Long before the reign of complex animals, the Earth's crust stretched and thinned, creating a marine basin in what is now central India. The Vindhyan basin was flooded with sea water, and subsequently filled with sediments. But it is unclear just when these events happened. Radiometric ages from zircons and diamonds, as well as the orientation of magnetic grains, suggest that the basin began to fill between 1.7 and 1 billion years ago. However, purported fossils hidden in the lower rock layers hint at the presence of simple animals that are consistent with a much later origin for the layers — in the Ediacaran or Cambrian periods (about 640 to 500 million years ago).

Credit: © 2009 PNAS

The ensuing debate has seen some researchers question whether these 'fossils' are actually of biological origin, whereas other groups have suggested that the radiometric ages reflect the source of the sediments, rather than the creation of the basin's lower layers. Stefan Bengtson of the Swedish Museum of Natural History and his colleagues have jumped into the fray, with new fossil specimens and new ages for the surrounding rocks (Proc. Natl Acad. Sci. USA doi: 10.1073/pnas.0812460106; 2009).

The miniscule fossils are mostly contained within phosphorous-rich nodes. The nodes revealed the remains of bacterial colonies, which generally show up as small clusters of filaments that form distinctive shapes. The group also found segmented tubes, less than 200 μm in diameter, which could be the remnants of algae. But they failed to find the most contentious microfossils from previous studies: embryos from primitive multicellular animal life. Instead, they suggest that the tiny spherical shapes are air bubbles produced by bacterial activity, trapped in the sticky film covering the microbial mats. Overall, they contend that the fossil assemblage probably represents a pre-Ediacaran ecosystem.

Direct dating of the phosphate-rich material surrounding the fossils using lead isotopes minimized potential sources of contamination. Their analyses converged on an age of roughly 1.65 billion years — well in line with previous mineral-based estimates — leading the group to conclude that the rocks, and the fossils within, are indeed from the Palaeoproterozoic era.

Although the rocks no longer seem to reveal early animals, Bengston and colleagues are quick to point out that the exceptional preservation of the fossils provides a unique window into life 1.6 billion years ago. Interpretation of the segmented tubes as algae would push back the earliest appearance of multicellular eukaryotes by up to 600 million years, and the bacterial clumps show a rare glimpse of pre-Cambrian calcifying cyanobacteria — proving that there is still much to learn from these controversial rocks.