Question

I was watching a lecture by Richard Dawkins earlier today and he mentioned that

Answer 1

All currently existent forms of life (and all their ancestors) are thought to be descended from a single life form known as LUCA (Last Universal Common Ancestor). Within those organisms, all organisms that share a common ancestor more recent than LUCA are said to be phylogenetically related. These organisms are (ideally - see note) grouped together by scientists into monophyletic taxa.

Therefore, ideally, each taxon represents a group of organisms that have descended from a single common ancestor, not shared by any other taxon. Taxa are defined at various levels including, but not limited to, Kingdom, Phylum, Class, Order, Family, Genus and Species, with organisms at each level having a more recent common ancestor than the ones preceding it. For example, for humans, a superficial phylogenetic classification would look somewhat like this:

• Kingdom: Animalia (shares a common ancestor with all multicellular, eukaryotic, heterotrphic, motile organisms)
• Phylum: Chordata (shares a common ancestor with all animals that possess a spinal cord)
• Class: Mammalia (shares a common ancestor with all chordates that produce milk and are warm-blooded)
• Order: Primates (shares a common ancestor with all monkey-like mammals)
• Family: Hominidae (shares a common ancestor with all the great apes)
• Genus: Homo (shares a common ancestor with relatively recent humanoid species)
• Species: sapiens (shares a common ancestor with all humans)

As you can see, this means that all humans share a common ancestor with all other humans, but going further back, they also share a common ancestor with all primates, all mammmals and all vertebrates. Eventually, all animals share a common ancestor, and if you go as far back as LUCA, so does all life.

Note on taxonomy and phylogenetics: Many modern taxa are (to the best of our knowledge) monophyletic, meaning all organisms in a given taxon share a unique common ancestor. However, there remain several taxa that are paraphyletic, because they were described before modern genetic methods became available, or because no one has got down to reclassifying them or (in at least one case, reptiles and birds), because they are so widely used that changing the classification could (and probably would) cause a lot of confusion. So, when using taxonomic classifications to deduce phylogenetic relationships, a bit of caution must be exercised to ensure that the taxon one is talking about is indeed monophyletic.