The Definition and Description of a "Transitional"

Loudmouth writes:The reality of the matter is that we have very nice transitional fossils for the transition between reptile and mammal, land mammal and whale, ape-like ancestor and man, etc.

Australopithecus ramidus (mid-Pliocene, 4.4 Ma) -- A recently discovered very early hominid (or early chimp?), from just after the split with the apes. Not well known. Possibly bipedal (only the skull was found). Teeth both apelike and humanlike; one baby tooth is very chimp-like. (White et al., 1994; Wood 1994)
Australopithecus afarensis (late Pliocene, 3.9 Ma) -- Some excellent fossils ("Lucy", etc.) make clear that this was fully bipedal and definitely a hominid. But it was an extremely ape-like hominid; only four feet tall, still had an ape-sized brain of just 375-500 cc (finally answering the question of which came first, large brain or bipedality) and ape-like teeth. This lineage gradually split into a husky large-toothed lineage and a more slender, smaller- toothed lineage. The husky lineage (A. robustus, A. boisei) eventually went extinct.
Australopithecus africanus (later Pliocene, 3.0 Ma) -- The more slender lineage. Up to five feet tall, with slightly larger brain (430-550 cc) and smaller incisors. Teeth gradually became more and more like Homo teeth. These hominds are almost perfect ape- human intermediates, and it's now pretty clear that the slender australopithecines led to the first Homo species.
Homo habilis (latest Pliocene/earliest Pleistocene, 2.5 Ma) -- Straddles the boundary between australopithecines and humans, such that it's sometimes lumped with the australopithecines. About five feet tall, face still primitive but projects less, molars smaller. Brain 500-800 cc, overlapping australopithecines at the low end and and early Homo erectus at the high end. Capable of rudimentary speech? First clumsy stone tools.
Homo erectus (incl. "Java Man", "Peking Man", "Heidelberg Man"; Pleist., 1.8 Ma) -- Looking much more human now with a brain of 775-1225 cc, but still has thick brow ridges & no chin. Spread out of Africa & across Europe and Asia. Good tools, first fire.
Archaic Homo sapiens (Pleistocene, 500,000 yrs ago) -- These first primitive humans were perfectly intermediate between H. erectus and modern humans, with a brain of 1200 cc and less robust skeleton & teeth. Over the next 300,000 years, brain gradually increased, molars got still smaller, skeleton less muscular. Clearly arose from H erectus, but there are continuing arguments about where this happened.
One famous offshoot group, the Neandertals, developed in Europe 125,000 years ago. They are considered to be the same species as us, but a different subspecies, H. sapiens neandertalensis. They were more muscular, with a slightly larger brain of 1450 cc, a distinctive brow ridge, and differently shaped throat (possibly limiting their language?). They are known to have buried their dead.
H. sapiens sapiens (incl. "Cro-magnons"; late Pleist., 40,000 yrs ago) -- All modern humans. Average brain size 1350 cc. In Europe, gradually supplanted the Neanderthals.

Loudmouth writes:Also, look at the photos above and focus on the brain case. You will notice that humans obviously have bigger craniums than the earlier fossils.

NosyNed writes:How come when they are laid out in time various features are closer to the current norms when they are nearer in time and further from those norms when they are further away in time? (With some variance since I don't see that any feature would constantly and smoothly approach the current state)?

Jazzns writes:When you see them all lined up it there is a nice gradient and it is very hard to draw the line between what is an ape and what is a human.

Loudmouth writes:As you can see, A. afarensis and A. africanus fall well outside normal ranges for living humans, as do A. robustus and A. boisei.

Loudmouth writes:But as Jazzns says in the post above, why would we only find these deformities as a function of geologic depth? Why don't we find normal, modern human/ape skulls at the same depth?