The first major diversification of multicellular eukaryotic organisms corresponds to the time of the thawing of snowball Earth.
The first multicellular organisms were colonies.
Some cells in the colonies became specialized for different functions.
Such specialization can be seen in some prokaryotes.
For example, certain cells of the filamentous cyanobacterium Nostoc differentiate into nitrogen-fixing cells called heterocysts, which cannot replicate.
The evolution of colonies with cellular specialization was carried much further in eukaryotes.
A multicellular eukaryote generally develops from a single cell, usually a zygote.
Cell division and cell differentiation help transform the single cell into a multicellular organism with many types of specialized cells.
With increasing cell specialization, specific groups of cells specialized in obtaining nutrients, sensing the environment, etc.
This division of function eventually led to the evolution of tissues, organs, and organ systems.
Multicellularity evolved several times among early eukaryotes.
Animal diversity exploded during the early Cambrian period.
Most of the major phyla of animals appear suddenly in the fossil record in the adaptive radiation known as the Cambrian explosion.
Cnidarians (the phylum that includes jellies) and poriferans (sponges) were already present in the late Precambrian.
However, most of the major groups (phyla) of animals make their first fossil appearances during the relatively short span of the Cambrian period’s first 20 million years.
Molecular evidence suggests that animal phyla originated and began to diverge between 1 billion and 700 million years ago.
At the beginning of the Cambrian, these phyla suddenly and simultaneously increased in diversity and size.
Plants, fungi, and animals colonized the land about 500 million years ago.
The colonization of land was one of the pivotal milestones in the history of life.
There is fossil evidence that cyanobacteria and other photosynthetic prokaryotes coated damp terrestrial surfaces well more than a billion years ago.
Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-15