The search for Adam’s ancestors
by Elaine Kennedy
Evolutionary biologists are convinced that humans are descendants of ape-like creatures. In spite of a number of disputes over theories of ape-human lineages, paleoanthropologists concur. Christian response to these assertions has been varied. Some Christian organizations agree with the scientific community about the origin of humans but maintain that at some time in the past human beings acquired an immortal soul, moral sense, and/or the ability to reason. Others, including Seventh-day Adventists, accept the Genesis account as the record of a historical event.
Where did Adam come from? Was he fashioned from the dust of the ground by an intelligent Creator, or did he descend from an ape-like creature? We know what the Bible says. Does the “book of nature” agree?
Determining what is human
Although some pet owners might argue the point, traits such as esthetics and moral sense, free choice, and complex speech set humans apart from animals.1 Extinct human-like skeletons cannot provide us with this type of information. Since scientists are not able to talk to the organisms that are alleged to be our ancestors to determine just how human they were, researchers rely on structural features of the fossil bones and genetic information in modern apes and humans.
Modern humans are distinguished by several skull features. Three notable characteristics can be easily recognized: (1) At the front of the lower jaw, modern humans have a part of the jaw bone that protrudes to form the chin. (2) The angle of the face is very flat because modern humans lack a muzzle and have a non-receding forehead. (3) The upper portion of the skull in modern humans is wider than the base of the skull. Determining whether a fossil skeleton is a modern human does not appear to be too difficult.
Hominid is the name given to the bipedal primates, including all of the species in the genera Australopithecus and Homo. The australopithecines include the genus Australopithecus and, for some researchers, Paranthropus. The hominines refer to the members of the genus Homo.
The australopithecines are divided into two groups, based on body type: (1) The gracile, small-boned, more fragile ape-like forms include A. ramidus (the most recent australopithecine find, currently proposed as the fossil closest to the “missing link” or common ancestor of apes and humans); A. afarensis (a “community” of fossils has been found; one 40 per cent complete skeleton is popularly known as “Lucy”); and A. africanus (the “Taung Child,” named for the locality near which it was found).
(2) The robust ape-like forms include A. aethiopicus (a skeleton with some distinct traits found in A. afarensis, known as “Black Skull”), A. robustus, and A. boisei. Some researchers place all of the robust forms in the genus Paranthropus.2
The genus Homo, to which humans belong, has a number of species assigned to it: H. habilis (fragmentary material of a small species found near stone tools, known as “Handy Man”); H. rudolfensis (gracile skull and bone material notably larger than H. habilis, even though it was formerly assigned to that species); H. erectus (more than 200 fossil individuals popularly referred to as erectines, including Java Man and Peking Man); H. ergaster (skull and bone material formerly assigned to the erectines and now distinguished by lower jaw and tooth structure as a separate species, known as “Turkana Boy”); H. heidelbergensis (“Rhodesian Man”, an archaic H. sapiens previously identified as an erectine, sometimes listed as H. sapiens heidelbergensis, a subspecies of H. sapiens; the species has a larger cranial capacity than the erectines); H. neanderthalensis (a robust species commonly pictured as a “Cave Man”, skeletal remains frequently display evidence of trauma, sometimes listed as H. sapiens neanderthalensis); and finally, Homo sapiens or Homo sapiens sapiens (modern humans).3
In the search for human origins, three major groups of scientists—paleoanthropologists, evolutionary phylogeneticists, and molecular anthropologists—approach the problem from three very different perspectives. Paleoanthropologists focus on physical features of the hominid skeletons and on tool use. Evolutionary phylogeneticists describe the similarities or relatedness of organisms. Molecular anthropologists emphasize protein and DNA similarities among the hominids.
Some hominid physical features. Paleoanthropologists are scientists who study exclusively human origins. In comparisons of skeletal structures or morphological traits in the hominids, they believe they have found several hominine and ape-like features in these fossils. One of the most important species of the australopithecines, Australopithecus afarensis, exhibits these features.
Australopithecus afarensis, a hominid also popularly known as “Lucy,” has a hip joint, that is not quite ape-like, not quite human. While it seems clear that the australopithecines were not knuckle-walkers like modern apes, the hip joints were rotated forward enough that they are not confused with modern human hip joints. (One of the criteria that has been used to identify the genus Homo is a fully upright walking posture.) Another structure pointed out by paleoanthropologists as evidence for an australopithecine link between apes and humans is the curvature of the finger and toe bones. The australopithecine fingers and toes are not as straight as human fingers and toes but the knuckle is not as simple as the chimpanzee’s.4 A number of these somewhat simian, somewhat human features in the limbs of the australopithecines have been identified. In addition, the decrease in the size of the teeth from the front of the mouth to the back is a trait similar to the arrangement of teeth in the hominines.5
Human/ape-like features also are found in the genus Homo. Homo habilis, or “Handy Man,” is included in the genus Homo primarily because the fossil material was found associated with “tools.” In addition, H. habilis has a jaw that is very human-like; however, its body skeleton resembles an australopithecine. The specimens assigned to Homo rudolfensis are included in the genus Homo because the skeletal structure is very human-like; but the face and teeth look like robust australopithecines.6 Paleoanthropologists divide the erectines into two species, based on the jaws and teeth, African location, and smaller brain capacity of H. ergaster relative to the erectines from Asia.7
Several diagrams have been constructed to demonstrate the proposed ancestral lineage of the hominids. The diagrams differ because the paleoanthropologists do not agree on the specific physical features that should be used to identify ancestral relationships, timing of divergence, and placement of new skeletal finds.8
Hominid relationships. Phylogeneticists use cladistic methods (cladograms) to describe relationships among organisms. Cladograms are diagrams that arrange organisms in groups having shared characters, describing organisms in terms of sister rather than ancestral relationships, in a hierarchical form. In developing cla-dograms, phylogeneticists make three basic assumptions: (1) The features or characters that make up the database can be arranged in a hierarchical structure; (2) the data or characters selected accurately represent the organisms; and (3) there has been little or no loss of defining characters.9 A cladogram describing possible relationships among hominids is shown in Figure 1.10
Some characters used to develop the cladogram appear in species in a different order than the majority of the characters defining the cladogram. Phylogeneticists select the cladogram with the least number of out-of-order characters to develop “best fit” diagrams; consequently, there is some disagreement over which characters best describe the organisms and how they should be arranged in the hierarchy.
After using cladistics to identify hierarchical relationships, numerous researchers incorporate this information into hypotheses and develop phylogenetic schemes depicting ancestral relationships for hominids. By 1993, at least six major phylogenetic schemes had been proposed for the hominids. Since the discovery of A. ramidus, a seventh scheme has been proposed. Much of the shuffling of species in these diagrams represents disputes over the validity of attributing to human evolution the various traits found in the skulls and teeth of the specimens.
The hominid molecular relationships. Some anthropologists have studied molecular similarities among modern apes and humans to develop hypotheses about hominid lineages. Some of these researchers assume that mutations and changes occur at a constant rate in DNA. Numerous studies, spanning more than 30 years, have tried to determine when various living species diverged from related species, based on the assumption of relatively constant rates of change in DNA, a “molecular clock.”11 Interpretations based on the “molecular clock” imply human origins occurred millions of years ago and assume that there is a link between apes and humans. The time spans postulated for the ape-human divergence range from 5 to 7 million years ago. However, the assumed mutation rates used to calculate these ages were challenged more than 30 years ago by Morris Goodman and more recently by Wen-Hsiung Li.12 The issues they have raised allow one to question the validity of the method.
Others have narrowed their field of inquiry and compared the mitochondrial DNA among human races, hypothesizing that the human line can be traced to a single African population.
The hominid “lineage”
Australopithecines. In the australo-pithecine group, A. ramidus (the most recent find) and A. afarensis (Lucy) are both considered ancestral (Figure 2), whereas A. africanus (Taung Child), listed as recently as 1993 as ancestral (Figure 3), continues to be disputed as part of the direct line.13
Hominines (See Figure 2). In the Homo genus, H. habilis (Handy Man) remains problematic but is listed outside the human line by Wood, and inside the human line by McHenry.14 The gracile form of H. rudolfensis once replaced H. habilis in the human lineage but now is also excluded by some workers. H. erectus (Peking Man, Java Man) should be currently listed as “offline” according to Tattersall (see Fig. 3)15 due to the fact that a portion of its skull structure is too robust.16 Some researchers list H. ergaster as one of the preferred “links,” although others still consider H. ergaster as a separate species and continue to include these organisms with the erectines and in the ancestral line. Finally, H. heidelbergensis is regarded as ancestral to both modern humans and the neanderthals (Figs. 2 and 3).17
Evolutionary hypotheses falsified
Figure 2 illustrates some current paleoanthropological conclusions in regard to ancestor-descendent relationships for hominids. The common ancestor for hominids and apes is still missing. A. ramidus, A. afarensis, erectines and H. heidelbergensis are all clearly listed as “links” in the lineage.
Using standard scientific methods, researchers test their hypotheses, rejecting those ideas shown to be false. In studies of human evolution, standard scientific methods may not be adhered to by some workers. For example, A. afarensis has unique traits that actually preclude them from being included among our ancestors. One cladistic study identified 69 traits that are expressed differently among the species in the “human lineage.” Of these, only 45 support the preferred evolutionary hypothesis.18
The remaining 24 characters contradict this preferred evolutionary hypothesis. The preferred hypothesis was selected by the researchers as representing the probable path of “human evolution” because it had been falsified the least number of times. As a result—and to their credit—other researchers have questioned the validity of A. afarensis as a human ancestor.19 The reversal in robustness that occurs with the inclusion of H. erectus in the “lineage” is another factor that is inconsistent with the current hominid evolutionary hypothesis.
What does the “book of nature” tell us? All hominid evolutionary hypotheses have been falsified. To be fair, this does not rule out the evolutionary theory (new specimens may be discovered to resolve the conflicts); therefore, it is not appropriate to announce to the world that “Evolution has been refuted,” based on the incongruence of current hypotheses.
If all of the hominid evolutionary hypotheses have been falsified, how does one interpret the fossil material? Comments by Wood illustrate what can be perceived as a blending of characters: “Whereas H. habilis sensu stricto [in the strict sense] is hominine with respect to its masticatory complex [mouth or jaws], it retains an essentially australopithecine postcranial skeleton [body]. Homo rudolfensis, on the other hand, apparently combines a later Homo-like postcranial skeleton [body] with a face and dentition [teeth] which are adaptively analogous to those of the ‘robust’ australopithecines, especially P. boisei.” Many characters that occur together in the australopithecines and in the hominines represent a mosaic of traits. Some Christians would interpret these organisms to have resulted from degeneration of the human form due to the entrance of sin. Another interpretation restricts the term human to anatomically modern humans and assigns the rest of the fossils to non-human created kinds. A broader interpretation of the term human would accept at least some of the fossils as other created subspecies of humans. Whatever these creatures were, it is obvious that there are problems with almost any interpretation of these fossils. Given the current database, caution is warranted. Indeed, it would be premature to draw any definitive conclusions with regard to the origins of these organisms and their relationship to the Genesis record.20
Elaine Kennedy (Ph.D., University of Southern California) is a geologist at the Geoscience Research Institute. She is the author of several articles, including “The Intriguing Dinosaur,” (Dialogue 5:2). Her address: Geoscience Research Institute; Loma Linda University; Loma Linda, CA 92350, U. S. A.
Notes and references
1. For an earlier discussion of the topic in this journal see David Ekkens, “Animals and Humans: Are They Equal?” (Dialogue, 6:3, pp. 5-8).
2. F. E. Grine, “Australopithecine Taxonomy and Phylogeny: Historical Background and Recent Interpretation,” in The Human Evolution Source Book, R. L. Ciochon and J. G. Fleagle, eds. (Englewood Cliffs, New Jersey: Prentice-Hall, 1993), pp. 198-210. Also, I. Tattersall, The Human Odyssey (New York: Prentice-Hall, 1993), p. 191.
3. Information on the australopithecines and hominines was obtained from the following sources: Tattersall, p. 191; M. H. Nitecki and D. V. Nitecki, Origins of Anatomically Modern Humans (New York: Plenum Press, 1994), p. 341; M. L. Lubenow, Bones of Contention (Grand Rapids, Mich.: Baker Books, 1987), p. 295.
4. R. L. Susman, J. T. Stern, Jr., and W. L. Jungers, “Arboreality and Bipedality in the Hadar Hominids,” Folia Primatologica, 43 (1984), pp. 113-156.
5. J. T. Robinson, “The Origin and Adaptive Radiation of the Australopithecines,” in Evolution and Hominization, G. Kurth, ed. (Stuttgart: Verlag, 1962), pp. 150-175.
6. B. Wood, “Origin and Evolution of the Genus Homo,” Nature, 355 (1992), pp. 783-790.
8. Grine, pp. 198-210.
9. R. H. Brady, “Parsimony, Hierarchy, and Biological Implications,” in Advances in Cladistics, vol. 2, Platnick and Funk, eds. (New York: Columbia University Press, 1983), pp. 49-60.
10. R. R. Skelton, H. M. McHenry, and G. M. Drawhorn, “Phylogenetic Analysis of Early Hominids,” Current Anthropology, 27 (1986), pp. 21-35.
11. M. Hasegawa, H. Kishino, and T. Yano, “Dating of the Human-Ape Splitting by a Molecular Clock of Mitochondrial DNA,” Journal of Molecular Evolution, 22 (1985), pp. 160-174.
12. See A. Gibbons, “When It Comes to Evolution, Humans Are in the Slow Class,” Science, 267 (1995), pp. 1907, 1908.
13. Wood, B. 1992. Reprinted in The Human Evolution Source Book, R. L. Ciochon and J. G. Fleagle, eds. (Englewood Cliffs, New Jersey: Prentice-Hall, 1993), pp. 319-331.
14. H. M. McHenry. Tempo and Mode in human evolution: Proceedings of the National Academy of Sciences, 91 (1994), pp. 6780-6786.
15. See I. Tattersall, The Human Odyssey.
16. The inclusion of H. erectus in the hominid “lineage” represents a reversal in the robustness of the skulls; from the gracile form of A. afarensis to robust structure of the H. erectus skull and back to the gracile skull structure of humans.
17. T. White, G. Suwa, and B. Asfaw, “Australopithecus ramidus, a New Species of Early Hominid from Aramis, Ethiopia” Nature, 371 (1994), pp. 306-312. B. Wood, “Origin and Evolution of the Genus Homo,” Nature, 355 (1992), pp. 783-790. F. E. Grine, “Australopithecine Taxonomy and Phylogeny: Historical Background and Recent Interpretation,” in The Human Evolution Source Book, pp. 319-331, 198-210. See also Tattersall, p. 151.
18. Skelton, McHenry, and Drawhorn, pp. 21-35.
20. I would like to express my appreciation to Dr. Lee Spencer and to the Geoscience Research Institute staff for their advice during the final stages of this essay.