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USGS CMG InfoBank: First Appearance of Hard Shelled Organisms

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Comment: 08:59 - 10:00 (01:01)
Source: Annenberg/CPB Resources - Earth Revealed - 11. Evolution Through Time
Keywords: "Dee Trent", "hard part", Cambrian, Precambrian, mutation, oxygen, evolution, jellyfish, earthworm, atmosphere

Our transcription: The significance of the Cambrian Explosion is that this marks the first appearance, the widespread appearance, of hard shelled organisms.
Undoubtedly, some of the earlier organisms were Precambrian, mutated and evolved hard parts, but for some reason those hard parts were an adverse thing, probably having to do with availability of oxygen.
Also, those earlier creatures just absorbed oxygen through the tissues of their skin.
If you can imagine a jellyfish or an earthworm, it has a large surface area, through which it can absorb oxygen.
A hard part would inhibit that.
In the early part of the Earth's history, we had much less oxygen in the atmosphere than we do now.
By the Cambrian, it may have been then necessary for animals to have hard parts in order to survive because of other animals eating them.
So if you have a hard shell, you were less attractive and harder to eat, so you might survive a little better than something like an earthworm or a jellyfish.

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