It is generally agreed that the Greek letter San developed from the Phoenician Tṣadē, but I'm not sure I see the graphical similarity. The Phoenician form does bear some resemblance to the Greek form
and even to the more common Greek form
, but the resemblance doesn't seem that great to me, and there is evidence to suggest that the Greek alphabet was developed at a time when Tṣadē was drawn more like a Y, as shown on the Izbet Ṣartah abecedary:
How did the shape evolve?