Descent with modification means that all life on Earth probably came from one common ancestor – a single-celled organism – We just have to speculate and create models for what it may have looked like, how it lived and how it evolved into today's modern cell.
So model we do and a recent paper uses mathematical modeling to speculate that life's Last Universal Common Ancestor (LUCA) had a 'leaky' membrane, which, if would, would help scientists answer two of biology's biggest questions:
1. Why all cells use the same bizarre, complex mechanism to harvest energy
2. Why two types of single-celled organism that form the deepest branch on the tree of life – bacteria and archaea – have completely different cell membranes