Astro was an astrology student from ancient Mesopotamia (modern Iraq). He was using an apparently inactive volcano as an observation point to study the night sky. However, the volcano eventually erupted and Astro was there during the eruption. He tried to escape, but he was instead sealed in a nearby cave. Astro was mutated by the explosion, gaining shape-shifting powers. He remained trapped in the cave for thousands of years, in an apparent state of inertia.
In the modern era, a new eruption of the volcano released Astro from his captivity. He was shocked at realizing what happened to him. Lashing out in anger, he destroyed several Middle Eastern cities. After calming down, he started traveling the globe. He eventually settled in the United States of America.
Despite his powers keeping him alive for so long, Astro still had a need to eat and drink to remain alive. This forced him to find a job and start earning an income. He uses his expertise in astrology to be hired as an astrology columnist by the Daily Bugle. He used a feminine form to assume the identity of "Jane Virgo".
Astro used the super-villain identity of "Mr. Zodiac" in a series of clashes with Spider-Man, while using the identity of "Jane Virgo" to befriend Peter Parker. Peter shared his theories about Mr. Zodiac with "Jane", and eventually noticed that the villain seemed to change plans according to the information only Peter and Jane knew about. After injuring Mr. Zodiac in a battle, Spidey noticed a similar injury on "Jane". Allowing him to deduce the villain's identity.
After Peter became aware of her true identity, Jane kidnapped Peter's acquaintance of "Ben". She started blackmailing Peter over Ben's safety. By this point, Peter had noticed that the villain required several minutes of concentration for his/her powers to properly work. He contacted Ben and had him intentionally disrupting Jane's concentration. With Astro's powers unavailable to him due to the constant disruptions, these powers stopped keeping him alive. Astro/Jane's body collapsed into mere dust.
Powers and Abilities
Transformation: Astro could take the form of all 12 zodiac signs. The animal zodiacs gave him/her various monstrous forms, while the Virgo sign allowed Astro to transform into a young woman (Jane Virgo). He was using this feminine form in his civilian identity.
Advanced Longetivity: Astro's powers kept him/her alive for several thousands years, though he spend most of this time inert in the cave.
Expertise in traditional astrology, writing skills as a regularly published newspaper columnist.
Transformation Dependency: Astro needed to change forms at least twice within a 24-hour-day, for his/her body to keep working properly. The transformations required intense concentration for several minutes. If these concentration was disrupted, the transformation could not be performed and Astro's body ceased working properly.
Irrationality: His/her villainy was apparently caused by feelings of anger over his/her mutation. The victims of this anger had actually nothing to do with the mutation, suggesting that Astro's thinking processes were irrational.