Wrong, the first fossils with a hard shell appeared 580 million years ago. Body impressions, though understandably quite rare, go back much further than the Cambrian. The oldest is over 2 billion years old:<quoted text>
Here is one to throw out there;
The oldest potential fossil evidence for life is fossil bacteria from the Apex Chert of Australia (3.46bya), though these fossils are currently embroiled controversy and may not represent traces of life. The next oldest fossils are well-accepted fossil bacteria and bacterial mats (stromatolites) from South Africa that date to 3.3bya. Thus, the oldest fossil prokaryotes date to 3.3 to 3.5bya.
Meaning it took roughly 1 billion years for any life to even form.
For nearly the next 1-2 billion years, rocks from the Archean have no multicellular life at all, just prokaryotes. The oldest eukaryote fossils are acritarchs dating to about 1.75byr. Then for another 1000 million years(1 billion), there is still no evidence of multicellular life.
So roughly we are down to 750mya and still no evidence of multicellular life. The first fossils resulting in multicellular life are only 580mya and I say “only” for it is a short time for the 4 billion years that had already passed. So we go through roughly 4 billion years with no multicellular life and then in only 550 million years it just all of a sudden ran amuck, exploded and went boom! Producing millions of species. Isn’t that the theory?
The ability to develop a hard shell was probably due to either continued changes of the atmosphere from on of almost zero oxygen to one of roughly 20% oxygen. And the associated chemical changes that went with a different atmosphere.