Attack Of The Protists

Friday, November 21, 2008

Gromia sphaerica takes a stroll on the ocean floor.

When we think of single-celled organisms, we invariably think of tiny little beasties that can only be seen under a microscope. However, there are single celled organisms that are visible to the naked eye, as the above image shows. These organisms are part of a group of organisms called protists. Algae is another example of a protist. As you can see, some are so big - the size of a grape - that they can leave tracks! These huge one-celled creatures were recently discovered on the ocean floor near the Bahamas by biologist Mikhail Matz and colleagues from The University of Texas at Austin.

It is these tracks, as a matter of fact, that have biologists who study evolution intrigued. Fossil grooves and tracks found from the Precambrian era, as early as 1.8 billion years ago, have always been attributed to early evolving multicellular animals. Says Matz, "If our giant protists were alive 600 million years ago and the track was fossilized, a paleontologist unearthing it today would without a shade of doubt attribute it to a kind of large, multicellular, bilaterally symmetrical animal. We now have to rethink the fossil record."

By "bilaterally symmetrical", he means that most animals, from humans to insects, can be roughly divided into halves that are mirror images.  Bilateral animals appeared in the fossil record in the early Cambrian about 542 million years ago, quickly diversifying into all of the major animal groups, or phyla, still alive today. This rapid diversification, known as the Cambrian explosion, puzzled Charles Darwin and remains one of the biggest questions in animal evolution to this day. 

Very few fossils exist of organisms that could be the Precambrian ancestors of bilateral animals, and even those are highly controversial. That's because lifeforms during that ancient time were soft-bodied, and unable to leave much of a fossil. Also, since that time billions of years ago, much of Earth's original rock has been metamorphosed, obscuring their origins. Fossil traces (such as tracks) are the most accepted evidence of the existence of these proto-animals.

With their giant deep-sea protist find, Matz and his colleagues argue that fossil traces alone cannot be used  as evidence that multicellular animals were evolving during the Precambrian, slowly setting the stage for the Cambrian explosion. As a matter of fact, he states ""I personally think now that the whole Precambrian may have been exclusively the reign of protists.  Our observations open up this possible way of interpreting the Precambrian fossil record." So, these protists may be the ultimate "living fossils" - essentially unchanged for almost two billion years.

So how do these blobby giant protists leave tracks? How do they travel? While they haven't yet been observed in action, Matz says the protists probably move by sending leg-like extensions, called pseudopodia, out of their cells in all directions. The pseudopodia then grab onto mud in one direction and the organism rolls that way, leaving a track.

Actually, that's remarkably similar to how I get out of bed in the morning.

Source: Current Biology