A significant chapter in human evolutionary history concluded with the demise of one of our closest surviving relatives, Homo floresiensis, often colloquially termed “the hobbit” due to its diminutive stature. The enigma surrounding its extinction, following an existence spanning over a million years on the remote volcanic terrain of Flores Island, Indonesia, has persisted for an extended period.
Emerging findings now point towards an epoch of severe aridity, commencing approximately 61,000 years ago, as a potential catalyst for their disappearance.
Our recent investigation, disseminated today in Communications Earth & Environment, meticulously reconstructs a narrative of ecological prosperity followed by significant decline. We have assembled the most comprehensive climate record to date for the specific locale inhabited by these ancient hominins.
It appears that H. floresiensis, along with a principal quarry species—a dwarf elephant—were compelled to relocate from their established territories by a protracted drought lasting millennia. This displacement may have led them into direct encounters with the considerably larger Homo sapiens.
An island characterized by extensive cave systems
The unearthing of H. floresiensis in 2003 fundamentally reshaped our understanding of human origins. These small-statured, small-brained hominins, standing merely 1.1 meters tall, possessed the capacity for crafting stone implements. Improbably, they are believed to have reached Flores without the benefit of maritime technology.
Skeletal fragments and lithic artifacts attributed to H. floresiensis were discovered within Liang Bua cave, nestled in a secluded valley in the island’s highlands. These remnants have been dated to the period between 190,000 and 50,000 years ago.
Presently, Flores experiences a monsoon-driven climate, characterized by heavy precipitation during the summer wet season (predominantly from November to March) and reduced rainfall during the drier winter months (May to September).
However, during the last glacial epoch, considerable fluctuations in both the volume of precipitation and its seasonal timing would have been prevalent.
To ascertain the nature of the rainfall patterns, our research collective investigated a cavern situated 700 meters upstream from Liang Bua, identified as Liang Luar. Fortuitously, within the depths of this cave, a stalagmite was discovered that had undergone continuous growth throughout the period of H. floresiensis‘s disappearance.
As stalagmites develop incrementally from dripping water, their evolving chemical makeup serves as a proxy for past climatic conditions.
Palaeoclimatologists employ two primary geochemical methodologies for reconstructing historical precipitation from stalagmites. Analysis of a specific oxygen isotope, denoted as d18O, provides insights into shifts in monsoon intensity. Concurrently, the ratio of magnesium to calcium reveals the overall volume of rainfall.
By integrating these measurements from identical samples, meticulously synchronized to specific temporal markers, we were able to reconstruct seasonal and annual precipitation quantities. This holistic approach yielded unparalleled understanding of interseasonal climatic variability.
Our findings illuminated three distinct climatic phases. The region experienced conditions wetter than present throughout the year between 91,000 and 76,000 years ago. From 76,000 to 61,000 years ago, the monsoon exhibited pronounced seasonality, with increased rainfall in the summer and arid conditions in the winter. Subsequently, between 61,000 and 47,000 years ago, the climate became significantly drier during the summer, mirroring conditions currently observed in southern Queensland.
The hobbits’ movements were dictated by their prey
Thus, we had a precisely dated record of substantial climatic transformations, but what was the corresponding ecological impact, if any? It became imperative to establish an accurate chronology for the fossil evidence of H. floresiensis at Liang Bua.
An unexpected solution emerged from our analysis of d18O isotopes in the fossilized tooth enamel of Stegodon florensis insularis, an extinct dwarf relative of extant elephants.
Juvenile dwarf elephants constituted a primary food source for the hobbits, as evidenced by cut marks discovered on bones recovered from Liang Bua.
Remarkably, the d18O signature observed in the Liang Luar stalagmite and in teeth extracted from increasingly deep sedimentary strata at Liang Bua exhibited a precise correlation. This synchronicity enabled us to accurately date the Stegodon fossils and the contemporary H. floresiensis remains.
The refined chronological framework indicated that approximately 90% of the pygmy elephant skeletal remains date to the period between 76,000 and 61,000 years ago, coinciding with the highly seasonal “Goldilocks” climate regime. This environment may have been optimal for the dwarf elephants’ grazing and, consequently, for H. floresiensis to hunt them. However, both species experienced a near-total disappearance as the climate transitioned to a drier state.
The concurrent decline in rainfall, pygmy elephants, and hobbits strongly suggests that resource scarcity played a pivotal role in what appears to have been a gradual desertion of the Liang Bua site.
As the climate grew more arid, the primary dry-season water source, the diminutive Wae Racang river, likely diminished to insufficient levels, depriving the Stegodon of adequate freshwater. The animals may have then migrated away from the region, with H. floresiensis following suit.
Could a volcanic event have also played a role?
The most recent traces of Stegodon fossils and stone tools discovered in Liang Bua are overlaid by a conspicuous stratum of volcanic ash, estimated to be approximately 50,000 years old. It remains undetermined whether a proximate volcanic eruption served as a “coup de grâce” in the decline of the Liang Bua hobbits.
The initial archaeological evidence attributed to Homo sapiens is situated stratigraphically above the ash layer. Therefore, while definitive knowledge of whether H. sapiens and H. floresiensis interacted is unattainable, recent archaeological and genetic data both indicate that H. sapiens were undertaking island-hopping expeditions across Indonesia towards the supercontinent of Sahul by at least 60,000 years ago.
Should H. floresiensis have been compelled by environmental pressures to depart from their sanctuary and venture towards the coastline, interactions with modern humans may have occurred. If this were the case, could competition, novel pathogens, or even direct predation have then been decisive contributing factors?
Irrespective of the ultimate causal agents, our investigation furnishes a foundational framework for subsequent research aimed at elucidating the extinction of the distinctive H. floresiensis within the context of profound climatic shifts.
The fundamental reliance on freshwater availability for the demise of one of our human relatives underscores the precarious nature of humanity’s evolutionary journey and highlights the significant ramifications that altering precipitation patterns can precipitate.
