Editor’s note: We are delighted to present a series by geologist Casey Luskin asking, “Do Fossils Demonstrate Human Evolution?” This is the fifth post in the series, which is adapted from the recent book, The Comprehensive Guide to Science and Faith. Find the full series here.

After realizing that Homo habilis could not serve as a link between Homo and Australopithecus, two paleoanthropologists lamented that “this muddle leaves Homo erectus without a clear ancestor, without a past.”¹ Indeed, it is difficult to find fossil hominins to serve as direct transitional forms between the ape-like australopithecines and the first human-like members of Homo. The fossil record shows abrupt changes that correspond to the appearance of our genus Homo about two million years ago. 

From its first appearance, Homo erectus was very human-like, and differed markedly from prior hominins that were not human-like. Yet Homo erectus appears abruptly, without apparent evolutionary precursors. An article in Natureexplains: 

The origins of the widespread, polymorphic, Early Pleistocene H. erectus lineage remain elusive. The marked contrasts between any potential ancestor (Homo habilis or other) and the earliest known H. erectus might signal an abrupt evolutionary emergence some time before its first known appearance in Africa at ~1.78 Myr [million years ago]. Uncertainties surrounding the taxon’s appearance in Eurasia and southeast Asia make it impossible to establish accurately the time or place of origin for H. erectus.²

A 2016 paper likewise admits, “Although the transition from Australopithecus to Homo is usually thought of as a momentous transformation, the fossil record bearing on the origin and earliest evolution of Homo is virtually undocumented.”³ While that paper argues that the evolutionary distance between Australopithecus and Homo is small, it concedes that the lineage that led to Homo is “unknown.”⁴

Unique and Previously Unseen 

Early members of Homo, namely Homo erectus, show unique and previously unseen features that contributed to this “abrupt” appearance. The technical literature observes an “explosion,”⁵ “rapid increase,”⁶ and “approximate doubling”⁷ in brain size associated with the appearance of Homo. Wood and Collard’s major Science review found that only a single trait of one hominin species qualified as “intermediate” between Australopithecus and Homo: the brain size of Homo erectus.⁸ However, this one trait of brain size does not necessarily indicate that humans evolved from less intelligent hominids. Intelligence is determined largely by internal brain organization, and is much more complex than the singular dimension of brain size.⁹ Christof Koch, president of the Allen Institute for Brain Science, observes that “total brain volume weakly correlates with intelligence … brain size accounts for between 9 and 16 percent of the overall variability in general intelligence.”¹⁰ Because of this, brain size is not always a good indicator of evolutionary relationships.¹¹ In any case, erectus had an average brain size within the range of modern human variation. A few skulls of “intermediate” size do not demonstrate that humans evolved from primitive ancestors.

Much like the explosive increase in skull size, a study of the pelvis bones of australopithecines and Homo found “a period of very rapid evolution corresponding to the emergence of the genus Homo.”¹² One Nature paper noted that early Homo erectus shows “such a radical departure from previous forms of Homo (such as H. habilis) in its height, reduced sexual dimorphism, long limbs, and modern body proportions that it is hard at present to identify its immediate ancestry in east Africa.”¹³ 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.¹⁴

“A Real Acceleration of Evolutionary Change”

Noting these many differences, 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 required 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.¹⁵

These rapid and unique changes are termed “a genetic revolution” in which “no australopithecine species is obviously transitional.”¹⁶

For those not constrained by an evolutionary paradigm, it is not obvious that this transition took place at all. The stark lack of fossil evidence for this hypothesized transition is confirmed by three Harvard paleoanthropologists:

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.¹⁷

As for the “good news,” they 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.”¹⁸

Before and After, but No Transition

In other words, the fossil record shows ape-like australopithecines (“before”), and human-like Homo (“after”), but not fossils documenting a transition between them. In the absence of intermediates, we’re left with inferences of a transition based strictly upon the assumption of evolution — that an undocumented transition must have occurred somehow, sometime, and someplace. They assume this transition happened, even though we do not have fossils documenting it.

The literature thus admits the “abrupt appearance”¹⁹ of early Homo and calls the origin of our genus “an enduring puzzle.”²⁰ The great evolutionary biologist Ernst Mayr recognized these problems:

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.²¹

Another commentator proposed that the evidence implies a “big bang theory” of the appearance of Homo.²² This large, unbridged gap between the ape-like australopithecines and the abruptly appearing human-like members of genus Homo challenges evolutionary accounts of human origins.

Next, “Human Origins: All in the Family.”

Notes

1. Walker and Shipman, Wisdom of the Bones, 134.
2. Berhane Asfaw et al., “Remains of Homo erectus from Bouri, Middle Awash, Ethiopia,” Nature 416 (March 21, 2002), 317-320.
3. William Kimbel and Brian Villmoare, “From Australopithecus to Homo: The Transition that Wasn’t,” Philosophical Transactions of the Royal Society B 371 (2016), 20150248. 
4. Kimbel and Villmoare, “From Australopithecus to Homo: The Transition that Wasn’t.”
5. Stanley A. Rice, Encyclopedia of Evolution (New York: Checkmark, 2007), 241. 
6. Franz Wuketits, “Charles Darwin, Paleoanthropology, and the Modern Synthesis,” Handbook of Paleoanthropology, 97-125, 116. 
7. Dean Falk, “Hominid Brain Evolution: Looks Can Be Deceiving,” Science 280 (June 12, 1998), 1714.
8. Specifically, Homo erectus is said to have intermediate brain size, and Homo ergaster is said to have a Homo-like postcranial skeleton with a smaller, more australopith-like brain size.
9. Terrance Deacon, “Problems of Ontogeny and Phylogeny in Brain-Size Evolution,” International Journal of Primatology 11 (1990), 237-282. See also Terrence Deacon, “What Makes the Human Brain Different?,” Annual Review of Anthropology 26 (1997), 337-357; Stephen Molnar, Human Variation: Races, Types, and Ethnic Groups (Upper Saddle River, NJ: Prentice Hall, 5^th ed., 2002), 189.
10. Christof Koch, “Does Brain Size Matter?,” Scientific American Mind (January/February, 2016), 22-25.
11. See Wood and Collard, “The Human Genus.”
12. Marchal, “New Morphometric Analysis of the Hominid Pelvic Bone.”
13. Robin Dennell and Wil Roebroeks, “An Asian Perspective on Early Human Dispersal from Africa,” Nature 438 (Dec. 22/29, 2005), 1099-1104.
14. John Hawks, Keith Hunley, Sang-Hee Lee, and Milford Wolpoff, “Population Bottlenecks and Pleistocene Human Evolution,” Molecular Biology and Evolution 17 (2000), 2-22.
15. Hawks et al., “Population Bottlenecks and Pleistocene Human Evolution.”
16. Hawks et al., “Population Bottlenecks and Pleistocene Human Evolution.”
17. Daniel E. Lieberman, David R. Pilbeam, and Richard W. Wrangham, “The Transition from Australopithecus to Homo,” Transitions in Prehistory: Essays in Honor of Ofer Bar-Yosef, eds. John J. Shea and Daniel E. Lieberman (Cambridge, MA: Oxbow, 2009), 1
18. Lieberman et al., “The Transition from Australopithecus to Homo.”
19. Alan Turner and Hannah O’Regan, “Zoogeography: Primate and Early Hominin Distribution and Migration Patterns,” in Handbook of Paleoanthropology, 623-642.
20. Kimbel, “Hesitation on Hominin History.”
21. Ernst Mayr, What Makes Biology Unique?: Considerations on the Autonomy of a Scientific Discipline (Cambridge, UK: Cambridge University Press, 2004), 198.
22. “New Study Suggests Big Bang Theory of Human Evolution,” University of Michigan News Service (January 10, 2000), http://www.umich.edu/~newsinfo/Releases/2000/Jan00/r011000b.html (accessed October 26, 2020).