Recently I mentioned that physicist Stephen Barr was wrong to call the origin of humanity “gradual.” One of the citations I noted recounts the non-gradual origin of humans. It is a 2000 paper in the journal Molecular Biology and Evolution by Hawks et al. I have cited this paper in the past but haven’t mentioned it in quite some time, so I’d like to add a few additional comments.
The lead author is John Hawks, a professor of anthropology at the University of Wisconsin–Madison, who has a popular — and often very insightful — blog on paleoanthropology. A couple of years ago, someone alerted me to a post by Hawks in 2024 where he protested the use of quotes from Hawks et al. (2000), similar to those cited in my post, to discuss the “sudden” appearance of the human body plan in the fossil record. Professor Hawks did not seem to be specifically responding to me in his blog, but because I have used this quote in the past, I wanted to make some comments on it.
How I’ve Quoted Hawks et al. (2000)
First, I want to make it clear that to my knowledge Hawks and his colleagues do not support intelligent design and are entirely committed to standard evolutionary explanations of human origins. I’ve absolutely never said or implied otherwise.
The fact that they are coming from a completely pro-evolution perspective is what makes this paper’s comments so significant. My previous post’s citations of my 2023 review of human origins in the journal Religions is a good example of how I’ve cited this paper. Here’s a slightly more expanse quotation of the paper from my chapter in the 2022 open access book Science and Faith in Dialogue from the South African academic publisher Aosis:
An important study in the Journal of Molecular Biology and Evolution found that Homo and Australopithecus have major differences including not only brain size but also body height, dental anatomy, degree of cranial buttressing, vision and respiratory-related anatomy. The article concludes (Hawks et al. 2000):
“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 behaviour.” (p. 3)
The study thus described the origin of our genus Homo as, ‘a real acceleration of evolutionary change from the more slowly changing pace of australopithecine evolution’, and stated that such a transformation would have required radical changes (Hawks et al. 2000):
“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.” (p. 4)
These rapid and unique changes are strikingly characterised as ‘a genetic revolution’ where ‘no australopithecine species is obviously transitional’ (Hawks et al. 2000, p. 4).
What Did Professor Hawks Say?
Hawks’s 2024 response claimed that these sorts of quotations from the paper are “out of context”:
This morning a reader alerted me that a quote from one of my scientific papers was making the rounds of a creation science interest group on Facebook. Here’s the quote:
“No gradual series of changes in earlier australopithecine populations clearly leads to the new species [Homo sapiens], and no australopithecine species is obviously transitional.”
I know I have a lot of readers who have run across selective quotation before. It’s a common and obviously misleading tactic: take a long passage and quote a single sentence out of context. In this case the sentence on its own seems to suggest that our species, Homo sapiens, did not evolve in a gradual series of changes from earlier populations of Australopithecus.
Again, to my knowledge Hawks was not responding directly to me, but I have used that quote before so it’s worth responding to what he wrote. Is the quotation out of context? Does it misrepresent the paper, as Hawks alleges? Below I will go through the paper in detail and show that the answer is absolutely not — in fact the quotation is an accurate representation of what the paper says the fossil material shows.
A Plain Archaeological and Paleontological Analysis
The paper is ultimately about whether genetic evidence can corroborate the fossil and archaeological evidence regarding human origins, and what the evidence implies for models about the pace and tempo of the origin of Homo sapiens. The paper begins by affirming that the best way to get “direct” evidence about the past is through “archaeological and paleontological sources”:
Unlike indirect methods based on population genetics, archaeological and paleontological sources provide direct evidence about the past that can be independently compared with genetic inferences.
The paper then proceeds to provide extensive analysis of the archaeological and paleontological evidence. Soon thereafter, in a section titled “A 2-Myr Bottleneck,” it explains what the paleontological evidence shows. It’s quite straightforward in its explanation:
A hominid speciation is documented with paleoanthropological data at about 2 MYA by significant and simultaneous changes in cranial capacity and both cranial and postcranial characters. This marks the earliest known appearance of our direct ancestors. The new species has been called Homo erectus or Homo ergaster by some authors. Following others (Jelinek 1978; Aguirre 1994; Wolpoff et al. 1994), we call this emerging evolutionary species early Homo sapiens, as it begins an unbroken lineage leading directly to living human populations. The first specimens are humanity’s earliest known direct ancestors.
We, like many others, interpret the anatomical evidence to show that early H. sapiens was significantly and dramatically different from earlier and penecontemporary australopithecines in virtually every element of its skeleton (fig. 1) and every remnant of its behavior (Gamble 1994; Wolpoff and Caspari 1997; Asfaw et al. 1999; Wood and Collard 1999). Its appearance reflects a real acceleration of evolutionary change from the more slowly changing pace of australopithecine evolution.
So what have we seen here? The paper is evaluating the paleontological and paleoanthropological evidence. It notes that the when early Homo appears — whether we call it Homo erectus or Homo ergaster or “early H. sapiens” — it was “significantly and dramatically different” from the australopithecines. It was so different that the differences included “virtually every element of its skeleton” and “every remnant of its behavior.” Keep in mind that they are simply reviewing what the paleontological evidence shows.
The paper then goes deeper into explaining the differences between these early members of Homo and the australopithecines. They list four, as follows:
(1) changing brain size (larger, especially longer vault, with a broad frontal bone and an expanded parietal association area; neural canal expansion); (2) changing dental function (more anterior tooth use, greater emphasis on grinding and less on crunching) as reflected in broader faces and larger nuchal areas; (3) development of a cranial buttressing system to strengthen the vault, including vault bone thickening and prominent tori; and (4) dramatic expansion of body height (estimated average weights double) and numerous changes in proportions.
They then note that there are additional differences between humans and australopithecines. Consider this next passage carefully:
These, and other changes involving the visual and respiratory systems, reflect significant adaptive differences for the new species and give us important insight into the mode of speciation because they seem to happen all together, at the time of its origin.
In other words, an entirely new type of organism — early Homo — appears that is significantly different from the earlier australopithecines, and these “changes … seem to happen all together, at the time of its origin”.
They are not discussing some genetics hypothesis or hypothetical model here. They’re just discussing what the hard fossil evidence seems to show.
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; some of its characteristics are unique, such as the very large body sizes and long legs described below, while others can be found in isolation in various different Pliocene and penecontemporary hominid species.
“A Genetic Revolution”
Next, in a section of the paper titled “A Genetic Revolution,” the authors compare two possible evolutionary models for the emergence of Homo sapiens: “whether early H. sapiens is a consequence of rapid speciation with significant founder effect or the result of a long, gradual process of anagenic change.” They argue that the fossil evidence better supports the “rapid” speciation model:
The first explanation, cladogenesis, is suggested by the fact that no gradual series of changes in earlier australopithecine populations clearly leads to the new species, and no australopithecine species is obviously transitional. This may seem to be an unexpected statement, because for 3 decades habiline species have been interpreted as being just such transitional taxa, linking Australopithecus through the habilines to later Homo species. But with a few exceptions, the known habiline specimens are now recognized to be less than 2 Myr old and therefore are too recent to be transitional forms leading to H. sapiens.
This is very significant: effectively they see the Homo body plan appearing rapidly in the fossil record, which shows “no gradual series of changes in earlier australopithecine populations clearly leads to the new species, and no australopithecine species is obviously transitional.” This is their direct assessment of the fossil evidence.
Now they put their own evolutionary interpretation on this evidence by saying it suggests “rapid speciation.” That’s fine. But the raw evidence shows the abrupt appearance of humanlike Homo. To accommodate this model, they have to appeal to very rapid genetic changes, which they describe as follows:
Our interpretation is that the changes are sudden and interrelated and reflect a bottleneck that was created because of the isolation of a small group from a parent australopithecine species. In this small population, a combination of drift and selection resulted in a radical transformation of allele frequencies, fundamentally shifting the adaptive complex; in other words, a genetic revolution.
This is a standard evolutionary model of speciation: part of a population becomes isolated, and experiences a genetic bottleneck where rapid genetic change can occur. But it’s all being driven by the fossil evidence which shows rapid change and a lack of transitional forms.
They then document a variety of anatomical traits, including the occiput and innominate bones, that are strikingly different between australopithecines and what they call early Homo sapiens, stating: “earliest remains exemplify the significance and magnitude of the newly evolved differences.” They summarize this evidence as follows:
In sum, the earliest H. sapiens remains differ significantly from australopithecines in both size and anatomical details. Insofar as we can tell, the changes were sudden and not gradual.
To explain one more time: what they see in the fossil record is a new species that differs significantly from previous species, and there are not transitional forms documenting that transition. So they conclude that the changes are “sudden and not gradual.”
Behavioral Analysis
The paper then offers an argument based upon behavior that the new Homo species must have undergone “a genetic reorganization” (or as they also infer, a bottleneck). However, this inference of behavioral changes is based upon the large morphological changes they observe. Two main lines of morphological differences between “earliest H. sapiens” and earlier australopithecines are cited: body size and brain size. They write:
Body size is a key element in the behavioral changes reflected at the earliest H. sapiens archaeological sites because of the locomotor changes that large body size denotes and the increased metabolic resources it requires. Moreover, the marked increase in brain size for early H. sapiens has significant metabolic consequences, because the human brain, which is 2% of the body weight, uses some 20%–25% of its metabolic energy.
The larger body size and brain size would have required “additional energy” and they suggest it came from humans eating meat. Here’s how they put it:
the body size increase from the australopithecines would require a 40%–45% increase in the total energy expenditure of early H. sapiens. They suggest that if this evolutionary change were associated with a shift to a more human-like foraging strategy, it would mean that the energy expenditure increase may have been even greater, perhaps as much as 85% greater than that for australopithecines, because of the locomotor requirements. The payoff for early H. sapiens populations, and the source of the additional energy, was in the higher-quality diet with its concentrated energy sources and the predictable use of more resources provided by the newly developed hunting, gathering, and scavenging strategy.
The question becomes how did this radical change happen? They propose that there was a speciation event, where a small population of australopithecines became isolated and a genetic reorganization took place — the “bottleneck” referenced earlier. Here’s how they put it:
These behavioral changes are far more massive and sudden than any earlier changes known for hominids. They combine with the anatomical evidence to suggest significant genetic reorganization at the origin of H. sapiens, and from this genetic reorganization, we deduce that H. sapiens evolved from a small isolated australopithecine population and that small population size played a significant role in this evolution.
This is one way to look at it, but it’s really not my point. My point is that the raw data they have shown reflects dramatic changes between Australopithecus and early Homo. They infer that it implies a rapid speciation event, and that’s fine. But the raw data they are citing is what I’m most interested in — not their interpretation. That raw data shows abrupt large changes, and that’s why I cite this paper.
The balance of the paper explores the population dynamics that could have led to such a bottleneck: variables such as the effective breeding size or the length of time the bottleneck lasted, and how the population expanded after the bottleneck. They look at mtDNA and nuclear DNA (from both sex chromosomes and autosomes). They consider the possibility that there was selection on human populations as well. It’s a very long and complex analysis, but the discussion and conclusion at the end of the paper clearly summarizes its findings:
A population size bottleneck early in the evolution of the H. sapiens lineage, perhaps at its origin some 2 MYA, has significant explanatory power… All the currently available genetic, paleontological, and archaeological data are consistent with a bottleneck in our lineage more or less at about 2 MYA.
That “bottleneck” of course refers to a speciation event when Homo sapiens is said to have appeared in a “sudden” manner.
Now the paper does claim to refute a “recent” bottleneck within the past couple of hundred thousand years. But it supports a bottleneck coincident with the abrupt appearance of the humanlike body plan in the fossil record about 2 Mya. However, the paper does strongly support both an abrupt speciation event and a severe population bottleneck at around 2 Mya as the best explanation for the origin of our species. Here is the quote I used indicating that the paleontological evidence shows that the human species arose abruptly:
No gradual series of changes in earlier australopithecine populations clearly leads to the new species [Homo sapiens], and no australopithecine species is obviously transitional.
My use of it is not “out of context” as Hawks says. He claims that “the sentence on its own seems to suggest that our species, Homo sapiens, did not evolve in a gradual series of changes from earlier populations of Australopithecus.” Hawks may believe that we evolved in a series of gradual changes from earlier populations of Australopithecus. But his paper says that the fossil evidence shows otherwise — as I noted, it says the fossil evidence suggests the changes between Australopithecus and Homo were “sudden and abrupt.”
If you don’t believe me, wait until my next post where I ask a (hopefully) unbiased source — Grok — about this paper’s argument.