It evolved in animals because it is advantageous. It is known that even the simplest worms can make a difference between the part of their own body and the body of the other worm or the rest of the environment: they would eat another worm but not their own tail. Worms eating their body are typically not having an advantage (but the local optimums of scarifying a part of the body can of course still develop). So the "subjective experience" of distinguishing between "me" and "anything else" allows the living things to protect themselves from the rest of the environment.
Then if you question "why are there living things" the answer is again that they evolved exactly because the property of these units "preserving" themselves and duplicating that behavior -- these entities which don't have such properties remain lifeless and don't reproduce.
In short, a "subjective experience" is still a "simple" emerging property of the complex enough living organisms.
I intentionally quote "simple" as for us it is "of course" not simple, even if it can be reduced to such a simple explanation. The reason for that is again known today: the existence of life as we know it today is a result of the 14 billion years of the development of the environment during which effectively each moment and every detail happening just a bit different could have resulted in what we know as our environment not existing in this very form we know today and therefore "we" as we understand us to be wouldn't exist too.
Even starting from the basic "building blocks": today we know that for the atoms in our body to exist, they had to be produced in some exploding star, for all elements except for hydrogen which was produced shortly after the Big Bang (so only hydrogen atoms in our bodies are as old as the Big Bang, it's so amazing to be able to know that!). The stars from which we are made had to "happen" to "develop" "just right" for us to be able to exist. In the areas of the Universe where they developed a little different, we know that we can't search for the life like ours. So we are in "just right" part of the Universe, even if we know that there are many parts that could also be "just right." But the other "just right" places will still develop something not exactly the same like our place here even if there would be a lot of common properties. They aren't "we." We is the result of everything that happened in the part of the Universe relevant to us since the Big Bang.
At the end, all "why" questions have sense only in a predefined context where both the one asking and the one answering agree that the answer is satisfying. If the one asking asks knowing that no answer will be satisfying, or no shared context is known, there's seldom point in asking:
"Q: If you get hold of two magnets and you push them you can feel this pushing between them. Turn them the other
way and they slam together. Now what is it, the feeling between those two magnets?
RF: What do you mean what's the feeling?
Q: Well, there's something there isn't it?
I mean that's the sensation that
there's something there when you
push these two magnets together.
RF: Listen to my question: what is the meaning, when
you say that there's is a
feeling, of course you feel it, now what
do you want to know?"
Q: What I want to know is,
what's going on between the other
bits of these two bits of metal.
RF: They repel each other.
Q: Well then, what does that mean,
or why are they doing that
or how are they doing it?
RF: You ask...
Q: I'm not saying.. I think that's a
perfectly reasonable question.
RF: Of course it's reasonable,
it's an excellent question.
The problem that you're asking: You see, when you ask why
something happens, how does a person answer why something
happens? For example: "Aunt Minnie is in hospital." "Why?"
"Because she slipped, she went out, she slipped on
the ice and broke her hip." -- that satisfies
the people. It satisfies them, but it
wouldn't satisfy someone who came from
another planet and knew nothing about that. The first-graders understand why: "when you
break your hip you go to the hospital."
"How do you get to the hospital with that
hip, when the hip is broken?" "Well, because her
husband seeing that she has had her hip broken
has called the hospital up and sent
somebody to get her." All that is understood
by people and when you explain "a why"
you have to be in some framework that
you allow for something to be true otherwise
you're perpetually asking why.
"Why did the husband call up the hospital?" "Because
husband is interested in his wife's
welfare." Not always, some husbands aren't
interested in their wife's welfare, when
they're drunk and they're angry and... So
you begin to get a very interesting
understanding of the world and all its
complications. If you try
to follow anything up you go deeper and
deeper in various directions. For example
you'd go: "Why did she slip on the ice? Well,
ice is slippery, everybody knows that, no
problem, but I asked why is ice
slippery"..."
(an extended quote ends)
So if you'd say "but the subjective experience that a worm has is not my subjective experience" the answer is "of course it isn't."
If you'd claim that something would have to be able to tell you about its subjective experience for you to accept that it is like yours then you'd exclude those humans who can't talk, etc. The language is of course an emerging property, most of people can agree with that. At the end, these who try to claim a specific emerging property among the living organisms as something "very special" ("the feeling of me being different from the rest") somehow want to exclude that specific emerging property from all the rest, and they have some special motivation to do so. But it's still just an emerging property.
It evolved in animals because it is advantageous. It is known that even the simplest worms can make a difference between the part of their own body and the body of the other worm or the rest of the environment: they would eat another worm but not their own tail. Worms eating their body are typically not having an advantage (but the local optimums of scarifying a part of the body can of course still develop). So the "subjective experience" of distinguishing between "me" and "anything else" allows the living things to protect themselves from the rest of the environment.
Then if you question "why are there living things" the answer is again that they evolved exactly because the property of these units "preserving" themselves and duplicating that behavior -- these entities which don't have such properties remain lifeless and don't reproduce.
In short, a "subjective experience" is still a "simple" emerging property of the complex enough living organisms.
I intentionally quote "simple" as for us it is "of course" not simple, even if it can be reduced to such a simple explanation. The reason for that is again known today: the existence of life as we know it today is a result of the 14 billion years of the development of the environment during which effectively each moment and every detail happening just a bit different could have resulted in what we know as our environment not existing in this very form we know today and therefore "we" as we understand us to be wouldn't exist too.
Even starting from the basic "building blocks": today we know that for the atoms in our body to exist, they had to be produced in some exploding star, for all elements except for hydrogen which was produced shortly after the Big Bang (so only hydrogen atoms in our bodies are as old as the Big Bang, it's so amazing to be able to know that!). The stars from which we are made had to "happen" to "develop" "just right" for us to be able to exist. In the areas of the Universe where they developed a little different, we know that we can't search for the life like ours. So we are in "just right" part of the Universe, even if we know that there are many parts that could also be "just right." But the other "just right" places will still develop something not exactly the same like our place here even if there would be a lot of common properties. They aren't "we." We is the result of everything that happened in the part of the Universe relevant to us since the Big Bang.
At the end, all "why" questions have sense only in a predefined context where both the one asking and the one answering agree that the answer is satisfying. If the one asking asks knowing that no answer will be satisfying, or no shared context is known, there's seldom point in asking:
See Feynman's take on "why questions":
https://www.youtube.com/watch?v=Dp4dpeJVDxs
(an extended quote starts)
"Q: If you get hold of two magnets and you push them you can feel this pushing between them. Turn them the other way and they slam together. Now what is it, the feeling between those two magnets?
RF: What do you mean what's the feeling?
Q: Well, there's something there isn't it? I mean that's the sensation that there's something there when you push these two magnets together.
RF: Listen to my question: what is the meaning, when you say that there's is a feeling, of course you feel it, now what do you want to know?"
Q: What I want to know is, what's going on between the other bits of these two bits of metal.
RF: They repel each other.
Q: Well then, what does that mean, or why are they doing that or how are they doing it?
RF: You ask...
Q: I'm not saying.. I think that's a perfectly reasonable question.
RF: Of course it's reasonable, it's an excellent question. The problem that you're asking: You see, when you ask why something happens, how does a person answer why something happens? For example: "Aunt Minnie is in hospital." "Why?" "Because she slipped, she went out, she slipped on the ice and broke her hip." -- that satisfies the people. It satisfies them, but it wouldn't satisfy someone who came from another planet and knew nothing about that. The first-graders understand why: "when you break your hip you go to the hospital." "How do you get to the hospital with that hip, when the hip is broken?" "Well, because her husband seeing that she has had her hip broken has called the hospital up and sent somebody to get her." All that is understood by people and when you explain "a why" you have to be in some framework that you allow for something to be true otherwise you're perpetually asking why. "Why did the husband call up the hospital?" "Because husband is interested in his wife's welfare." Not always, some husbands aren't interested in their wife's welfare, when they're drunk and they're angry and... So you begin to get a very interesting understanding of the world and all its complications. If you try to follow anything up you go deeper and deeper in various directions. For example you'd go: "Why did she slip on the ice? Well, ice is slippery, everybody knows that, no problem, but I asked why is ice slippery"..."
(an extended quote ends)
So if you'd say "but the subjective experience that a worm has is not my subjective experience" the answer is "of course it isn't."
If you'd claim that something would have to be able to tell you about its subjective experience for you to accept that it is like yours then you'd exclude those humans who can't talk, etc. The language is of course an emerging property, most of people can agree with that. At the end, these who try to claim a specific emerging property among the living organisms as something "very special" ("the feeling of me being different from the rest") somehow want to exclude that specific emerging property from all the rest, and they have some special motivation to do so. But it's still just an emerging property.