This is really a question of definition. At what point do you say that something is an animal?
The earliest complex multi-cellular life that we know of are the Gabon fossils, found in West Africa last year. Last time I read about them they weren’t entirely sure if there were multi-celled creatures, or just colonies of lots of single-celled creatures stuck together.
Hello Maranna! The very first living creatures on earth with single-celled microbes, that lived over 3.5 billion years ago. Things we would call animals arose much much later. Multicellular organisms first appeared around 1.2 billion years ago. These organisms mostly consisted of cell colonies of limited complexity as Drew stated. The first true animal lineages to diverge (if you define an animal as a multicellular organism – Drew is right definitions are important!!) were sponges and comb jellies. Now this doesn’t tell us that the first animal was a sponge or a comb jelly, but it does tell us that comb jellies and sponges are closely related to what that first animal was. So we can find out a lot about the first animals by studying these simple creatures!
The honest answer is we don’t know but our best guess based on fossil evidence is as the others have said sponges and comb jellies. Life almost certainly started as single celled organisms but personally I don’t class them as an animal! Who knows though, we might find a fossil that is even earlier animal.