Human Evolution

One of the major goals of paleoanthropol-ogy—the study of human evolution—is to identify hominin species and to reconstruct a chronological sequence and evolutionary relationships among them. Hominin is a colloquial term used to indicate membership to the Tribe Hominini in which humans and their immediate ancestors are currently classified. This is accomplished by sorting the morphological variation within and between hominin fossil materials, taking into account idiosyncratic, geographic, and geologic parameters. Studying human evolution requires training in human and nonhuman primate anatomy, geology, ecology, taphonomy, and evolutionary biology in order to evaluate critically the fossil material and the depositional context in which it was recovered. Unfortu nately, despite the enormous amount of work that goes into recovering fossil humans, the often scanty nature of fossils makes many inferences tenuous at best. This was particularly true in the early twentieth century.

Prior to 1950, inferences about evolutionary history construed from a sparse fossil record led some researchers to believe that the human lineage extended back into the Early Miocene (roughly 14 to 17 million years ago). This paradigm, and others like it, was based primarily on Darwin's suggestion in Descent of Man (1872) that humans were closely related to the apes—something Linnaeus had recognized a century earlier. Darwin predicted that one should look where apes live (in the jungles of Africa) if human fossils were to be discovered. He also predicted that these fossils would prove that relatively large brains evolved long before bipedal adaptations. This simple but brash hypothesis would sadly mislead human evolutionary studies for more than forty years. In 1914 leading British scientists accepted Piltdown man (a purported fossil with a humanlike cranium and apelike jaws and teeth) as the earliest human ancestor, simply because they believed in Darwin's theoretical tenet of an evolutionary link. Unfortunately, a common thread among paleoan-thropologists at that time was to assume that human evolution is a unique situation, and the missing link mindset was deeply ingrained. Piltdown man seemed to be their perfect candidate for the missing link between apes and humans—perhaps too perfect. Despite having at their disposal the Javanese Trinil fossil human (Homo erectus), and a little later, the South African australopithecines (that is, the Taung child Australopithecus africanus), many scientists clung to Piltdown man as the earliest human ancestor. At the time of its discovery in 1925, the Taung child was dismissed by many as an ancient ape. Similarly, many dis missed the Trinil fossil calvaria as an extinct species of giant gibbon.

By the mid-1950s, sites from South Africa, east Africa, and Europe were generating much more fossil human material. After sober assessments of the fossils were made, the implications became clear. None of the earlier hominin phylogenetic claims could be supported. Pilt-down was exposed to be a hoax—the doctored remains of a human and orang-utan; the Taung child dated to roughly 2.5 million years ago; and the Trinil calvaria dated to 750,000 years ago. With the elimination of Piltdown from the hominin family tree, both of these fossils became widely accepted as human ancestors. Even so, the human fossil record was far from being a clear picture to stand back and admire. Certain paleoanthro-pologists (known as splitters) had just entered a period when every fossil discovery generated a newly described species. The human fossil record became a junkyard littered with dozens of species, and it was becoming increasingly difficult to flesh out humanity's prehistory. This muddle in the middle needed to be addressed, and it took a lecture on human evolution by world-renowned evolutionary biologist Ernst Mayr to make the necessary changes. Although paleoanthropology was unfamiliar territory for the biologist, he quickly lumped many hominin species together under a single name, bringing a breath of fresh air to the field. It became very fashionable thereafter for paleoanthropologists (now known as lumpers) to do the same.

The 1960s and 1970s will be remembered as a period of enlightenment for human evolutionary studies. Protein synthesis research in primates enabled scientists to propose a molecular clock that could predict the amount of time needed to achieve the number of changes observed in certain proteins sampled from living primates. If the molecular clock is cor rect, the origin of the ape/human clade occurred only 5 to 7 million years ago, a much later divergence date than the previously proposed 14 to 17 million years. Despite ongoing arguments that the number of molecular changes observed in protein structures was not constant over time (or that most structural changes were neutral and have no reproductive recourse), researchers in the early 1970s uncovered bipedal apelike hominins from the Afar region of Ethiopia. These fossils led to the scientific naming of a new hominin species Australopithecus afarensis, dated at 3 million years. They were then the oldest known human ancestors and quickly became the basal stock to all later hominins. In just one field season, A. afarensis offered clarity to the longstanding question of whether the molecular clock really worked. In other arenas, many were convinced that no more than a single hominin species could have existed on earth at any given time, especially after the lumpers were finished with the fossils. This single species concept resembled earlier tendencies to perceive human evolution as being unique among mammals, but in fact, it fell short of accurately describing the nature of the variability observed in the human fossil record.

Over the next twenty-five years, hundreds more fossil human discoveries will offer fuel to feed the fires touted by contemporary lumpers and splitters. Although contemporary research and concepts quickly negated the single species hypothesis, they did little to clear up issues of modern human origins. In its place comes the long-winded debate of Multiregional Evolution versus the Out of Africa hypothesis. The Out of Africa theory firmly argues that all modern humans descended from a single African population. This is often referred to as the Mito-chondrial Eve theory by some, because genetic tests were used to derive the time frame in which these African descendants would have replaced all other extant humans—a continuation of research similar to that started with the protein molecular clock. On the other hand, the Multiregional Evolution hypothesis argues simply that no one single ancient population is responsible for the origin of modern human populations.

Despite minor differences of opinion, it is clear that the human fossil record is not unique when compared with the evolution of other mammalian taxa. Hominid phylogeny is richly diversified, comprising numerous genera and multiple species—in upwards of eighteen different species—some of which shared ecological niches in both time and space. At present the multiethnic varieties of modern humans occupy the only surviving branch of what appears to have been a densely bushy phylo-genetic shrub in the past (see Figure 1). Until very recently, the oldest hominins known were dated to 4.5 million years ago. However, newly announced fossil discoveries from East Africa may now extend our evolutionary history back in time to 6 million years ago. The human fossil record, which is discussed in some detail below, provides ample evidence of a rich biodiversity with multiple branching events coinciding with several extinctions. Recent modern humans just happen to occupy the last branch of a dying tribe. The worst possible reality for anyone to grasp is that extinction is inevitable, and we are the last of the hominins.

Orrorin tugenensis. Announced in 2001 by a joint Kenyan and French team, thirteen fragmented fossils belonging to at least five individuals were collected from four localities in the Tugen Hills of Kenya. Fossils include a few teeth, fragmentary leg, and arm and hand bones. Sedimentary analyses indicate an age of 6 million years, making this the oldest known fossil human. The paleoenviron-ment is reported as having been open wood land at the time of deposition, with patches of dense forest. Many remain skeptical because of the fragmentary nature of the fossils, as well as the reported geologic age.

Ardipithecus ramidus. Initially announced in 1994 as Australopithecus ramidus by a joint Ethiopian and U.S. team, this group of fragmentary fossils was renamed Ardipithecus in a brief erratum in a 1995 issue of Nature. Very little is known about this group, except that it was collected from the Middle Awash region, Ethiopia, and is reported to have thin enameled dentition and bipedal capabilities. Initial response suggests that this group is not a direct ancestor to later hominins. Initial sedimentary analyses indicated an age of 4.4 million years. A more recent and interesting announcement by an Ethiopian team member in 2001 extended Ardipithecus into the Miocene at 5.8 million years ago. Eleven specimens were collected, and they are purported to exhibit enough dental variability to name a new subspecies A.r. kadabba—thought by most to be nothing short of suspiciously miraculous. The paleoenvironment appears to have been woodland.

Kenyanthropus platyops. Announced in 2001 by joint Kenyan and British teams, this find consists of a largely complete reconstructed cranium (and two previously collected mandibles) collected from the Nachukui Formation near Lomekwi, West Turkana, Kenya. Sedimentary analyses indicated an age of 3.5 million years and a paleoenvironment of mosaic habitats, but predominantly woodland.

Kenyanthropus rudolfensis. Formally known as Homo rudolfensis, fossil cranium ER 1470 appears to share many facial characters with K. platyops. Discovered in the late 1970s, 1470 was initially thought to be a male member of Homo habilis dated at 1.8 million years ago; it has a cranial capacity of 750 ml. Other habi-line finds prompted researchers to reassign this peculiar specimen to a different taxon (H. rudolfensis). Always considered a morphological sore thumb, when platyops was announced, researchers were quick again to accept the Kenyanthropus assignment.

Australopithecus anamensis. Announced in 1994 by a joint Kenyan and U.S. team, fragmentary fossils were collected from Allia Bay and Kanapoi localities in Kenya. Fossils include partial maxillae, mandibles, and proximal tibia. Dated at 4.2 million years ago, this species exhibits many primitive traits, making it a welcomed precursor to A. afarensis.

Australopithecus afarensis. Initially announced in 1978 by an international team, this group of hominins was collected from the Afar and Middle Awash Region, Ethiopia, and Laetoli, Tanzania. Fossils now include several jaws and teeth, largely complete crania and partially complete individuals (including the famous "Lucy"), as well as several individuals of mixed age and sex from the same site; they are considered a family unit catastroph-ically killed by flood. Sedimentary analyses indicate a lake and wooded paleoenviron-ment dated to 3.2 million years ago. Postcra-nial material suggests bipedal capabilities, and fossil footprints from Laetoli found in 1976 by Mary Leakey support bipedal behavior. Also helped to show that Darwin was wrong about big brains preceding bipedalism.

Australopithecus bahrelghazali. Announced in 1996 by a joint French and British team, a fossil mandible from this group was collected from Bahr el Ghazal, Chad. Only a partial mandible exists, dated to 3.4 million years ago. Very little is known about this specimen, except that it extends the known range of human ancestors westerly a few thousand kilometers.

Australopithecus africanus. Announced in 1925 by Raymond Dart, a subadult partial face and natural endocast were collected by miners from the Taung cave, South Africa. The Taung child holds the distinction of being the first australopithecine named. This group is known from several hundred fragmentary and partial crania, and the largely complete Sts 5— (so-called Mrs. Ples.)

Australopithecus garhi. Announced in 1999 by an international team, fossils were collected from the Middle Awash Region of Ethiopia. Fossils include partial crania, fragmentary limb bones, and several teeth. Sedimentary analyses indicate freshwater lake and open woodland, dated to 2.5 million years ago. A. garhi receives the distinction of being the oldest hominid associated with stone tools and cut-marked bone.

Australopithecus (Paranthropus) aethiopicus. Initially announced in 1968 by a French team, a toothless fossil mandible was collected near the Omo River in the Shungura Formation; taxa currently recognized for a largely complete skull collected from West Turkana, Kenya, in 1985 by an international team. The fossil, known as the "black skull" because it is stained by manganese-rich minerals, is striking for its tall sagittal crest and projecting face.

Australopithecus (Paranthropus) robustus. Initially announced in 1938 by Robert Broom, an adult partial fossil skull was collected at Kromdrai, South Africa. This species distinction is related to Broom's tracking down a schoolboy who had found a molar that raised Broom's interest. Because of dating difficulties with South African dolomite caves, the estimated age for this species is 1.5 to 2 million years. This species is known from hundreds of fragmentary specimens, as well as fairly complete crania and mandibles with dentition.

Australopithecus (Paranthropus) boisei. Announced in 1959 by Louis and Mary Leakey, a fossil was collected from Lower Bed 1, Olduvai Gorge, Tanzania. A massively built cranium with huge teeth (similar to those of

A. robustus), this fossil was initially called Zin-janthropus (Arabic/Greek for East African Man), species nomen, pertaining to a colleague funding the project. Students of paleo-anthropology know this fossil by the nickname "nut-cracker man," because of its huge cheek teeth. Dated to 1.8 million years ago, Olduvai Gorge was thought to be lake margin at the time of deposition.

Homo habilis. Announced in 1964 by Leakey and others, this fossil group was collected in Lower Beds at Olduvai Gorge, Tanzania, and East Turkana, Kenya. Fossils include complete crania and dozens of jaws and teeth. A partial individual collected much later by a U.S. team provides evidence for a more boreal way of life than previously predicted. Fossils are distinguished from those of other Homo fossils by the small size of the cranium and by dentition. Once thought to be the female to ER 1470. Cranial capacity is not more than 600 ml.

Homo ergaster. Fossils previously touted as Homo erectus were collected from East Africa in 1976 and 1985 (and maybe South Africa in 1969 from a museum find). Fossils include two complete adult crania and a fairly complete subadult individual collected from East and West Turkana, Kenya. The subadult has the distinction of being the most complete fossil human discovered from Africa.

Homo erectus. Initially announced in 1892 by French doctor Eugene Dubois working for the Dutch East Indies Company, this group has a rich history. Known from dozens of skull caps and partial mandibles from China and Java, a wide range of environments are reported for H. erectus. It is dated between 1.8 million years ago and 53,000 years ago. Many remain skeptical regarding the dates. Moreover, many feel that H. erectus is a taxonomic dumping ground, meaning that it has too wide a range of variability in skeletal morphology to be just one species. Distinction

Skull of a Homo neanderthalensis found in the Bouffna Bonneval Cave in the Sourdoire River Valley near the village of Chapelle-aux-Saints, 40 km southeast of Brive-la-Qaillarde, Correze, France. The skull is commonly called "La Chapelle" and is dated to about 40 thousand years ago. (KAREN/CORBIS)

Skull of a Homo neanderthalensis found in the Bouffna Bonneval Cave in the Sourdoire River Valley near the village of Chapelle-aux-Saints, 40 km southeast of Brive-la-Qaillarde, Correze, France. The skull is commonly called "La Chapelle" and is dated to about 40 thousand years ago. (KAREN/CORBIS)

for Peking Man fossils were lost during World War II. The only thing left are fossil casts made by Franz Weidenreich prior to the Japanese invasion of Peking.

Homo antecessor. Announced in 1997 by a Spanish team of researchers, this species is thought by some to be the common ancestor to modern humans and Neanderthals. Their species diagnosis is based primarily on a unique combination of craniodental traits that collectively are different from those of other known Homo fossils. These include a combination of primitive and derived "modern" features that show none of the derived features of later Neanderthals. So far, the fossils representing this species all come from a site called Gran Dolina in Spain, and they date from about 800,000 years ago.

Homo heidelbergensis. Discovered in 1907 by a workman at the Mauer quarry in Heidelberg,

Germany, the type specimen for this species is the Mauer mandible. Fossils included in this species are found in both Europe and Africa and date to the Middle Pleistocene, ranging from about 500,000 to 250,000 years ago. Some researchers have proposed that Homo heidelbergensis, more primitive than modern humans and lacking many derived features of Neanderthals, represents the common ancestor to both modern humans and Neanderthals. However, with the discovery of Homo ante-cessor fossils, its phylogenetic position is currently being debated.

Homo neanderthalensis. Announced in 1864 by William King, a partial fossil skeleton was collected from inside a small cave situated about 45 feet above the Dussel River in the Neander Valley, Haan, Germany. The discovery was made in the summer of 1856 by two quarrymen. The Neander fossil became the type specimen of a newly recognized extinct human species, Homo neanderthalensis. Neanderthals lived approximately 150,000 to 30,000 years ago in a variety of temperate paleoenvi-ronments. Since William King's taxonomic assessment, numerous other fossils found throughout Europe and the Middle East have been attributed to this species, including those previously discovered from Engis in 1830 and Gibraltar in 1848. Important advances in ancient nuclear and mitochondrial DNA analyses have provided solid genetic support for what William King had proposed more than a century earlier—that Neanderthals are taxonomically distinct from modern humans. Homo sapiens—Nosce te ipsum.

—Ken Mowbray

See also: Homo Sapiens; Physical Anthropology; Primates; Great Apes

Bibliography

Johanson, Donald, and Blake Edgar. 1996. From Lucy to Language. New York: Simon and Schuster; Schwartz, Jeffrey, and Ian Tattersall. 2000. Extinct Humans. Boulder, CO: Westview; Tattersall, Ian. 1995. The Fossil Trail: How We Know What We Think We Know about Human Evolution. New York: Oxford University Press; Tattersall, Ian. 1998. Becoming Human: Evolution and Human Uniqueness. New York: Harcourt Brace; Tattersall, Ian. 1999. The Last Neanderthal: The Rise and Success and Mysterious Extinction of Our Closest Living Relative. Boulder, CO: Westview; Wolpoff, Milford. 1999. Paleoanthropology, 2d ed. Boston: McGraw-Hill.

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