If all adult human beings should be treated equally, then there must be something equal about all adult human beings -- something truly the same that we share. But it can't be age, size, cognitive abilities, athleticism, appearance or IQ. We all differ when it comes to those things. In fact, there's only one thing we all have in common: a human nature (i.e., we are all human).
And we have that human nature from the time we begin to exist as a zygote (and then as an embryo, fetus, newborn, toddler and so on). So the very thing that makes each of us equal in fundamental dignity (our humanity) is what gives unborn human beings that same dignity.
The killing of abortion and embryo-destructive research is a violation of that dignity, and a rejection of the principle of human equality.