Today, scientists like Pohorille have come a long way in understanding how a random collection of atoms and molecules could have slowly, over billions of years, combined into the complex living systems that we know today. They've pieced together much of the puzzle. They know how the first cell membrane formed. They know how the simplest living creature, the bacterium, came into being. They've traced the evolution of bacteria, plus or minus a few gaps in the fossil record, to the formation of higher animals and eventually to humans.
Scientists have few clues to go on.
Fossil records from this era are incomplete, and researchers are still debating whether the stuff of life zoomed to Earth on meteorites or bubbled up from deep-sea thermal vents. Wherever life came from, scientists believe it appeared on Earth around 3.8 billion years ago. They think the key ingredients were nonliving molecules like carbon dioxide and methane. Early attempts to re-create life by combining these ingredients in a chemistry beaker, however, failed miserably. Life has a highly ordered structure.