The animal kingdom groups an enormous variety of organisms, from some small ones like sponges and ants, to other very large ones, like elephants and whales. Different shapes, colors, structures and complexity for approximately one million known species that inhabit our planet.
Despite the huge differences, there are a series of general characteristics shared by all of the animals that inhabit the Earth. They are all eukaryote multicellular organisms, meaning, their cells have a defined nucleus.
In addition, these same cells participate in specific tasks and, in more complex organisms, form tissues and organs. Most have a nervous system that enables them to receive and respond to a variety of stimuli, as well as a reproductive system with specialized cells to perpetuate the species.
But despite counting with important survival systems, no animal has been capable of overcoming the latent threat of their definite extinction. No species is safe, and the vulnerability of many goes up day after day, mainly due to the great impact, accentuated as of the Industrial Revolution, that human activities have been causing the planet.
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