The presence of occupations at Alexandria which predated Alexander was proposed by researchers analysing lead, and was reported in April last year. They proposed that there had been a bronze age settlement dated to the IV dynasty and an iron age settlement dated to about 1000-800 BC. The latest discoveries are summarized on the above site:
Now scientists have discovered hidden underwater traces of a city that existed at Alexandria at least seven centuries before Alexander the Great arrived, findings hinted at in Homer's Odyssey and that could shed light on the ancient world. . . .
Alexandria was known to have developed from a settlement known as Rhakotis, or Râ-Kedet, vaguely alluded to as a modest fishing village of little significance by some historians. Seven rod-shaped samples of dirt gathered from the seafloor of Alexandria's harbor now suggest there may have been a flourishing urban center there as far back as 1000 B.C.
Coastal geoarchaeologist Jean-Daniel Stanley of the Smithsonian's National Museum of Natural History and his colleagues used vibrating hollow tubes to gently extract three-inch-wide rods of sediment 6 to 18 feet long (2 to 5.5 meters) from up to 20 feet (6.5 meters) underwater. Collecting these samples underwater proved challenging. "Alexandria now is home to as many as 4 million people, and we were in the unfortunate position of having to deal with their discharge — human waste, municipal waste, industrial waste — which got released into the harbor," Stanley said. "It's not funny, but you have to sort of laugh."