'Human', like any biological category, is fuzzy. If we find a specimen of Homo habilis, we can determine from the bone morphology that it's a hominid, but we need measurements of the brain case and other bones to decide on the classification. And, of course, like all fuzzy classifications, biologists disagree. Some like to create a new species for every fossil, others are 'lumpers'. But what they'll probably all agree on is the fossil is close to the direct lineage of modern humans.
The Australopithecines
Humans, apes, and monkeys are members of the Order Primates. Under evolution, all primates are related and the chimpanzee is the closest living relative to humans, and humans are descended from a common ancestor they shared with chimpanzees (see Figure 3). There is essentially no fossil evidence of the supposed evolutionary ancestors of chimpanzees and other living apes,2 however there are some species believed by evolutionists to be ancestors, or close relatives of the ancestors of humans. The majority of "hominid" fossils have been divided into two taxonomic categories: the genus Australopithecus and the genus Homo (which includes our species, Homo sapiens).
Australopithecines (literally meaning "southern ape") are a genus of extinct hominids that lived in eastern Africa (see Figure 4) from about 4.2 million years ago (Ma) until about 1 Ma.10 Some evolutionists think they are ancestral to humans (see Figure 9), however it has also been argued they are a "side-branch" of the line that led to humans, and not direct human ancestors.12 The four most common species are Australopithecus afarensis, Australopithecus africanus, Australopithecus robustus, and Australopithecus boisei.13 The two smaller and "gracile" forms, africanus and afarensis (the species which includes the famous fossil "Lucy", see Figure 5) are thought by evolutionists to be those most closely related to humans (see Figure 9).
Australopithecines stood about 1-1.5 m in height and had relatively small brains between 370 and 515 cubic cm (cc)14, 15--a range that extends only slightly beyond the brain size of a chimpanzee (see Table 1). Though there are fossils creating a general grade of increasing skull sizes from Australopithecus into modern Homo, the fossil record indicates that about 2 Ma, skull sizes began a "dramatic trajectory" that ultimately resulted in an "approximate doubling in brain size."14 This "rapid evolution" is not uncommon with regards to the origins of characteristics of the genus Homo.
The australopithecine mode of locomotion has been a point of controversy. Many evolutionists believe they were "bipedal" (i.e. walked on two legs). Early studies thought the pelvis of australopithecines was a clear-cut precursor to Homo-like bipedality,16 while many later studies of australopithecine locomotion found it to be different from that of modern apes, but also very different from that of humans--a distinct mode of locomotion.12, 17 One study found sharp differences between the pelvic bones of australopithecines and Homo, and, lacking intermediate fossils, proposed a period of "very rapid evolution corresponding to the emergence of the genus Homo."18 Other recent studies have found that the handbones of Lucy are similar to those of a knucklewalking ape,19, 20 and that their inner ear canals, responsible for balance and related to locomotion, resemble small inner-ear canals of the great apes rather than larger canals found in humans and other members of the genus Homo.21 The most common consensus is that australopithecines were adapted for both tree-climbing and at least semi-upright walking,25 walking differently from humans and living apes.50
However, australopithecines were apes and were very different from humans. One reviewer said that ecologically speaking, australopithecines "may still be considered as apes."23 Harvard paleoanthropologist William Howells mentioned that the arboreal bipedalism of Lucy was "successful in serving Lucy's purposes," but "not something simply transitional"50 to the locomotion of modern humans. These are important clues as to whether or not australopithecines were fully bipedal hominids and ancestral to humans.
http://www.ideacenter.org/contentmgr/showdetails.php/id/1146
I see why it would be difficult to define humans.