We have a fairly informative biography of Shelob in the chapter Shelob's Lair in The Two Towers.
There agelong she had dwelt, an evil thing in spider-form, even such as once of old had lived in the Land of the Elves in the West that is now under the Sea, such as Beren fought in the Mountains of Terror in Doriath, and so came to Luthien upon the green sward amid the hemlocks in the moonlight long ago. How Shelob came there, flying from ruin, no tale tells, for out of the Dark Years few tales have come. But still she was there, who was there before Sauron, and before the first stone of Barad-dur; and she served none but herself, drinking the blood of Elves and Men, bloated and grown fat with endless brooding on her feasts, weaving webs of shadow; for all living things were her food, and her vomit darkness. Far and wide her lesser broods, bastards of the miserable mates, her own offspring, that she slew, spread from glen to glen, from the Ephel Duath to the eastern hills, to Dol Guldur and the fastnesses of Mirkwood. But none could rival her, Shelob the Great, last child of Ungoliant to trouble the unhappy world.
Some information of her mother, Ungoliant, is also mentioned in The Silmarillion1:
In Avathar, secret and unknown save to Melkor, dwelt Ungoliant, and she had taken spider's form, and was a weaver of dark webs. It is not known whence she came, though among the Eldar it was said that in ages long before she had descended from the darkness that lies about Arda, when Melkor first looked down in envy upon the light in the kingdom of Manwe.
So we can conclude that both were evil "things" in a spider's form. Even if she did have a human form (which is 100% unlikely either way), this is not shown in canon material.
1 There is barely any mention of Shelob in The Silmarillion.