It doesn’t appear that anybody has noticed yet, but 2021 marks a rather important anniversary in the history of science and western civilization. It was 2,500 years ago this year that a philosopher named Anaxagoras arrived in Athens, Greece.

Nobody held any celebrations at the time, either. But it was nonetheless an important historical and intellectual landmark. Before Anaxagoras, ancient Greek science (or to be less anachronistic, natural philosophy) hadn’t actually been practiced much in Greece itself. Natural philosophy originated early in the sixth century B.C. at the Greek settlement Miletus in Ionia, the western coast of modern-day Turkey. A second branch of primordial Greek science soon took root in southern Italy after one Ionian, a math fan named Pythagoras, moved there.

Anaxagoras, born in the Ionian town of Clazomenae, was the first natural philosopher to reside in Athens and promote the Ionian philosophical outlook there. As the science historian George Sarton wrote, Anaxagoras “introduced the scientific spirit into Athens.” Soon after, Athens became the western world’s center of philosophical inquiry, as the triumvirate of Socrates, Plato and then Aristotle established philosophy as an essential component of civilized intellectual discourse.

To be honest, there is some doubt about the exact date of Anaxagoras’ move to Athens. But the biographer of philosophers Diogenes Laertius wrote that Anaxagoras began to do philosophy in Athens at the age of 20, and says he was 20 years old when the Persian king Xerxes attacked Greece — and that was 480 B.C., 2,500 years ago. (You might think that 2021 would make that 2,501 years ago, but only if you forgot that there was no year 0, so you have to subtract one year from the calculation.)

It’s possible that during his time in Athens, Anaxagoras met the young Socrates, but the direct link to Socrates and his philosophical descendants was through the philosopher Archelaus. Anaxagoras “was the first to stimulate Archelaus the Athenian to practice philosophy,” wrote the famous physician Galen. And Archelaus was the teacher of Socrates, who taught Plato, who taught Aristotle, whose influence dominated science for two millennia.