Darwin believed that all organisms on Earth are united into a single tree of life by common decent. In other words, Darwin believed that all organisms on Earth arose from one common ancestor. Over time, this common ancestor divided and produced additional cells. Eventually, the cells evolved and became adapted to their environments via natural selection. Thus, the diversity amongst today’s living organisms arose.
The first cell was thought to be a simple single-celled prokaryote about 3.8 billion years ago. One theory is that the first cell was the result of self-replicating RNA becoming enclose in a phospholipid membrane.
The endosymbiotic theory suggests that eukaryotic cells arose from symbiotic relationships amongst prokaryotes.
Today, phylogenetic trees are used to display suspected evolutionary relationships amongst the organisms that are currently found on Earth.