well it has not realy been an ivention, its been an observation of the skies gradually became scientific theory may seem mysterious, but no more mysterious than mankind's gradual understanding over thousands of years of any other chain of cause and affect events.
perhaps the neanderthals great-grandchildren began looking more closely at the movements of those mighty deities of the skies and to link those movements with events on earth. maybe they began to notice that a child born when mars was prominent was very different indeed from a child born when venus was at its most brilliant. perhaps these great-grandchildren even began to record celestial events on their primitive stone tablets - creating the first albeit rudimentary, astrological tablets. but a genuine astrological system couldn't be defined (and refined) until the invention of writing, mathematics and a good calendar. then man could do more than just admire the skies, he could actually count and map the stars and use them, mathematically and systematically, to track the movements of the sun, moon and planets throughout our sky.
it was the babylonians who after many centuries produced a reliable calendar and began to devise the zodiac which we are familiar with today. by 235 BC an ancient astrologer could tell an expectant mother that, if your child is born when venus comes forth and jupiter has set, his wife will be stronger than he. slowly over the centuries, astrological theory became more complicated and sophisticated. it spread from babylonia via egypt to greece. where the greeks (being very good at math) contributed a great deal to its development. when astrology reached rome, it really took off. there it was used both by ordinary people and the emperors, who consulted the zodiac for political as well as personal reasons.