If Rome had never fallen, the Industrial Evolution would have occurred slowly and naturally, and probably been in full swing by 700 AD, or whatever calendar the Romans would have maintained, powered flight possibly by 800, and following the current historical timeline, spaceflight less than 70 years later, putting humans on the moon over a thousand years earlier. Being Romans, they would not have waited and would have pushed on to Mars soon after colonizing the lunar surface. Christianity would have been suppressed, Islam might never have existed, and there would be three major Earth powers; the Roman Empire, stretching across Europe and the Atlantic into the New World, both North and South continents, and all through Africa, down to the Cape; the Chinese Empire, most likely occupying the same space it does today, or less, if it had been conquered by the third major power, the Russian Empire, covering Asia from the North Pole to the tip of India, and from the Great Wall (if it had been built) to somewhere in present day Western Russia, where a shaky peace treaty with the Romans marks the delineation of Europe and Asia.
