Hortensia de los Santos
Author, Researcher, Theorist

Reevaluating Hominin Diversity

Environmental Adaptations Following Catastrophic Events Rather than Separate Species

Abstract

The traditional interpretation of the hominin fossil record classifies various Homo species,such as Homo erectus, Homo neanderthalensis, and Homo floresiensis, as distinct evolutionary branches.

This paper proposes an alternative explanation:

that many of these hominin groups were not separate species but rather highly adapted populations of Homo sapiens responding to extreme environmental pressures following catastrophic events, such as the hypothesized Younger Dryas impact event. We examine morphological, genetic, and environmental evidence to support the claim that these adaptations parallel modern human diversity rather than indicating speciation. If a similar event occurred today, surviving human populations could exhibit extreme differentiation in body structure and genetics, potentially misleading future scientists into defining them as separate species.

This reevaluation urges caution in species classification and highlights the importance of understanding environmental selective pressures.

Introduction

The study of human evolution has long been dominated by the classification of multiple Homo species based on skeletal and morphological differences. However, these distinctions may not reflect true speciation but rather environmental adaptations of a single lineage of Homo sapiens.

This paper explores an alternative hypothesis:

• that catastrophic events, such as the Younger Dryas impact hypothesis, periodically reshaped human populations, forcing rapid adaptation to extreme environments.
• These adaptations, over thousands of years, produced the physical variations seen in the fossil record, leading to misclassification as separate species.
• If a cataclysm occurred today, how would surviving populations evolve? Would their skeletal structures differ so dramatically that future scientists might mistakenly classify them as separate species?
• By exploring modern human diversity and the effects of environmental pressures, we argue that past hominins may represent adaptations rather than separate evolutionary branches.

1. The Problem of Hominin Species Classification

• The classification of early Homo species has largely been based on skeletal and cranial morphology, with limited DNA evidence available for many extinct groups. However, species definitions in paleoanthropology remain highly debated, especially given the sparse fossil record.

  1.1 The Biological Species Concept and Its Limitations
  • According to the biological species concept, species are defined by their inability to produce fertile offspring.
  • However, we now know that:
    • Neanderthals and Denisovans interbred with modern humans, contributing up to 5% of the DNA of some modern populations.
    • This contradicts their classification as fully separate species since they produced viable, fertile offspring.
    • If Neanderthals and Denisovans were reclassified today, they might be considered subspecies or regional adaptations rather than distinct species.
  1.2 Extreme Variation in Modern Humans
  • Modern human populations display striking physical diversity, yet all belong to a single species (Homo sapiens):
  • The Dinka people of South Sudan can reach heights of over 6'5", whereas the Aka pygmies of Central Africaaverage 4'10".
  • Skull shapes vary widely across populations, from long and narrow (dolichocephalic) to broad and round (brachycephalic).
  • If only fossilized remains of these groups were found, paleoanthropologists might classify them as different species.
  • This raises the question: Are ancient hominin "species" actually the result of similar environmental variations rather than true speciation?

2. The Role of Catastrophes in Human Evolution

• Throughout prehistory, cataclysms, such as asteroid impacts, volcanic eruptions, and climate shifts, have acted as evolutionary bottlenecks, reducing genetic diversity and accelerating adaptation.
• 2.1 The Younger Dryas Catastrophe and Its Effects
  • The Younger Dryas Impact Hypothesis suggests that around 12,800 years ago, a cosmic impact triggered widespread fires, flooding, and climate shifts, leading to mass extinctions. Such events:
    • Would have wiped out most human populations, leaving only small, isolated groups to repopulate.
    • Could have induced genetic bottlenecks, intensifying unique physical traits in survivors.
    • Would have created environments that favored different physical adaptations as cold-adapted populations in ice-covered areas vs. tropical-adapted populations in warm refuges.
    • If a similar event occurred today, we might see extreme divergences in human form within a few thousand years, potentially leading future scientists to mistakenly define new "species."

3. Case Studies: Are These "Species" Actually Environmental Variants?

• 3.1 Thick bones, short limbs, and large nasal cavities suggest adaptations to Ice Age Europe.
  • Interbreeding with modern humans indicates they were not fully separate species.
  • If isolated cold-adapted humans existed today, they might develop similar robust features.
• 3.2 Homo Floresiensis: An Example of Island Dwarfism?
  • The "Hobbit" of Flores was only 3 feet tall, with a small brain (~426 ㎤ ).
  • Similar dwarfism is seen in modern species (elephants, deer) when isolated on islands.
  • If a modern human group were stranded on an island for 50,000 years, would their descendants be classified as a separate species?
• 3.3 Homo Naledi: Cave Adaptive Traits?
  • Small-brained hominin (~465 to 610 ㎤ ) found deep in South African caves.
  • Possible underground adaptation, with curved fingers and a mix of primitive and modern traits.
  • If humans were forced to live underground after a solar storm or nuclear winter, could they evolve similar skeletal changes?
  • These cases support the idea that many hominin groups may not be separate species but localized adaptations of Homo sapiens under extreme environmental pressures.

5. Implications for Paleoanthropology

• If this hypothesis is correct, it fundamentally changes how we interpret human evolution:
  • Many named hominin species may actually be extreme variants of Homo sapiens.
  • Cataclysmic events may have repeatedly forced rapid, localized adaptations, leading to morphological differences.
  • Future discoveries may require a reclassification of species, viewing them instead as subspecies or adaptive populations.
  • This model aligns with the observed high adaptability of modern humans, who thrive in diverse environments without becoming separate species.

6. Conclusion

• The traditional view that Homo erectus, Neanderthals, Denisovans, and other hominins were separate species may be an oversimplification.
• Instead, these groups likely represent environmentally adapted populations of a single, resilient human lineage that repeatedly diversified after catastrophic bottlenecks.
• If a global disaster occurred today, isolated survivors would undergo similar adaptations, potentially misleading future scientists into creating artificial species classifications.
• The implications of this hypothesis call for a reassessment of the hominin fossil record and greater caution in defining new species based on limited evidence.