The Geological Elevator
The Qiongzhou Strait, separating Hainan Island from Guangdong's Leizhou Peninsula, is a shallow channel, averaging just 44 meters deep and less than 20 kilometers at its narrowest point. This narrow gap in the South China Sea is a geologically recent feature. During the Last Glacial Maximum (LGM), from about 26,500 to 19,000 years ago, vast quantities of Earth's water were locked up in continental ice sheets. This caused global sea levels to fall by as much as 120 to 135 meters below their present-day positions.
This dramatic drop in sea level completely exposed the floor of the strait, transforming it into a broad, low-lying plain. This created a continuous land bridge connecting what was then the proto-Hainan island to the Asian mainland. This land connection wasn't a fleeting event; it persisted for thousands of years, a geological elevator that repeatedly rose and fell with the planet's glacial cycles. As the LGM ended and the planet warmed, melting ice sheets caused the seas to rise again. The land bridge was progressively flooded, and Hainan became a true island once more when the strait fully opened around 8,500 years ago.
A Lost World of Megafauna
The exposure of the land bridge provided an important migration corridor for plants and animals. The most spectacular of these migrants were the Pleistocene megafauna of the Ailuropoda-Stegodon fauna complex, a collection of large mammal species characteristic of southern China during this period. Fossil evidence confirms that animals now long extinct from the region, such as the elephant-like Stegodon orientalis and various rhinoceros species (Rhinoceros sinensis), crossed this bridge.
These large herbivores were part of a diverse ecosystem that included giant pandas (Ailuropoda baconi), giant tapirs (Megatapirus augustus), and other species whose ranges were dictated by the changing climate and landscape. The presence of their fossils records a vanished world, one where Hainan was not an isolated tropical island but an integrated part of a mainland ecosystem supporting a much greater diversity of large animals.
The eventual submergence of the land bridge isolated these populations, leading to unique evolutionary paths for Hainan's flora and fauna. The corridor that once allowed giants to walk to the island was closed by the rising sea, preserving a biological snapshot of the Pleistocene. Archaeological finds from sites like Luobi Cave and the newly discovered Nanhuo River site show that early modern humans also used this corridor, with evidence of their presence on the island dating back tens of thousands of years.