The study focused on the Kalibiuk and Kaliglagah Formations near the village of Bentasari in Central Java. These rock formations are especially important because they contain fossils older than the earliest known remains of Homo erectus in Java. Long before modern cities and rice fields appeared, this region was part of a dynamic coastal environment shaped by rivers, tides, monsoon climates, and volcanic activity.

To uncover the story hidden inside the rocks, the researchers collected dozens of sediment samples from a one hundred meter section of exposed strata. Inside each sample were fossil pollen grains, spores from ferns, freshwater algae, and tiny fragments of ancient plant tissue. These microscopic remains act like environmental fingerprints. Different plants produce distinct forms of pollen and spores, allowing scientists to identify what kinds of vegetation once dominated the region.

Palynology, the study of fossil pollen and spores, may sound highly specialized, but it offers one of the most powerful tools for reconstructing ancient environments. Pollen grains are remarkably durable. Once buried in mud or sediment, they can survive for millions of years. By identifying the types and abundance of pollen in different layers of rock, researchers can track environmental changes through time with remarkable precision.

What Harsanti Morley and Robert Morley discovered was a landscape that shifted repeatedly between marine and freshwater conditions. In the older parts of the sequence, the coastline was dominated by extensive mangrove forests. The pollen record showed overwhelming evidence of mangrove trees from the Rhizophoraceae family, accompanied by Sonneratia trees, mangrove palms, and salt-tolerant ferns.

These mangroves formed part of a huge tropical delta system where rivers met the sea. Some areas were muddy tidal flats, while others supported dense mangrove stands growing directly at the site where sediments accumulated. The evidence even suggests that sea levels rose and fell in repeating cycles, causing the coastline to advance and retreat over time.

The researchers believe the sediments captured two major environmental cycles linked to variations in Earth’s orbit. During the early Pleistocene, global climate patterns were strongly influenced by changes in the tilt of Earth’s axis, known as obliquity cycles. Each cycle lasted roughly forty one thousand years. As climate and sea levels shifted, the delta expanded and contracted, creating new habitats across the landscape.

One of the most striking discoveries involved the role of the mangrove fern Acrostichum. In some layers, spores from this fern became extraordinarily abundant, suggesting that it temporarily dominated large stretches of the delta plain. Today, Acrostichum often flourishes in disturbed coastal environments, and its abundance in the fossil record may point to periods of ecological disruption.

As conditions became more freshwater influenced, the environment changed dramatically. The younger parts of the sequence recorded freshwater swamps filled with plants such as Jussiaea, Pandanus, Polygonum, and Hanguana. Open pools of water supported algae, while grasses and sedges spread across the wetlands.

Yet the most surprising feature of the ancient landscape may have been what existed beyond the swamps. The pollen record showed enormous quantities of grass pollen, along with charred fragments of grass cuticle. These fragments provide evidence of repeated burning. According to the researchers, the surrounding lowlands were probably covered by open grassy savanna rather than dense tropical rainforest.

This conclusion challenges older ideas that early Pleistocene Java was dominated by lush lowland rainforest. Instead, the evidence points toward a strongly seasonal climate with long dry periods that allowed fires to spread across grasslands. Trees certainly existed, especially along rivers and in scattered forest patches, but open vegetation appears to have been widespread.

At the same time, the mountains in the distance supported cooler montane forests. Pollen from podocarps, oaks, and other upland trees drifted into the delta system from higher elevations. The result was a landscape of extraordinary ecological diversity. Coastal mangroves bordered freshwater swamps, open savannas stretched across the lowlands, and forested volcanic mountains towered above everything.

The study also provides clues about the animals that once inhabited the region. Earlier fossil discoveries from the Kaliglagah Formation included unusual species such as pygmy hippopotamuses and giant tortoises. The swampy, grassy environment reconstructed from the pollen evidence may have provided ideal habitat for these animals.

The researchers even suggest that some of the freshwater swamps may have been repeatedly disturbed by large herbivores. Certain plants found in the pollen record, including Jussiaea and Polygonum, often thrive in disturbed wetland environments today. Ancient animals moving through the swamps could have helped maintain these open wetland habitats.

Perhaps most importantly, the findings help scientists better understand the environmental setting that existed just before and during the arrival of early humans in Java. Homo erectus eventually became one of the most successful early human species, spreading far beyond Africa into Asia. Understanding the environments these early humans encountered is essential for reconstructing how they adapted and survived.

The Bentasari study suggests that early humans entering Java may have encountered a mosaic of open grasslands, wetlands, mangroves, and volcanic uplands rather than endless rainforest. Open environments could have provided opportunities for hunting, scavenging, and movement across the landscape.

Another important discovery involved a fossil spore called Stenochlaenidites papuanus. This ancient plant type disappeared from the Sunda region around one and a half million years ago. Its presence in the Bentasari sediments confirms that the deposits belong to a very early chapter of the Pleistocene. The same spore has been found at a few other important fossil localities in Java, helping scientists compare the ages of different sites across the island.

The work also highlights how much information can be preserved in microscopic fossils that most people would never notice. A grain of pollen smaller than the width of a human hair can reveal whether ancient coastlines supported mangroves, whether grasslands burned during droughts, or whether mountain forests flourished nearby.

Studies like this remind us that Earth’s environments are constantly changing. Coastlines shift, climates fluctuate, ecosystems expand and disappear, and species adapt or vanish. The tropical landscapes of ancient Java were shaped by forces that still operate today, including sea level change, climate cycles, volcanic activity, and fire.

At the same time, the research carries a modern message. Many of the ecosystems reconstructed in the study, especially mangroves and freshwater swamps, are now heavily damaged or disappearing across Southeast Asia. Understanding how these environments evolved over immense stretches of time can help scientists appreciate their resilience, but also their vulnerability.