Human Fossil Record—Diverse or Distorted?
The question is a legitimate one, and one that should be asked more frequently. Do paleontologists take into consideration skeletal variations when they report the latest “missing link”? After all, anyone who has ever sat in a room with more than 200 individuals of various ages, and from various cultures, realizes that skulls (and body sizes) come in a variety of shapes and forms. But, on occasion, impatient paleontologists are expectantly eager to place the next branch on the family tree. Some of these overzealous scientists have been rebuked by University of California (Berkeley) paleontologist Tim White, as he attempts to rein in the tendency of fossil hunters to classify every find as a new species.
In the March 28, 2003 issue of Science, White authored an article titled “Early Hominids—Diversity or Distortion?,” in which he quoted two former Harvard professors—Ernst Mayr, who once described hominid taxonomy as a “bewildering diversity of names,” and George Gaylord Simpson, who lamented “the chaos of anthropological nomenclature” (White, 2003, 299:1994). White then pointed out that many paleoanthropologists herald each new fossil as evidence for biodiversity, thus pointing to a “bushy” hominid tree. But, as White noted: “Whether judged from fossil evidence or zoological considerations, the metaphor of an early hominid bush seems seriously misplaced” (p. 1994). He then offered as an example last summer’s announcement of the African “Toumai” hominid cranium from Chad, which “was enthusiastically greeted as ‘the tip of an iceberg of taxonomic diversity during hominid evolution 5-7 million years ago.’ The same author even predicted a Late Miocene ‘African ape equivalent of the Burgess Shale.’ How could a single fossil from a previously unknown period warrant such claims?” (p. 1994, emp. added).
How indeed? How can scientists make bold claims with just a single specimen, especially when they consider the normal variation found in a room of 200 people? White inquired:
New hominid fossils are routinely given new species names such as Ardipithecus ramidus, Australopithecus anamensis, Australopithecus garhi, and Homo antecessor. At the same time, long-abandoned names such as H. heidelbergensis and H. rhodesiensis have recently been resurrected. Textbook authors and publishers eagerly adopt these taxa. But does the resultant nomenclature accurately reflect early hominid species diversity? (p. 1994).
He then went on to make a statement that should echo throughout the halls of academia:
To evaluate the biological importance of such taxonomic claims, we must consider normal variation within biological species. Humans (and presumably their ancestors and close relatives) vary considerably in their skeletal and dental anatomy. Such variation is well documented and stems from ontogenetic, sexual, geographic, and idiosyncratic (individual) sources (p. 1994, parenthetical items in orig., emp. added).
The fact is, even within our own families, we find huge variations.
White has launched an all-out attack on those who are quick to name new species. One of the first bombs he dropped was aimed at the famous Leakey family (or, more accurately, at Richard Leakey’s wife, Meave). White questioned the heritage of a Kenyanthropus platyops (the “flat-faced man of Kenya”)—a fossil discovered by Meave Leakey and her colleagues and reported in the March 22, 2001 issue of Nature (see Leakey, 2001). White believes this is just one more example of scientists being too quick to give us a bushy family tree.
In an article titled “Flat-faced Man in Family Feud” posted on the Nature Science Update portion of Nature’s Web site on March 28, 2003, Rex Dalton noted that White believes it was “geology, not genes” that “gave the Flat-faced Man his distinctive looks” (2003). In other words, White is suggesting that, over time, fine-grained rock invaded tiny cracks in the skull and distorted its shape in an irregular way. White has seen the Kenyanthropus fossil, but has not studied it in depth. His explanation for the unusually flat face is based on skulls of pigs that, White remarked, were “flattened and narrowed by geological deformation, not natural selection.” And White is not alone in his assessment. Elwyn Simons, who studies primate evolution at Duke University, concurred: “The evidence may not support the description of a new genus” (as quoted in Dalton, 2003).
Dalton noted that Bernard Wood, a hominid specialist of George Washington University in Washington, D.C., agrees that geological processes altered the skull. Dalton also pointed out that Leakey’s team knew that. Wood noted: “What is at issue is whether that alteration materially affects if this is a new genus” (as quoted in Dalton). White went on to say:
There are two questions to be asked in considering whether the fossil constitutes evidence of early hominid species diversity. First, are the described morphological differences from the A. anamensis to A. afarensis lineage real, or are they merely artifacts of postmortem fossilization processes? Second, does the putatively new morphology lie outside the expected range of phenotypic variation of this lineage? (p. 1995).
His first point is illustrated in the Science article by a sequence of pictures of pig skulls that almost anyone would consider to be separate species. Yet experts know the skulls are all from the same species. Geological processes, as it turns out, distorted the skulls. After burial, these skulls were crushed, extruded, and otherwise modified, sometimes in nonlinear and asymmetric ways. In illustrating his second point, White showed two very different-looking skulls of modern female chimpanzees. One skull was narrow, the other broad; one profile had a pronounced slant, while the other was compressed. The teeth, brow ridges, skullcap, and eye sockets were vastly different—yet both these specimens belonged to the same species, and are even the same sex. White remarked: “This variation is normal in a single sex of an extant species; even more variation is present in other extant ape species” (p. 1995).
The phrase, “a picture speaks a thousand words,” comes to mind as one reads Dr. White’s Science article. The pig skulls are grossly distorted, and the two bonobo skulls show incredible variation. Entire cranial regions are different between the two. Given the fact that we know geological conditions distort fossils, and given the fact that we see so much variation within species, one must wonder how many “missing links” have been “created” using only one skull—a skull that was either damaged by geological conditions, or simply was a variation of a human?
Dalton, Rex (2003), “Flat-faced Man in Family Feud,” Nature Science Update, [On-line] URL: http://www.nature.com/nsu/030324/030324-10.html, March 28.
Leakey, Meave, et al. (2001), “New Hominin Genus from Eastern Africa Shows Diverse Middle Pliocene Lineages,” Nature, 410:433-440, March 22.
White, Tim (2003), “Early Hominids—Diversity or Distortion?,” Science, 299:1994-1996, March 28.