Faith & Science
Traditional or Not? Assessing William Lane Craig’s Model on Adam and Eve
Editor’s note: In a multipart series, Casey Luskin is reviewing a new book by philosopher William Lane Craig. Look here for the full review so far.
It is possible to reconcile a view of Adam and Eve as a historical couple who were ancestors of humanity with mainstream science. This is perhaps the major message of William Lane Craig’s 400+ page book, In Quest of the Historical Adam, and it is well argued.
To summarize his main conclusion: Craig identifies Adam and Eve as members of the species Homo heidelbergensis. His primary arguments for this proposal are (1) that Neanderthals, Denisovans, and Homo sapiens all show advanced cognitive ability, and therefore must all be descendants of Adam and Eve. If this is correct, Craig infers, then Adam and Eve must have belonged to a species ancestral to those various groups — and mainstream paleoanthropology has proposed that Homo heidelbergensis could have fit into that position in the human family tree. Moreover, he finds that (2) Homo heidelbergensis is also thought to have had a large brain and could have been highly intelligent, consistent with this species having been created in the image of God. Further, when Craig considers the genetic evidence, (3) multiple analyses have found that modern-day human genetic diversity could have arisen from an initial pair that lived at least ~500,000 years ago, and this includes the precise time range when Homo heidelbergensis is thought to have lived. This conclusion may not necessarily be true, but it’s plausible.
Is This a “Traditional” Adam and Eve?
But what kind of Adam and Eve is Craig defending? Is it the “traditional” Adam and Eve where they are miraculously created de novo by God and are our sole genetic progenitors? It’s not clear.
At times Craig claims to defend a “traditional” view of the “original human pair, miraculously created by God a few thousand years before Christ. They are the parents of the entire human race, so that every human being who has ever lived on the face of this planet is descended from them.” (p. xii) That description certainly incorporates some very important aspects of the “traditional” view. But as we saw in the previous installment, his model accepts many evolutionary assumptions, and as we will see in a subsequent post Craig at points defends human-ape common ancestry — making it difficult to see how his model fits with Adam and Eve having been “miraculously created de novo.”
Craig’s description of Adam and Eve would be closer to the traditional view if we added the word “sole” before the word “parents.” And indeed, at points Craig seems to affirm this possibility as well, acknowledging that the scientific evidence supports “a bottleneck of two” and an initial pair who were “our sole genetic progenitors” (p. 353). This is all good.
But does Craig require or even advocate that Adam and Eve are our “sole genetic progenitors”? It’s not clear. He mentions the work of Joshua Swamidass who has promoted the “interbreeding” or “admixture” hypothesis where Adam and Eve are not our sole genetic ancestors because their offspring interbred with other hominids that evolved from a common ancestor we shared with apes. This is called the “genealogical Adam and Eve” hypothesis where Adam and Eve are genealogical ancestors of everyone alive today — though they are not our sole genealogical or genetic ancestors. As Craig writes:
Some have appealed to interbreeding with other evolved hominin species in order to explain how Adam and Eve could have been de novo creations from inanimate material and yet their descendants bear such striking genetic similarity to chimpanzees, including broken pseudogenes that have ceased their original function. (p. 378)
At this point Craig seemingly turns away from this hypothesis because it implies humans mating with non-human hominids—tantamount to bestiality:
No such appeal to interbreeding is necessary if we envision Adam and Eve as emerging from a hominin population that shared common ancestry with chimpanzees and other great apes. Indeed, on the view proposed here, Adam and Eve could be our sole genetic progenitors, whose descendants never fell into bestial relations with nonhuman hominins or at least produced no descendants from such liaisons. (p. 378)
Nonetheless, Craig does allow at various points the possibility that Adam and Eve’s descendants “may or may not have interbred” with these nonhuman hominins (p. 354) and even says “the existence of a historical Adam and Eve need not imply their sole genetic progenitorship … we do not have any idea whether there were offspring of such unions [between Adam and Eve’s descendants and other hominins] that had genetic input into the human line.” (p. 355) In his conclusion he again allows and even anticipates this possibility, writing: “If there were sexual encounters with nonhuman hominins, these would be cases of bestiality, contrary to God’s will for humanity, though not entirely surprising for a fallen race.” (p. 378) So Craig seems quite open to the interbreeding hypothesis after all.
Pseudogenes as Non-Functional Junk
The underlying problem for Craig’s model is his assumption that pseudogenes are non-functional “broken” junk DNA that he thinks show we have some genetic connection with apes in our DNA. He thinks this because God would not create broken genes in two different species; the broken DNA implies we share a common ancestor. Once he accepts this assumption (which will be critiqued in Part 6 of this review), there seem to be two options for him to explain how humans came to have “broken pseudogenes” in our DNA:
(1) Allow for miraculous and de novo creation of Adam and Eve, but their progeny interbreed with an evolved population of hominids not created in the image of God, donating their “broken pseudogenes” to Adam and Eve’s descendants.
(2) Propose that Adam and Eve are directly descended from some common ancestor we share with apes (from which they obtained these “broken pseudogenes”), and they are our sole genetic ancestors.
The problem with Craig (at times) allowing option (1) is that then Adam and Eve are not our sole genetic ancestors — a thesis he claims to defend in the book. The problem with Craig (at times) allowing option (2) is that then Adam and Eve evolved naturally and were not specially or miraculously created de novo — a thesis he also (at times) seems to want to defend in the book. Moreover, if Craig wants to defend both the claims that Adam and Eve were miraculously created by God AND are our sole genetic ancestors — i.e., if he wants to defend the traditional Adam and Eve — then I don’t see how he can either affirm human-ape common ancestry or the mixture of Adam’s line with non-human hominids — both claims he affirms at various points in his book.
In other words, I don’t know how Craig can affirm (A) the miraculous and de novo creation of Adam and Eve, (B) Adam and Eve as our sole genetic ancestors, (C) human-ape common ancestry, and (D) allow human mixture with non-human primates all in a single model. (A) is incompatible with (C), and (B) is incompatible with (D). Moreover, Craig at times says he wants to affirm a “traditional” Adam and Eve, where (A) and (B) are necessary components of any “traditional” model. But (C) and (D) are both incompatible with a traditional Adam and Eve, because under (C) Adam and Eve aren’t specially created, and under (D) they aren’t our sole genetic ancestors.
As a result, I’m having trouble making sense of exactly what his model holds. And it seems I’m not alone. A review of the book in journal Science expressed similar confusion:
we eventually learn that his model allows for an unspecified amount of admixture from other hominin lineages into the descendants of Adam, eliminating the need for such a tight bottleneck. A more clearly stated hypothesis in this section would have saved the reader time and frustration.
Perhaps Craig allows some ambiguity so that both those who want a truly “traditional” Adam and Eve and those who want an “evolutionary” Adam and Eve are comfortable with his argument. But I am someone who wants to know if a “traditional” Adam and Eve are possible, and I hope this review helps to clarify some of that ambiguity.
While I believe that Craig has shown that even evolutionary science allows Adam and Eve could have been our “ancestors” in some sense, evolutionary science requires that they are not our “sole ancestors” and/or they weren’t miraculously created de novo. So I don’t think Craig has shown that evolutionary science is compatible with a “traditional” Adam and Eve, because I don’t think such a reconciliation is possible.
For me the solution to the quandary — which also allows us to restore a traditional Adam and Eve (A and B above) — is to recognize the compelling and rapidly growing body of evidence that pseudogenes are not “broken” DNA but have function — a thesis I will defend in Part 6 of this review. This removes the need to find some way to bring DNA we share with apes into our genome through some strictly natural mechanism (options 1 or 2 above)
But I’m also very interested in paleoanthropology. So let’s set aside the question of exactly what Craig means by his model. Instead let’s ask: Does Craig identify potential candidates in the fossil record for a traditional Adam and Eve who could have been specially created by God as our sole genetic ancestors? Here, I think the answer is yes — and I take it as a praiseworthy accomplishment and an important contribution from his book.
Mainstream Doubts about Homo heidelbergensis
Many are praising Craig for reconciling Adam and Eve with “mainstream” science, and I find his proposal that Adam and Eve could have been represented by something like Homo heidelbergensis to be convincing and plausible. But it’s worth noting that mainstream science does not have a consensus view on whether Craig’s candidate species for Adam and Eve — Homo heidelbergenesis — is a valid taxonomic group. Homo heidelbergensis has been heavily criticized in the paleoanthropology community. A recent article in Evolutionary Anthropology called the species “poorly defined and variably understood,” adding that this taxon “should be abandoned altogether, as it has been poorly defined and used inconsistently.”1 The article continued:
Multiple, often contradictory views on what constitutes H. heidelbergensis make this taxon particularly misleading. Even to non-specialists (e.g., biologists working in other realms, Palaeolithic archaeologists, etc.) H. heidelbergensis represents either (and sometimes paradoxically both) the generalized Middle Pleistocene hominin, or a chronospecies of Neanderthals. Within the palaeoanthropological community, the taxon’s ambiguity has contributed to complex and some-times hard-to-follow discussions: in a single paper, one can find numerous descriptions of the taxon with incompatible hypodigms. More troublingly, newly discovered Middle Pleistocene hominin fossils that cannot easily be assigned to Homo erectus, H. neanderthalensis, or early H. sapiens, still tend to be lumped into this one-size-fits-all taxon, often with a sensu lato qualifier to indicate a nonspecific morphology of a Middle Pleistocene hominin. Alternatively, they are assigned more general or descriptive names like “archaic H. sapiens,” “mid-Pleistocene Homo,” or “Homo sp.,” which do little to convey their evolutionary position.
Indeed, the authors commented on the mid-Pleistocene, the epoch that heralds the appearance of Homo heidelbergensis, and noted that “human evolution during this age is poorly understood, a problem that paleoanthropologists call ‘the muddle in the middle.’”2
Another issue is with Craig’s identification of Homo heidelbergensis as the progenitor of modern humanity is that not all paleoanthropologists consider it to be the most recent common ancestor of Homo sapiens, Neanderthals, and Denisovans, as the article in Evolutionary Anthropology explains:
This taxon was previously considered as the most recent common ancestor (MRCA) of LP [late Pleistocene] hominins, or minimally, the common ancestor of the African and European lineages (i.e., H. sapiens and Neanderthals, respectively). Since the MRCA of the modern human and Neanderthal lineages has been pushed further back in time toward the late Early Pleistocene or very early Middle Pleistocene, the specimens currently assigned to H. heidelbergensis sensu latocannot be considered representatives of the MRCA. This is a particularly pertinent point given that the split between African and Eurasian hominins has been recently proposed to be earlier than the split between the Denisovan and Neanderthal lineages. As such, H. heidelbergensis sensu lato can no longer be considered the root of all African and European hominin lineages.
These criticisms of Homo heidelbergenesis are important to note but they are not necessarily fatal to Craig’s conclusion. After all, fossil hominid relationships are notoriously difficult to interpret (within an evolutionary framework) and there are just about as many opinions on these topics as there are workers in the field. So the fact that the particular team that authored this particular paper doesn’t see H. heidelbergensis as ancestral to the species Homo sapiens, Neanderthals, and Denisovans doesn’t mean Craig’s thesis is necessarily wrong. The data is ambiguous and other teams of mainstream paleoanthropologists would undoubtedly agree with Craig that this is a real species, and very important in the human lineage!
It should be emphasized immediately that each of the “species” supposedly descended from Homo heidelbergensis — Homo sapiens, Neanderthals, Denisovans — are (or were) highly similar both morphologically and genetically. Craig repeatedly notes that the three species could interbreed and produce fertile offspring. Likewise, the Evolutionary Anthropology paper notes, “The extent and frequency of interbreeding among hominin terminal branches in the LP has been well established and recent research indicates that interbreeding can be observed in the Middle Pleistocene.” The article recognizes “strong and growing evidence of migrations as well as gene flow between” Neanderthals and Homo sapiens. This raises questions about whether they ought to even be called separate “species.” As a paper in Annual Review of Anthropology notes, “Neandertals and Denisovans were part of the biological species Homo sapiens.”3 Thus, moving from a “species” like Homo heidelbergensis to modern humans (or to Neanderthals, or to Denisovans) would entail microevolution, or small-scale change within a species.
Selling Homo erectus Short?
In order to explain the high degree of genetic, morphological, intellectual, and behavior similarities between modern humans (Homo sapiens), Neanderthals, and Denisovans, Craig must locate Adam and Eve within a species that can serve as an ancestor to all of these groups. He prefers Homo heidelbergensis as the best candidate species for Adam and Eve, and disfavors Homo erectus. Perhaps Craig is correct, but we should be very careful in excluding Homo erectus from Adam’s line (and the human species). Indeed, the Evolutionary Anthropology paper mentioned above cites other literature4 and proposes that the common ancestor of humans, Neanderthals, and Denisovans pre-dated H. heidelbergensis, and was something that lived much earlier, probably more like Homo erectus.
The name Homo erectus means “upright man,” and according to an article in Nature, it was the “earliest species to demonstrate the modern human semicircular canal morphology,”5 a feature related to the mode of locomotion. Below the neck erectus was extremely similar to us,6 leading an Oxford University Press volume to note that erectus was “humanlike in its stature, body mass, and body proportions.”7 Indeed, several paleoanthropologists have classified Homo erectus as belonging within our own species.8 Lucy discoverer Donald Johanson suggests that if erectus were alive today, it could reproduce with modern humans, meaning we would be members of the same species.9
Craig’s primary argument against erectus being descended from Adam is that the brain size is simply too small to allow for advanced cognitive abilities. When trying to determine what sorts of traits would rule out an individual from being a human, he writes, “a brow ridge would hardly seem sufficient to disqualify a species of Homo from being reckoned human, but a small braincase plausibly would, given the correlation between brain size and cognitive capacity.” (p. 258) Craig may have a point, but it can only be taken so far. While the erectus brain size is on average smaller than that of modern humans, its brain size is nonetheless within the range of modern human variation.10 Moreover, many paleoanthropologists have argued that the correlation between brain size and intelligence is unclear.
Leading paleoanthropologists Bernard Wood and Mark Collard observe: “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.”11 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.12 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.”13 Anthropologist Stephen Molnar likewise observes, “The size of the brain is but one of the factors related to human intelligence.”14 Christof Koch, president of the Allen Institute for Brain Science in Seattle, writes that “total brain volume weakly correlates with intelligence … brain size accounts for between 9 and 16 percent of the overall variability in general intelligence.”15 As a demonstration, Koch offers the examples of the Russian novelist Ivan Turgenev (1818-1883) who had a brain weight of 2,021 grams compared with the French novelist Anatole France (1844-1924) who had a brain weight of 1,017 grams. Turgenev’s brain was literally twice the mass of France’s brain, yet both were acclaimed novelists!
Craig correctly observes that archaeological evidence for H. erectus’s intelligence and tool building capacity is limited, but the Cambridge Encyclopedia of Human Evolution observes, “It is very difficult judging whether a particular kind of early human had developed a language, society or art.”16 Nonetheless, we can make certain inferences, a method Craig uses elsewhere for studying intelligence in other ancient hominids.17 Homo erectus remains have been found on islands, where the most reasonable explanation is that they arrived by boat. Karl Wegmann of North Carolina State University said, “We all have this idea that early man was not terribly smart. The findings show otherwise—our ancestors were smart enough to build boats and adventurous enough to want to use them.”18 Daniel Everett, professor of global studies at Bentley University, argues that Homo erectus “sailed to the island of Crete and various other islands. It was intentional: they needed craft and they needed to take groups of twenty or so at least to get to those places.”19 According to Everett, this means:
Erectus needed language when they were sailing to the [Indonesian] island of Flores. They couldn’t have simply caught a ride on a floating log because then they would have been washed out to sea when they hit the current. They needed to be able to paddle. And if they paddled they needed to be able to say ‘paddle there’ or ‘don’t paddle.’ You need communication with symbols not just grunts.20
In an essay in Aeon, he continues:
[S]ailing demonstrates a level of cognitive development rivalling even that of modern humans. The erectus accomplishment of paddling together across one of the strongest ocean currents in the world … required not only cooperation, but also corrections, instructions and commands. Few detailed instructions or corrections can be given without language.21
While there are certainly paleoanthropologists who disagree, there is good circumstantial evidence to make an inference that erectus too was highly intelligent.
Another reason Craig discounts Homo erectus as Adam and Eve’s species is its teeth — specifically, the rate of tooth development as understood from enamel growth patterns in fossilized hominid teeth. In scientific terms, teeth can be plastic and aren’t always a good way to determine relationships, even within primates and hominids. As a paper in Proceedings of the National Academy of Sciences stated:
[O]ur results show that the type of craniodental characters that have hitherto been used in hominin phylogenetics are probably not reliable for reconstructing the phylogenetic relationships of higher primate species and genera, including those among the hominins.22
Theologically speaking, with all of the incredible abilities of humans that reflect our creation in God’s image, it’s difficult to see why the rate at which enamel accumulates on the surface of teeth during the first two years of a child’s life can hardly be taken as good evidence of whether that child bears the Imago Dei and is a descendant of Adam.
Teeth certainly reflect an organism’s diet, lifestyle, and habits, but they are just one variable. A better measure of the erectus lifestyle could be seen in its total energy expenditure (TEE) value, a complex variable related to body size, diet quality, and food-gathering activity. One study found that TEE value, “increased substantially in Homo erectus relative to the earlier australopithecines,” beginning to approach the very high TEE value of modern humans.23 Researchers believe the earliest members of Homo erectus were “more similar to modem humans than to the earlier and contemporaneous australopithecines,” due to their “larger relative brain sizes, larger bodies, slower rates of growth and maturation, dedicated bipedal locomotion, and smaller teeth and jaws,”24 and “major changes” in diet and foraging behavior.25 Together, these features reflect “a lifestyle that was more similar to that of modern humans than that inferred for earlier and contemporaneous hominins.”26
Again, it’s possible that Craig’s thesis is correct and Adam and Eve belonged to Homo heidelbergensis rather than Homo erectus. I want to make this clear: his thesis is plausible, well-argued, and worth taking very seriously! But all of these members of Homo are highly similar — so similar, as noted above, that some paleoanthropologists have considered them all to belong to the same species, our own, Homo sapiens. The point is therefore not that Craig is wrong, but that there are many viable options for Adam and Eve, and it’s difficult to say exactly which one is right. Perhaps this bolsters the compatibility of Adam and Eve with the scientific evidence to a higher degree than even Craig argues in his book.
- Mirjana Roksandic, Predrag Radovi, Xiu-Jie Wu, and Christopher J. Bae, “Resolving the ‘muddle in the middle’: The case for Homobodoensis sp. nov.,” Evolutionary Anthropology (2021), DOI: 10.1002/EVAN.21929
- The University of Winnipeg, “New species of human ancestor named: Homo bodoensis,” ScienceDaily (October 28, 2021), https://www.sciencedaily.com/releases/2021/10/211028143648.htm
- John Hawks, “Significance of Neandertal and Denisovan Genomes in Human Evolution,” Annual Review of Anthropology, Vol. 42: 433-449 (2013) (“Neandertals and Denisovans were part of the biological species Homo sapiens.”).
- Aida Gómez-Robles, “Dental evolutionary rates and its implications for the Neanderthal–modern human divergence,” Science Advances,” Science Advances, 5: eaaw1268 (2019).
- Fred Spoor, Bernard Wood, and Frans Zonneveld, “Implications of early hominid labyrinthine morphology for evolution of human bipedal locomotion,” Nature 369 (June 23, 1994): 645-648.
- See Sigrid Hartwig-Scherer and Robert D. Martin, “Was ‘Lucy’ more human than her ‘child’? Observations on early hominid postcranial skeletons,” Journal of Human Evolution (1991) 21, 439-449.
- William R. Leonard, Marcia L. Robertson, and J. Josh Snodgrass, “Energetic Models of Human Nutritional Evolution,” in Evolution of the Human Diet: The Known, the Unknown, and the Unknowable, ed. Peter S. Ungar (Oxford, UK: Oxford University Press, 2007), 344-359.
- Jan Jelínek, “Homo erectus or Homo sapiens?” In Recent Advances in Primatology, Volume Three: Evolution, edited by D.J. Chivers and K.A. Joysey, 419-429. London: Academic Press, 1978; Emilio Aguirre, “Homo erectus and Homo sapiens: One or More Species?” In 100 Years of Pithecanthropus: The Homo erectus Problem 171 Courier Forschungsinstitut Seckenberg, edited by Jens Lorenz, 333-339. Frankfurt: Courier Forschungsinstitut Senckenberg, 1994; Milford H. Wolpoff, Alan G. Thorne, Jan Jelínek, and Zhang Yinyun. “The Case for Sinking Homo erectus: 100 Years of Pithecanthropus is Enough!” In 100 Years of Pithecanthropus: The Homo erectus Problem 171 Courier Forschungsinstitut Seckenberg, edited by Jens Lorenz, 341-361. Frankfurt: Courier Forschungsinstitut Senckenberg, 1994; Eric Delson, “One skull does not a species make.” Nature 389 (October 2, 1997): 445-46; John Hawks, Keith Hunley, Sang-Hee Lee, and Milford Wolpoff. “Population Bottlenecks and Pleistocene Human Evolution.” Journal of Molecular Biology and Evolution 17 (2000): 2-22.
- Donald C. Johanson and Maitland Edey, Lucy: The Beginnings of Humankind (New York: Simon & Schuster, 1981).
- See S. Molnar, Races, types, and ethnic groups: the problem of human variation, p. 57 (1975); S. Molnar, Human Variation Races Types and Ethnic Groups, p. 65 (2nd ed., 1983); C. G. Conroy, et. al., “Endocranial Capacity in an Early Hominid Cranium from Sterkfontein, South Africa,” Science, 280:1730-1731 (1998); Bernard Wood and Mark Collard, “The Human Genus,” Science, 284:65-71 (1999).
- Bernard Wood and Mark Collard, “The Human Genus,” Science, 284 (April 2, 1999): 65-71.
- 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”).
- 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.
- Christoph Koch, “Does Brain Size Matter?,” Scientific American Mind (January/February, 2016), 22-25.
- C. B. Stringer, “Evolution of Early Humans,” in Cambridge Encyclopedia of Human Evolution, 241.
- For example, Craig infers from Neanderthal hunting strategies that it was capable of “extremely careful planning, coordination, and discussion” (p. 294, quoting Thieme, 2007). On this point he is probably correct, but we can we not make similar inferences based upon erectus migration strategies?
- Jørn Madsen, “Who Was Homo erectus,” Science Illustrated (July/August 2012): 23.
- Quoted in Nicola Davis, “Homo erectus may have been a sailor – and able to speak,” The Guardian (February 19, 2018), https://www.theguardian.com/science/2018/feb/20/homo-erectus-may-have-been-a-sailor-and-able-to-speak.
- Quoted in Nicola Davis, “Homo erectus may have been a sailor – and able to speak,” The Guardian (February 19, 2018), https://www.theguardian.com/science/2018/feb/20/homo-erectus-may-have-been-a-sailor-and-able-to-speak.
- Daniel Everett, “Did Homo erectus speak?,” Aeon, February 28, 2018, https://aeon.co/essays/tools-and-voyages-suggest-that-homo-erectus-invented-language.
- Mark Collard and Bernard Wood, “How reliable are human phylogenetic hypotheses?,” Proceedings of the National Academy of Sciences (USA), 97 (April 25, 2000): 5003–06.
- William R. Leonard and Marcia L. Robertson, “Comparative Primate Energetics and Hominid Evolution,” American Journal of Physical Anthropology 102 (February, 1997): 265-81. See also Leslie C. Aiello and Jonathan C. K. Wells, “Energetics and the Evolution of the Genus Homo,” Annual Review of Anthropology 31 (2002): 323-38.
- Aiello and Wells, “Energetics and the Evolution of the Genus Homo” (emphases added).
- William R. Leonard, J. Josh Snodgrass, and Marcia L. Robertson, “Effects of Brain Evolution on Human Nutrition and Metabolism,” Annual Review of Nutrition 27 (2007): 311-27; William R. Leonard, Size Counts: Evolutionary Perspectives on Physical Activity and Body Size From Early Hominids to Modern Humans,” Journal of Physical Activity and Health 7 (2010): S284-98; Leonard et al., “Energetics and the Evolution of Brain Size in Early Homo.”
- Leslie C. Aiello and Jonathan C. K. Wells, “Energetics and the evolution of the genus Homo,” Annual Review of Anthropology(2002), 31:323-338.
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