A Big Bang Theory of Homo
There aren't any good candidates.
Many paleoanthropologists have cited Homo habilis, dated at about 1.9 mya,87 as a transitional species between the australopithecines and our genus Homo. But there are many questions about what exactly habiline specimens were. In the words of Ian Tattersall, an anthropologist at the American Museum of Natural History, the species is "a wastebasket taxon, little more than a convenient recipient for a motley assortment of hominin fossils."88 As recently as 2009, Tattersall reaffirmed this view, writing with Jeffrey Schwartz that habilis represents "a rather heterogeneous assemblage, and it is probable that more than one hominid species is represented."89
Penn State University paleoanthropologist Alan Walker explains how severe the disagreements are surrounding this species: "[T]his is not a matter of some fragmentary fossils that are difficult to agree on. Whole crania are placed by different people in different species or even genera."90 One reason for the disagreements is that the quality of the fossils is often poor. As Walker puts it, "[d]espite the number of words published on this species... there is not as much bony evidence as we would like."91
Ignoring these difficulties and assuming that H. habilis existed as the species many claim it was, there is a chronological consideration that precludes it from being ancestral to Homo. Habiline remains do not predate the earliest fossil evidence of true members of Homo, which appear about 2 mya. As a consequence, H. habilis could not have been a precursor to our genus.92
Morphological analyses further confirm that habilis makes an unlikely candidate as an "intermediate" or "link" between Australopithecus and Homo. An authoritative review paper titled "The Human Genus," published in Science in 1999 by leading paleoanthropologists Bernard Wood and Mark Collard, found that habilis is different from Homo in terms of body size, body shape, mode of locomotion, jaws and teeth, developmental patterns, and brain size, and should be reclassified within Australopithecus.93 A 2011 article in Science similarly noted that habilis "matured and moved less like a human and more like an australopithecine," had a dietary range "more like Lucy's than that of H. erectus."94 Like the australopithecines, many features of habilis indicate they were more similar to modern apes than to humans. According to Wood, habilines "grew their teeth rapidly, like an African ape, in contrast to the slow dental development of modern humans."95
An analysis in Nature of the ear canals of habilis similarly found that its skull is most similar to baboons and suggested the fossil "relied less on bipedal behaviour than the australopithecines."96 The article concluded that "[p]hylogenetically, the unique labyrinth of [the habilis skull] represents an unlikely intermediate between the morphologies seen in the australopithecines and H. erectus."97 Additionally, a study by Sigrid Hartwig-Scherer and Robert D. Martin in the Journal of Human Evolution found that the skeleton of habilis was more similar to living apes than were other australopithecines like Lucy.98 They concluded: "It is difficult to accept an evolutionary sequence in which Homo habilis, with less human-like locomotor adaptations, is intermediate between Australopithecus afaren[s]is ... and fully bipedal Homo erectus."99 Elsewhere, Hartwig-Scherer explained "expectations concerning postcranial similarities between Homo habilis and later member of the genus Homo could not be corroborated."100
To the contrary, she explains, habilis "displays much stronger similarities to African ape limb proportions" than even Lucy.101 She called these results "unexpected in view of previous accounts of Homo habilis as a link between australopithecines and humans."102
Without habilis as an intermediate, it is difficult to find fossil hominins to serve as direct transitional forms between the australopithecines and Homo. Rather, the fossil record shows dramatic and abrupt changes that correspond to the appearance of Homo.
A 1998 article in Science noted that at about 2 mya, "cranial capacity in Homo began a dramatic trajectory" that resulted in an "approximate doubling in brain size."103 Wood and Collard's review in Science the following year found that only a single trait of one individual hominin fossil species qualified as "intermediate" between Australopithecus and Homo: the brain size of Homo erectus.104 However, even this one intermediate trait does not necessarily offer any evidence that Homo evolved from less intelligent hominids. As they explain: "Relative brain size does not group the fossil hominins in the same way as the other variables. This pattern suggests that the link between relative brain size and adaptive zone is a complex one."105
Likewise, others have shown that intelligence is determined largely by internal brain organization, and is far more complex than the sole variable of brain size. As one paper in the International Journal of Primatology notes, "brain size may be secondary to the selective advantages of allometric reorganization within the brain." 106 Thus, finding a few skulls of intermediate size does little to bolster the case that humans evolved from more primitive ancestors.
As with brain size, a study of the pelvic bones of australopithecines and Homo proposed "a period of very rapid evolution corresponding to the emergence of the genus Homo."107 In fact, a paper in the Journal of Molecular Biology and Evolution found that Homo and Australopithecus differ significantly in brain size, dental function, increased cranial buttressing, expanded body height, visual, and respiratory changes and stated: "We, like many others, interpret the anatomical evidence to show that early H. sapiens was significantly and dramatically different from... australopithecines in virtually every element of its skeleton and every remnant of its behavior."108
Figure: Brain size is not always a good indicator of intelligence or evolutionary relationships. Case in point: Neanderthals had a larger average skull size than modern humans. Moreover, skull size can vary greatly within an individual species. Given the range of modern human genetic variation, a progression of relatively small to very large skulls could be created by using the bones of living humans alone. This could give the misimpression of an evolutionary lineage when in fact it is merely an interpretation of data, guided by preconceived notions of what happened.
Illustration: Jonathan Jones
Noting these many changes, the study called the origin of humans, "a real acceleration of evolutionary change from the more slowly changing pace of australopithecine evolution" and stated that such a transformation would have included radical changes: "The anatomy of the earliest H. sapiens sample indicates significant modifications of the ancestral genome and is not simply an extension of evolutionary trends in an earlier australopithecine lineage throughout the Pliocene. In fact, its combination of features never appears earlier."109
These rapid, unique, and genetically significant changes are termed "a genetic revolution" where "no australopithecine species is obviously transitional."110 For anyone who considers the evidence unconstrained by an evolutionary paradigm, what is also not obvious is that this transition took place at all. The lack of fossil evidence for the hypothesized transition is confirmed by Harvard paleoanthropologists Daniel E. Lieberman, David R. Pilbeam, and Richard W. Wrangham, who provide a stark analysis of the lack of evidence for a transition from Australopithecus to Homo:
Of the various transitions that occurred during human evolution, the transition from Australopithecus to Homo was undoubtedly one of the most critical in its magnitude and consequences. As with many key evolutionary events, there is both good and bad news. First, the bad news is that many details of this transition are obscure because of the paucity of the fossil and archaeological records.111As for the "good news," they still admit: "[A]lthough we lack many details about exactly how, when, and where the transition occurred from Australopithecus to Homo, we have sufficient data from before and after the transition to make some inferences about the overall nature of key changes that did occur."112
In other words, the fossil record provides ape-like australopithecines, and human-like Homo, but not fossils documenting a transition between them.
In the absence of fossil evidence, evolutionary claims about the transition to Homo are said to be mere "inferences" made by studying the non-transitional fossils we do have, and then assuming that a transition must have occurred somehow, sometime, and someplace.
Again, this does not make for a compelling evolutionary account of human origins. Ian Tattersal also acknowledges the lack of evidence for a transition to humans:
Our biological history has been one of sporadic events rather than gradual accretions. Over the past five million years, new hominid species have regularly emerged, competed, coexisted, colonized new environments and succeeded--or failed. We have only the dimmest of perceptions of how this dramatic history of innovation and interaction unfolded...113Likewise, evolutionary biologist Ernst Mayr recognized our abrupt appearance when he wrote in 2004:
The earliest fossils of Homo, Homo rudolfensis and Homo erectus, are separated from Australopithecus by a large, unbridged gap. How can we explain this seeming saltation? Not having any fossils that can serve as missing links, we have to fall back on the time-honored method of historical science, the construction of a historical narrative.114As another commentator proposed, the evidence implies a "big bang theory" of the appearance of our genus Homo.115
Illustration: Adapted from Wikimedia Commons work in the public domain.
[Editor's Note: This is an excerpt from Chapter 3, "Human Origins and the Fossil Record," of the new book Science and Human Origins, co-authored by Ann Gauger, Douglas Axe, and Casey Luskin. For details, see Discovery Institute Press.]
[87.] F. Spoor, M. G. Leakey, P. N. Gathogo, F. H. Brown, S. C. Antón, I. McDougall, C. Kiarie, F. K. Manthi, and L. N. Leakey, "Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya," Nature, 448 (August 9, 2007): 688-91.
[88.] Ian Tattersall, "The Many Faces of Homo habilis," Evolutionary Anthropology, 1 (1992): 33-37.
[89.] Ian Tattersall and Jeffrey H. Schwartz, "Evolution of the Genus Homo," Annual Review of Earth and Planetary Sciences, 37 (2009): 67-92. Paleoanthropologists Daniel E. Lieberman, David R. Pilbeam, and Richard W. Wrangham likewise co-write that "fossils attributed to H. habilis are poorly associated with inadequate and fragmentary postcrania." Daniel E. Lieberman, David R. Pilbeam, and Richard W. Wrangham, "The Transition from Australopithecus to Homo," in Transitions in Prehistory: Essays in Honor of Ofer Bar-Yosef, eds. John J. Shea and Daniel E. Lieberman (Cambridge: Oxbow Books, 2009), 1. See also Ann Gibbons, "Who Was Homo habilis--And Was It Really Homo?," Science, 332 (June 17, 2011): 1370-71 ("researchers labeled a number of diverse, fragmentary fossils from East Africa and South Africa 'H. habilis,' making the taxon a 'grab bag... a Homo waste bin,' says paleoanthropologist Chris Ruff of Johns Hopkins University in Baltimore, Maryland").
[90.] Alan Walker, "The Origin of the Genus Homo," in The Origin and Evolution of Humans and Humanness, ed. D. Tab Rasmussen (Boston: Jones and Bartlett, 1993), 31.
[92.] See Spoor et al., "Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya," 688-91; Seth Borenstein, "Fossils paint messy picture of human origins," MSNBC (August 8, 2007), accessed March 4, 2012, http://www.msnbc.msn.com/id/20178936/ns/technology_and_sciencescience/t/fossils-paint-messy-picture-human-origins/.
[93.] Wood and Collard, "The Human Genus," 65-71.
[94.] Gibbons, "Who Was Homo habilis -- And Was It Really Homo?," 1370-71.
[95.] Wood's views are described in Gibbons, "Who Was Homo habilis--And Was It Really Homo?," 1370-71. See also Wood and Collard, "The Human Genus," 65-71.
[96.] Spoor, Wood, and Zonneveld, "Implications of early hominid labyrinthine morphology for evolution of human bipedal locomotion," 645-48.
[98.] Hartwig-Scherer and Martin, "Was 'Lucy' more human than her 'child'? Observations on early hominid postcranial skeletons," 439-49.
[100.] Sigrid Hartwig-Scherer, "Apes or Ancestors?" in Mere Creation: Science, Faith & Intelligent Design, ed. William Dembski (Downers Grove: InterVarsity Press, 1998), 226.
[103.] Dean Falk, "Hominid Brain Evolution: Looks Can Be Deceiving," Science, 280 (June 12, 1998): 1714 (diagram description omitted).
[104.] Specifically, Homo erectus is said to have intermediate brain size, and Homo ergaster has a Homo-like postcranial skeleton with a smaller more australopithecine-like brain size.
[105.] Wood and Collard, "The Human Genus," 65-71.
[106.] Terrance W. Deacon, "Problems of Ontogeny and Phylogeny in Brain-Size Evolution," International Journal of Primatology, 11 (1990): 237-82. See also Terrence W. Deacon, "What makes the human brain different?," Annual Review of Anthropology, 26 (1997): 337-57; Stephen Molnar, Human Variation: Races, Types, and Ethnic Groups, 5th ed. (Upper Saddle River: Prentice Hall, 2002), 189 ("The size of the brain is but one of the factors related to human intelligence").
[107.] Marchal, "A New Morphometric Analysis of the Hominid Pelvic Bone," 347-65.
[108.] Hawks, Hunley, Lee, and Wolpoff, "Population Bottlenecks and Pleistocene Human Evolution," 2-22.
[111.] Lieberman, Pilbeam, and Wrangham, "The Transition from Australopithecus to Homo," 1.
[113.] Ian Tattersall, "Once we were not alone," Scientific American (January, 2000): 55-62.
[114.] Ernst Mayr, What Makes Biology Unique?: Considerations on the Autonomy of a Scientific Discipline (Cambridge: Cambridge University Press, 2004), 198.
[115.] "New study suggests big bang theory of human evolution" University of Michigan News Service (January 10, 2000), accessed March 4, 2012, http://www.umich.edu/~newsinfo/Releases/2000/Jan00/r011000b.html.