Princeton scientists find earliest evidence of animal life

At first, Adam Maloof thought they were just mud chips.

But after seeing more and more of the red flecks in an ancient section of rocky mountains in South Australia, he noticed they came in odd shapes: tiny rings, anvils, and wishbones.

The Princeton University geologist and colleague Catherine V. Rose wondered: Could the flecks, dated to 650 million years ago, be the remnants of living things?

After months of sophisticated analysis back in the United States, the scientists made a dramatic announcement Tuesday:

They think the odd shapes are the fossils of sponge-like creatures - the earliest animals yet discovered by more than 70 million years.

In their paper in the journal Nature Geoscience, Maloof, Rose, and colleagues do not claim they have found conclusive proof of animal life. But they, and other experts who have seen their results, are having a hard time imagining what else the curious, centimeter-wide blobs could be.

Andrew H. Knoll, a Harvard University paleontologist familiar with the findings, praised the authors for their painstaking work and predicted it would inspire further discoveries.

"While I think it falls short of absolute proof, it tells me that a whole bunch of people ought to get out and scour rocks of that age," Knoll said.

Precious few fossils have been found from before 550 million years ago, largely because the earliest animals were soft-bodied and did not preserve well. Yet scientists have been able to trace life back to that era with molecular "clocks" - working backward from the genomes of modern species to deduce when earlier creatures emerged.

And molecular-clock estimates for the earliest sponges, it turns out, coincide with the age of the physical specimens that the Princeton team found in Australia.

Maloof, an assistant professor in Princeton's department of geosciences, called the match "really gratifying."

"With new technologies, and literally just discovery-based science - walking around in the mountains - we're discovering more and more fossils that are slowly filling in the gaps," he said.

The specimens offer new insight - and raise questions - about a prehistoric era that scientists are only starting to understand.

Not long after these sponge-like creatures appeared on Earth, the planet is thought to have plunged into a global ice age - "snowball earth."

Cold-tolerant bacteria and algae survived, but it is unclear how animals could have managed. Some scientists have proposed the existence of "refugia" - havens where temperatures were not quite as cold, perhaps near volcanoes.

Animals are relative latecomers to the story of life. The first single-celled beings are thought to have lived 3.8 billion years ago. The first multicellular creatures - algae - came along about 1.2 billion years ago.

Animals, including sponges, are multicellular, too. But they have other distinguishing characteristics: They have more than one kind of cell, and each cell contains a nucleus. Unlike plants, they do not have rigid cell walls.

Maloof said he spent nearly a year figuring out how to penetrate the secrets of his rock samples. The reddish flecks were of the same density as the surrounding rock, so traditional tools such as chisels and X-rays were no good.

Eventually, the team members hit on a labor-intensive method, using a milling machine to grind away ultrathin slices of the rock, taking images of each layer as they went. Nearly 500 such images were then reassembled into a three-dimensional picture using computers, with the help of Situ Studio, an architectural design and fabrication firm in Brooklyn, N.Y.

Many of the flecks were fragmented, but at least two complete specimens emerged on the computer screen: asymmetrical blobs, less than the width of a man's thumb, crisscrossed with canal-like tunnels like those found in sponges.

Maloof's team sorted through all sorts of explanations for what the specimens might be: algae, tunnels burrowed by worms, perhaps some chemical process that had nothing do with living things. One by one, these other possibilities were ruled out. Algae, for example, do not have interior tubes as large as the ones in Maloof's samples.

The team also enlisted the help of Douglas Erwin, a senior scientist at the Smithsonian National Museum of Natural History, who was initially skeptical.

"Most paleontologists when they see something like this are suspicious," Erwin said. "I had to sort of grudgingly concede that Adam might actually have something."

The sponge-like phenomena were found in 2007, embedded in rocks that were underwater long ago.

The Princeton geologists had traveled to Australia to study ancient climate; they were not even looking for fossils.

Now, Maloof is determined to find more.

He also plans to build a facility at Princeton to automate and speed up the grinding and imaging. The first time, it took 14 days to get through an inch and a half of rock.

Now that his reddish flecks have a new meaning, others will surely join him in the hunt, said the Smithsonian's Erwin:

"It gives us something to look for."