This group of unique organisms, though, they themselves went extinct.
They were relatively short-lived, and
they were replaced by a group of organisms that were Metazoans that had skeletons.
And the skeletons at first were all exoskeletons.
They were on the outside of the body of the organism.
And then, as time evolved through the Phanerozoic, then we saw the advent of
Metazoans being able to synthesize and secrete a skeleton that was interior.
And we call those vertebrates.
So, we move from an invertebrate Earth to an Earth that had both vertebrates and
invertebrates.
And we see how that structural design feature allowed them
to really do well in these evolving Earth environments,
both in the terrestrial environment and in the oceans.
Within that grouping of geological time that goes from the 543
million year Cambrian explosion through the modern, we call that the Phanerozoic.
And that Phanerozoic is broken into three primary groups of what we call Fauna.
And the Fauna are the groupings of animals.
The first grouping of animals was called the Cambrian Fauna.
The second grouping of animals was called the Paleozoic Fauna.
And the third grouping of animals was called the Modern Fauna.
And then, the changes sequentially going from the Cambrian Fauna
into the Paleozoic Fauna into the Modern Fauna, it was benchmarked.
It was truncated.
t was defined by Earth processes.
One of the most important of which was meteor impacts.
So, when large meteors slammed into the Earth,
they caused a lot of organisms to go extinct.
And each time those massive extinction events took place,
then nature would respond by, once the catastrophe took place and
some geological time went forward, then new organisms evolved and moved into those
ecological zones that were previously inhabited by another type of organism.
And we see that there's this basic threefold structure of having organisms
respond to new and open ecospace in which they could radiate and
evolve, and then have the whole system reset by meteor impacts.
So, the Cambrian Fauna, the Paleozoic Fauna and the Modern Fauna.
And that's the template of the evolution of multicellular
organisms with skeletons, both internal and external.
That defines and sculpts the shape of one of the primary three
branches of life, which are the Eukarya.
Moving forward from here, we want to then start to look at the idea that
that Eukarya branch in history was also interfacing all the time,
not only with a changing planet, but also with microbes, bacteria and archaea.