Multicellular fossils may be the world's oldest.

Sunlight filters through shallow water, dappling tiny blades of brown algae that cling to the soft seafloor. Small spores drift from pores and settle to the bottom. This tranquil scene could take place today--except that these particular algae no longer exist.

Their fossils do, however, and like an ocean current, scientific uncertainty swirls around them.

In the Oct. 27 Science, two researchers at the Chinese Academy of Geological Sciences in Tianjin announced the discovery of 300 of the earliest fossils of multicellular organisms. Each less than an inch long, the 1.7 billion-year-old fossils lie in sedimentary rock that constitutes the Tuanshanzi Formation. Earth's movements pushed the rock to the surface near Jixian, east of Beijing.

Authors Zhu Shixing and Chen Huineng suggest that the organisms are brown algae. They point to similarities in modern brown algae such as Laminaria, a much larger, multicellular seaweed that lives in shallow coastal waters. Like Laminaria, some of the Tuanshanzi fossils have an apparent holdfast for clinging to rocks, an algal stem, and one or more wide blades.

Scanning electron microscope photos reveal another similarity: The fossils have porelike openings ringed with what appear to be cells.

But nonmulticellular organisms--rafts of blue-green algae, for example--can yield similar images, says the University of Montreal's Hans J. Hofmann, a paleontologist familiar with the Tuanshanzi layers. "Interpreting photos is always difficult," he adds, "even though the dating of the fossils may be fine."

Uranium-lead isotope dating places surrounding rock at around 1.7 billion years old, somewhat less than half the age of Earth. But that "makes the fossils a good 800 million years older than the next organisms that look clearly like true multicellular algae," says Harvard University algologist Andrew H. Knoll.

Researchers have long hunted for early examples of multicellular plants or animals. Later fossils of clearly multicellular algae abound: They appeared late in Precambrian time, some 800 million years ago. Much earlier fossils of single-celled organisms, twice as old as the Tuanshanzi samples, also exist.

But finding fossils from the intervening period, when the jump to multicellularity probably occurred, poses "a problem," Knoll says. Volcanic heat and land shifts may have destroyed many remnants of early multicellular organisms. Even so, scientists are concerned that so few fossils have been found.

"I think they're true fossils," says Hofmann, "but I'm not convinced that they are truly multicellular."