Do the fossils found at the sites explored by Louis and Mary Leakey and the sites explored by Donald Johanson represent several hominidspecies or only one
Do the fossils found at the sites explored by Louis and Mary Leakey and the sites explored by Donald Johanson represent several hominidspecies or only one?
Viewpoint: Yes, the fossils found by Louis and Mary Leakey and by Donald Johanson represent several hominid species.
Viewpoint: No, the hominid fossils found and named by Donald Johanson and Louis and Mary Leakey represent a single species of Australopithecine or very early Homo.
Modern studies of the relationship between the ancestors of the great apes and modern humans involve many disciplines, such as paleoanthropology, historical geography, archaeology, comparative anatomy, taxonomy, population genetics, and molecular biology. Although insights gained by the genetic analysis of human and nonhuman lineages have provided new insights into human evolution, the fossilized remains of human ancestors still provide the most valuable clues to the past. Unfortunately, hominid fossils are rare and generally quite fragmentary. Complete skulls and skeletons are uncommon, and identifying and classifying bits of bones and teeth to determine their relationship to other ancient specimens involves formidable challenges. Many subtle characters must be used in the analysis of fragmentary remains. The major sites of discovery of the most ancient hominid fossils have been in Africa: Kenya, South Africa, Tanzania, and Ethiopia.
When Charles Darwin published On the Origin of Species in 1859, he hinted that his theory of evolution by means of natural selection might throw some light on the origins of human beings. It was only with great reluctance that he finally explored this most controversial aspect of his theory in Descent of Man, and Selection in Relation to Sex (1871). Other evolutionists had written about man's place in nature and the survival of the fittest in human society, but Darwin realized that his contemporaries were not ready for a rigorous analysis of human evolution as a purely biological process. In Descent of Man, Darwin argued that human beings, like every other species, had evolved from previous forms of life by means of natural selection. According to Darwin, all the available evidence indicated that "man is descended from a hairy, tailed, quadruped, probably arboreal in its habits." The evidence available to Darwin did not, however, allow him to reach any specific conclusions about the time, place, or identity of the first humans. Although studies of cultural anthropology and paleontology were very limited in his time, Darwin's views on human evolution were remarkably perceptive. He suggested that the ancient ancestor of modern human beings was related to that of the gorilla and the chimpanzee. Moreover, he predicted that the first humans probably evolved in Africa between the Eocene and Miocene eras. Wrestling with the crucial theme of the development of human intelligence, Darwin pointed out that differences in body size must be taken into account when evaluating the significance of absolute differences in brain size.
Since Darwin established the basic framework for the study of human evolution, scientists have searched for physical evidence of the most ancient ancestors of modern humans and their closest relatives, the chimpanzee and the gorilla. Going beyond morphology and taxonomy, paleoanthropologists now employ the techniques of molecular biology, the analysis of genetic similarities and differences, new methodologies in archeological excavation, and insights from sociobiological studies of primates and hunter-gatherer societies. Moreover, scientists now generally accept the concept that the evolutionary history of the primates was more like a "bush" with many branches (some of them evolutionary dead-ends) than a ladder leading directly to modern Homo sapiens.
The chimpanzee and the gorilla are clearly the living animals most closely related to modern humans. Indeed, studies of nuclear and mitochondrial DNA suggest that humans and chimps might have shared a common ancestry, after divergence from the gorilla lineage. Based largely on differences between the DNA of modern apes and humans, the last common ancestor of humans and chimpanzees presumably lived in Africa about 6 million years ago. Fossil evidence for the species that existed at the time the human lineage separated from that of the great apes is, unfortunately, very fragmentary.
The South African physical anthropologist and paleontologist Raymond Dart made the first substantive discovery of human ancestors in Africa as early as 1924, when he identified the famous Taung fossils as Australopithecus africanus (South African Ape-man). The most exciting subsequent twentieth-century discoveries of ancient human ancestors are associated with the work of Kenyan anthropologist Louis Leakey and his anthropologist wife Mary, and that of the American anthropologist Donald Johanson. Working primarily at sites in Olduvai Gorge and Laetoli in Tanzania, Mary and Louis Leakey identified many hominid fossils, including Proconsul africanus (an extinct Miocene primate) in 1948, Australopithecus boisei (originally called Zinjanthropus boisei, or Nutcracker Man) in 1959, Homo habilis (Handy man) in 1960-1963, and a remarkable trail of fossilized hominid footprints preserved in volcanic ash. Johanson's most important discovery was the unusually complete skeleton of a primitive australopithecine (usually referred to as Lucy) in the Afar region of Ethiopia in 1974. In addition to proclaiming that he had found a new species, which he called Australopithecus afarensis, Johanson claimed that Mary Leakey's Laetoli fossils were actually members of this species. Many scientists objected to Johanson's designation of a new species and the relationship between his Afar fossils and Mary Leakey's Laetoli specimens remains controversial.
Several new hominid finds were announced at the beginning of the twenty-first century. As usual, the identification and classification of these fragments of bone provoked intense debate among paleoanthropologists. In 2000, French paleoanthropologists Brigette Senut and Martin Pickford discovered a set of fossil fragments in the Tugen Hills of Kenya. They claimed that the fossils represented a new species, which they called Orrorin tugenensis (Original man, Tugen region). A few months later, a report by Yohannes Haile-Selassie raised questions about the hominid status of O. tugenensis, and announced the 1997 discovery of an early form of A. ramidus. Further discoveries will, no doubt add new insights into the history of human evolution, and create new disputes among paleoanthropologists.
—LOIS N. MAGNER
Viewpoint: Yes, the fossils found by Louis and Mary Leakey and by Donald Johanson represent several hominid species.
Theories of human evolution are increasingly framed in terms of insights gained by the genetic analysis of human and nonhuman primate lineages. Using differences in DNA to estimate how long humans and chimps have been separate lines, scientists suggest that humans separated from the apes about 5 to 8 million years ago. Nevertheless, debates about the traditional source of information, i.e., the fossil evidence, are still complicated by the paucity of the evidence and disagreements about palaeontological systems of classification. Although thousands of hominid fossils have been collected, many specimens consist of only bits of bone or a few teeth. Indeed, it is sometimes said that all known hominid fossils could fit into one coffin. The discovery of any hominid fossil specimen, no matter how fragmentary, inevitably serves as the basis for endless speculation. Nevertheless, according to the eminent eighteenth-century French anatomist Baron Georges Cuvier (1769-1832), through knowledge of the comparative anatomy of living animals, many insights into the form and function of extinct creatures can be reconstructed by the analysis of a few bones.
Long Baseline Experiments
Most scientists agree that the fossil record provides evidence of 10 to 15 different species of early humans, but the relationships among these ancient species and their relationship, if any, to modern humans remains uncertain and controversial. The classification of various species of early humans, and the factors that influenced evolution and extinction are also subjects of debate. The conventional criteria for allocating fossil species to a particular genus have often been challenged for being ambiguous, inappropriate, and inconsistently applied. Arguments about the identity of fossil remains are complicated by evidence that suggests hominid species may have been quite variable, with some of the apparent variability due to sexual dimorphism, a characteristic often found among living nonhuman primates. Given the interesting variations found in hominid fossils, many paleoanthropologists agree that human remains exhibiting a unique set of traits should have a new species name.
The story of the search for the human ancestors and the debates about the relationships among the various species included in the catalog of early hominids is, in large part, the story of the Kenyan anthropologist Louis Leakey and his wife, paleoanthropologist Mary Leakey. The Leakeys stimulated and inspired many paleoanthropologists, including American Donald Johanson, to search for human ancestors and explore the relationship between humans and other primates. Few people have had more impact on the modern era of paleoanthropology than Leakey, the patriarch of a remarkable multi-generational family of anthropologists. The Leakeys were largely responsible for convincing scientists that the search for human ancestors must begin in Africa. When Louis Leakey began hunting for fossil hominids in the early 1930s, most anthropologists believed that early humans had originated somewhere in Asia because of previous discoveries of human fossils in Java (now Indonesia) and China. Leakey's son Richard Leakey, his wife Meave, and their daughter Louise, are carrying on with the work begun by Louis and Mary Leakey.
The Leakeys were not, of course, the first scientists to discover ancient human ancestors in Africa. The South African paleontologist Raymond Dart was one of the first to recognize the existence of the fossilized remains of primitive, but bipedal human ancestors. In 1925, Dart discovered the skull of an extinct primate at Taung, South Africa. According to Dart, the creature was not an ape, but it walked upright. Because its brain was only about 28 cu in (450 cc), too small for admission to the genus Homo, Dart established a new genus, Australopithecus (Southern apeman). He named the primitive creature Australopithecus africanus. Dart's contemporaries generally rejected his claims until Robert Broom, another South African paleontologist, discovered many more A. africanus skulls and other bones. Although brain size was originally considered the key to human evolution, many paleontologist now consider the evolution of bipedalism, the ability to walk on two legs, to be one of the most critical early differences between the human and the ape lineages. Habitual bipedalism, as opposed to the ability to stand upright like chimps, requires many anatomical adaptations, in both the upper and lower body. Such changes involve the pelvic bone, hip joints, leg bones, toes, S-shaped cure of the spine, and the position of the foramen magnum. Australopithicines did, however, have curved, elongated fingers and elongated arms, which suggests that, in addition to walking upright, they climbed trees like apes.
The discovery that brought worldwide attention to the Leakeys occurred in 1959 at Olduvai gorge in Tanzania, when Mary Leakey found the skull of a creature originally called Zinjanthropus boisei (East African man). Informally the fossil became known as Nutcracker Man, because of its robust skull and huge teeth. The specimen was an almost complete cranium, with a brain size of about 32 cu in (530 cc). The specimen was estimated to be dated about 1.8 million years ago. Today, this species is known as Australopithecus boisei.
During the 1960s the Leakeys and their son Jonathan discovered fossils remains that seemed to represent the oldest known primate with human characteristics. Leakey challenged contemporary ideas about the course of human evolution and established a new species name for these fossils—Homo habilis (Handy man). Louis Leakey thought that H. habilis was a tool-making contemporary of the australopithecines, with a brain size of about 43 cu in (700 cc). His designation of these remains as a new species belonging to the genus Homo was very controversial. Some critics argued that Leakey's H. habilis was based on insufficient material and that the remains in questions were actually a mixture of A. africanus and H. erectus. Others questioned the age of the fossil and concluded that Leakey's fossil should have been classified as a rather large-brained Australopithecus, rather than a small-brained Homo. Although H. habilis was originally very controversial, Leakey's designation of this new species was subsequently widely accepted, because the brain size was above the range for the australopithicines. Moreover, there were significant characteristics of the feet, the ratio of the length of the arms to the legs, and the shape and size of the molar teeth, premolar teeth, and jaws of H. habilis that distinguish it from those of contemporary australopithicines. Differences in the body size of various H. habilis specimens suggest a striking degree of sexual dimorphism.
In 1975 Mary Leakey discovered the jaws and teeth of at least 11 individuals at Laetoli, 30 mi (48 km) south of Olduvai Gorge. The fragments were found in sediments located between deposits of fossil volcanic ash dated at 3.35 and 3.75 million years. At the time these were the oldest known hominid fossils. Mary classified these remains as H. habilis. Mary made another remarkable discovery in 1978, a trail of fossilized hominid footprints that had been preserved in volcanic ash at the Laetoli site. The footprints seemed to be those of two adults and a child, probably made about 3.5 million years ago. The footprints definitely prove that australopithicines regularly walked bipedally, but the discovery led to a major controversy about the identity of the hominid species that made them. According to some anthropologists, the species Mary Leakey was studying at Laetoli was not H. habilis, but a new hominid species, A. afarensis, that Donald C. Johanson had recently discovered at Hadar, Ethiopia.
Donald Johanson is one of the best-known American paleoanthropologists and the founder of the Institute of Human Origins, a nonprofit research institution devoted to the study of pre-history. While working at Hadar in the Afar region of Ethiopia from 1972 to 1977, Johanson discovered hominid remains that were dated as 2.9 to 3.3 million years old. One of his finds was a small but humanlike knee, the first example of a hominid knee. He made his most famous discovery in 1974, the partial skeleton of a female australopithecine, popularly known as Lucy. The skeleton has been dated between 4 and 3 million years ago and was almost 40% complete, making Lucy the oldest, most complete human ancestor ever assembled. In 1975 Johanson's team found a collection of fossils at a single site that seemed to be the remains of some 13 individuals. The collection was nicknamed the First Family. Eventually still more hominid fossils were discovered, along with stone tools.
After analyzing the fossils with Timothy White, Johanson came to the conclusion that all the Afar fossils belonged to a new species. In 1978 Johanson and White named the new species Australopithecus afarensis. These discoveries and Johanson's interpretation created a major controversy among paleoanthropologists. Critics claimed that slight differences did not justify a new species name and said Johanson's A. afarensis should be considered a geographical subspecies of A. africanus. Johanson and his supporters argue that the anatomical differences between A. afarensis and other hominids are qualitatively and quantitatively beyond the normal variation found within a species. In addition to being older, Johanson pointed out that that the brain case of A. afarensis was smaller than that of H. habilis and A. africanus. There were also significant differences in the teeth, jaws, fingers, foot, and leg bones. Johanson argues that the differences between specimens justified assigning the Afar fossils to a new species.
Much of the controversy about creating a new species designation arose when Johanson argued that his Afar specimens belonged to the same species as Mary Leakey's Laetoli fossils. Based on apelike characteristics of the teeth and skull shared by no other fossil hominid, Johanson and White assigned both the Laetoli and Afar remains to A. afarensis. They claimed that this new species was more ancient and more primitive than any other hominid fossil. Mary and Richard Leakey criticized Johanson for proclaiming a new species too quickly, and suggested that the fossils could be a mixture of several different species. Other anthropologists, however, agreed that the features pointed out by Johanson were significant enough to distinguish the Afar and Laetoli fossils as different species.
In 1994 Meave Leakey found teeth and bone fragments similar to a fossil arm bone that had been discovered in 1965. In 1995 Meave classifed all these remains as belonging to a new species, Australopithecus anamensis. This very primitive australopithicene had an apelike skull, but leg bones apparently adapted to bipedalism. Ironically, A. anamensis appears to be quite similar to A. afarensis, the species that was the subject of a dispute between Mary Leakey and Johanson.
Despite the ambiguities involved in identifying and naming ancient ancestors, there is general agreement that the earliest human ancestors were the australopithecines. The most significant features distinguishing australopithecines from the apes were their small canine teeth and bipedalism. Members of this group appear to be the first mammals anatomically adapted for habitually walking on two legs. However, they had a brain size of about 24-34 cu in (400-550 cc), a low cranium, and a projecting face. The most primitive australopithecines are now placed in the genus Ardipithecus. In addition to the genus Australopithecus, some anthropologists have adopted the category Paranthropus.
At the beginning of the twenty-first century, several new fossil hominids discoveries were announced, and, as usual, greeted by debate about their identity and their relationship to other ancient ancestors. The announcement of the discovery of new hominid remains in 2001 sparked renewed controversy about the earliest hominid ancestors, as well as those of the chimpanzee. In 1990, French paleoanthropologists Brigitte Senut and Martin Pickford discovered a set of fossil fragments in Kenya 6 million years old, which they dubbed Millennium Man. Senut and Pickford classified the fossils as belonging to a new species, which they called Orrorin tugenensis (Original man, Tugen Hills region). One aspect of the controversy has been attributed to prior conflicts between Richard Leakey and Pickford. Nevertheless, the bones and teeth do show an interesting combination of features, which separate Orrorin from the australopithecines.
Within months of the report on O. tugenensis, Yohannes Haile-Selassie, of the University of California, Berkeley, announced a new find that cast doubt on the hominid status of Orronin and supported hominid status for A. ramidus, a species discovered in 1994 by an international team led by paleoanthropologist Timothy White. The fossils, found in the Middle Awash area of Ethiopia, are estimated to be between 5.2 and 5.8 million years old. Haile-Selassie argues that they represent an early form of A. ramidus, and appear to be from a hominid species closely related to the common ancestor of chimpanzees and humans. The debate about the status of O. tugenensis and A. ramidus could, therefore, provide insight into the lineage of chimpanzees and hominids.
Debates about hominid fossils have been the one constant in the rapidly changing field of paleoanthropology. The conflict between the Leakeys and Johanson is well known, but disputes continue into the twenty-first century with the discovery of each new fossil.
—LOIS N. MAGNER
Viewpoint: No, the hominid fossils found and named by Donald Johanson and Louis and Mary Leakey represent a single species of Australopithecine or very early Homo.
Ever since Charles Darwin published his book On the Origin of Species (1859), in which he speculated on a common ancestor for humans and other members of the primate family, scientific imaginations have been fired by the possibility of finding evidence of such a common ancestor. Not long after Darwin's book was published, the fossilized remains of early humans began turning up when intrepid adventurers, and later scientists, retrieved them from the earth where they had lain for hundreds of thousands, or even millions, of years. Paleoanthropologists now agree that it was about 12 million years ago when what became the modern apes diverged from the primate line that became modern humans. The fossils found by Leakey family of paleoanthropologists and Donald Johanson come from a period of from 3.5 to 1.5 million years ago, not old enough to be considered the true common ancestor of prehumans and apes. However, the finds of both the Leakeys and Johanson represent crucial evidence of the sequence of human evolution.
Crises of Category
In the twentieth century, the problem in tracing human origins through an evolutionary process consistent with Darwin's model became a question of how to classify fossils with regard to their relationship to each other and with modern humans. Once science was able to reliably date fossils, this task became easier. However, science is a process that requires creating categories, making comparisons, testing, and taking leaps of logic, as well as leaps of faith. The need for categorization has often created scientific disagreement. Each of these disagreements might be called "a crisis of category." Trying to understand the path of human evolution by accounting for human and prehuman fossils has been plagued by many such crisis.
In 1925, South African anatomist Raymond Dart discovered and named an extinct hominid (a primate that walks upright). The primate was not an ape, nor was it a modern or premodern human that could fit into the category "Homo," or human, because its brain was too small. Following the scientific convention of using genus and species to name a biological organism, Dart established a new genus, calling his discovery Australopithecus (Southern Apeman), with the species name africanus. In some respects, Australopithecus africanus was a functional "missing link" because it demonstrated bipedal, or humanlike, upright locomotion, but was topped with a very small, apesized brain, about 27 cu in (450 cc), the size of a small grapefruit. Because scientists then thought that in the evolution of humans a large brain came before bipedal locomotion, Dart's discovery created a crisis of category that lasted almost 25 years.
While the genus Australopithecus was rejected by many scientists, then and for another generation, hominid fossils clearly bigger than Dart's Australopithecus began showing up in South Africa, and Australopithecus robustus soon emerged. A generation later, Louis and Mary Leakey—who had found their share of A. robustus specimens in East Africa at Olduvai Gorge—convinced the scientific world that they had found the earliest member of the genus Homo. Louis Leakey maintained that his bigger-brained Homo habilis, had a brain big enough at 43 cu in (700 cc) to be considered human and, once more, his find used stone tools. So, he was granted the first find in the Homo genus and named a new species, habilis. Many argued then that Leakey's Homo habilis (Handy Man) was nothing more than a large-brained australopithecine. Whether it used stone tools was—and still is—debated.
Whether one believes that H. habilis was notably different from the australopithecines depends whether one is a "lumper" or "splitter." Lumpers prefer to ignore small differences in fossil samples, lumping them into the same species. Splitters focus on slight variations and use them to justify naming new species.
The evolutionary biologist Ernst Mayr (1904- ) cautioned against the careless naming of new species. He had seen what has been called a "species naming frenzy" during the 1950s. Mayr, an extreme lumper who recommended lumping Australopithecus and early Homo, said that brain size should be minimized in the species debates. Paleoanthropologists C. Loring Brace (1930- ) and Milford Wolpoff (1942- ) felt that the Homo genus should have started with a brain size of around 73 cu in (1,200 cc), rather than the 43 cu in (700 cc) allowed Louis Leakey when he named H. habilis.
The human evolution landscape remained somewhat quiet after the classification of H. habilis until 1974, when Donald Johanson discovered the fossil remains of the oldest, smallest, and most primitive-looking australopithecine. It had a brain about 18 cu in (350 cc), or about the size of a softball. Johanson's discovery (later called Lucy) was made in the Hadar region of Ethiopia, in north east Africa, some 1,500 mi (2,414 km) to the north of where the Leakeys were finding hominid fossils in Kenya and Tanzania.
Lucy caused a "crisis of category" that has not abated. At issue is whether Johanson's discovery and naming of A. afarensis represents a different species than previously discovered A. africanus, A. robustus, or even H. habilis.
In response to the question under discussion—no, from the perspective of a lumper, the fossils found by the Leakeys and Johanson are not distinct, but represent geographical sub-species of A. africanus/robustus. This lumper's argument suggests that the morphological (structural) differences between the specimens represent normal variation within a species, variations wide enough to include all species of australopithecines and H. habilis.
C. Loring Brace and a South African paleoanthropologist, Phillip Tobias, took a lumper position in the controversy over whether A. afarensis was a new species or represented normal variation within a species. Brace said that the anatomical differences between various hominid fossils found in East Africa and South Africa were the result of variation within a single species. Tobias suggested that Lucy was a sub-species of A. africanus.
In addition to being older, what made A. afarensis different from H. habilis and A. africanus/robustus were a smaller brain case, differences in dentition, and some differences in finger and leg bones. Remembering that the Olduvai/Laetoli specimens were found more than 1,000 miles (1,610 km) south of Hadar and were significantly older, the lumper's perspective suggests that, over time, the geographical barriers to mixing gene pools, the genetic material of all the members of a given population, can produce great variation with a species. These differences may not only be in size but in function, because over great amounts of time, evolution is adaptive to local environments. Even if the local ecological environments at Hadar and Laetoli/Olduvai were similar more than two million years ago, there could still be local variation in terms of hominid morphology.
Mayr thought that there would have been a less serious category crisis if Johanson had named Lucy Australopithecus johansonensis, rather than name it for a region. Mayr, who felt that regional morphological variation was important to keep in mind, recommended "suppressing" the name Johanson had chosen. Mary Leakey also took great issue over the suggestion that Johanson's A. afarensis from Ethiopia, rather than her H. habilis or A. africanus, was once walking around on her site in Tanzania 1,500 mi (2,414 km) away.
Tobias, who worked closely with Louis Leakey, agreed with Mayr. From the beginning, Tobias said that Johanson's fossils from Afar that he named A. afarensis and the hominids from Laetoli in Tanzania were of the same genus and species—A. africanus. Tobias would have preferred that Johanson name his find as a sub-species, such as A. africanus aethiopicus (since it was discovered in Ethiopia), while the Laetoli finds from Tanzania he said should be A. africanus tanzaniensis.
It is important to note that even today there are phenotypic differences, i.e., observable physical differences, such as skin color, hair texture, shape of the nose and eyes, in the modern human populations in Ethiopia and Tanzania. In general, Ethiopian males are much shorter and smaller boned than males in Tanzania, many of whom are quite tall by world standards. These differences are the result of millennia of environmental adaptation, as well as the product of the genetic traits available in gene pools. Gene pools can be restricted by geographical boundaries, as well as by cultural preferences, in the choice of reproductive mate.
What Determines a Species?
The argument over whether Johanson and the Leakeys have been finding the same species or different species of hominid comes down to the question: "By what does science judge speciation?" Biologists have always had difficulty when defining species. Generally, species are defined as similar individuals that breed mostly among themselves. While species also denotes morphological and biological stability, a species also has the ability to change over many generations. Most importantly, biologists contend that species change occurs within a species, so drawing the species line is difficult. Biologists consider species the units of evolution.
To determine whether similar or different species of several million-years-old hominids have been turning up in East and South Africa and in Ethiopia, paleoanthropologists can employ three concepts—cladistics, phylogeny, and scenario. A clade is a group of species recently derived from a common ancestor. A cladistic study compares traits and groups of traits. Cladistic studies of the australopithecines and H. habilis have been inconclusive, as well as confusing. In some cladistic studies, A. robustus and H. habilis share more primitive characteristics than they do with the other australopithecines, either A. afarensis or A. africanus. Other groupings of traits make A. afarensis and H. habilis look more alike.
Once a cladistic comparison is made, and similarities and dissimilarities are agreed upon, scientists may create a phylogeny, or a natural history of change that can show how one species may have evolved into the next. Finally, a scenario is developed. A scenario is a kind of story about evolution that seeks to explain which species was ancestral to which other, and why and how change occurred. Lumpers tend to be liberal in cladistic studies, allowing for wide variation within species. Splitters prefer to add up differences and then name a new species if they judge the total differences warrant. Using cladistics, phylogeny, and scenario, it is not perfectly clear that A. afarensis, A. africanus, A. robustus, and H. habilis represent distinct species. In additon, paleoanthropologists still argue over the design of their phyletic "branching" on the early human and prehuman family tree.
Phyletic Gradualism and Punctuated Equilibrium
Encephalization, or increasing brain size, plays a large part in how the fossils "hang" from the family tree. How and why brain size increased and at what speed are two questions that not only need answers, but also have helped create many a crisis of category. Paleoanthropologists recognize two evolutionary speeds—phyletic gradualism, where change is slow and gradual—and punctuated equilibrium, where change is rapid and dramatic when a new species arises by splitting off from a lineage. How these two evolutionary speeds hinder or help the species-naming game needs consideration.
The phylogenic and scenario approach to hominid speciation favors phyletic gradualism. However, when looked at cladistically, the reading of our fossil record might favor punctuated equilibrium. For splitters, the fossil record shows greatly different forms showing up relatively quickly, with no or few transitional forms or species in between. This approach begs the question: Are the "breaks" in the fossil record real, or have we not yet found enough transitional forms to fill all in the gaps between, for example, A. afarensis and H. habilis? Lumpers visualize evolution as occurring slowly and gradually.
Other Pressures Affecting the Naming of Species
Finally, the question of whether the Leakeys and Donald Johanson were finding a different species or the same needs some nonscientific consideration. The scientific literature on human evolution—and specifically species discovery and naming—does not shed light on the fact that successful species naming brings with it fame and, perhaps more importantly, research money. The internal politics of paleoanthropology have a real role in the species naming process. Louis Leakey was successful in naming Homo habilis because his specimens had larger brains than australopithecines, big enough to be called human, he said. In addition, Leakey convinced the scientific world that H. habilis used primitive stone tools. However, what especially helped Leakey's case that H. habilis was "human," was his influence within the scientific community and that community's willing acknowledgment of his many decades of hard work. In some respects, science rewarded Leakey by agreeing that his discovery was the earliest known Homo. Had H. habilis just been a bit smaller brained, had Leakey not convinced science that H. habilis used tools, or had Leakey not been so highly regarded, his H. habilis may have been entered into the paleontology books as only a big-brained, and perhaps, tool-making australopithecine.
When Johanson, who proved that A. afarensis was more than a million years older than hominid fossils found in Kenya and Tanzania, and also showed that Lucy was more "primitive" than other australopithecines, he won most, but not all, the hearts in the scientific community. Splitters awarded him the new species; lumpers would rather have not.
In conclusion, what can be said with surety is that from about 3 million to about 1.5 million years ago, there were bipedal primates in Africa, primates we call australopithecines, whose brain size was less than one-half to one-third that of modern humans. Whether these primates represent the same or distinct species depends on whether one is a lumper or a splitter, and how the crisis of category is resolved. Science is a conservative discipline and thus should err on the side of caution, which means lumping the fossils finds of the Leakeys and Johanson until further data and research suggest otherwise.
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Genetic changes that can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring.
One of the earliest defining human traits, the ability to walk on two legs.
Opening at the bottom of the skull through which the spinal cord passes in order to join the brain.
Term originally used by the German scholar Georg Bauer (Georgius Agricola) in the sixteenth century to refer to anything dug out of the earth. Eventually the term was restricted to the preserved remains of previously living animals or plants.
The genetic material of all the members of a given population.
Term traditionally applied to species in the fossil record that seem to be more closely related to modern humans than to apes. The term originally referred only to species of humans, when only humans were included in the family Hominidae. Recent genetic evidence suggests that humans, chimpanzees, and gorillas are so closely related that hominid should refer to the family Hominidae, which includes Homo and Pan.
Modern humans and their ancestors; used by some classification systems as a replacement for the older term hominid.
Subfamily containing the ancestral and living representatives of the African ape and human branches.
Scientific study of human evolution; a subfield of anthropology, the study of human culture, society, and biology.
Term traditionally applied to animals in the fossil record that seem to be closer to the apes than to modern humans.
Group of organisms belonging to the same species and sharing a particular local habitat.
Differences in size or other anatomical characteristics between males and females.
In reference to animals that reproduce sexually, refers to a group whose adult members interbreed and produce fertile offspring. Each species is given a unique, two-part scientific name.
A NEW ERA OF GENUS AND SPECIES NAMING FRENZY?
Almost 50 years ago, the evolutionary biologist Ernst Mayr warned against being too zealous in naming new species in the quest for answers on human evolution. Mayr urged scientists to become conservative lumpers, not splitters, in creating categories. However, new fossil finds must be categorized and their place in human history accounted for.
New answers about human origins continue to unravel in Ethiopia, an area that yielded Australopithecus afarensis (Lucy) in 1974, at almost 4 million years old, the oldest recognized human ancestor. New fossil finds in Ethiopia have occasioned new genus and species designations—Ardipithicus ramicus (1997), Orrorin tugenensis (2000), and most recently, a subspecies of Ardipithicus ramicus was crowned Ardipithicus ramicus kadabba. Dated by the volcanic material in which it was embedded, kadabba, is about 5.8 million years old, older than the previous Ardipithicus by more than a million years.
The significance of kadabba was presented in a paper published in the journal Nature in July 2001. Ethiopian graduate student Yohannes Haile-Selassie, who discovered kadabba, was working out of the University of California, Berkeley, under Timothy White, a former collaborator of Donald Johansen, who discovered Lucy. Haile-Selassie found fossilized teeth, toes, part of a jaw bone and pieces of collar bone. He says that the fossil teeth and toes of kadabba point to the primate as having been an upright, bipedal walker. If kadabba was bipedal, this find pushes the known beginnings of bipedalism back another million years or so. However, kadabba fossils are few, and the conclusions that can be drawn from them are therefore sketchy.
Even if kadabba is found to have been bipedal, its place in human evolution may not be conclusive. There may have been more than one bipedal primate in Africa six million years ago, and discovering which bipedal primates may have marched in the parade toward becoming human is still a difficult question to answer.