Homo naledi and the Dating Dilemma

The discovery of Homo naledi continues to generate discussion in paleoanthropology, not merely for the size of the assemblage but also for the profound implications it has on evolutionary theory. Since its first announcement, Homo naledi has remained in public discourse for over a decade and is repeatedly invoked as reshaping the way human evolution is conceptualized. While this attention underscores the perceived importance of the discovery, it also highlights critical problems in both its interpretation and the methods used to establish its antiquity.

Shifting Evolutionary Frameworks

One reason the Homo naledi discovery is significant is because it challenges earlier evolutionary assumptions. The narrative of human origins has been repeatedly revised to accommodate findings that do not conform to established expectations. From a scientific standpoint, a hypothesis that is continually reshaped to fit new, contradictory evidence loses predictive power. This raises questions about the reliability of evolution as a testable framework, since each revision creates a moving target for critique.

The dating of the Homo naledi fossils exemplifies this difficulty. Paleoanthropologist Lee Berger initially hoped to date the remains to around two million years ago, which would have allowed him to claim discovery of the earliest intentional burials.¹⁾Berger, et al., “Homo naledi, a new species of the genus Homo from the Dinaledi chamber, South Africa,” eLife, September 10, 2015, p. 24 However, the dating methods did not confirm this expectation.

Inconsistencies in the Dating Results

The first public acknowledgment of the fossil dates appeared in Berger’s book Almost Human. There, he reported the fossils as “less than 450,000 years old,” later clarifying them as “between 450,000 and 250,000 years ago.”²⁾Lee R. Berger, and John Hawks, Almost Human: The Astonishing Tale of Homo naledi and the Discovery That Changed Out Human Story, National Geographic Partners (Washington, DC: 2017), p. 217. Yet, the source of the 450,000-year figure remains obscure, as official reports consistently listed younger ranges: “Homo naledi lived sometime between 335 and 236 thousand years ago, making it relatively young”³⁾Natasha Joseph, interview with John Hawk, “More secrets of human ancestry emerge from South African caves,” The Conversation, May 9, 2017; http://www.cbsnews.com/news/homo-naledi-human-ancestry-secrets-south-african-caves/. or, more generally, around 300,000 years.

Jean O’Micks and Dr. Timothy L. Clarey criticized Berger’s claims:

Nearly all of the U-Th dates of the teeth and the carbon dates of the bones indicated an age less than 100,000 years old (Clarey 2017; Dirks et al. 2017). Why Berger and his team chose the older dates is not explained in his book or even adequately explained in their paper (Dirks et al. 2017).⁴⁾Jean O’Micks, and Dr. Timothy L. Clarey, “Book Review of Almost Human, the Astonishing Tale of Homo naledi and the Discovery that Changed our Human Story, by Lee Berger and John Hawks,” Answers Research Journal, 10 (2017), p. 192

In Berger and John Hawks’ subsequent book, Cave of Bones, electron spin resonance analysis of three teeth yielded dates of 104,000 years, 139,000 years, and 335,000 years. Uranium-series dating of flowstones gave an age of 243,000 years (±7,000). Based on these findings, the team settled on a range of 236,000–335,000 years.⁵⁾Lee Berger and John Hawks, Cave of Bones: a true story of discovery, adventure, and human origins, National Geographic Partners, LLC (Washington, D.C.: 2023), pp. 60-61

Yet, they also noted:

The laboratory carrying out radiocarbon analysis of three naledi bones fragments said that two of the fragments were estimated to be less than 35,000 years old.⁶⁾Lee Berger and John Hawks, Cave of Bones: a true story of discovery, adventure, and human origins, National Geographic Partners, LLC (Washington, D.C.: 2023), p. 60.

Thus, dating results spanned from under 35,000 years to 335,000 years. Despite this, Berger once suggested 450,000 years, though no supporting data was ever published for this figure.

Assumptions Underlying Dating Techniques

The peer-reviewed report outlining the dating procedures repeatedly emphasized that calculations were based on assumptions, as seen in the following admissions:

• “calculated ages are based on the assumption of continuous diffusion…”⁷⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 19
• “Ages were calculated assuming closed system behaviour (CS), and compared with age results assuming continuous diffusion models.”⁸⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 45.
• “In calculating the ages… for all samples assuming… Therefore, in calculating the ages we have assumed… an estimate has to be made for the burial history of the teeth as well as the more general background radiation in the cave.”⁹⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 45.
• “In all calculations we have made the following assumptions: (i) post-230Th [thorium-230] daughter elements are in equilibrium in dental tissues, which is the standard assumption in ESR [Electron Spin Resonance] dating; and (ii) complete (effective) burial of the samples occurred (i.e., infinite matrix assumption for the gamma dose rate measured in sediment).”¹⁰⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 46.
• “The OSL [Optically Stimulated Luminescence] results are more difficult to interpret than the US-ESR [Uranium-Series Electron Spin Resonance] results, not only because we have to assume model parameters to estimate the environmental dose rate, but also because we have few constraints on the origin and provenance of the quartz grains that were sampled…. Thus, the MAM [Minimum Age Model] age is likely to provide a maximum age estimate for the Unit 1 sediments, and must be viewed with caution.”¹¹⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, pp. 28-29.
• “a major issue is whether 222Rn [Radon-222] degassing was a process that operated continuously over the past 300 ka or not…. escape of 222Rn [Radon-222] (half-life 3.82 days) is likely…. although it seems unlikely that an extreme degassing environment similar to the 80% measured today was maintained during the entire history of burial.”¹²⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 28.
• “we have no reason to prefer one age result over another.”¹³⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 25.
• “the extremely complex nature of sedimentary cave fill in many cave systems involving repeated cycles of deposition, erosion and reworking, leading to complex and sometimes contradictory age results.”¹⁴⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, pp. 29-30.

Such statements reveal the speculative character of the dating efforts, undermining confidence in the published range.

Environmental Complications: Water Contamination

The reliability of these results is further weakened by the influence of water. Researchers admitted that high uranium levels in sample 1767 were “probably the result of its greater exposure to water,” concluding:

Given these factors, we do not trust the reliability of the age results for sample 1767 and have excluded this tooth from the final age estimates for the fossils.¹⁵⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 28.

But if one sample could be compromised by water exposure, how could the rest of the assemblage remain unaffected after allegedly lying in a damp cave for 300,000 years?

Other teeth were similarly compromised. Sample 1788 was described as “highly bleached and brittle and appears affected by water action,”¹⁶⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 38. yet was still used to generate one of the “best age estimates.”¹⁷⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 34. Researchers acknowledged, “However, the dose rate can vary over time as U can be highly mobile in wet environments, and can move in and out of teeth.”¹⁸⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 41. Such admissions cast doubt on all dating results from the site.

Radiocarbon dating also demonstrated the issue. Three Homo naledi bone fragments yielded ages of 33,000 and 35,500 years, though researchers dismissed these results as reflecting calcite contamination.¹⁹⁾Dirks, P., et al. “The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa,” eLife, May 9, 2017, p. 26. Nonetheless, the findings highlight the confusion introduced by environmental interference.

Lee Berger himself admitted the fragile condition of the bones, writing in Almost Human:

We were lucky to have found them before erosion destroyed the deposits altogether.²⁰⁾Lee R. Berger, and John Hawks, Almost Human: The Astonishing Tale of Homo naledi and the Discovery That Changed Out Human Story, National Geographic Partners (Washington, DC: 2017), p.  218.

This acknowledgment directly contradicts his team’s denial that water transport could explain how the bones entered the chamber.

Bone Assemblage and Missing Evidence

The state of the remains further supports the possibility of water transport. Paleoanthropologist Aurore Val observed that only 10.8% of bones from an estimated 15 individuals were recovered, amounting to 2,757 missing elements. Despite claims of articulated remains, Val noted that only four articulated groups were found (a hand, a foot, a mandible with skull, and an ankle), far fewer than in other hominin discoveries. She concluded:

Despite the impression given by Berger et al., … the majority of the fossils are isolated and fragmented.²¹⁾Aurore Val, “Deliberate body disposal by hominins in the Dinaledi Chamber, Cradle of Humankind, South Africa?” Journal of Human Evolution, Vol. 96 (2016), p. 2.

If intentional burial were responsible, why are nearly 90% of the bones missing? Such evidence better supports explanations involving natural forces such as water transport.

Other studies have compared the Dinaledi assemblage with known archaic burial sites. The findings indicated a closer resemblance to naturally accumulated remains, including scavenged or water-transported deposits, than to deliberate burials:

Nevertheless, the recurrent clustering of the DC [Dinaledi Chamber] assemblage with the disturbed and carnivore-consumed samples and, in particular, the naturally accumulated bone sample of cave baboons, is intriguing.²²⁾Egeland, C.P., et al “Hominin skeletal part abundances and claims of deliberate disposal of corpses in the Middle Pleistocene,” PNAS (May 1, 2018) Vol. 115, No. 18, p. 4604.

Reassessing the Homo Designation

Dr. Tim Clarey of the Institute for Creation Research has argued that water transport provides the most plausible explanation for how the bones entered the chamber.²³⁾Dr. Tim Clarey, “Disposal of Homo naledi in a possible Deathtrap or Mass Mortality Scenario,” Journal of Creation, (August 2017) 31(2):61-70. If so, the interpretation of Homo naledi as intentionally burying its dead collapses. Since the Homo designation has been defended primarily on inferred behavior, not anatomical distinctiveness, removing this behavioral claim undermines the justification for placing naledi within the genus Homo.

The case of Homo naledi demonstrates the fragility of evolutionary interpretations that must be continually revised to fit unexpected data. The uncertain dating, the compromised condition of the remains, and the speculative assumptions behind the conclusions suggest that the celebrated claims of ceremonial burial and great antiquity are not supported by the evidence.

For a more detailed treatment of the dating issues, the bone assemblage, and the broader debate over Homo naledi, see my book: Is It Human? Assessing the Homo naledi Debate, available here.