the huge dinosaurs could not survive.
Caring for the Young
About 200 million years ago a warm-blooded vertebrate evolved from the reptiles and diversified into a new class of animals that would eventually bring forth our ancestors, the primates. The females of these warm-blooded animals no longer enclosed their embryos in eggs but instead nourished them inside their own bodies. After birth the young were relatively helpless and were nursed by their mothers. Because of this distinctive behavior, which includes nursing with milk secreted from mammary glands, this class of animals is known as “mammals.” Around 50 million years later another lineage of warm-blooded vertebrates, the newly evolved birds, also began to feed and teach their vulnerable offspring.
The first mammals were small nocturnal creatures. Whereas the reptiles, unable to regulate their body temperatures, were sluggish during the cool nights, the mammals evolved the ability to maintain their body warmth at relatively constant levels independent of their surroundings and thus remained alert and active at night. They also transformed some of their skin cells into hair,
which insulated them further and allowed them to migrate from the tropics to colder climates.
The early primates, known as prosimians (“premonkeys”), evolved in the tropics around 65 million years ago from nocturnal, insect-eating mammals that lived in trees and looked somewhat like squirrels. Today’s prosimians are small forest animals, mostly nocturnal and still living in trees. To jump from branch to branch at night, those early insect-eating tree dwellers developed keen eyesight, and in some species the eyes shifted gradually to a frontal position, which was crucial to developing three-dimensional vision—a decisive advantage for judging distances in trees. Other well-known primate characteristics that evolved from their tree- climbing skills are clinging hands and feet, flat fingernails, opposable thumbs, and big toes.
Unlike other animals, the prosimians were not anatomically specialized and therefore were always threatened by enemies. However, they made up for their lack of specialization by developing greater dexterity and intelligence. Their fear of enemies, constant running and hiding, and active night life encouraged cooperation and led to the social behavior that is characteristic of all higher primates. In addition, the habit of protecting themselves by making frequent loud noises gradually evolved into vocal communication.
Most primates are insect eaters or vegetarians, feeding on nuts, fruits, and grasses. At times, when not enough nuts and fruits were available in the trees, the early primates would have left the protective branches and come down to the ground. Looking anxiously for enemies over tall grasses, they would assume an upright posture for brief moments before returning to a crouched position, as baboons still do today. This ability to stand upright, even for short moments, represented a strong selective advantage, as it allowed the primates to use their hands for gathering food, wielding sticks, or throwing rocks to defend themselves. Gradually their feet became flatter, their manual dexterity increased, and the use of primitive tools and weapons stimulated brain growth, and thus some of the prosimians evolved into monkeys and apes.
The evolutionary line of the monkeys diverged from that of the
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THE WEB OF LIFE
prosimians around 35 million years ago. Monkeys are diurnal animals, generally with flatter and more expressive faces than those of prosimians, and they usually walk or run on four legs. Around 20 million years ago the line of the apes split from that of the monkeys, and after another 10 million years our immediate ancestors, the great apes—orangutans, gorillas, and chimpanzees— came into their own.
All apes are forest dwellers, and most of them spend at least some of the time in trees. Gorillas and chimpanzees are the most terrestrial of the apes, traveling on all fours by “knuckle walking”—that is, leaning on the knuckles of their forelimbs. Most apes are also able to walk on two legs for short distances. Like humans, apes have broad, flat chests, and arms capable of reaching up and backward from the shoulder. This enables them to move in trees by swinging from branch to branch arm over arm, a feat of which monkeys are not capable. The brains of the great apes are much more complex than those of monkeys, and thus their intelligence is far superior. The ability to use and, to a limited extent, even make tools is characteristic of the great apes.
Around 4 million years ago a chimpanzee species in the African tropics evolved into an upright walking ape. This primate species, which became extinct a million years later, was quite similar to the other great apes, but because of its upright gait it has been classified as a “hominid,” which, according to Lynn Margulis, is unjustified on purely biological grounds:
Objective scholars, if they were whales or dolphins, would place humans, chimpanzees, and orangutans in the same taxonomic group. There is no physiological basis for the classification of human beings into their own family. . . . Human beings and chimps are far more alike than any two arbitrarily chosen genera of beetles. Nonetheless, animals that walk upright with their hands
dangling free are aggrandizingly defined as hominids . . . not
47
apes.
257
The Human Adventure
Having followed the unfolding of life on Earth from its very beginnings, we cannot help feeling a special sense of excitement when we arrive at the stage where the first apes stand up and walk on two legs, even though this may not be justified scientifically. As we learn how reptiles evolved into warm-blooded vertebrates who care for their young; how the first primates developed flat fingernails, opposable thumbs, and the beginnings of vocal communication; and how the apes developed humanlike chests and arms, complex brains, and tool-making capabilities, we can trace the gradual emergence of our human characteristics. And when we reach the stage of upright walking apes with free hands, we feel that now the human evolutionary adventure begins in earnest. To follow it closely, we must shift our time scale once more, this time from millions of years to thousands.
The upright walking apes, which became extinct around 1.4 million years ago, all belonged to the genus Australopithecus. The name, derived from the Latin australis (“southern”) and the Greek pithe\os (“ape”), means “Southern ape” and is a tribute to the first discoveries of fossils belonging to this genus in South Africa. The oldest species of these Southern apes is known as Australopithecus afarensis, named after fossil finds in the Afar region in Ethiopia that included the famous skeleton called “Lucy.” They were lightly built primates, perhaps 4.5 feet tall, and probably as intelligent as present-day chimpanzees.
After almost 1 million years of genetic stability, from around 4 to around 3 million years ago, the first species of Southern apes evolved into several more heavily built species. These included two early human species that coexisted with the Southern apes in Africa for several hundred thousand years, until the latter became extinct.
An important difference between human beings and the other primates is that human infants need much longer to pass into childhood, and human children longer again to reach puberty and adulthood, than any of the apes. Whereas the young of other
|
Human Evolution |
|
|
Years Ago |
Stages of Evolution |
|
4 million |
Australopithecus afarensis |
|
3.2 million |
“Lucy” (Australopithecus afarensis) |
|
2.5 million |
several Australopithecus species |
|
2 million |
Homo habilis |
|
1.6 million |
Homo erectus |
|
1.4 million |
Australopithecines become extinct |
|
1 million |
Homo erectus settles in Asia |
|
400,000 |
Homo erectus settles in Europe Homo sapiens begins to evolve |
|
250,000 |
archaic forms of Homo sapiens Homo erectus becomes extinct |
|
125,000 |
Homo neanderthalensis |
|
100,000 |
Homo sapiens fully evolved in Africa and Asia |
|
40,000 |
Homo sapiens (Cro-Magnon) fully evolved in Europe |
|
35,000 |
Neanderthals become extinct; Homo sapiens remains the single surviving human species |
mammals develop fully in the womb and leave it ready for the outside world, our infants are incompletely formed at birth and utterly helpless. Compared with other animals, human infants seem to be born prematurely.
This observation is the basis of the widely accepted hypothesis that the premature births of some apes may have been decisive in triggering human evolution. 48 Because of genetic changes in the timing of development, the prematurely born apes may have retained their youthful traits longer than others. Ape couples with those characteristics, known as neoteny (“extension of the new”), would have given birth to more prematurely born children, who would have retained even more youthful traits. Thus an evolutionary trend may have been started that eventually resulted in a relatively hairless species whose adults in many ways resemble the embryos of apes.
259
According to this hypothesis, the helplessness of the prematurely born infants played a crucial role in the transition from apes to humans. These newborns required supportive families, which may have formed the communities, nomadic tribes, and villages that became the foundations of human civilization. Females selected males who would take care of them while they nursed and protected their infants. Eventually the females no longer went into heat at specific times, and since they could now be sexually receptive at any time, the males caring for their families may have changed their sexual habits as well, decreasing their promiscuity in favor of new social arrangements.
At the same time, the freedom of the hands to make tools, wield weapons, and throw rocks stimulated the continuing brain growth that is characteristic of human evolution and may even have contributed to the development of language. As Margulis and Sagan describe it:
By throwing rocks, and stunning or killing small prey, early humans were catapulted into a new evolutionary niche. The skills necessary to plot the trajectories of projectiles, to kill at a distance, were dependent on an increase in the size of the left hemisphere of the brain. Language abilities (which have been associated with the left side of the brain . . .) may have fortuitously accompanied such an increase in brain size. 49
The first human descendants of the Southern apes emerged in East Africa around 2 million years ago. They were a small slender species with markedly expanded brains, which enabled them to develop tool-making skills far superior to those of any of their ape ancestors. This first human species was therefore given the name Homo habilis (“skillful human”). By 1.6 million years ago Homo habilis had evolved into a more robust and larger species, whose brain had expanded further. Known as Homo erectus (“upright human”), this species persisted well over a million years and became far more versatile than its predecessors, adapting its technologies and ways of life to a wide range of environmental conditions. There are indications that these early humans may have gained control of fire around 1.4 million years ago.
Homo erectus was the first species to leave the comfortable African tropics and migrate into Asia, Indonesia, and Europe, settling in Asia around 1 million years ago and in Europe around 400,000 years ago. Far away from their African homeland, the early humans had to endure extremely harsh climatic conditions that had a strong impact on their further evolution. The entire evolutionary history of the human species, from the emergence of Homo habilis to the agricultural revolution almost 2 million years later, coincided with the famous ice ages.
During the coldest periods sheets of ice covered large parts of Europe and the Americas, as well as small areas in Asia. These extreme glaciations were interrupted repeatedly by periods during which the ice retreated and gave way to relatively mild climates. However, large-scale floods, caused by the melting of the ice caps during the interglacial periods, were additional threats to animals and humans alike. Many animal species of tropical origin became extinct and were replaced by more robust, woolly species—oxen, mammoths, bison, and the like—which could withstand the harsh conditions of the ice ages.
The early humans hunted those animals with stone axes and spearheads, feasted on them by the fire in their caves, and used the animals’ furs to protect themselves from the bitter cold. Hunting together, they also shared their food, and this sharing of food became another catalyst for human civilization and culture, eventually bringing forth the mythical, spiritual, and artistic dimensions of human consciousness.
Between 400,000 and 250,000 years ago Homo erectus began to evolve into Homo sapiens (“wise human”), the species to which we modern humans belong. This evolution occurred gradually and included several transitional species, which are referred to as archaic Homo sapiens. By 250,000 years ago Homo erectus was extinct; the transition to Homo sapiens was complete around 100,000 years ago in Africa and Asia and around 35,000 years ago in Europe. From that time on, fully modern humans have remained as the single surviving human species.
While Homo erectus gradually evolved into Homo sapiens, a different line branched off in Europe and evolved into the classic
261
Neanderthal form around 125,000 years ago. Named after the Neander Valley in Germany, where the first specimen was found, this distinct species persisted until 35,000 years ago. The unique anatomical features of the Neanderthals—they were stocky and robust, with massive bones, low sloping foreheads, heavy jaws, and long, protruding front teeth—were probably due to the fact that they were the first humans to spend long periods in extremely cold environments, having emerged at the onset of the most recent ice age. The Neanderthals settled in southern Europe and Asia, where they left behind signs of ritualized burials in caves decorated with a variety of symbols and of cults involving the animals they hunted. By 35,000 years ago they had either become extinct or had merged with the evolving species of modern humans.
The human evolutionary adventure is the most recent phase in the unfolding of life on Earth, and for us, naturally, it holds a special fascination. However, from the perspective of Gaia, the living planet as a whole, the evolution of human beings has been a very brief episode so far and may even come to an abrupt end in the near future. To demonstrate how late the human species arrived on the planet, the Californian environmentalist David Brower has devised an ingenious narrative by compressing the age of the Earth into the six days of the biblical creation story. 50
In Brower’s scenario the Earth is created on Sunday at midnight. Life in the form of the first bacterial cells appears on Tuesday morning around 8:00 a.m. For the next two and a half days the microcosm evolves, and by Thursday at midnight it is fully established, regulating the entire planetary system. On Friday around 4:00 P.M., the microorganisms invent sexual reproduction, and on Saturday, the last day of creation, all the visible forms of life evolve.
Around 1:30 a.m. on Saturday the first marine animals are formed, and by 9:30 a.m. the first plants come ashore, followed two hours later by amphibians and insects. At ten minutes before five in the afternoon, the great reptiles appear, roam the Earth in lush tropical forests for five hours, and then suddenly die out around 9:45 p.m. In the meantime the mammals have arrived on the Earth
in the late afternoon, around 5:30, and the birds in the evening, around 7:15.
Shortly before 10:00 p.m. some tree-dwelling mammals in the tropics evolve into the first primates; an hour later some of those evolve into monkeys; and around 11:40 p.m. the great apes appear. Eight minutes before midnight the first Southern apes stand up and walk on two legs. Five minutes later they disappear again. The first human species, Homo habilis, appears four minutes before midnight, evolves into Homo erectus half a minute later, and into the archaic forms of Homo sapiens thirty seconds before midnight. The Neanderthals command Europe and Asia from fifteen to four seconds before midnight. The modern human species, finally, appears in Africa and Asia eleven seconds before midnight and in Europe five seconds before midnight. Written human history begins around two-thirds of a second before midnight.
By 35,000 years ago the modern species of Homo sapiens had replaced the Neanderthals in Europe and evolved into a subspecies known as Cro-Magnon—named after a cave in southern France—to which all modern humans belong. The Cro-Magnons were anatomically identical to us, had fully developed language, and brought forth a veritable explosion of technological innovations and artistic activities. Finely crafted tools of stone and bone, jewelry of shell and ivory, and magnificent paintings on the walls of damp, inaccessible caves are vivid testimonies to the cultural sophistication of those early members of the modern human race.
Until recently archaeologists believed that the Cro-Magnons developed their cave art gradually, beginning with rather crude and clumsy drawings and reaching their height with the famous paintings at Lascaux around 16,000 years ago. However, the sensational discovery of the Chauvet cave in December 1994 forced scientists to radically revise their ideas. This large cave in the Ardeche region of southern France consists of a maze of underground chambers filled with over three hundred highly accomplished paintings. The style is similar to the art at Lascaux, but careful radiocarbon dating has shown that the paintings at Chauvet are at least 30,000 years old. 51
The figures, painted in ocher, hues of charcoal, and red hema-
THE UNFOLDING OF LIFE
263
tite, are symbolic and mythological images of lions, mammoths, and other dangerous animals, many of them leaping or running across large panels. Specialists in ancient rock art have been amazed by the sophisticated techniques—shading, special angles, staggering of figures, and so on—used by the cave artists to portray motion and perspective. In addition to the paintings, the Chauvet cave also contained a wealth of stone tools and ritualistic objects, including an altarlike stone slab with a bear skull placed on it. Perhaps the most intriguing find is a black drawing of a shamanistic creature, half human and half bison, in the innermost, darkest part of the cave.
The unexpectedly early date of those magnificent paintings means that high art was an integral part of the evolution of modern humans from the very beginning. As Margulis and Sagan point out:
Such paintings alone clearly mark the presence of modern Homo sapiens on earth. Only people paint, only people plan expeditions to the rear ends of damp, dark caves in ceremony. Only people bury their dead with pomp. The search for the historical ancestor of man is the search for the story-teller and the artist. 52
This means that a proper understanding of human evolution is impossible without understanding the evolution of language, art, and culture. In other words, we must now turn our attention to mind and consciousness, the third conceptual dimension of the systems view of life.