Like several other characters in the DCU (Superman and Lobo, I'm looking at you two), Martian Manhunter has his powers as a result of being the last of his species. J'onn Jonnz was a scientist and a scholar on Mars, and had a wife and daughter. Then a cataclysm occurred--Hronmeer's Curse, which is the source of the Martian weakness to fire. A psychic and fiery holocaust wiped out the entire Martian species--J'onn was only spared by a freak accident. A human scientist named James Erdell had built a time (and space as it turns out) machine that he did not completely understand; at the last second, J'onn was pulled one thousand years into the future and hundreds of thousands of miles away from his home, to Earth. One look at the Martian he had summoned gave Erdell a fatal heart attack, leaving J'onn J'onnz utterly alone on Earth; he could not get back to the place and time he had come from, and all his people had been dead for an aeon. He proceeded to use his natural abilities to impersonate and learn about humanity. Eventually he developed such an empathy and respect for humans that he decided to use his awesome powers to defend them. About those powers. The green Martians were a superhuman species; they were so highly evolved that they naturally possessed superhuman powers. A typical green Martian had a lifespan of at least three thousand years; telepathy; telekinesis; invisibility; intangibility; and superhuman strength, stamina, and speed. On Mars, having such abilities made you normal--but on Earth, they made you very powerful. Of course, J'onn isn't the typical Martian. He's had decades to practice and refine his abilities, until he has become super-powerful even for a Martian. He has learned to use his telekinesis to fly and fire kinetic force beams from his eyes, for example. He has also mastered shapeshifting and telepathy to a degree that few Martians ever have. The green Martians were a peaceful and reflective people, and rarely had the opportunity to go on decade's worth of limit-testing galaxy-spanning adventures alongside the world's greatest heroes. Nothing like time and practice to help a Martian realize his potential. Martian Manhunter has adopted the Justice League as a surrogate family, and takes the team very seriously; he has been a member of every single incarnation, even during times when bigger names like Superman and Batman couldn't be bothered. One of his most important goals is the proper training and education of the "next generation' of heroes--J'onn plans for the long term, and has a strong fatherly desire to pass on the torch to worthy successors. J'onn is like a more mature, more responsible Superman--an ultrapowerful hero who cares for and protects the whole world, and not just the status quo of his hometown and his reporter friends.