By around 3.5 billion years ago, a living entity had evolved with a genome that consisted of recipes for making RNAs and proteins – the last universal common ancestor of all life. At least 100 genes can confidently be traced all the way back to LUCA, says Eugene Koonin of the National Institutes of Health in Bethesda, Maryland, who studies the evolution of life, and LUCA probably had more than 1000 genes in total.

LUCA had a lot of the core machinery still found in all life today, including that for making proteins. Yet it may have been quite unlike life as we know it today. Some researchers believe that LUCA wasn’t a discrete, membrane-bound cell at all but rather a mixture of virus-like elements replicating inside some non-living compartment, such as the pores of alkaline hydrothermal vents.

Michael Le Page, “A brief history of the human genome,” New Scientist, 17 September 2012