Overview: Welcome to Your Kingdom
Biologists have identified 1.3 million living species of animals.
Estimates of the total number of animal species run far higher, from 10 to 20 million to as many as 100 to 200 million.
Concept 32.1 Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers
There are exceptions to nearly every criterion for distinguishing an animal from other life forms.
However, five criteria, taken together, comprise a reasonable definition.
Animals are multicellular, ingestive heterotrophs.
Animals take in preformed organic molecules through ingestion, eating other organisms or organic material that is decomposing.
Animal cells lack cell walls that provide structural support for plants and fungi.
The multicellular bodies of animals are held together by extracellular structural proteins, especially collagen.
Animals have other unique types of intercellular junctions, including tight junctions, desmosomes, and gap junctions, which hold tissues together.
These junctions are also composed of structural proteins.
Animals have two unique types of cells: nerve cells for impulse conduction and muscle cells for movement.
Most animals reproduce sexually, with the diploid stage usually dominating the life cycle.
In most species, a small flagellated sperm fertilizes a larger, nonmotile egg.
The zygote undergoes cleavage, a succession of mitotic cell divisions, leading to the formation of a multicellular, hollow ball of cells called the blastula.
During gastrulation, part of the embryo folds inward, forming layers of embryonic tissues that will develop into adult body parts.
The resulting development stage is called a gastrula.
Some animals develop directly through transient stages into adults, but others have a distinct larval stage or stages.
A larva is a sexually immature stage that is morphologically distinct from the adult, usually eats different foods, and may live in a different habitat from the adult.
Animal larvae eventually undergo metamorphosis, transforming the animal into an adult.
Animals share a unique homeobox-containing family of genes known as Hox genes.
All eukaryotes have genes that regulate the expression of other genes.
Many of these regulatory genes contain common modules of DNA sequences called homeoboxes.
All animals share the unique family of Hox genes, suggesting that this gene family arose in the eukaryotic lineage that gave rise to animals.
Hox genes play important roles in the development of animal embryos, regulating the expression of dozens or hundreds of other genes.
Hox genes control cell division and differentiation, producing different morphological features of animals.
Hox genes in sponges regulate the formation of channels, the primary feature of sponge morphology.
In more complex animals, the Hox gene family underwent further duplication.
In bilaterians, Hox genes regulate patterning of the anterior-posterior axis.
The same conserved genetic network governs the development of a large range of animals.
Concept 32.2 The history of animals may span more than a billion years
Various studies suggest that animals began to diversify more than a billion years ago.
Some calculations based on molecular clocks estimate that the ancestors of animals diverged from the ancestors of fungi as much as 1.5 billion years ago.
Similar studies suggest that the common ancestor of living animals lived 1.2 billion to 800 million years ago.
The common ancestor was probably a colonial flagellated protist and may have resembled modern choanoflagellates. Neoproterozoic Era (1 billion–542 million years ago)
Although molecular data indicates a much earlier origin of animals, the oldest generally accepted animal fossils are only 575 million years old.
These fossils are known as the Ediacara fauna, named for the Ediacara Hills of Australia.
Ediacara fauna consist primarily of…