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

If humans evolved from ape-like creatures, what were the transitional species between the ape-like hominins discussed earlier in this series and the truly human-like members of the Homo genus found in the fossil record? There aren’t any good candidates.

The Demise of Homo habilis

Many have cited Homo habilis (literally “handy man”) as a tool-using species that was a transitional “link” between the australopithecines and Homo.¹ But its association with tools is doubtful and appears driven mainly by evolutionary considerations Anthropologist Ian Tattersall calls it “a wastebasket taxon, little more than a convenient recipient.² for a motley assortment of hominin fossils.”³ Ignoring these difficulties and assuming habilis was a real species, chronology precludes it from being ancestral to Homo: habiline remains postdate the earliest fossil evidence of the genus Homo.⁴

Morphological analyses further confirm that habilis makes an unlikely “intermediate” between Australopithecus and Homo — and show habilis doesn’t even belong in Homo. An authoritative review in Science by Bernard Wood and Mark Collard found that habilis differs from Homo in terms of body size, shape, mode of locomotion, jaws and teeth, developmental patterns, and brain size, and should be reclassified within Australopithecus.⁵ A study by Sigrid Hartwig-Scherer and Robert D. Martin in the Journal of Human Evolution found the skeleton of habilis was more similar to living apes than were other australopithecines like Lucy.⁶ They conclude, “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.”⁷ Alan Walker and Pat Shipman similarly called habilis “more apelike than Lucy” and remarked, “Rather than representing an intermediate between Lucy and humans, [habilis] looked very much like an intermediate between the ancestral chimp-like condition and Lucy.”⁸ Hartwig-Scherer explains that habilis “displays much stronger similarities to African ape limb proportions” than Lucy — results she calls “unexpected in view of previous accounts of Homo habilis as a link between australopithecines and humans.”⁹

The Link Resurrected?

The news media might be heavily biased toward evolution, but at least it is predictable. Whenever a new hominin fossil is discovered, reporters seize the opportunity to push human evolution. Thus it was no surprise when news outlets buzzed about the latest “human ancestor” after a new species, Homo naledi, was unveiled in 2015. 

CNN declared, “Homo naledi: New Species of Human Ancestor Discovered in South Africa.”¹⁰ The Daily Mail reported, “Scientists Discover Skull of New Human Ancestor Homo Naledi.”¹¹ PBS pronounced, “Trove of Fossils from a Long-Lost Human Ancestor.”¹² And so on.

The find is striking because it represents probably the largest cache of hominin bones — many hundreds — ever found. In a field where a single scrap of jaw ignites the community, this is a big deal. But do we know that Homo naledi is a human ancestor, as news outlets declared? Dig into the details, and the answer again is no.

The primary claim about Homo naledi is that it was a “transitional form” or “mosaic” — a small-brained, upright-walking hominin with a trunk similar to the australopithecines, but with human-like hands and feet. But the technical material shows that even some of those supposedly human-like traits have unique features:

• The hands showed “a unique combination of anatomy”¹³ including “unique first metacarpal morphology,”¹⁴and long, curved fingers that suggest naledi was, unlike humans, well-suited for “climbing and suspension.”¹⁵
• Its foot “differs from modern humans in having more curved proximal pedal phalanges, and features suggestive of a reduced medial longitudinal arch,” giving it an overall “unique locomotor repertoire.”¹⁶ The foot shows, again, that unlike humans, it was “likely comfortable climbing trees.”¹⁷

The technical papers also reveal “unique features in the femur and tibia” — making a hindlimb that “differs from those of all other known hominins.”¹⁸ As for the head, “Cranial morphology of H. naledi is unique…”¹⁹ Sound familiar? Whatever it was, overall naledi appears quite unique. 

Indeed, the discoverers of naledi called it “a unique mosaic previously unknown in the human fossil record.”²⁰Such terminology should raise a red flag. In the parlance of evolutionary biology, “mosaic” usually means a fossil has a suite of traits that are difficult to fit into the standard evolutionary tree. That is the case here.

In 2010, some of the same scientists who discovered and promoted naledi — a team led by Lee Berger of the University of Witwatersrand — were promoting a different hominin species, Australopithecus sediba, as the intermediate du jure between the australopithecines and Homo. However, sediba and naledi differ in important ways that make them unlikely partners in an evolutionary lineage. Specifically, sediba (classified within Australopithecus) had an advanced “Homo-like pelvis,”²¹ “surprisingly human teeth,”²² and a “human-like” lower trunk,²³ whereas naledi — placed within Homo — bears an “australopith-like” and “primitive” pelvis,²⁴ “primitive” teeth, and a “primitive or australopith-like trunk.”²⁵ An australopithecine with apparently advanced Homo-like features seems a poor candidate to evolve into a member of Homo with primitive australopith-like versions of those same features. Thus, although both sediba and naledi have been said to be a human ancestor — by some of the same people, no less — evolutionarily speaking, traits are evolving in the wrong direction. As one news outlet put it: “Each [sediba and naledi] has different sets of australopith-like and human-like traits that can’t be easily reconciled on the same family tree.”²⁶

Problems with Chronology and Morphology

In any case, sediba cannot be ancestral to Homo because, like habilis, it postdates the origin of our genus and has the wrong morphology.²⁷ Based upon fossil chronology, a 2019 study found that the likelihood that sediba is a human ancestor is less than 0.001.²⁸ Commenting on sediba, Harvard’s Daniel Lieberman said, “The origins of the genus Homoremain as murky as ever,”²⁹ and Donald Johanson remarked, “The transition to Homo continues to be almost totally confusing.”³⁰

Another dubious claim about naledi is that it intentionally buried its dead — a testimony to its supposedly human-like intellect. Burying dead in the cave where it was found would require shimmying through a steep, narrow crevice while dragging a body a long distance in the dark — a physically challenging task for any hominin of any level of intelligence. For many reasons, multiple scientists — including two of Berger’s colleagues at the University of Witwatersrand — dispute the intentional burial hypothesis.³¹ Alison Brooks of George Washington University observed that claims of intentional burial are “so far out there that they really need a higher standard of proof.”³²

But the deathblow to claims for Homo naledi as an ancestral or transitional fossil is its age. When first published, naledi‘s promoters suggested, on the basis of evolutionary considerations rather than geological evidence, that it lived 2–3 million years ago. But at that time the fossils hadn’t been dated geologically. Carol Ward of the University of Missouri warned, “Without dates, the fossils reveal almost nothing about hominin evolution.”³³ This didn’t stop paleoanthropologists from speculating, predicting that naledi lived 2–3 million years ago and “represents an intermediate between Australopithecus and Homo erectus.”³⁴ In 2017, Homo naledi‘s remains were dated to the “surprisingly” and “startlingly young” age of 236,000–335,000 years³⁵ — an order of magnitude younger than the age predicted by evolutionary considerations, and far too young to be ancestral to our species. Anthropologist James Kidder candidly admitted, “Nearly everyone in the scientific community thought that the date of the Homo naledi fossils, when calculated, would fall within the same general time period as other primitive early Homo remains. We were wrong.”³⁶

Many cautioned against the hype over naledi,³⁷ and its trajectory resembles other hominins for which hyped claims of transitional or ancestral status eventually failed. When evaluating media claims of the newest human ancestor, a dose of healthy skepticism is warranted.

“Next, The Big Bang Origin of Homo.”

Notes

1. See Alan Walker and Pat Shipman, Wisdom of the Bones: In Search of Human Origins (New York: Alfred Knopf, 1996), 133.
2. Jeffrey Schwartz and Ian Tattersall, “Defining the Genus Homo,” Science 349 (August 28, 2015), 931-932.
3. Ian Tattersall, “The Many Faces of Homo habilis,” Evolutionary Anthropology 1 (1992), 33-37.
4. See F. Spoor et al., “Implications of New Early Homo Fossils from Ileret, East of Lake Turkana, Kenya,” Nature 448 (August 9, 2007), 688-691; Seth Borenstein, “Fossils Paint Messy Picture of Human Origins,” NBC News (August 8, 2007), https://www.nbcnews.com/id/wbna20178936 (accessed October 26, 2020).
5. Wood and Collard, “The Human Genus”; see also Mark Collard and Bernard Wood, “Defining the Genus Homo,” in Handbook of Paleoanthropology, 2107-2144.
6. Sigrid Hartwig-Scherer and Robert Martin, “Was ‘Lucy’ More Human than Her ‘Child’? Observations on Early Hominid Postcranial Skeletons,” Journal of Human Evolution 21 (1991), 439-449.
7. Hartwig-Scherer and Martin, “Was ‘Lucy’ More Human than Her ‘Child’?”
8. Walker and Shipman, Wisdom of the Bones, 132, 130.
9. Sigrid Hartwig-Scherer, “Apes or Ancestors?” Mere Creation: Science, Faith and Intelligent Design, ed. William A. Dembski (Downers Grove, IL: InterVarsity, 1998), 226.
10. David McKenzie and Hamilton Wende, “Homo naledi: New Species of Human Ancestor Discovered in South Africa,” CNN (September 10, 2015), http://www.cnn.com/2015/09/10/africa/homo-naledi-human-relative-species/ (accessed October 26, 2020).
11. Rachel Reilly, “Is This the First Human? Extraordinary Find in a South African Cave Suggests Man May Be Up to 2.8 Million Years Old,” Daily Mail(September 10, 2015), http://www.dailymail.co.uk/sciencetech/article-3228991/New-species-ancient-human-discovered-Fossilised-remains-15-bodies-unearthed-South-African-cave.html (accessed October 26, 2020).
12. “Trove of Fossils from a Long-Lost Human Ancestor Is Greatest Find in Decades,” PBS Newshour (September 10, 2015), https://www.pbs.org/newshour/show/new-family (accessed October 26, 2020).
13. University of the Witwatersrand, “The Hand and Foot of Homo naledi,” ScienceDaily (October 6, 2015), http://www.sciencedaily.com/releases/2015/10/151006123631.htm (accessed October 26, 2020).
14. Berger et al., “Homo naledi, a New Species of the Genus Homo from the Dinaledi Chamber, South Africa,” eLife 4 (2015), e09560.
15. Tracy Kivell et al., “The Hand of Homo naledi,” Nature Communications 6 (October 6, 2015), 8431.
16. W.E.H. Harcourt-Smith et al., “The Foot of Homo naledi,” Nature Communications 6 (October 6, 2015), 8432.
17. American Museum of Natural History, “Foot Fossils of Human Relative Illustrate Evolutionary ‘Messiness’ of Bipedal Walking,” ScienceDaily(October 6, 2015), http://www.sciencedaily.com/releases/2015/10/151006131938.htm (accessed October 26, 2020).
18. Berger et al., “Homo naledi, a New Species of the Genus Homo.”
19. Berger et al., “Homo naledi, a New Species of the Genus Homo.”
20. Harcourt-Smith et al., “The Foot of Homo naledi.”
21. Kate Wong, “First of Our Kind,” Scientific American (November 1, 2012), http://www.scientificamerican.com/article/first-of-our-kind-2012-12-07/ (accessed October 26, 2020). See also Brandon Bryn, “Australopithecus sediba May Have Paved the Way for Homo,” AAAS News (September 8, 2011), http://www.aaas.org/news/science-australopithecus-sediba-may-have-paved-way-homo (accessed October 26, 2020).
22. Ann Gibbons, “A Human Smile and Funny Walk for Australopithecus sediba,” Science 340 (April 12, 2013), 132-133. See also Nadia Ramlagan, “Human Evolution Takes a Twist with Australopithecus sediba,” AAAS News (April 11, 2013), http://www.aaas.org/news/science-human-evolution-takes-twist-australopithecus-sediba (accessed October 26, 2020).
23. Peter Schmid et al., “Mosaic Morphology in the Thorax of Australopithecus sediba,” Science 340 (April 12, 2013), 1234598; Charles Choi, “Humanity’s Closest Ancestor Was Pigeon-Toed, Research Reveals,” LiveScience (April 11, 2013), https://www.livescience.com/28656-closest-human-ancestor-was-pigeon-toed.html (accessed October 26, 2020).
24. Caroline Vansickle et al., “Primitive Pelvic Features in a New Species of Homo,” The 85th Annual Meeting of the American Association of Physical Anthropologists, 2016, http://meeting.physanth.org/program/2016/session39/vansickle-2016-primitive-pelvic-features-in-a-new-species-of-homo.html (accessed October 26, 2020).
25. Berger et al., “Homo naledi, a New Species of the Genus Homo.”
26. Ed Yong, “6 Tiny Cavers, 15 Odd Skeletons, and 1 Amazing New Species of Ancient Human,” The Atlantic (September 10, 2015), http://www.theatlantic.com/science/archive/2015/09/homo-naledi-rising-star-cave-hominin/404362/ (accessed October 26, 2020).
27. Andrew Du and Zeresenay Alemseged, “Temporal evidence shows Australopithecus sediba is unlikely to be the ancestor of Homo,” Science Advances5 (May 8, 2019), eaav9038; Tim White, “Five’s a Crowd in Our Family Tree,” Current Biology 23 (February 4, 2013), R112-R115; William Kimbel, “Hesitation on Hominin History,” Nature 497 (May 30, 2013), 573-574; Gibbons, “Human Smile and Funny Walk for Australopithecus sediba”; Gibbons, “Who Was Homo habilis?” Science 332 (June 17, 2011), 1370-1371; Nicholas Wade, “New Fossils May Redraw Human Ancestry,” New York Times (September 8, 2011), http://www.nytimes.com/2011/09/09/science/09fossils.html (accessed October 26, 2020); John Noble Wilford, “Some Prehumans Feasted on Bark instead of Grasses,” New York Times (June 27, 2012), http://www.nytimes.com/2012/06/28/science/australopithecus-sediba-preferred-forest-foods-fossil-teeth-suggest.html (accessed October 26, 2020).
28. Du and Alemseged, “Temporal evidence shows Australopithecus sediba is unlikely to be the ancestor of Homo.” 
29. Carl Zimmer, “Yet Another ‘Missing Link,’” Slate (April 8, 2010), http://www.slate.com/articles/health_and_science/science/2010/04/yet_another_missing_link.single.html (accessed October 26, 2020).
30. Michael Balter, “Candidate Human Ancestor from South Africa Sparks Praise and Debate,” Science 328 (April 9, 2010), 154-155.
31. See Kate Wong, “Debate Erupts over Strange New Human Species,” Scientific American (April 8, 2016), http://www.scientificamerican.com/article/debate-erupts-over-strange-new-human-species/ (accessed October 26, 2020); Tanya Farber, “Professor’s Claims Rattle Naledi’s bones,” Sunday Times (April 24, 2016), http://www.timeslive.co.za/sundaytimes/stnews/2016/04/24/Professors-claims-rattle-Naledis-bones (accessed October 26, 2020); Aurore Val, “Deliberate Body Disposal by Hominins in the Dinaledi Chamber, Cradle of Humankind, South Africa?,” Journal of Human Evolution 96 (2016), 145-148.
32. Kate Wong, “Mysterious New Human Species Emerges from Heap of Fossils,” Scientific American (September 10, 2015), http://www.scientificamerican.com/article/mysterious-new-human-species-emerges-from-heap-of-fossils/ (accessed October 26, 2020). 
33. Quoted in Yong, “6 Tiny Cavers.”
34. University of Colorado Anschutz Medical Campus, “Ancient ancestor of humans with tiny brain discovered,” ScienceDaily (September 10, 2015), https://www.sciencedaily.com/releases/2015/09/150910084610.htm (accessed October 26, 2020).
35. University of the Witwatersrand, “Homo naledi’s surprisingly young age opens up more questions on where we come from,” ScienceDaily (May 9, 2017), https://www.sciencedaily.com/releases/2017/05/170509083554.htm (accessed October 26, 2020). See also Dirks et al., “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife 6 (2017), e24231.
36. James Kidder, “What Homo Naledi Means for the Study of Human Evolution,” BioLogos (May 30, 2017), http://biologos.org/blogs/guest/what-homo-naledi-means-for-the-study-of-human-evolution (accessed October 26, 2020).
37. See Chris Stringer, “Human Evolution: The Many Mysteries of Homo naledi,” eLife 4 (2015), e10627; Daniel Curnoe, “What About Homo naledi’s Geologic Age?,” Phys.org (September 15, 2015), http://phys.org/news/2015-09-opinion-homo-naledi-geologic-age.html (accessed October 26, 2020).