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This PDF is a scholarly critique and clarification This PDF is a scholarly critique and clarification published in the Journal of Human Evolution (2005), written by anthropologists Kristen Hawkes and James F. O’Connell. It examines and challenges a high-profile claim that human longevity is a recent evolutionary development, supposedly emerging only in the Upper Paleolithic. The document argues that the method used in the original study is flawed and does not accurately measure longevity in fossil populations.
Through comparative primate data, demographic theory, and paleodemographic evidence, the authors demonstrate that fossil death assemblages do not reliably reflect actual population age structures, and therefore cannot be used to claim that modern humans only recently evolved long life.
🔶 1. Purpose of the Article
This paper responds to Caspari & Lee (2004), who argued:
Older adults were rare in earlier hominins (Australopiths, Homo erectus, Neanderthals).
Long-lived older adults first became common with Upper Paleolithic modern humans.
This increase in longevity contributed to modern human evolutionary success.
Hawkes and O’Connell show that these conclusions are unsupported, because the age ratio Caspari & Lee used is not a valid measure of longevity.
🔶 2. Background: The Original Claim
Caspari & Lee analyzed fossil teeth using:
Third molar (M3) eruption to mark adulthood.
Tooth wear to classify “young adults” vs. “old adults.”
Calculated a ratio of old-to-young adult dentitions (OY ratio).
Their findings:
Fossil Group O/Y Ratio
Australopiths 0.12
Homo erectus 0.25
Neanderthals 0.39
Upper Paleolithic modern humans 2.08
They interpreted the dramatic jump in the OY ratio for modern humans as evidence of a major increase in longevity late in human evolution.
🔶 3. Main Argument of the Authors
Hawkes and O’Connell argue that:
⭐ The OY ratio does NOT measure longevity.
Even if ages are correctly estimated, the ratio is strongly influenced by:
Preservation bias (older bones deteriorate more)
Estimation errors (tooth wear ages are imprecise)
Non-random sampling of deaths
Archaeological context (burial practices, living conditions)
Thus, high or low representation of older adults in a fossil assemblage may reflect postmortem processes, not real lifespan differences.
🔶 4. Key Evidence Provided
⭐ A. Cross-primate comparison
The authors calculate OY ratios for:
Japanese macaques
Chimpanzees
Modern human hunter-gatherers
Despite huge differences in their real lifespans:
Macaques live ≈ 30 years
Chimpanzees ≈ 40–50 years
Humans ≈ 70+ years
Their O/Y ratios are nearly identical:
Species O/Y Ratio
Macaques 0.97
Chimpanzees 1.09
Humans 1.12
This proves that if the metric worked, there would be very little variation in OY ratios—even between species with very different longevity.
Therefore, the extreme fossil ratios (e.g., 0.12 to 2.08) cannot reflect real lifespan differences.
How old is human longevity
⭐ B. Paleodemographic Problems
The paper explains why skeletal assemblages almost never reflect real population age structures:
Age estimation errors (especially for adults)
Poor preservation of older individuals’ bones
Non-random sampling of deaths (cultural, ecological, and taphonomic factors)
Even large skeletal samples cannot be assumed to represent living populations.
How old is human longevity
🔶 5. Theoretical Implications
If Caspari & Lee’s OY ratios were valid, they would contradict:
Stable population theory
Known mammalian life-history invariants
Primate patterns linking maturity age with lifespan
Since all primates show a fixed proportional relationship between age at maturity and adult lifespan, drastic jumps in the OY ratio are biologically implausible.
Instead, the variation seen in fossil OY ratios most likely reflects sample bias, not evolutionary change.
🔶 6. Final Conclusion
Hawkes and O’Connell conclude:
❌ The claim that human longevity suddenly increased in the Upper Paleolithic is unsupported.
❌ Fossil age ratios do not measure longevity.
✔ Differences in OY ratios across fossil assemblages reflect archaeological and preservation biases, not biological evolution.
They emphasize that interpreting fossil age structures requires extreme caution, and that modern demographic and primate comparative data provide essential context for understanding ancient life histories.
⭐ Perfect One-Sentence Summary
This PDF demonstrates that the fossil tooth-wear ratio used to claim a late emergence of human longevity is not a valid measure of lifespan, and that differences across fossil assemblages reflect sampling and preservation biases—not real evolutionary changes in human longevity.... |