what is the single most important evolutionary change that occured in the animal kingdom?

Answer 1

An adaptation specific to animals so not in plants or fungi. That must be the evolutionary step that split the animals from the plants ~1.6 billion years ago. Plants are autotrophs while animals are heterotrophs. ~20% of the ancestral animal genes originated after animals diverged from plants and fungi. This led to all subsequent body specializations. Once both became multicellular the body plans organization utterly diverged in response to their respective trophs.

Animals have highly specialized body plans with complex organ systems. They are physically more complex than other kingdoms because they are heterotrophic so needed to augment their mobility to actively seek their resources, they need sensory functions. Autotrophs acquire their resources without directed movement and rely on distribution of functions throughout the body to maximize exposure to their food source, usually sunlight. So one intense selective pressure has been towards efficient acquisition of those resources.
Animal body complexity has increased to produce enormous variation of specialized organs from differentiating germ tissue layers. "Animals divide into two groups, sponges and eumetazoans. The eumetazoans consist of comb jellies, cnidarians such as sea anemones, and bilaterians, which include everything else: limpets, lions, lobsters, and us."
So I see that first first step towards general body organization resulting in symmetry, anteroposterior axis, dorsoventral axis, germ layers, segmentation, neurons, and skeletons as the pivotal delineating step to becoming distinctly multicellular animals. This pivotal change occurred in these first novel genes & set the stage for the evolution of highly organized tissues, notably nerves and muscles.

Answer 2

The single most important evolutionary change occured, I think, when we were very much not like "animals" at all - the invention of DNA - the ability for the reproducing cell to keep an informational record encoded in chemical base-pairs of the ongoing evolutionary experiment from one generation to the next. If then we speak of just "animals," I'd say the development of sexual dimorphism. The existence of two reproducing sexes really mixes up the gene pool and often creates resistance to genetic anomalies and disease that would not otherwise exist.

Answer 3

Life began some 3.5billion years ago and, once it had started, evolution changed it over time. All species alive today have gone through 3.5billion years of evolution and are adapted to their environment. Consequently all species have evolved for the same time for the same reasons and continue to evolve. No species has undergone the "most evolutionary change", certainly not humans. There is nothing special about us. Remove humans from the planet and everything else will get on fine without us, probably better.

Answer 4

I'd say the development of a third body tissue layer, or germ layer.
In development, the simplest animals - sponges, have only one germ layer, and jellyfish, and corals have two germ layers (they are "diploblastic"): the endoderm and ectoderm.
The more complex animals are triploblastic - with an endoderm, mesoderm, and ectoderm. The development of the mesoderm allowed the formation of an internal body cavity, the coelum, which paved the way towards the circulatory system, and many of the internal organs.

Answer 5

In regards to human evolution?

I think this is a vague question. But maybe my guess would be aerobic respiration, which allowed us to produce more ATP... with more useable energy more things are possible...

Answer 6

Amniotic egg,
Language,
Cephalization.