Human Evolution [Part II]
[EDITOR’S NOTE: Part I of this two-part series appeared in the September issue. Part II follows below and continues, without introductory comments, where the first article ended.]
Any attempt at constructing an evolutionary family tree from molecular data faces serious questions, but at least there is no shortage of test material. The veins of every human, chimp, or other target of study provide a veritable gold mine of information for protein and DNA analysis. Genetics and molecular biology, with their detailed reports of chemical sequences, also lend an air of objectivity and precision. Nonetheless, such studies deal with only the presumed heirs of an eons-long process.
GENES VERSUS BONES
At this point we can turn to the traditional workers in this field—paleoanthropologists and paleontologists—and appraise their collection of bones, tools, and other artifacts. These largely sterile samples are not good candidates for DNA or protein analysis, and so there is room for disagreement between the experts. Many paleontologists try to incorporate molecular evidence into their interpretations of the fossil evidence, but some fundamental problems remain unresolved.
Two Evolutionary Models
Perhaps the most vigorous example of this debate centers on the origin of modern humans. The molecular evidence is, if in no other instance, unanimous in suggesting a common origin for all human populations. Of these groups, Africans show far more genetic variation than non-Africans (i.e., Asians, Europeans, Native Americans, Pacific Islanders, et al.). Molecular biologists explain this greater variability by suggesting that African populations have had the most time to accumulate mutations and diverge from each other. Africa, then, is supposed to represent the ancient cradle from which all other populations have emerged (e.g., Cann, et al., 1987; see also Major, 1992).
This out-of-Africa model rocketed into public consciousness a few years ago with talk of a so-called mitochondrial Eve. In this case, the molecular data came not from the main DNA of the cell’s nucleus, but from tiny strands residing in the mitochondria (the cell’s “energy factories”). Theoretically, children inherit all this DNA from their mother, because sperm lack mitochondria. Relying solely on the maternal line, geneticists traced the family tree back to a hypothetical woman nicknamed “Eve.” Of course, the popular media could not resist the proximity of biblical metaphor to evolutionary speculations.
However, naysayers within the scientific community questioned the validity of the whole exercise. Alan Templeton and others (1992) have shown that other trees with non-African roots are possible, but that the variation among these computer-generated solutions is so great as to negate far-ranging conclusions based on mitochondrial data. This merely reinforces our general suspicion of the evolutionary premises behind the tree-constructing exercises.
Most criticisms come from paleontologists who object to the out-of-Africa theory on the basis of fossil evidence (e.g., Thorne and Wolpoff, 1992). In their multiregional model, several populations of Homo sapiens evolved independently in different parts of the world. They leave open the possibility that the immediate forbear, Homo erectus, may have had a common origin in Africa. However, they believe that people today reflect a variety of features bequeathed by different ancestral populations of H. erectus. For example, Milford Wolpoff argues that the classic protruding brow ridges of Neanderthal skulls from Krapina, Croatia, are visible in only slightly less pronounced form among relatively recent remains in the same area. To him, this demonstrates a mixing of distinctive local traits and general human features borne on migrations from many different areas. Indeed, several sites around the Middle East and Europe show Neanderthals living side-by-side with groups bearing somewhat modern features (sometimes referred to as either archaic sapiens or Homo heidelbergensis). Hence, multiregional advocates look incredulously on the idea that African emigrants could remain isolated genetically from neighboring populations of H. erectus or H. neanderthalensis.
A CREATIONIST INTERPRETATION
Both views contain a kernel of truth. For example, creationists would agree with the out-of-Africa model tenet that humans share a recent common ancestry, but also would agree with the multiregional model on a continuity between ancient H. erectus and H. sapiens populations. However, creationists would argue that many of these Homo species represent ancient and living variations of a created human kind and, most important, that humans did not evolve from an ape-like creature. In the following sections, I would like to attempt a distinction between genuinely human fossils, and the fossils of extinct ape species.
Variation in Fossil and Modern Humans
We would recognize a Neanderthal walking the streets of New York or Paris by prominent brow ridges, low forehead, flat skull, weak chin, jutting midfacial region, very large nose, forward-sloping face, and short, muscular limbs—to name some of the more visible characteristics (Stringer and Gamble, 1993, pp. 76-77). The skull of H. erectus shared many of the Neanderthals’ features, but with flatter brow ridges and a less prominent midfacial region. Some H. erectus skeletons were short and stocky like the Neanderthals, but one specimen—a nine- to eleven-year-old boy from West Turkana, Kenya—was tall and slender (Andrews and Stringer, 1993, p. 242). Cranial volume varied from 850 to over 1100 milliliters for H. erectus, and 1250 to over 1740 ml for Neanderthals. One specimen of H. heidelbergensis had an estimated volume of 1300 ml. The average for modern humans is 1350 ml, but we exhibit a broad range of 700 to 2200 ml (Lubenow, 1992, p. 138).
All the Homo species mentioned so far had some vocal capacity, as indicated by the arched shape of the base of their skulls (Leakey, 1994, pp. 130-133). Other mammals have a flat skull base and a very limited capacity for vocalization. Again, there is some variation among the fossil human types that does not follow a clear evolutionary pattern. Neanderthals, for instance, appear to have had a much flatter skull base than H. erectus. This may have limited their speech, but to what extent, we do not know. Unfortunately, the fossil record has not preserved the soft tissues of the vocal apparatus (the pharynx, larynx, tongue and lips). Other evidence (such as brain size, tool technologies, and deliberate burials) suggests that the Neanderthals were capable, thinking beings.
In general, skeletal proportions, the angularity of the face, and the shape of the brain case varied considerably among fossil humans (e.g. Figure 1). Yet differences, every bit as dramatic, occur among modern humans. Watusis today would not miss a Mbuti pygmy who strolled into their village, and an Inuit would stand out at a gathering of Australian aborigines.
Figure 1. The most likely candidates for fossil humans. From top to bottom: archaic H. sapiens (Qafzeh 9); Neanderthal (the “Old Man” of La Chapelle-aux-Saints); and Homo erectus (Sangiran 17). From Tattersall, 1995. Bars show scale of 1 cm.
Despite obvious facial features (Figure 2), both H. erectus and appear to fit within a distinct human kind. Although some specimens show a mixture of traits, there is no clear lineage from, say, H. erectus to H. sapiens. In fact, the fossil record suggests that they were contemporaries and, in some cases, neighbors (Stringer and Gamble, 1993, p. 137). The different species names are convenient for evolutionary discussions, but there is no evidence of reproductive isolation. Marvin Lubenow is one creationist who sees no problem including all these forms within a highly variable created human kind (1992, pp. 120-143).
Figure 2. Picture inspired by Earnest Hooten’s claim that no one would notice fossil men walking down modern streets if they were dressed in formal attire. Characters represent archaic H. sapiens (top right), Neanderthal (top), and H. erectus (bottom left and right).
Problematic Transition from Apes to Humans
As we have just seen, all human fossils possess fairly large brains in relation to their body size. Chimps, however, have relatively small brains, averaging around 400 ml. Humans also show a distinctive upright posture. In 1891, when Eugene Dubois found a skullcap, tooth, and leg bone in Trinil, Java, he named it Pithecanthropus erectus (“upright ape-man”). Later, as much better examples came to light, paleontologists recognized their humanness and changed the genus name to Homo. Hence, the transition from apes to humans represented a shift in posture and a four-fold increase in cranial volume.
Supposedly, the first critical step in this transformation took place when a small-brained animal—an australopithecine (“southern ape”)—began to walk upright (Figure 3). Of course, many animals are able to walk on two legs, but humans are the only modern primates that rely almost exclusively on this bipedal form of locomotion. However, a growing collection of fossil finds has enabled a closer scrutiny of different hominid species and the claims surrounding them. In particular, these studies have thrown doubt on the bipedalism of Australopithecus africanus, and its evolutionary dead-end cousin, Paranthropus.
Figure 3. The most unlikely candidates for fossil humans. From top to bottom: Homo habilis (KNM-ER 1813); Australopithecus africanus (Sts 5); and Australopithecus afarensis (reconstruction from unassociated fragments). From Tattersall, 1995. Bars show scale of 1 cm.
In order to walk upright, humans need good balance. A crucial part of this “sixth sense” resides in the bony labyrinth of the inner ear, which often is preserved in fossil remains. Fred Spoor and his colleagues (1994) used this information, and new technology in the form of CT scans, to compare the labyrinth of modern humans, great apes, and fossil hominids. Their results show a clear divide between H. erectus and H. sapiens on one side, and great apes, A. africanus, and Paranthropus robustus on the other.
Other recent evidence contrary to bipedalism includes:
- chimp-proportioned arm bones in A. afarensis (Kimbel, et al., 1994);
- chimp-like thumbs in A. afarensis more suited to tree climbing than tool making (Susman, 1994). This study identifies human-like thumbs in P. robustus, but this bone may belong to H. erectus instead (Aiello, 1994);
- a nonhuman gait in “Lucy,” one of the most famous specimens of A. afarensis, based on ratio of leg size to foot size (as reported by Oliwenstein, 1995).
- ape-like features in foot bones belonging to A. africanus or another contemporary hominid (Clarke and Tobias, 1995); and
- human-like limb proportions in A. afarensis, but ape-like limb proportions in its successors, A. africanus and Homo habilis. One researcher went as far to suggest that A. afarensis was a failed experiment in ape bipedalism, and should be consigned to a side branch of the human evolutionary tree (as reported by Shreeve, 1996).
The overall picture is one in which alleged ape-men derail the evolutionary process by returning to the trees. This assumes, of course, that A. afarensis was fully bipedal in the first place. One piece of evidence offered in support of this view comes from the well-known footprints in volcanic ash at Laetoli. Radiometric methods dated these tracks to 3.7 million years ago, which places the deposit within the supposed time span of A. afarensis. Apart from suspicions we may entertain about such dates, there is no proof that these tracks were made by anything other than fully modern humans. After analyzing the footprints of 70 Machiguenga Indians from Peru, and examining the available fossil toe bones, Russell H. Tuttle concluded that the ape-like feet of A. afarensis could not have made the Laetoli tracks (Bower, 1989).
Figure 4. One evolutionary “best guess” of hominid evolution (from Tattersall, 1995, p. 234).
This leaves the transition from the very ape-like A. africanus to the fully human H. erectus entirely in the hands (or is that feet?) of H. habilis (Figure 4). As noted previously, H. habilis possessed the same ape-like limb proportions as A. africanus. In fact, the whole issue of its place among Homo is highly contentious, and the species has become a dumping ground for strange and out-of-place fossils. Some paleontologists have tried to impose some order by reassigning australopithecine-like specimens to Homo rudolfensis, and the most modern-looking specimens to “early African H. erectus” or Homo ergaster (to which some would assign the Turkana boy). Apart from a small difference in brain size between australopithecines (less than 550 ml) and habilines (around 500-650 ml), there are no other compelling reasons to divide them among two genera. The same cannot be said about the gap between habilines and H. erectus. The latter have much larger brains (at least 848 ml, if we count the Turkana boy), well-developed stone tools, definite upright stance, and speech capabilities. Tattersall confesses that there is only a weak link between H. habilis and H. ergaster (1995, p. 232). Andrews and Stringer offer a similar opinion, stating:
The relation between habilis and erectus is unclear. It is widely assumed that the first gave rise to the second, but since there seem to be at least two kinds of habilis, whose toolmaking skills could be independent of their successors’, there is no obvious continuity (1993, p. 242).
The debate between creation and evolution centers constantly on a sort of “half empty, half full” argument. Evolutionists draw on molecular and fossil evidence to establish a genealogical connection between humans and living apes. They emphasize the similarities, and credit differences to the vagaries of natural selection. Any shared attribute (whether genetic, morphological, or behavioral) is used as an indicator of common ancestry; the degree of similarity is used to assign an alleged ancestor to a place on the “family tree.” For their part, creationists emphasize the differences, and credit similarities to God’s use of a common design. So which of these carries the day: similarities or differences?
As we have seen, the molecular evidence is very limited in providing proof of relatedness between distant relatives. The 1% difference between chimp and human DNA really is significant, and many protein comparisons fail to support the alleged evolutionary tree. Likewise, the fossil record establishes a clear difference between humans and apes, with no good candidates for transitional forms. Overall, the argument for relatedness based on similarity is void of reasonable proof.
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