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The Cambrian Explosion
For its first four billion years, earth was populated, initially, only by prokaryotic organisms, and later by very simple metazoan life forms. This was the Precambrian Period, and there were no rabbits.
But about 540 million years ago, the diversity of marine animals suddenly expanded dramatically, an event known as the Cambrian Explosion or (less dramatically) the Cambrian Radiation.
Recall that the (relatively) rapid diversification of life forms from a single ancestor is known as adaptive radiation.
The Cambrian Explosion saw the evolution of modern animal phyla--and even more that are now extinct--from simple, ancestral forms.
Molecular clock studies suggest that...
the last common ancestor of Animalia and Fungi existed approximately a billion years ago.
the last common ancestor of all animals existed between 800-675 million years agoM
Fossil evidence does not go back that far (only about 550 million years), but that doesn't mean the ancestors didn't exist at an earlier time.
Eumetazoans are defined by gastrulation and the presence of true embryonic germ layers (endoderm, ectoderm, mesoderm) that develop into true tissues.
Eumetazoans can be broadly divided into two main groups, based on body similarity and number of germ layers:
radially symmetrical and diploblastic (seen in the earliest, most ancient animals) OR
bilateral symmetrical and triploblastic (seen in the more recently derived animals)
Radially Symmetrical Animals
These diploblastic organisms are among the simplest of modern animals.
In the radially symmetrical animals we first see...
true plane of symmetry: radial (sometimes modified as biradial
ectoderm (becomes the epidermis in the adult)
endoderm (becomes the gastrodermis in the adult)
mesogloea (gelatinous layer between epidermis and gastrodermis)
Note: When mesogloea contains cellular components, it is known as mesenchyme. A few derived cnidarians have mesenchyme.
These simplest of all animals are of uncertain evolutionary affinity. They are little more than a jelly-like plate of interdependent cells exhibiting the beginnings of radial symmetry.
There is only one recognized extant species, Trichoplax adhaerens.
Paired rows of cilia along the body axis beat in waves to propel the animal through the water.
Their tissues are colorless and translucent, with the same refractive index as water.
This makes them quite cryptic in their natural habitat, and well concealed from predators.
But when the light catches them just right...