- Domain - Eukaryota, a group that includes organisms with a nucleus. Thus, bacteria are excluded from this group already.
- Kingdom - Animalia. 'Animals' are a hard group to draw a strict border around, but they are all multicellular heterotrophs with tissues developing from embryonic layers. (A heterotroph is an organism that gets its food from other organisms - either directly or indirectly.)
- Phylum - Chordata. Chordates are animals that have, for at least some of their development, a hollow dorsal nerve cord, a notochord (a longitudinal rod between the digestive tube and the nerve cord) and pharyngeal slits. The latter are a bunch of slits in the pharyngeal region; in fish these develop into gills, but in humans they are used to construct parts of the head and neck. 'Vertebrates' are a sub-phylum of the chordates, and are defined as chordates that have a head and a backbone. This added neurological complexity allowed vertebrates to become active predators in many cases.
- Class - Mammalia. In between chordates and mammals are lots of subgroups. We've already mentioned the vertebrates, but others include tetrapods (with limbs and feet), and amniotes (tetrapods that have a terrestrially-adapted egg). Mammals are amniotes that have hair and produce milk.
- Order - Primates are an odd group that share characteristics reflecting their ancestral time in and around trees. Such features include opposable thumbs (allowing for a precision grip), forward-facing eyes (allowing for better 3D vision, which is important in negotiating high branches safely), well-developed cerebral cortex (ditto, but probably also reflecting their 'advanced' social behaviours), etc.
- Family - Hominidae is the group of great apes, including humans, orangutans, gorillas and chimps.
- Genus - Homo. This genus of upright, bipedal hominoids is some 2.5 million years old, and begins (rather arbitrarily) with Homo habilis. The only remaining member of this group are us - the rest, from 'habilis' to the Neanderthals, have gone extinct.
- Species - Sapiens. In what can only be described as a cruel joke, we classified ourselves as sapiens - Latin for 'wise'! I suppose there were no dissenters at the meeting...
One point has to be made at this juncture. Such a linear list can give the mistaken impression that evolution 'leads to us' - that we are the culmination of billions of years of painstakingly-slow evolutionary processes. This is not true, and no serious biologist has belived this to be the case for about 65 years now. The tree of life is not a linear chain of progressions, but a branching bush - there's a common beginning, but successive branchings mean that all living species are represented by one of the outer leaves. We are not the 'most advanced' in any general way. Sure, our brains may be the best in the animal kingdom, on the whole, but other animals have better eyes, others perform much more complicated biochemical reactions, and still others have more impressive... trunks. The list goes on and on.Bertrand Russell put it well, if much more sarcastically:
Since evolution became fashionable, the glorification of Man has taken a new form. We are told that evolution has been guided by one great Purpose: through the millions of years when there were only slime, or trilobites, throughout the ages of dinosaurs and giant ferns, of bees and wild flowers, God was preparing the Great Climax. At last, in the fullness of time, He produced Man, including such specimens as Nero and Caligula, Hitler and Mussolini, whose transcendent glory justified the long painful process. For my part, I find even eternal damnation less incredible, and certainly less ridiculous, than this lame and impotent conclusion which we are asked to admire as the supreme effort of Omnipotence. And if God is indeed omnipotent, why could He not have produced the glorious result without such a long and tedious prologue?