A Ball on the Seafloor

In the summer of 2007 during an expedition off the Bahamas, a team of scientists made an unusual discovery. On the seafloor more than 2,000 feet down, a remote submersible vehicle recorded video of what one of the researchers, Mikhail V. Matz of the University of Texas, described as a "brainless, eyeless, colorless ball completely covered in mud."

What's more, the researchers discovered that these balls, which were about an inch in diameter, appeared to have left tracks on the seafloor, as if they were rolling slowly under their own power.

Now, in a paper in Current Biology, Dr. Matz and colleagues have described their finding in more scientific terms: it's a giant amoeba of the genus Gromia, a transparent envelope of protoplasm with a water-filled center that helps it maintain its spherical shape. And the researchers say the creature does actually roll, pulling itself along by exuding bits of protoplasm from apertures in its surface that latch onto the seafloor and consume nutrients.

But this self-propelled living golf ball is more than a curiosity. The researchers realized that its tracks were very similar to grooves found in seafloor fossils dating back more than 550 million years. So the rolling amoeba casts doubt on scientists' understanding of how life on Earth diversified.

Many scientists have argued that multicellular organisms that have two halves that mirror each other occurred before the so-called Cambrian explosion of diverse life forms 542 million years ago. One of the best arguments for this was the fossilized tracks. It seemed clear that only a complex, bilaterally symmetric creature could maneuver under its own power and leave such tracks.

But the Gromia organism is unicellular and not bilaterally symmetric, and yet it leaves very similar tracks. "This is really a hard hit for the school of thought that animals slowly evolved in the Precambrian," Dr. Matz said.

Will

The University of Texas