You are sitting in the centre field-level front-row seat of a large football stadium. On your right is your mom, and beside her, your grandfather. You know them well, you say hi. But beside him is his mom, and her father, and his father, and his mother, and her mother, and her mother, and her father, and his father, and on, and on, and on - an uninterrupted line of your ancestors snaking their way all around first row, then the second row, then the third row, and on and on until they fill the entire stadium.
You don't know these people, but at first they look very familiar, except they wear funny clothes. They're obviously conscious and intelligent and deserving of legal personhood just like you. But you look a few rows up and they start to look off, kinda weird. Whether Neanderthal or Denisovan, they're not quite like you any more. The further up you look, the less humanlike they look, until you look at the top row and see your great^100,000-grandmother Lucy. She's bipedal, but not human anymore. If you saw her in a zoo rather than at the top of the stadium, you would never think that she is conscious, or deserves the same legal rights as you.
So where in this stadium did intelligence and consciousness arise? Is there a single ancestor you could plausibly point to and say, "This person deserves legal personhood, she's conscious, but her mom, no she is not a person". It's impossible, the boundaries of intelligence are too fuzzy. The best you can do is point to some rather large group and say that somewhere in there, the rate of change added up enough to make some kind of difference.
As we wonder whether an AI is "alive", when they will become conscious or intelligent enough to deserve legal protection, it's useful to remember that the answer will probably be at least as hard. A definitive answer may well be impossible, it will be fuzzy and we'll reach broad consensus slowly.