Divers dig deep for the hole truth about our ancestors

Fossils from deep saltwater 'caves' show how human settlement has always upset the natural order, reports Roger Highfield

A pioneering study of life recorded in giant sinkholes that extend far below sea level has shown scientists the catastrophic impact that ancient communities had on their environments.

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The Great Blue Hole at Lighthouse Reef Atoll, Belize

By swimming down into "blue holes" - vertical caves in the landscape - scientists can travel back in time, exploring a treasure trove of fossils and gaining a new understanding of the prehistoric ecosystem.

The holes are so named because of the dramatic contrast between the dark blue saltwater of their depths and the lighter blue of the freshwater shallows around them. The holes, such as the Great Blue Hole at Lighthouse Reef Atoll, Belize, can sink to depths of more than 650ft and are typically found in the Caribbean on the Bahama Banks, as well as on and around the Yucatán Peninsula.

Expert divers and scientists recently carried out the first sophisticated excavation on a hole in the Bahamas, which provided a rare glimpse of life before humans arrived. The cave on Great Abaco Island, known as Sawmill Sink, contains a host of well-preserved fossils, many from species that are now extinct. These include the first entire skeletons found in the West Indies of a 2,500-year-old tortoise and an unusual crocodile that roamed the land, along with bones from a lizard, snakes, bats and 25 species of birds, not to mention abundant plant fossils. Diver Brian Kakuk, a consultant for the Bahamas National Museum, discovered the first Sawmill Sink fossil - the giant tortoise shell, which was in excellent condition.

But long before tourists arrived in the Bahamas, ancient humans had taken up residence on this archipelago off the coast of Florida. The subsequent disappearance of species from the fossil record offers stark evidence that the arrival of humans permanently changed - and eliminated - life on what had been isolated islands.

"The climate and the environmental conditions back then weren't much different from those of today," explains David Steadman, who works at the Florida Museum of Natural History. "The big difference is us. When people got to the island, there was probably nothing easier to hunt than tortoises, so they cooked and ate them. And they got rid of the crocodiles, probably because it's tough to have kids playing at the edge of a village where there are terrestrial crocodiles running around."

Radiocarbon analysis dates the human bones - the most recent discovery is a tibia - found at Sawmill Sink at between 1,000 and 4,200 years old. The fossils are the best?preserved of any found in the Bahamas, because the deep saltwater layer of the sinkhole contains no oxygen, which would normally feed the bacteria and fungi that cause bones to decay.

"The fossils from Sawmill Sink open unparalleled opportunities for much more sophisticated work than ever in reconstructing the ancient plant and animal communities," Steadman says. "They help us to understand not only how individual species evolve on islands but also how these communities changed with the arrival of people, because we know that changes in the ecosystem are much more dramatic on islands than on continents."

The fossil site is especially valuable because of the presence of fossilised plants as well as animals: leaves, twigs, flowers, fruits, seeds, pollen and spores. For example, the bracken ferns suggest that the island once boasted a regenerative, fire-swept landscape of open grasslands. "For the first time in the West Indies, we have plant fossils right in with the vertebrates, so we can reconstruct the habitats in a much more sophisticated way," says Steadman.

Other blue holes could yield further palaeontological and archaeological riches. Admittedly, exploring them is hazardous: depth, tight spaces, vision-clouding particles and toxic layers of hydrogen sulphide can make the sinkholes dark and dangerous places to dive. But that won't put off the scientists for long.