What is the principle called that all living things have an ancestor?

Answer 1

'Biogenesis is the belief that living things come only from other living things, e.g. a spider lays eggs, which develop into Spiders.'

There's a Wikipedia article on it. I stole that quote from the article.

It was an advance on the earlier theory of 'spontaneous generation', which claimed that flies arose from rotting meat, and various creatures just 'popped out of the Earth'.

Biogenesis suffers from the big problem that at some stage there must have been at least one first living thing that did not have an ancestor. Disregarding creation myths, non-living matter produced life. The process by which that happened is called abiogenesis. We don't actually know which of the various possible processes actually did the job; science has more than half a dozen plausible alternatives. Four billion years after the event we can't say which one did it. However, in 2003, The J. Craig Venter Institute created a synthetic version of the bacteriophage, PhiX 174, life from non-life.

Answer 2

The principle is called common descent, which states that all living organisms share a common ancestor through evolution. This idea forms the foundation of the theory of evolution by natural selection.