Jul 02 2010

Oldest Multicellular Life

Nature recently reported the finding of what may be the oldest examples of multicellular life – 2.1 billion years old. These are centimeter-sized “cookie” shaped fossils that show an internal structure that may reflect the complex organization of a multicellular creature.  If true, this would push back the oldest example of multicellular life by about 200 million years.

Multicellular life, like plants and animals, did not really take off until the Cambrian Explosion – about 530 million year ago. At this time all of the basic body plans that make up modern life emerged. It is not clear exactly how long it took for all of this diversity to evolve, but it may have been as little as 5 million years. This rapid expansion was at least partly fueled by a rise in atmospheric oxygen.

However, the Cambrian Explosion is also partly a fossil artifact. It represents the first occurrence of multicellular life with hard parts that fossilize – a turning on of the fossil record more than of plants and animals. This creates the question of what the real history of multicellular life was prior to the development of hard parts. We have a few foggy windows into the pre-Cambrian era, and more questions than answers. What evidence we have found likely represents random pieces to the puzzle that happened to leave fossil remnants we have discovered, rather than anything approaching a complete picture of pre-Cambrian life.

One example is the Ediacara fauna – the first clear large multicellular creatures from 565-543 million years ago. However, these creatures have significant differences in form from later multicellular life – they exist mainly as sheets of cells, rather than three dimensional structures. They may represent an experiment in multicellular design that failed, or at least was pushed aside by later Cambrian life. It is still somewhat controversial what the relationship between the Ediacara biota and later Cambrian life is.

There are hints in the fossil record of even earlier complex life forms, but their true nature remains a mystery. The Nature article references Grypania spiralis, a 2 billion year old fossil (although some references say 2.1 billion years old) that looks like a spiral over a centimeter long. It is not clear if it is a large bacterium, a colony of bacteria, or a eukaryotic alga. Fossils like Grypania also raise the question of whether or not these early fossils were just colonies of single-celled creatures or true multicellular creatures, or if colonies were in fact a stepping stone to true multicellularity.

Now we have another example of early life – the Gabon fossils. The authors of this new paper write:

The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signaling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45–2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.

As I said above, the Cambrian explosion occurred during a period of increased oxygen in the atmosphere – but that was the second such increase. A previous increase occurred about 2.45-2.32 billion years ago, right before the time of the Gabon fossils. This is only suggestive, but may indicate a similar expansion of life at this time.

The authors are also obviously addressing the question of whether or not these Gabon “cookies” were colonies or multicellular creatures or precursors, suggesting that their structure indicates aspects of true multicellularity. If these fossils do represent multicellular creatures, that raises the question of whether or not they were the ancestors to Cambrian creatures or represent another early experiment and dead end.

This is an awesome find, and it is interesting that multicellular creatures may stretch so far back into the history of life on earth – a full 1.6 billion years prior to their explosion in the fossil record.

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