|Tsuneaki Asai's protoplasts|
Why are there animals?
Or rather, why are there multi-cellular creatures?
Life has been on Earth for three and a half thousand million years, give or take. For either two and a half, or three thousand million years, that life has been in the form of single celled creatures.
The standard answer is, well, there are advantages to being a multi-cellular creature. It means you are bigger than a single celled predator, which makes it harder for you to be eaten. It means that you can glom together, share resources, communicate about the environment. For the same reason that a cell has a cell wall, producing a contained environment that it can fashion into whatever suits its chemistry best, so a multicellular creature can do the same, only bigger, with more possibilities. It means you can form a gut, and take full advantage of the things you consume. It means you can specialize your cells, so they can perform different tasks better - better than a general purpose cell. So there are all sorts of advantages.
And then, it turns out, many single celled organisms tended to cooperate in communities long before multi-cellular creatures appeared. All the specialized adaptions in your cells are taken from pre-adaptive structures used by single cells. The little linkage proteins that your bones and muscles use, the molecular hooks that hold you together, the ion channels nerves use to communicate, the internal protein sequences ready to interpret self from non-self, all of these exist in single celled creatures. They are used for community relations, for cooperation, which, despite the adage about Nature being red in tooth and claw, is more often than not what life is about.
So why did it take so long? Well, we are told oxygen is the key. Collagen, a very important protein that holds us all together, requires large amounts of oxygen to be made. And oxygen has not been around in large quantities for very long. Only (only, ha!) the last billion years or so.
Okay, okay, so that's the standard explanation. Me? I've a different take. Have a look at this video:
Did you watch it? Now what do you think?
Well, look at the picture at the top. For all practical purposes, that picture might as well be a video. Not much happnin'. And then you got the video of the cats and crows. Some difference.
Here's my take. The reason animals (and plants) are around is because single celled life is BORING!