TEMPE, Ariz. — We’re not in the Garden of Eden anymore.

Darwin speculated that life began in a warm pond on the primordial Earth. Lately other scientists have suggested that the magic joining of molecules that could go on replicating might have happened in an undersea hot spring, on another planet or inside an asteroid. Some astronomers wonder if it could be happening right now underneath the ice of Europa or in the methane seas of Titan.

Two dozen chemists, geologists, biologists, planetary scientists and physicists gathered here recently to ponder where and what Eden might have been. Over a long weekend they plastered the screen in their conference room with intricate chemical diagrams through which electrons bounced in a series of interactions like marbles rattling up and down and over bridges through one of those child’s toys, transferring energy, taking care of the business of nascent life. The names of elements and molecules tripped off chemists’ tongues as if they were the eccentric relatives who show up at Thanksgiving every year.

They charted the fall of meteorites and the rise of oxygen on the early Earth and evidence in old rocks that life was here as long as 3.5 billion years ago. The planet is only a billion years older, but estimates vary on when it became habitable.

In front of a 2,400-member audience one night they debated the definition of life — “anything highly statistically improbable, but in a particular direction,” in the words of Richard Dawkins, the evolutionary biologist at Oxford. Or, they wondered if it could be defined at all in the absence of a second example to the Earth’s biosphere — a web of interdependence all based on DNA.