07 June 2009

Pikaia gracilens

The Cambrian period around seven hundred million years ago was one of the most dramatic eras in the history of life on Earth. Before then, as far as we know, all organisms on Earth were microscopic. It was during the Cambrian that evolution finally overcame the hurdle of assembling large, complex, multi-cellular life forms. The result was a tremendous explosion in the number and variety of species, including the first true plants and animals. Some of the new organisms thrived and diversified into whole family trees of successor species, while others failed and died out. This period is actually referred to in biology as the Cambrian explosion.

One of these early life forms was pikaia gracilens, a little water-dwelling creature about an inch and a half in length, resembling a worm with a tail fin -- in fact, its fossils were initially misclassified as being those of some kind of worm. If you had been there at the time, it certainly would not have impressed you as being notably interesting compared with all the other odd new animals and plants you would have seen.

It was not a worm. It was the earliest common ancestor of all of the chordate animals -- that is, all fish, amphibians, birds, reptiles, and mammals.

Many species went extinct during the Cambrian explosion. It is probably pretty much a matter of chance that pikaia gracilens did not. It may have come very close to disappearing, perhaps many times. We'll never know.

If at some point pikaia gracilens had indeed died out, then all the species that later evolved from it -- all the fish, amphibians, birds, reptiles, and mammals in Earth's history -- would simply never have existed. There would have been no sharks, no mice, no elephants, no dinosaurs, no frogs, no eagles, no saber-toothed tigers, no whales, no dogs, no apes, no humans. Perhaps some other Cambrian life form would have diversified into such a vast plethora of species instead, but none of those species would have been like the ones we know. Perhaps eventually a species with enough intelligence to build a technological civilization would have evolved, but it would not have been us, or anything much like us.

How close did it come? Was there ever a time when there were only a hundred pikaia gracilens wriggling through the Cambrian sea? Was there ever a day when there were merely a dozen? Was there ever a moment when just one predator that zigged instead of zagged could have gobbled up the last few specimens in existence?

All that we are, and almost all that we know, might so easily never have been.


Blogger Ranch Chimp said...

Really enjoyed this post.....


08 June, 2009 05:50  
Blogger Zardoz said...

An interesting, if somewhat scary thought that a predator could have zagged instead of zigging thus changing the course of life on this planet! Of course, if that had happened, some other species would have made it to the top of the heap. But what would it have been? Speculation of this sort will keep my imagination going for quite a while! Thanks!

08 June, 2009 10:02  
Blogger vamp said...

Fascinating. Thank you little worm with a fin. You are now my god. ha All hail to finworm.

09 June, 2009 09:27  
Blogger by Michael Boh said...

That was awesome Infidel! Btw, "How the Earth was Made" has been one of my favorite shows lately. I think it's on the History Channel. I'm a big fan of the geologic history of Earth too - fascinating stuff!

09 June, 2009 16:35  
Blogger Infidel753 said...

Thanks for all the comments!

Vamp -- that may be the ultimate in ancestor worship.

09 June, 2009 17:02  
Blogger Rita said...

Thought provoking & informative post. :)

11 June, 2009 08:09  

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