That is a question all of us ponder now and again and is an issue that is taken particularly seriously by scientists. In recent years, a series of fossil finds have yielded some of the answers.
Sexual activity is, it seems, as old as the hills.
In the Barberton Hills of South Africa, in rocks 3.5 billion years old, the remains of what are thought to be the oldest bacteria have been found. Under the scanning electron microscope, they appear as circles and cylinders between one and five microns long. Tantalisingly, Dr Frances Weston of the University of Bologna in Italy thinks she has found cells there in the process of dividing. That is what simple cells do today - they split in two to reproduce. At some point, many millions of years ago after the Barberton rocks were formed, cells started to share their genetic material with a friend.