It depends on what you consider "person" to mean.
Most in the pro-life camp think it means "a being with human DNA".
But we would be able to recognize a person even if he didn't have human DNA. If we discovered a previously unknown island with people on it who through years of isolation were no longer sexually compatible with humans (and biologically they may be considered a different species), we would still consider them persons if they behaved as we wold expect a human to behave.
In fiction, we have no problems recognizing talking cows as persons, or intelligent aliens etc.
Many religions have some form of angels or demons that are not human, yet are persons. Even God would fit that bill.
So why do we recognize them as persons? Because we recognize in them the properties we value - sentience, sapience, emotion, feelings, etc.
When do humans first acquire these properties? It develops over time, but most of it is not meaningful until higher brain activity has started.