There are two momentous performances in the Darwinian horror fable I Am Legend. One is by the movie’s star, Will Smith, and the other by the visual effects that render a near-future New York City devastated by an incurable plague. Deserted cars choke the bridges. Tree roots protrude through the surface of Seventh Avenue. And Times Square bustles with herds of wild deer, stampeding through on the run from human plague survivors transformed by the virus into rabid mutant predators. Through this desolate landscape moves a single uninfected man, the scientist Robert Neville (Will Smith), who has lost his family to the plague and who now spends every waking hour searching for a cure. Based on Richard Matheson’s seminal 1954 sci-fi novel, I Am Legend has been adapted for the screen twice before, but this version, directed with subtlety and artfulness by Francis Lawrence, marks the first time an actor has been asked to play Neville as something more than God’s lonely, angry man. Smith, who plays many of his scenes solo save for the company of a German shepherd, is more than up for the task, and in what has been a pretty remarkable career up to now, this may be the performance that fully affirms him as one of the great leading men of his generation.