Yes, animals. Again we are not going to talk deeply to define what animal is or about the hierarchy of the species, how they have developed and ... We just want to get a feeling of if they all can be explained just by physics or not? Although both animals and plants are made of cells but their cells are not the same, it is because of the way they both evolved through the time. Surely no animal cell needs to have chloroplasts because they don't need to make their own food; instead, they generally digest food by digestion mechanism they have, and by food, we mean other animals or plants.
We defiantly can describe the way an animal body parts or organs work with physics, chemistry and part of the biology we have introduced by now. The thing that makes an animal different from a plant is that we think since they move and make decisions so there may be some supernatural phenomena in them which doesn't exist in plants. Now let us see if such a thing exists or not.
OK, we are not going to talk about why or how the first single cell came to life some 2-3 billion years ago at the moment. But just consider the time between 500 million years ago which is an estimation for existing of animals and that 2 billion years ago, it is 1.5-2.5 billion years.
In statistics and probability there is a saying like this "if you give enough time, the most unlikely probabilities will happen". It is a simple saying; there is also an "infinite monkey theorem" which says that if a monkey hits keys at random on a typewriter keyboard for an infinite amount of time, he will finally type a complete work of William Shakespeare.
This idea, not only makes us believe that having the first single cell life, some 2-3 billion years ago is not work of supernatural forces; but lets us also think that during the time, and being under the changing of environmental conditions, these single cells could evolve to some more complex organisms gradually too, just remember how complex molecules can come into existence from simple atoms.
Now let us back to what we said before, that animals can think or make a decision. Yes, they do, but do you think this is such an enormous supernatural phenomena that physics can't describe it? No, it isn't. Even most difficult decisions can be simplified to some small binary decisions. If you have done some programming before, you know what I mean. Now the question is how come an organism can make these simple decisions. First making decisions in nature are happen all the time, but they all obey physics laws. For example, if the weather gets cold and there are enough moisture in the atmosphere and some other conditions happen, we surely have snow. That is nothing than a logical statement like "A & B & C & ... => D" that can be described as some binary decisions too, the same thing can happen in the brain of an animal, which exactly happens, but more complex with electricity currents. If you had some electronic background, were familiar with flip-flops and some other logical elements you would understand these better.
But if you ask, who has helped the nature to build such a complex brain for animals? I'll say just "long passage of time", remember "infinite monkey theorem". There is also one other thing which has helped we have animals like what we have today, and that is the way evolution works. The evolution does not purely obey the probability laws; it is more efficient than pure randomness, and it takes less time for the evolution to evolve the first simple organism (last universal ancestor) to plants, animals, and humans.
We don't want to explain how evolution exactly works but as you see it is nothing but some chemistry of life and environmental conditions which can be explained by physics. Let us bring an example, how eye as an organ has been created. Assume there were some organisms used to live in oceans 3 or more billion years ago. These organisms used to convert light energy into chemical energy or food for themselves (photosynthesis) to do what they do as living organisms. During the time organisms that used to live near the ocean surface got more sunlight that those who used to live near the sea floor, the evolution has helped this organism to find the direction of sunlight gradually and even follow the sunlight to get the best of daylight. Having this ability is nothing but a simple eye, yes this simple eye helps the organism to know if there is a light at the direction it looks or not? Nothing more. But as time goes by, this simple eye learns also to distinguish the amount of light which is nothing but the ability to see the grayscale and gradually recognizing shapes ...
