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The mystery of the Cambrian explosion

Opabinia

550 million years ago, during the Cambrian period, animals appear suddenly in the fossil record. This spectacular phenomenon has been given the name of Cambrian explosion. Why did it happen then, and not before, has been, for over a century, one of the great mysteries of paleontology.

The evolution of life on Earth seems to have been rather discontinuous. Life is likely to have appeared 3,500 to 4,000 million years ago. Not much later, around 3,000 million years ago, photosynthesis appeared. Eukaryotes (cells with nuclei) emerged 2,000 million years ago. From then until the Cambrian explosion, nothing much seems to have happened for 1,500 million years. Then all the types of organization of the animals existing today appear suddenly. Why?

Isaac Asimov

The media have echoed a possible breakthrough that would explain the mystery. The proportion of oxygen in the atmosphere (and therefore in the sea) would have increased suddenly 550 million years ago. This, combined with the appearance of carnivores, would have caused the evolutionary growth of multi-cellular animals. The idea is not new. In 1976, Isaac Asimov published a popular article (Silent victory, reprinted in the book The planet that wasn?t, 1977) where he proposed a similar scenario.

The media usually presents scientific news for the general public in a way that leaves much to be desired. This particular piece of news is a good example. The media tend to hype, and present the news so as to cause the greatest possible impact, often conflicting with professional scientific publications, generally much more nuanced.

In this case, as usual, the media used sharp headlines: "A surge of oxygen explains the origin of the animals." They don?t bother to point out that it has not been proved that the oxygen surge did really happen. What we have is a new theory that combines two previous antithetical theories, and is based on a study of present Polychaete worms, which concludes that there are fewer species of carnivorous Polychaetes where there is less oxygen.

Trilobites

Anomalocaris

Professional scientific news are rarely so conclusive. In this case, other Paleobiologists suggest that, even if a sharp increase in oxygen had actually happened 550 million years ago, just around the evolutionary explosion of animals, we cannot be certain which of those two phenomena caused the other. It could very well be that, unlike what the new theory suggests, the evolution of animals would in some way have caused the increment of oxygen in the atmosphere. Or a different cause could have produced both effects simultaneously. The question is therefore much less clear than the way the media present it.

But let us assume that this theory is correct and actually solves the mystery of the Cambrian explosion. As always happens, the solution to a problem raises new questions, discovers new mysteries. If photosynthesis appeared 3,000 million years ago, why this sudden increase in the proportion of oxygen in the Earth's atmosphere, just 550 million years ago? We don?t know. That's what makes human progress interesting. Every solution to a problem always raises a new problem. Mysteries will never end.

The same post in Spanish

Manuel Alfonseca

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