HUMAN PHYLOGENY So far in this course we have examined the concept of evolution but with little reference to our own species, Homo sapiens. Unless we fit within the evolutionary paradigm, the concept of evolution has no ramifications for the study of human nature. Is there any evidence that our own species is the product of descent with modification from some ancestral species? The answer is a qualified "yes." Pre-evolutionary (creationist) taxonomists recognized our structural relationship to other animals and classified us in the Class Mammalia and Order Primates. As primates we share an affinity with prosimians (tarsiers and lemurs, pottos and galagos) and anthropoids (Old World monkeys, New World monkeys, and the great apes). Of these our closest taxonomic relatives are the great apes with whom we are included in the superfamily Hominoidea (hominoids) which consists of two different families: Pongidae (gibbons [subfamily Hylobatinae] and the orangutan, chimpanzee and gorilla [subfamily Ponginae]); and Hominidae (hominids) containing one living species - Homo sapiens. There is no question, however, that we differ from all other hominoids and this is why we are classified in a separate family. In contrast to all other primates our behavior is dominated by culture; so much so that we are said to belong to a new evolutionary regime (cultural rather than organic evolution) and are still viewed by some to be the product of special creation. We clearly possess abilities, e.g., speech, which set us apart from the rest of the animal kingdom. The gap separating us from other animals, even our taxonomic relatives, is simply too great to be bridged by some evolutionary process. Or is it? Could our taxonomic relatives also be our blood or genetic relatives? Fossil Evidence The fossil record provides evidence of species variation over time but until recently was of little value in documenting human phylogeny. In part this was due to the tendency to classify every discovery as a new species so that the sheer number of fossil species clouded the picture of human ancestry known today. The second was the absence of intermediate fossils suggesting a transition between the great apes and humans. The first problem was alleviated when typological thinking was replaced by populational thinking, i.e., when different fossils were viewed as geographic variants of the same species rather than as separate species or genera. Consequently, the number of described species was drastically reduced and so facilitated critical assessment of their relationships. The second problem still remains, presumably because any common ancestor of our line and that leading to the great apes was probably arboreal and so lived in habitats unlikely to produce fossils. Early hominoid fossils The oldest fossils believed to be ancestral to the modern hominoids are represented by several genera of dryopithecines which lived about 20 million years ago (mya). The dryopithecines, however, lacked the long arms, strong jaws and large canine teeth characteristic of pongids as well as the bipedalism of hominids. A later group, the ramapithecines found in India and Africa some 10-14 mya, resembled the orangutan especially in possessing a thin enamel coating on their teeth. This characteristic, found in our own species, was considered to be a specialized derivative of the thick enamel found in the African apes. The bias that we are more advanced than the African apes led to the evaluation of the orangutan as more advanced than its African cousins and more closely related to ourselves. The 1980 discovery in Pakistan of the sivapithecines with characters similar to both the orangutan and the ramapithecines led to a reassessment of hominoid phylogeny. Today it is believed that the ramapithecines and sivapithecines arose from dryopithecines and were either on the ancestral line leading to the hominids or were a side line, but in either case gave rise to the orangutans. Gibbons are believed to have diverged from the drypothecines before the appearance of the ramapithecines (see graph of hominoid phylogeny appended to these notes). Beyond this point no fossils exist which could be considered ancestral to both the African pongids and the genus Homo. Hominid fossils Despite the lack of fossil documentation of the ape-human split, tremendous advances have been made in discovering fossil species ancestral to ourselves. The first of these finds came in 1924 with the discovery of the "Taung Baby" skull by Raymond Dart of South Africa. Dart assigned this fossil to the species, Australopithecus africanus, "southern ape of Africa." As regards our own genus, analysis of fossils using populational thinking rather than typological thinking revealed that Java man (Pithecanthropus erectus) and Peking man (Sinanthropus pekinensis) were so closely related to African fossils of Homo erectus that they belonged to the same species. An almost complete skeleton of this species, found in Kenya by Alan Walker and Richard Leakey, gave unmistakable evidence of an affinity with ourselves and so H. erectus was interpreted as our immediate ancestor. Additional discoveries produced (1) other species of the genus Australopithecus (A. robustus and A. boisei), (2) the short-lived (for about one-half million years) Homo habilis, and (3) fossil humans: archaic Homo sapiens dated 500,000 years ago and anatomically modern Homo sapiens (Cro-Magnon man) about 100,000 years ago. The dating of these fossils supported the hypothesis that the first human was Homo habilis which appeared about 1.9 mya and gave rise to H. erectus about 1.7 mya, which in turn gave rise to H. sapiens about 0.5 mya. What was the relationship between this evolving line of the genus Homo and the australopithecines? An answer was provided in the 1970s through the discoveries of an international team of paleoanthropologists headed by Donald Johanson which unearthed a 40% complete skeleton of an australopithecine they nicknamed "Lucy." This expedition also discovered a number of other fossil australopithecines and announced to the world that they had discovered a new species, one ancestral to all other australopithecine species and also to the genus Homo. Since Lucy was shown to be bipedal (the distinguishing characteristic of the family Hominidae), the genus Australopithecus was included in this family along with the genus Homo. The Johanson-White model of hominid phylogeny Johanson and a colleague, Timothy White, concluded that this new species, A. afarensis - named after the Afar Triangle of Ethiopia where the fossils were found - was the ancestral hominid species for two reasons: (1) it predated all other hominids - about 3.5 mya, and (2) it possessed the primitive state for two characteristics found in both genera of hominids: a small cranial capacity (500 cc) and generalized dentition indicating an unspecialized, omnivorous diet. They hypothesized that A. afarensis evolved along two different lines by retaining one feature as primitive and specializing in the other. Later australopithecines indicate a trend of specialization towards a vegetarian diet as evidence by massive jaw bones and large molars. This trend culminated in the robust australopithecines which became extinct, presumably because they could not compete for food with early tool-using humans. These specialized australo- pithecines basically retained the small cranial capacity of A. afarensis. One model suggests that A. africanus evolved from A. afarensis and gave rise to A. robustus. Another australopithecine, A. boisei from central Africa, was believed to have evolved from A. robustus until Alan Walker discovered a fossil (KNM-WT 1700) which suggested that A. boisei may have evolved separately from A afarensis. The other line which evolved from A. afarensis retained unspecialized dentition and an omnivorous diet, but showed a remarkable trend in enlarged brain size as indicated by the average cranial capacity of the skull measured in cubic centimeters (cc): Homo habilis (700 cc), H. erectus (950 cc) and H. sapiens (1350 cc). Molecular Evidence The fossil record is silent regarding the date of the ape-human split but advances in molecular biology have been applied to this problem and offer evidence to fill the void. DNA, the genetic material, acts by producing proteins which interact during development to produce phenotypic traits. The amino acid sequence of proteins reflects the nucleotide sequence of DNA and similarities and differences in either offer a means of estimating genetic or phylogenetic relationship. Techniques were developed by molecular biologists which enable comparison of the genetic relationship between different species based on protein and/or DNA structural similarity. These results suggest that we share 98% of our genes with the chimpanzee and 97% with the gorilla. Furthermore, these molecular biologists estimated that humans diverged from chimpanzees 5-8 mya, from gorillas 8-10 mya, and from orangutans 15 mya. Although controversial, these conclusions indicate a degree of genetic relationship between humans and our closest living relatives (the chimpanzees) far closer than phenotypic comparisons suggest and a divergence time during phylogeny of less than 10 million years. Agreement between fossil and molecular evidence Until recently no direct fossil evidence existed to substantiate the claim of molecular biologists regarding the ape-human split, but analysis of Lucy's skeleton has provided indirect confirmation. We know that A. afarensis existed 3.5 mya and that this species was bipedal, hence, a hominid. Yet, Lucy's skeleton has some remarkable ape-like features and analysis of these features along with examination of comparative hominoid musculature prompted Susman and Stern to conclude that A. afarensis was half-way in manner of walking between apes and humans. If this is the case, then the ape-human split must have occurred about 7 mya (3.5 x 2) which agrees with the 5-8 mya date estimated by molecular biologists. Between 1992 and 1994 fragments (teeth, arm bones and skull parts) of 17 individuals were discovered in 4.4 million year old sediment in Aramis, Ethiopia (about 45 miles from the Lucy site). These fragments were assigned to a new species, Australopithecus ramidus, which extends human phylogeny another million years back into history. Preliminary analysis suggests that this species has characters intermediate between A. afarensis and chimpanzees and should this assessment hold up, it promises to shed further light on the ape-human split. Unfortunately no pelvis or leg bones have yet been found so classification of this new species as a hominid is not certain. The discovery site is believed to have been heavily wooded at the time these individuals lived and if it turns out that A. ramidus did walk upright, the current view that bipedality evolved in response to climatic change which produced open savannas will have to be discarded. Origin of Homo sapiens All evolutionary biologists believe that our species evolved from Homo erectus which is the hominid species most similar to us both in time and structure. The unanswered questions are how and where did this evolution occur. There are two major mechanisms that can account for the origin of a new species: phyletic evolution and speciation. Each has been suggested as the mechanism behind the origin of H. sapiens. Phyletic evolution (multiregional model) Fossil evidence suggests that Homo erectus evolved from H. habilis in Africa and then migrated to Asia and Europe. The multiregional or regional continuity origins model maintains that H. sapiens evolved gradually from each of these geographically separated populations of H. erectus. Thus, there was no single point of origin of our species. Some evolutionary force (selective pressure?) acted on all these populations in the same manner so that all reached the same evolutionary grade (structure) characteristic of our species. Gene flow between populations certainly could have facilitated this process because there existed considerable regional continuity among the populations of our ancestral species. Speciation (Noah's Ark or single origin model) An alternative model argues that our species evolved from a single African population of Homo erectus after the global migration had occurred. We then followed the same migration routes and subsequently came in contact with H. erectus only to replace this species through direct or indirect competition. Whether or not occasional interbreeding between the two species took place is unknown. Which of these two models is correct? Both the phyletic evolution and speciation models or hypotheses yield different predictions whereby they can be tested. Among them are the following. 1. Location of transitional fossils between the two species. The multiregional model predicts the existence of transitional forms in Europe, Asia, Africa and Australia since many different popu- lations of H. erectus were transformed into our species. The Noah's Ark model, on the other hand, predicts that transitional fossils should be found only in Africa. 2. Degree of interpopulational variation. Since the geographic distribution of H. erectus was widespread and subject to geographic isolation, the multiregional model predicts considerable variation among populations especially in comparing peripheral and central populations. The Noah's Ark model claims that only a few migrants left Africa to colonize the globe and so the differences among these populations should be small. Comparison of African and nonAfrican populations should reveal the largest amount of interpopulational difference. 3. Degree of intrapopulational variation. The multiregional origins hypothesis predicts that the most variable human populations should exist in the center of the species range with less variablilty in the peripheral populations, whereas the Noah's Ark hypothesis predicts that the largest amount of within-population varia- tion will be found in African populations which according to this model should be older. These two models are the focus of a hot debate today and unfortunately fossil data are too scarce to resolve the issue (prediction 1). That the oldest H. sapiens fossils are from Africa lends some support to the single origin by speciation claim, but the announcement in 1992 of two 350 thousand year old human skulls found in China with characteristics between H. sapiens and H. erectus provides fossil evidence for the multiregional model. Analysis of mitochondrial DNA (mDNA) and protein variation (polymorphism) has revealed very little interpopulation variation in modern humans and, in an often quoted study of human blood group variation, the population geneticist Richard Lewontin demonstrated that most genetic variation in humans is intrapopulational (85%), not interpopulational (prediction 2). Rebecca Cann while at the University of California (Berkeley) argued that Africa is our place of origin because the human population with the most intrapopulation variation in mDNA (prediction 3) is the Kung San of southern Africa. She also suggested that all existing mDNA can be traced back to a single female (Eve) or small group of females who lived in Africa some 200 thousand years ago but this claim has been recently discredited. Cultural Evolution The trend toward increased brain size in our species peaked some 30,000 years ago and has not proceeded any further. Large brain size, presumably correlated with a high level of intelligence, requires a large birth canal, hence pelvic size may have been the limiting factor to this trend. The brain is the only organ of the body which continues development after birth, but this development is not toward increased size; rather, it involves increased neuronal circuitry enhanced by external environmental stimuli. Consequently, human offspring are quite helpless at birth and require extensive parental care. During this period, however, they are capable of learning and it is learning that largely shapes their future behavior. Jared Diamond noted in the archaeological record a striking change about 35,000 years ago. Before then, human artifacts were remarkably similar worldwide. Afterwards, not only do previously existing tools begin to vary, but also new tools appear. Regional differences in tool-kind suggest local cultural adaptation. Diamond interprets this variation in human artifacts as evidence that about 35,000 years ago Homo sapiens became proficient in the use of language. The ability to communicate verbally resulted in an exchange of ideas and an explosion of creativity. Before this time the major mode of communication was imitation which explains the great similarity in cultural artifacts. It appears then that about 35,000 years ago our species entered a new sector of evolution, cultural or psychosocial evolution, and this new form of evolution has dominated our way of life ever since. Note, however, that we are still subject to organic evolution as differential reproduction still exists, especially in response to times of crisis, e.g., epidemic disease. But on the whole our evolution is in the realm of culture. Within this new realm (accelerated by the evolution of speech and the development of language) stored variability is in the form of memes (ideas), inventions, tradition, laws and custom (not genes); the vehicle of transmission is learning (not heredity); and the direction of cultural evolution is provided by imitation, cultural selection, foresight and planning (not natural selection). So we are unique as a species despite our taxonomic and evolutionary affinity with other hominoids and hominids; but this uniqueness resides mainly in our cultural, not biological, achievements and capabilities. Nevertheless, we cannot afford to overlook our biological past for, as we shall see, it still tugs at us and influences our behavior. HOMINOID PHYLOGENY MYA 0.1 (modern) H. sapiens 0.5 (archaic) H. sapiens 1.0 1.5 A. boisei H. erectus A. robustus 2.0 H. habilis 2.5 KNM-WT 1700 3.0 A. africanus 3.5 A. afarensis 4.0 4.5 A. ramidus 5.0 6.0 Pan 8.0 Gorilla 10.0 12.0 14.0 Pongo 16.0 Ramapithecines & Sivapithecines 18.0 20.0 Gibbons Dryopithecines 25.0 30.0 Old World monkeys (Baboons)