WIKIPEDIA:

In 1964, James Lovelock was among a group of scientists who were requested by NASA to make a theoretical life detection system to look for life on Mars during the upcoming space mission. When thinking about this problem, Lovelock wondered “how can we be sure that Martian life, if any, will reveal itself to tests based on Earth’s lifestyle?” [8] To Lovelock, the basic question was “What is life, and how should it be recognized?” When speaking about this issue with some of his colleagues at the Jet Propulsion Laboratory, he was asked what he would do to look for life on Mars. To this, Lovelock replied:

"I’d look for an entropy reduction, since this must be a general characteristic of life."

Thus, according to Lovelock, to find signs of life, one must look for a “reduction or a reversal of entropy.”