Animals are multicellular, heterotrophic eukaryotes that build tissues from embryonic layers.

It is difficult to list characteristics shared by all animals because virtually every criterion has exceptions.

When put together, however, numerous animal traits adequately define the group for our consideration. The mechanism of the feeding of animals differs from that of plants and fungi. Plants are autotrophic eukaryotes that produce organic compounds via photosynthesis. Fungi are heterotrophic organisms that grow on or near their food and eat by absorption (typically after releasing enzymes that break down the food outside their bodies).

Animals are eukaryotes, and they, like plants and the majority of fungi, are multicellular. Animals, unlike plants and fungi, lack the structural support of cell walls. Proteins beyond the cell membrane, on the other hand, offer structural support to animal cells and connect them to one another. Collagen, which is not present in plants or fungus, is the most prevalent of these proteins.

Most animals' cells are arranged into tissues, which are groupings of similar cells that work as functional units. Muscle tissue and neural tissue, for example, are in charge of moving the body and transmitting nerve impulses, respectively. The capacity to manipulate and convey nerve impulses is the foundation.

• The term blastula refers to a time period of the development of most animals, cleavage leads to the formation of a multicellular embryonic stage.

The majority of animals reproduce sexually, and the diploid stage typically dominates the life cycle. Unlike plants and fungi, sperm and egg cells are generated directly via meiotic division in the haploid stage.

A tiny, flagellated sperm fertilizes a bigger, nonmotile egg in most animal species, resulting in a diploid zygote. The zygote then experiences cleavage, which is a series of mitotic cell divisions with no cell growth in between. Cleavage occurs throughout the development of most animals and results in the creation of a multicellular embryonic stage known.

Biologists have discovered 1.3 million living animal species to far, with estimates of the true number running considerably higher. From corals to insects to crocodiles, this enormous variety comprises a stunning spectrum of morphological variation. According to much research, this tremendous variety arose within the last billion years.

For example, scientists discovered 710-million-year-old sediments bearing chemical evidence of steroids, which are now predominantly generated by a certain kind of sponges. Because sponges are animals, these “fossil steroids” indicate that animals existed 710 million years ago.

DNA studies typically support this fossil biochemical data.

Scientists studying how animals evolved from single-celled predecessors have discovered that the emergence of multicellularity necessitates the creation of novel mechanisms for cells to adhere (attach) and signal (communicate) to one another.

Researchers compared the genome of the unicellular choanoflagellate Monosiga brevicollis to that of typical animals in order to understand more about such processes. This study discovered 78 protein domains in M. brevicollis that were previously only known to exist in mammals.

A domain is a protein's important structural or functional area. M. brevicollis, for example, possesses genes that encode domains of certain proteins (known as cadherins) that play a role in cell adhesion.

The image attatched shows early signs of predation. This 550-million-year-old Cloudina fossil shows signs of being attacked by a predator that bore through its shell.

Animal evolution has lasted more than half a billion years (as shown in the image attatched).

Animals first appeared around 700 million years ago, according to fossil biochemical evidence and molecular clock studies.

According to genomic studies, critical milestones in the evolution of animals included novel methods of utilizing proteins encoded by genes present in choanoflagellates.

Animals can be distinguished by their "body plans."

Animals can be asymmetric, or they can have radial or bilateral symmetry. Animals that are bilaterally symmetrical have dorsal and ventral sides, as well as anterior and posterior ends.

Eumetazoan embryos can be diploblastic (two germ layers) or triploblastic (three germ layers) (three germ layers). A pseudocolor or a genuine coelom can be found in triploblastic animals having a body cavity.

Patterns of cleavage, coelom formation, and blastopore destiny frequently change between protostome and deuterostome development.