An After the big bang, the stars and planets were forming for billions of years. Earth formed about 4 billion years ago, and then life started about a half billion years later.

Scientists have a lot of ideas about how life might have started. Somehow, little molecules reacted to form big molecules, and bigger molecules, and organized structures of big molecules; and some of the molecules stored information for making more molecules, and some molecules helped the chemical reactions that made the food molecules that were needed to make more big molecules, and fatty lipid membranes formed to hold the molecules in containers.

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That question isn't easy to answer, since the big bang isn't directly involved in the existence of animals. It is indirectly involved; without the big bang, there would be no universe for animals to exist in, but there were about ten billion years between the big bang and the formation of the Earth, and then another four billion before the first animals appeared on Earth.

Animals appeared on Earth after a long period of evolution from single-celled organisms. The details of exactly when and how that happened are still largely unknown, although there is evidence that the Ediacaran period (about 630 to 540 million years ago) saw the evolution of a large number of relatively complex forms of life that probably included the first animals. By the early Cambrian (530 million years ago), animals had already diverged into the major lineages ("phyla") that we know today, although many of those lineages had yet to achieve their modern forms.