Published online 20 November 2008 | Nature | doi:10.1038/news.2008.1242

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Seabed tracks suggest new origin of animal life

First evidence that earliest fossils could be attributed to protozoans.

The discovery of tracks on the sea floor made by a giant single-celled organism could prompt palaeontologists to reassess the origins of animal life on Earth.

The tracks, found by biologists on a research dive around the Bahamas, measure up to 50 centimetres long and are thought to be made by Gromia sphaerica, a marshmallow-like protozoan measuring around 30 millimetres in diameter, as it moves along the sea floor. The researchers say the furrows bear a "remarkable resemblance" to Precambrian fossilized tracks, dating back to 1.8 billion years ago.

Gromia sphaericaCould these Precambrian fossilized tracks (left) have been made by a giant single-celled protozoan like G. sphaerica (pictured with a shrimp, right)?S. Jensen / M. Matz

These trails have commonly been attributed to early bilaterians — bilaterally symmetrical multicellular animals, from which today's huge diversity of macroscopic animal life on Earth is thought to have originated. But Mikhail Matz of the integrative biology department at the University of Texas in Austin, and lead author of the paper, says his team's discovery suggests that early trace fossils could have been made by single-celled protozoans instead.

That, he believes, calls for a rethink of how animal life first appeared on the planet.

Palaeontologists commonly accept that the Cambrian explosion — the sudden appearance of bilaterians 542 million years ago — is an artefact of the fossil record and that bilaterians existed long before the Cambrian era and evolved slowly. But because no animal fossil records from the early Precambrian period have been found — only fossil tracks — the true origins of multicellular animals is not known.

"Protozoans are under-estimated players in the history of macroscopic animal life," Matz says. "This is the first evidence that organisms other than multicellular animals can produce such traces. We now realize that they can be large, motile and trace-producing, and therefore may be responsible for much of the fossil record currently attributed to more familiar macroscopic organisms, such as multicellular animals."

"This possibility is especially important for inferring the shape of life on Earth over time, from the earliest evidence of macroscopic animal activity to the appearance of indisputable Cambrian fossils of multicellular animals," he adds.

Making tracks

The team did not directly observe the protists moving but say several observations they did make "virtually exclude" the possibility that the trails (also visible in this video) are due to currents or sediment slides moving the organisms around. They observed that the protist's body is nearly neutrally buoyant and a current dragging such an object across the sea floor would not make the tracks they observed. The tracks also successfully negotiate dips in the sea floor, and vary between different individuals. The team's work is published in Current Biology1.

Matz intends to study the protist in more detail because very little is known about its behaviour, physiology or ecology. The only other place it has been found is in the Arabian Sea. He hopes the findings will prompt researchers to now look for signs of a genetic mechanism that could provide evidence for the Cambrian explosion.

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Adolf Seilacher, a palaeontologist at Yale University in New Haven, Connecticut, says he is "very pleased" with the finding because it confirms his 2003 hypothesis that the earliest known animal fossils from the end of the Precambrian period were protozoan and not multicellular animals2. "I really hope this finding reopens the debate," he adds.

But Mary Droser, a professor of palaeobiology at the University of California in Riverside, says it is unlikely that protozoan tracks made during the Early Cambrian would be preserved because there would probably have been extensive disturbance of the seabed by plants and animals.

"However, the general assumption has been that these types of complex trace fossils had to have been made by bilaterians because we had no evidence to the contrary," adds Droser. "We can't say this anymore. The story has gotten more interesting." 

  • References

    1. Matz, M. et al. Curr. Biol. doi: 10.1016/j.cub.2008.10.028 (2008).
    2. Seilacher, A ., Grazhdankin, D . & Legouta, A . Paleontol. Res. 7, 43-54 (2003).
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