The fossil record

Until very recently, discussions about bird origins have centered almost exclusively upon one particular genus, Archaeopteryx, the earliest known bird. Archaeopteryx is now known from 7 specimens, the first discovered in 1855, and the most recent in 1992. Although Archaeopteryx is classified as a bird, it's anatomic details are much more similar to an earlier group of scaly, sometimes feathered, long-necked, bipedal, egg-laying vertebrates who possessed hollow bones, a furcula, digitigrade stance, and a back-turned pubis -- the therapod dinosaurs-- than they are to any modern birds. [Note: a fossil specimen of the therapod Oviraptor was recently found not only with eggs, but positioned on top of them in a typically avian brooding position!]

Specifically, Archaeopteryx displays an overwhelming similarity to one particular taxon of therapod dinosaurs, the dromaeosaurs (which includes the famous Cretaceous dinosaur, Velociraptor). The resemblance to dinosaurs is so close, in fact, that several Archaeopteryx specimens were originally misidentified as the contemporary dinosaur Compsognathus (Dingus and Rowe, p 120). This includes the earliest specimen, discovered in 1855, 4 years before Darwin published his theory of evolution. In 1964, John's Ostrom's discovery of the dromaeosaur Deinonychus provided an even closer approximation to the form of Archaeopteryx. Deinonychus, aside from its
larger size, is virtually identical to Archaeopteryx.

For example, Archaeopteryx retains a long tail of unfused vertebrae, almost identical to the tail of Deinonychus and other therapod dinosaurs. In modern birds, on the other hand, the tail is absent altogether, and the distal caudal vertebrae are fused into a stubby structure called a pygostyle.

Like all therapod dinosaurs, Archaeopteryx had noncuspate, pointed teeth. In modern birds, on the other hand, teeth are absent altogether, and the maxillary and premaxillary bones are instead covered with keratinous beak. Like therapod dinosaurs, Archaeopteryx retains curved raptorial claws on each of its 3 digits. No modern birds retains these 3 raptorial claws, although many birds display temporary embryonic claws (Nedin, 1999), and a few other birds, like the Hoatzin, retain 2 small claws during childhood. Like Deinonychus, Archaeopteryx and other early birds retain 3 unfused digits on the forelimb, whereas in modern birds these three digits are fused together. Like Deinonychus, Archaeopteryx possessed a peculiar, crescent-shaped wristbone called the semilunate carpal, which is formed by the fusion of distal carpals I and II. Only Archaeopteryx and dromaeosaurs like Deinonychus are known to share this bone. In modern birds, by contrast, distal carpal I is absent, and distal carpal II has been fused along with metacarpals I,II, and III into a single bone, the carpometacarpus. Like all dinosaurs, Archaeopteryx had a long neck attached at the back of the skull, whereas in birds it is attached at the bottom of the skull. Continuing on to the pelvis, we see that both Archaeopteryx and Deinonychus display an unfused ischium and ilium, and a "footed" pubis. In modern birds, the ilium and ischium are fused together, and the pubis lacks the "foot." In all of these characteristics and many others, Archaeopteryx clearly and consistently reveals its dinosaurian ancestry.

Table of archaeopteryx features

1 = present; * = present in some; ? = possibly present; x = absent

Dinosaurs Archae Birds
1 * 1 1
2 x 1 1
3 * 1 1
4 * 1 1
5 x x 1
6 x x 1
7 * x 1
8 * 1 x
9 1 1 x
10 1 1 x
11 1 1 x
12 1 1 x
13 1 1 x
14 1 1 x
15 1 1 x
16 6 6 11-23
17 1 1 x
18 1 1 *
19 1 1 x
20 1 1 x
21 1 1 x
22 1 1 x
23 1 1 x

It can be seen that Archae possesses many more characters which are present in dinosaurs and not in birds, than it does characters which are present in birds but not in dinosaurs. This is why Archae is a true transitional species, because it shares some characters which are diagnostic of one group whilst still retaining characters diagnostic of its ancestral group. Anyone who claims that Archae is 100% bird is wrong. Anyone who claims that Archae's skeleton is even predominantly bird- like is wrong. Anyone who claims Archae has a "totally birdlike" skull is wrong.

Humans are classified, zoologically, as Homo sapiens sapiens, a member of the primate family of mammals. Although the fossil record of early human evolution is very spotty, enough hominid material has been recovered to form at least a rough outline. According to the latest findings of paleoanthropologists, the immediate evolutionary predecessors of Homo sapiens are known as Homo erectus, whose skeletal remains have been found on several continents and are known by several different popular names such as "Java Man" and "Peking Man". Although Homo erectus walked upright like humans, used fire, and made crude stone tools, its brain capacity was somewhat smaller than that of a modern human, and it had much more primitive features in the face, such as heavy brow ridges and a receding chin.

It is generally believed that Homo erectus itself developed from Homo habilis, an upright form with even more primitive facial features and a still smaller brain size. Homo habilis was apparently the first tool maker, and thousands of crudely chipped lava cobbles have been found near its remains. Because Homo habilis was a toolmaker, it is generally considered to be the first of the true human family, and is the earliest species to be classified as Homo, or in the same genus as modern humans.

The earliest members of the human lineage are classified in the genus Australopithecus. Several species of Australopithecines have been found, and it is generally believed that a number of them are evolutionary offshoots, away from the human line, that specialized in a diet of tough vegetation and died out approximately one million years ago. Two species of the Australopithecines, however, are believed to lie in the direct human line. The most recent of these is Australopithecus africanus, which most paleoanthropologists consider to be the evolutionary ancestor of Homo habilis. (This interpretation is disputed by some authorities, who argue that H. habilis derived directly from the earlier Australopithecines. On the other hand, some authorities do not consider Homo habilis to be a valid species, and lump it together with A. africanus.)