Sorry to Ptah and to Fporretto for my quotation error above.

Ptah,

Yes, the fertilised egg and the eighty-year-old person that it develops into are the same kind of thing in terms of DNA. They are different stages of the same biological organism. However, they are not the same kind of thing from a moral perspective.

The moral status of something should not be based on what it might or probably will eventually become but on what it actually is now (including perhaps a record of its history). The point about abortions and stem cell research is that we can safely and effectively change what embryo cells can become.

It seems super-unlikely that a small lump of cells can think, hurt, make a choice, perceive colour, anything like that. The philosophy of mind emanating from The World accords with Karl Popper's theory of knowledge which implies that a mind develops by trial and error. For this trial and error to get underway you need ongoing interaction with a rich environment perceived by well-developed sense organs. Not much (in any) of this has happened before birth.

It's not so much

it doesn't look like a human being to me

It's more like

it doesn't interact like a human being to me

We don't yet have agreement on where to draw the line. The pro-lifers' DNA argument puts the line way too early. This wouldn't matter so much except for the fact that much conspicuous evil results from the sperm-meets-egg starting line or the related "ensoulment" idea. And also because we might be able to do so many good things with embryonic stem cells and by pre-screening frozen embryos. I don't see why we shouldn't eventually allow ourselves to alter germline DNA. Immunity to AIDS spliced in?