These slimy, hungry blobs may have been the Earth's first creepy crawlies

Scientists think they've found evidence of extremely ancient organisms that could crawland wriggle their way through the mud extraordinaryat a time when simple bacteria were previously thought to be the only living things on Earth.

The researchers discovered what appear to be fossilized tunnels boredbyhungry blobs creepingthrough the mud on the floor of a shallow sea 2.1 billion years ago, about a billion years before animals evolved.

Those may have been similar to modern day slime moldssingle-celled organisms that sometimes bunch together into a blob or "slug" to crawlin search of food, reports the international team led by Abderazak El Albaniofthe centre national de la recherche scientifiqueandtheUniversitdePoitiers in France. They published their findings in the Proceedings of the National Academy of Sciences earlier this week.

The tunnels appear near and between thick mats of bacteria that may have madea tasty lunch, said Luis Buatois, a University of Saskatchewan researcher who co-authored the paper.

"That's consistent with the idea of searching for food," he added. He and his wife, fellow University of Saskatchewan researcher M. Gabriela Mangano, are experts in "trace" fossils. While most fossils we're familiar with come directly from the bodies of organisms, trace fossils come from other evidence such as tracks or tunnels.

El Albani contacted Mangano and Buatois a few years ago for help figuring out the origin of strangeworm-likecordsof the mineral pyrite in some2.1 billion-year-old rocks. He had found them snaking among fossilized bacterial mats in a rock formation nearFranceville, Gabon, in Africa. El Albanishared detailed CT scans of the structures, some of which were as long and as thick as a pencil.

"We were immediately amazed by what he had," Buatois recalled.

Earliest multicellular organisms

El Albani proposed in a 2010 study that some of the fossils found in the same rock formation belonged to the earliest multicellular organisms suggesting such complex organisms evolved hundreds of millions of years earlier than previously thought.

What was striking about the worm-like fossils wasthat they seemed to cross different layers of sediment and appeared to have pushed the sedimentaround. That's quite different fromwhat's seen in specimens that are dead or immobile when buried they tend to sit in a single layer.

"We have evidence of organisms that were able to move. That's significant," Buatois said.

What was tricky was trying to imagine what kind of organism could have made the tunnels. They'reunusual, asthey change in width along their length and sometimes fuse together from the same direction impossiblefor a single organism such asa worm, but similar to the tracks left by slime molds made of blobs of cells that can join, separate and reshape themselves.

"The similarities are remarkable," Buatois said.

Watch as Princeton professor explores slime mold:

While they may have been similar to a slime molds, they were almost certainlyunrelatedslime molds didn't evolve until about 1.5 billion yearslater.

Pyrite proof?

The idea that the fossils were made by biological organisms is supported by geochemists such as Kurt Konhauser, a University of Alberta professor who also co-authored the study.

Organisms typically ooze slime as a lubricantas they tunnel through mud, Konhauser said. When the tunnel is later filled in with new sediment, bacteria eat that slime, producing a mineral called pyrite that's otherwise only found in volcanic rock: "You don't get pyrite unless you get bacteria eating organic carbon."

And pyrite is what the worm-like featureswere made of.

A big question that remains is whether the organisms that made the tunnels had a lasting impact on the evolution of living things.

Konhauser suspects they didn't: "Most likely, whatever it was went extinct."

That's because it lived at a very special time in Earth's history, shortly after photosynthetic bacteria evolved and filled the atmosphere with about half the concentration of oxygen we have today way more than there was before.

That didn't last, though. Oxygen levels soon plummetedand didn't rise again significantly until around 650 million years ago, which is when multicellular life really took off and became obvious in the fossil record.

Evolutionary biologists thinkthat meansoxygen was a requirementfor complex life.But then why did complex life not evolve thefirst time oxygen levels peaked?

"What this shows," Konhauser said, "is that in fact, it's quite likely that you did get complex life at that time."

Controversial discovery

Not everyone is convinced that the fossils show evidence of multicellular organisms or organisms that could move.

Juergen Schieber, a geology professor at Indiana University who has studied how slime trails left by burrowing organisms get fossilized, called both claims "dubious."

In an email, he agreed that organic matter would have been needed to form the pyrite cords, but suggested they were more likely to be rolled up bacterial mats.

Buatois said that was one possibilityhis team considered, but it didn't seem to match up with the fossils' positions relative to the layers of sediments.

"I know that it's a controversial issue," he said. "But we've been through every possible alternative explanation, and we rejected them all."