How writing systems descended from their ancestors.
Almost every alphabet in use today descends from a single invention around 1800 BCE: Proto-Sinaitic, devised by Semitic-speaking workers in Egypt who adapted hieroglyphs into a consonantal script. Through Phoenician it spread along Mediterranean trade routes, giving rise to Greek (and via Greek, Latin and Cyrillic), Aramaic (and via Aramaic, Arabic, Hebrew, Syriac, and — probably — Brahmi), and South Arabian (and via that, Ge'ez).
Other traditions arose independently: Chinese characters, Maya hieroglyphs, and a handful of 19th- and 20th-century inventions (Cherokee, Hangul, Vai, Bamum, Canadian Syllabics). This tree is a pedagogical simplification — derivation is often debated, and many scripts influence each other sideways rather than descending cleanly.
solid line = direct descentdashed line = likely / partial influencegreyed label = script no longer in current use