The rocks of south-east Oman have yielded chemical clues that appear to have put back the origin of multicellular animal life by tens of millions of years. Deposits analysed by scientists from the United States and Britain indicate there were thriving colonies of sponges at least 635 million years ago. Until now, biologists believed multicellular animal life originated about 580 million years before an array of creatures developed in an episode known as the Cambrian explosion.
Being soft-bodied, sponges do not tend to leave imprints behind as fossils, so instead researchers have relied on chemical analysis to detect them. As reported in the journal Nature, a molecule called 24-isopropylcholestane, produced by the breakdown of fat molecules in the sponges' cell membranes, is a key indicator of a sponge's presence. Colin Snape, one of the study's authors, insists substances found in the rock strata, which were being explored for their oil potential, are "absolutely non-ambiguous" for sponges.
"You can bet your bottom dollar if there was any ambiguity we wouldn't have got published in Nature," says Prof Snape, director of research in chemical and environmental engineering at the University of Nottingham in England. "Basically what we're doing is releasing biological markers - hydrocarbons that are distinctive to particular life forms." Prof Snape and his colleagues analysed powdered rock samples, while another group used uranium-lead dating techniques that use radioactive decay rates to date material from the drilling.
"In our process, once we get out biological markers, it's completely unambiguous that they are part of the rock and not something that's got in there by accident," says Prof Snape. "In oil strata, oils do migrate, so it's possible to get modern bitumen implanted in ancient rocks. "Our technique pulls out what is specifically associated with ancient rocks, rather than something that could have got in fortuitously much more recently."
Periods of glaciation during the Neoproterozoic era, which ran from one billion to 542 million years ago, are believed to have caused changes to ocean chemistry that allowed the sea floor creatures to evolve. Until now, scientists believed Earth was covered in ice until 635 million years ago, but the presence of the filter-feeding sponges indicates there were shallow areas not frozen over with enough dissolved oxygen to support life.
However, oxygen levels were low, so only small creatures with bodies of a few millimetres could have survived. "We believe we are converging on the correct date for the divergence of complex multicellular animal life on the shallow open floor between 635 and 750 million years ago," said Gordon Love, an assistant professor of earth sciences at the University of California Riverside and the report's senior author, in a statement.
Given the significance of the recent findings, it is sobering to remember that, had the rocks not been a potential source of petroleum deposits, they would not have been explored at all. The research was carried out in collaboration with Petroleum Development Oman. "The authorities wouldn't have gone out and done drilling just to get to these rocks if the oil industry wasn't interested commercially," Prof Snape says.
The discovery of chemical clues to the early evolution of sponges is only the latest recent important discovery in Arabia. Last year, a paper revealed that dinosaur tracks had been found 50km from Sanaa, the Yemeni capital. The tracks of a large bird-footed dinosaur or ornithopod and a herd of 11 sauropods are thought to date back 150 million years. At the time the discovery was announced, Dr Anne Schulp from the Maastricht Museum of Natural History said "practically nothing" was known about dinosaurs on the Arabian peninsula.
It all begs the question of what else remains to be unearthed in this part of the world. According to Andrew Hill, a professor and curator of anthropology at Yale University with extensive experience in the UAE, in Arabia there is "a lot to do in terms of simple discoveries". Indeed, he says the work that revealed the dinosaurs only scratched the surface of what could be found in the area. "It's not been very thorough there [in Yemen] - just two or three relatively short expeditions," he says. "There's lots to do."
In the Emirates, fossil sites have revealed a very different landscape to the one that exists today. About seven million years ago, during the Late Miocene period, there were sabre tooth cats, giraffes, gazelles, ostriches and at least two kinds of elephant as well as water-dwelling animals like fish, turtles and hippopotami. The rich vegetation included acacia trees and there were slow-flowing rivers.
Remains of life from that era have been discovered at 40 sites in Al Gharbia, all part of the geological strata known as the Baynunah Formation. As reported earlier this year, scientists want to investigate more of Al Gharbia's fossil sites before development of the area for tourism makes them inaccessible. Prof Hill says there is "huge potential" for more finds in the Baynunah Formation as "things turn up there all the time".
While Al Gharbia is on the verge of being developed, many of the coastal areas of the UAE have already been built upon. "I like to think there may be more along the coast but it's getting built up and it's hard to gain access to the places," Prof Hill says. "There could be stuff from different periods. I am sure there are areas here and there and possibly in other emirates." It is not just in Arabia where untold palaeontological riches may be waiting to be discovered. While he says east Africa has been examined reasonably well, Prof Hill says "very little" is known about the western part of the continent.
"West of the rift separating Uganda from Congo hasn't been looked at very well in any detail," he says. One issue that deters palaeontologists from exploring less well-known areas such as West Africa and parts of Arabia is funding. Scientists may be reluctant to try a new area because they risk not finding anything of value. "If you have a grant to find fossils, you go where the fossils are," Prof Hill says. "If you go to Cameroon with a lot of money and don't find anything, you won't get any more money."
Fortunately Prof Hill, who first came to the UAE a quarter of a century ago, still regularly comes to the UAE looking for fossils at the invitation of the Abu Dhabi Authority for Culture and Heritage. In fact, his most recent visit to the field ended only last month. He spent much of his time on Shuwayhat Island, which lies off the coast about 200km west of Abu Dhabi, searching for more Late Miocene fossils.
"While I was there, I didn't do any other exploring for other sites, other than trying to find other rodent sites," he says. "We got a lot of little rats. The fossils are hard to find - they're the size of a pin so you have to do a lot of sieving." firstname.lastname@example.org