Age Diameter Name Location (million years) Period (km)
Vredefort South Africa 2023 Precambrian 300
Sudbury Canada 1850 Precambrian 250
Bedout Australia 250 Permian 200
Chicxulub Mexico 65 Cretaceous 170
Manicougan Canada 214 Triassic 100
Popigai Russia 35 Tertiary 100
Chesapeake Bay United States 36 Tertiary 90
Acraman Australia 590 Precambrian 90
Puchezh-Katunki Russia 175 Jurassic 80
Morokweng South Africa 145 Jurassic 70
Kara Russia 73 Cretaceous 65
Beaverhead United States 600 Precambrian 60
Tookoonooka Australia 128 Cretaceous 55
Charlevoix Canada 357 Carboniferous 54
Kara-kul Tajikistan 5 Tertiary 52
Siljan Sweden 368 Devonian 52
Damage to trees caused by the Tunguska event. On June 0, 0, a huge fireball exploded in the sky over western Siberia, followed by an enormous explosion. The explosion flattened almost ,00 square miles (more than ,000 square km) of forest. The explosion occurred near the town of Vanavara, in the Podkamennaya Tunguska River valley. The explosion was probably caused by a meteorite about 0 feet (about 0 m) in diameter that broke up in the atmosphere, although no major fragments or craters have been found. This photo was taken in 0. (Courtesy of Novosti Press Agency/Science Photo
Library/Photo Researchers, Inc.)
by a silent wall of darkness advancing faster than sound. The impact would set off tsunamis, earthquakes, and volcanoes, and the hot ashes would ignite fires all around. If such an aster- oid hit today in Manson, a billion and a half people would die in the first day. Despite the magnitude of this impact, there are no known extinctions associated with the Manson event.
Some extraterrestrial impacts have occurred during recent history. An impact crater in southeastern Germany is only 2,200 years old, a date that matches descriptions in Roman records of fire falling from the sky. The Kaali event, a natural disaster that occurred about 350 b.c.e. in what is now Esto- nia and was preserved in legend, may have been an asteroid. An asteroid or comet, about 300 feet (100 m) in diameter, exploded above a remote area of Siberia on June 30, 1908. The object did not hit the ground but devastated over a half million acres of forest in the Tunguska region (see photo on page 30). Scientists refer to this impact as the Tunguska Event. While it is not known whether humans died in the explosion, a man’s clothing caught fire from it over 30 miles (50 km) away. Trees were flattened, radiating outward from the impact center. Debris from the explosion extended far into the upper atmosphere, reflecting sunlight for great distances. In Western Europe people were able to play tennis outside far into the night without artificial lighting because of it. If the impact had occurred over a densely populated region, the loss of life and property could scarcely be calculated.
Further Reading
Beatty, J. Kelly, ed. The New Solar System. Cambridge University Press, 1998.
Department of Geological Sciences, University of California, Santa Barbara. “Evidence of meteor impact found off Australian coast.” Available online. URL: http://beckeraustralia.crustal.ucsb.edu/. Accessed March 22, 2005.
Stone, Richard. “The last great impact on Earth.” Discover, Septem- ber 1996, 60–61.
australopithecines
Australopithecines were hominin apes that lived in Africa between about four million and one million years ago. Scientists universally recognize that some member of this group, perhaps yet to be discovered, was ancestral to modern humans.Two features distinguished australopithecines from other apes. First, unlike all other apes except the genus Homo, they walked upright on two legs (see bipedalism). Second, they differed from the genus Homo in that their brains were not consistently larger than those of modern apes such as chim- panzees and gorillas. Australopithecines may have used tools, such as sticks, or used rocks as tools; both of these are behav- iors seen in modern chimps. There is no evidence that they deliberately altered stones into tools.
Different anthropologists classify extinct hominins into different numbers of species and genera. In this entry, the term australopithecine is used to indicate hominins intermedi- ate between their pre-bipedal ancestors and the larger-brained genus Homo. As here used, the term australopithecine incor- porates the genera Ardipithecus, Australopithecus, Kenyan- thropus, Orrorin, Paranthropus, and Sahelanthropus. The
paragraphs below review the genera of australopithecines and related hominins that have been thus far discovered, in the approximate chronological order of their evolutionary appearance.
Sahelanthropus
The earliest bipedal ape currently known is Sahelanthropus tchadensis (“Sahel man that lived in Chad”), fossils of which were discovered in 2002 by a member of the research team of French anthropologist Michel Brunet in Chad, western Africa. Sahelanthropus lived six to seven million years ago (see photo above). DNA studies (see DNA [evidence for evolution]) suggest that chimpanzees and humans diverged from a common ancestor five million years ago. The structure of the fossil skull of Sahelanthropus suggests that this species walked upright. Evolutionary scientists have not yet deter- mined whether Sahelanthropus was a direct human ancestor. If in fact Sahelanthropus represents the common ancestral condition of both humans and chimpanzees, it is difficult to understand how chimpanzees would have lost bipedalism. Alternatively, the hominin and chimpanzee lines may have diverged earlier than the five million years suggested by the DNA studies. The discoverers claim that Sahelanthropus was a true human ancestor, and other australopithecines were not.
Orrorin
Orrorin tugenensis (“ancestor that lived in the Tugen Hills”) was discovered in Kenya, eastern Africa, in 2000 by pale- ontologists Martin Pickford and Brigitte Senut. The fossils are about six million years old. A leg bone, which includes
Skull of Sahelanthropus. Found in western Africa, this skull represents one of the earliest apes that was adapted to upright walking. (Courtesy
of Michel Brunet, Laboratoire de Géobiologie, Biochronologie et Paléontologie Humaine, Faculté des Sciences Fondamentales et Appliquées, Université de Poitiers, France)
the expanded end that connected with the knee joint, sug- gests that this hominin walked upright. Other limb bones, jaw fragments, and teeth were found as well. Orrorin pres- ents the same challenge to the interpretation of human evo- lutionary history as does Sahelanthropus: Bipedalism may have been present in the common ancestor of humans and chimpanzees, and it calls into question whether the later australopithecines were human ancestors. The place of ori- gin of the human lineage is likewise unclear: was it western Africa, where Sahelanthropus lived, or eastern Africa, where Orrorin lived? At the time, the extensive deserts of Africa had not yet formed, and the forests in which both Sahelan- thropus and Orrorin lived may have been continuous across the continent.
Ardipithecus
Ardipithecus ramidus (called “basic root ape” because it may represent the branch point or beginning of the human lin- eage) was discovered in Ethiopia in 1994 by anthropologist Tim White and associates. The fossils are about 4.4 million years old. The bones suggest that Ardipithecus was bipedal. Anthropologists have not agreed whether Ardipithecus was the ancestor of later australopithecines.
Australopithecus
The genus Australopithecus (“southern ape”) is represented by several species that lived between about four million and about two million years ago in various parts of Africa. The three earliest species were A. anamensis and A. afarensis, both named after the eastern African regions in which their fossils were found, and A. bahrelghazali (“Bahr el Ghazal,” the western African location at which the fossils were found). These species lived between four and three million years ago. Two other species, A. garhi (“surprise”) from eastern Africa and A. africanus (“African”) from southern Africa, lived between three million and two million years ago. The evolutionary relationships among these species are unclear. Genus Australopithecus is called the “gracile aus- tralopithecines” because they were small (three to four feet [one m] in height), had small faces, and jaws adapted to an omnivorous diet.
Australopithecus africanus was the earliest prehu- man hominin to be discovered. A South African anatomist (see Dart, Raymond) discovered this species in the 1920s. The especially striking fossil of the Taung child (see photo at right) showed a mixture of human and apelike features. Anthropologist Robert Broom found many other A. africanus specimens in the field. At first, British anthropologists dis- regarded Dart’s and Broom’s discoveries, primarily because they relied on Piltdown man as a guide to understanding human evolution. The characteristics of Piltdown man sug- gested that human brain size had begun to increase very early in human evolution, and that this transition had occurred in Europe. The Taung specimen suggested that ape skulls had begun assuming modern characteristics before any increase in brain size, and that this transition had occurred in Africa. Dart lived to see Piltdown man revealed as a hoax, and the
general acceptance of Australopithecus as a valid human ancestral genus, even if A. afarensis may not have been on the main line of human evolution.
Since the bones of A. africanus were found in asso- ciation with those of many prey mammals whose bones had been crushed, it was at first thought that A. africanus was a hunter. This is the image presented by writer Rob- ert Ardrey, upon which writer Arthur C. Clarke based the opening of the movie 2001: A Space Odyssey. However, a skull of a child of this species was found that had holes that exactly matched the species of leopard that was present in southern Africa at that time. According to the research of South African paleontologist C. K. Brain, the piles of crushed and broken bones, found in limestone caves, were apparently leftovers from leopard meals, rather than aus- tralopithecine hunts. Modern leopards eat their kills in trees, which often grow out of limestone caves in the arid South African landscape. A. africanus was the prey, not the predator.
One of the most famous australopithecine fossils, “Lucy,” was a nearly complete A. afarensis skeleton found in the early 1970s by Donald Johanson (see Johanson, Donald) (see figure on page 33). Since the discovery of Lucy, both small and large individuals of this species have been found, which may represent females and males of this species or may indicate that there was more than one species of australopithecine at that place and time. Because it was so nearly complete, the Lucy skeleton revealed a great deal about the movements of this species. The major adaptations for bipedalism were present in Australopithecus afarensis: The opening for the spinal cord was underneath the skull,
Skull of the “Taung child” Australopithecus africanus. Discovered by Raymond Dart in , this eventually directed attention of researchers to Africa as the place where humans evolved. The specimen consists of part of a juvenile skull and mandible and an endocast of the right half of the brain (seen at center left). It is about . million years old. (Courtesy of
Pascal Goetgheluck/Photo Researchers, Inc.)
and the pelvis was suitable for walking. The feet, however, appear to have been only partly adapted to upright walk- ing: the big toe was still at a noticeable angle. This suggests that Australopithecus lived mostly on the ground, walking upright, but frequently scrambled back into the trees when danger threatened. The relatively long arms and short legs of A. afarensis further suggest a mixture between arboreal
and ground life. It even suggests the possibility that upright posture began as an adaptation for shinnying up trees from brief visits to the ground and later allowed full adaptation to ground life.
The primarily upright locomotion of A. afarensis and/ or related australopithecines was confirmed by famous fos- silized footprints found by anthropologist Mary Leakey (see Leakey, Mary) at Laetoli in Tanzania (see photo below). At about three million years of age, the footprints were undoubtedly produced by A. afarensis or a closely related species. Their date is fairly certain, as the footprints were imbedded in volcanic dust, on which radiometric dating can be used.
Skeleton of “Lucy,” the Australopithecus afarensis discovered by Donald Johanson and associates. It is one of the most complete australopithecine skeletons that has been found, and it demonstrated that upright posture preceded increase in brain size in human evolution. (Courtesy of Science
VU/Visuals Unlimited)
Trail of hominin footprints in volcanic ash. This 00-foot (0-m) trail was discovered by Mary Leakey’s expedition at Laetoli, Tanzania, in . The footprints were probably made by Australopithecus afarensis individuals . million years ago. They demonstrate that hominins had already acquired the upright, free-striding gait of modern humans. The footprints have well-developed arches and the big toe does not diverge noticeably. They are of two adults with possibly a third set belonging to a child who walked in the footsteps of one of the adults. (Courtesy of John Reader/
In late 2006, two significant new Australopithecus dis- coveries were announced. A baby A. afarensis was found in Ethiopia by anthropologist Zeresenay Alemseged. The “Dikika Baby” fossil is even older than the Lucy fossil, and its shoulder blades indicated that this species retained some ability to swing in trees. The “Little Foot” australopithecine fossil, found in Sterkfontein, South Africa, had previously been represented only by a foot. In late 2006, the discovery of more of this same individual was announced.
Kenyanthropus
Kenyanthropus platyops (“flat-faced man of Kenya”) was dis- covered by anthropologist Meave Leakey. This species lived about three and a half million years ago and resembled Aus- tralopithecus in many ways, including upright posture. As its name suggests, it had a much flatter face than any other aus- tralopithecine, a feature it shares with Homo. For this reason, some anthropologists suggest that Kenyanthropus, rather than any species of Australopithecus, was the ancestor of humans. Paranthropus Three species of the genus Paranthropus (“alongside human”) were the “robust australopithecines.” They all lived in east- ern Africa and eventually became extinct without leaving evolutionary descendants. They contrasted with the gracile australopithecines in having huge jaws that allowed a diet of coarse vegetable materials. The first robust australopithecine to be discovered, by anthropologist Louis Leakey (see Leak- ey, Louis), was even called “Nutcracker man.” The large
teeth and prominent sagittal crest of robust australopithecine skulls indicated that they could chew powerfully. The sagittal crest, across the top of the skull, allowed attachment sites for large chewing muscles (see photo above). Chemical analysis of the tooth enamel of robust australopithecines suggests that they ate plants with the C4 photosynthetic pathway, prob-
ably grasses, or termites that had eaten grass (see isotopes). Detailed laser analysis of layers of tooth enamel in P. robus- tus specimens indicate that they were able to switch between food sources based on C4 plants and those based on C3 plants
such as fruits and nuts. The conclusion is that robust austra- lopithecines had dietary flexibility, and that their extinction was probably not due to the disappearance of food supplies. This conclusion raises the possibility that robust australopith- ecines were driven to extinction by competition with early modern humans, rather than by environmental changes. With their big jaws, the robust australopithecines may have looked fierce but were probably gentle, as are most herbivores. Paranthropus aethiopicus (“of Ethiopia”) lived about two million years ago. This species may have been the ancestor of P. robustus (“robust”) and P. boisei (named after Mr. Boise, a benefactor of Louis Leakey), which lived between two mil- lion and one million years ago. When climates change, species evolve and proliferate, but in many different ways. About two and a half million years ago, the climate of east Africa became even drier, with more grassland and less forest cover. This occurred at the beginning of the Pleistocene epoch, when the Earth began its ongoing cycle of ice ages. While Ice Ages caused very cold climates in the north, they caused dry climates in tropical areas such as those inhabited by the African hominins. According to paleontologist Elisabeth Vrba, this change, while not sudden, was sufficiently severe that many species of mammals (such as species of antelopes) became extinct, and others evolved that were better able to survive in open grasslands. This is also approximately when baboons evolved from arboreal monkeys. The hominins were no exception to this pattern. This was about the time of the last gracile australopithecines, which apparently evolved in two different directions. Some of them evolved into a more humanlike form, with larger brains and use of tools. These early humans, which may have been several species, are often referred to as Homo Habilis. Oth- ers evolved into the robust australopithecines.
A half dozen or more hominin species may have lived in Africa all at the same time, about two million years ago. It is presently impossible to determine which of them may have been ancestral to modern humans, although some (such as Paranthropus) can be dismissed. The last of the australopithecines were the robust australopithecines, which did not become extinct until after the evolution of Homo. Homo and Paranthropus lived at the same time and general location. Evolutionary biologist Stephen Jay Gould suggested that, rather than a ladder of upward progress, the human evolutionary story more closely resembles a bush, with branches leading in different directions, only one of which became the modern genus Homo (see progress, concept of).
This skull of a robust australopithecine (Paranthropus boisei) that lived in Africa about . million years ago has a sagittal crest across the top, which served as an attachment surface for large chewing muscles. The sagittal crest and the large molars indicate that the robust australopithecines primarily ate coarse vegetation. (Courtesy of Pascal
Goetgheluck/Science Photo Library)
Further Reading
Alemseged, Zerxenay, et al. “A juvenile early hominin skeleton from Dikaka, Ethiopia.” Nature 443 (2006): 296–301.
Johanson, Donald, and Maitland Edey. Lucy: The Beginnings of Humankind. New York: Warner Books, 1982.
———, Lenora Johanson, and Blake Edgar. Ancestors: In Search of Human Origins. New York: Villard Books, 1994.
Johnson, Patrick, and Scott Bjelland. “Kenyanthropus.” Available online. URL: http://www.kenyanthropus.com. Accessed March 22, 2005.
———. “Toumai Sahelanthropus tchadensis.” Available online. URL: http://www.sahelanthropus.com. Accessed March 22, 2005. Klein, Richard G., and Blake Edgar. The Dawn of Human Culture:
A Bold New Theory on What Sparked the “Big Bang” of Human Consciousness. New York: John Wiley, 2002.
Kreger, C. David. “Australopithecus/Ardipithecus ramidus.” Avail- able online. URL: http://www.modernhumanorigins.com/ramidus. html. Accessed March 22, 2005.
———. “Orrorin tugenensis.” Available online. URL: http://www. modernhumanorigins.com/lukeino.html. Accessed March 22, 2005.
Leakey, Richard, and Roger Lewin. Origins Reconsidered: In Search of What Makes Us Human. New York: Doubleday, 1992.
Sloan, Christopher P. “Found: Earliest Child—3.3 million year old bones discovered. National Geographic, November 2006, 148–159.
Smithsonian Institution, Museum of Natural History, Washington, D.C., USA. “Australopithecus afarensis: Composite reconstruc- tion.” Available online. URL: http://www.mnh.si.edu/anthro/ humanorigins/ha/afarcomp.htm. Accessed March 22, 2005. Sponheimer, Matt, et al. “Isotopic evidence for dietary variability in
the early hominin Paranthropus robustus.” Science 314 (2006): 980–982.
Tattersall, Ian, and Jeffrey H. Schwartz. Extinct Humans. New York: Westview Press, 2000.
Walker, Alan, and Pat Shipman. The Wisdom of the Bones: In Search of Human Origins. New York: Knopf, 1996.
Walker, Joanne et al. “U-Pb isotopic age of the StW 573 hominid from Sterkfontein, South Africa.” Science 314 (2006): 1592– 1594.
bacteria, evolution of
Of all the cells and organisms in the world, bacteria and archaebacteria are the: (1) smallest, (2) simplest, (3) most numerous, (4) most metabolically diverse, and (5) oldest. Although for many decades scientists classified archaebacteria in the same category as bacteria, most now rec- ognize archaebacteria to be an evolutionary lineage as distinct from the bacteria (also called eubacteria, or true bacteria) as they are from the complex organisms (see tree of life).Smallest. The typical eubacterial or archaebacterial cell