The Weird Youth of the Animal Kingdom (Slide Show)


Paleontologists have found traces of animal life dating back at least 635 million years. Those earliest animals may have been like today’s sponges, rooted to the sea floor and filtering food particles from the water.

Over the next 100 million years or so, new kinds of animals emerged. Some were recognizable members of living groups of animals, while others were so bizarre that paleontologists suspect they belonged to long-extinct lineages. And then, around 520 million years ago, the fossil record of animals starts to roar like a firehose switched from a trickle to full blast. Many of the oldest known members of living animal groups–including our own–appear during the Cambrian Period. But the Cambrian fossil record is also rife with forms only distantly related to animals on Earth today, some of which were so weird that the sight of a reconstruction of the creatures made scientists burst out laughing.

Many people have become familiar with this period of evolution through Steven Jay Gould’s 1989 influential book, Wonderful Life. In the 24 years since then, scientists have learned a lot more about the Cambrian. Two of the leading experts on the period, Doug Erwin of the Smithsonian Institution and James Valentine of Berkeley have collaborated on a new book, The Cambrian Explosion: The Construction of Animal Biodiversity, in which they synthesize evidence, both old and new, about this exceptional chapter in animal evolution.

The so-called Cambrian Explosion probably had many fuses. Erwin and Valentine explain how the Earth was undergoing drastic changes in the millions of years leading up to the flowering of the animal kingdom, with global ice ages and a burst of oxygen flooding the oceans. The stage was set for big, active creatures to evolve. As predators emerged, their prey became better defended with spikes and shields; the predators in turn became even deadlier. The animals changed their environment–burrowing animals, for example, pierced the sea floor with countless tunnels. As the environment changed, new kinds of animals evolved that could occupy new niches. The animal kingdom became both physically and ecologically complex.

Written By: Carl Zimmer
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  1. I think the Cambrian explosion is exaggerated too often. It’s based on an unusual fossil deposit which includes the soft tissue fossils of extinct animals. This is incredibly rare. But fossils show, at the very least, the latest that a species or lineage could exist at, and they’re not the only means of measuring the relationships between lineages.

    For instance, if you measure using the genetic clock the splitting of the lineages between chordates and fellow deuterostomes like the common starfish, you find that the separation would already have occurred about 742 million years ago, long before the Cambrian explosion. You also find that arthropods like Drosophila and chordates like Homo sapiens could possibly trace their ancestry back to a couple of hundred million years before the Cambrian, that chordate ancestors and those of comb jellies like Venus’ girdle could have split off over a billion years ago, and that plants like the giant sequoia split off about 1.3 billion years ago.

    There are also some details in The Ancestor’s Tale on the Cambrian “Explosion”, under the Velvet Worm’s tale. My basic point is that, instead of arising quickly, the ancestors of modern phyla were already going far along their separate ways when the likes of Wiwaxia, Hallucigenia, and Anomalocaris came along.

  2. Ask a creationist to name the day of the Cambrian Explosion, which they sometimes consider another name for the six days of genesis. Give the margin of error as a percentage, and explain the source of their claim for accuracy.

  3. In reply to #2 by psych(ot)ic:

    If a Creationist told me that any god had created these, I would have to ask what it was taking at the time.

    that eternal theological dilema. if god created everything, who was his dealer?

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