The following are not my words but observed differences between the (substantial) remains of this specimen and modern humans.
The skeleton shows evidence of small skull capacity akin to that of apes and of bipedal upright walk akin to that of humans, supporting the debated view that bipedalism preceded increase in brain size in human evolution.
One of Lucy's most striking characteristics is a valgus knee,[17] which indicates that she normally moved by walking upright. The femoral head of the knee was small and the femoral neck was short, both primitive characteristics. Her greater trochanter, however, was clearly derived, being short and human-like rather taller than the femoral head. The length ratio of her humerus to femur was 84.6% compared to 71.8% for modern humans and 97.8% for common chimpanzees, indicating that either the arms of A. afarensis were beginning to shorten, the legs were beginning to lengthen, or both were occurring simultaneously. Lucy also had a lumbar curve, another indicator of habitual bipedalism. Lucy likely had non-pathological (physiologic) flat feet, not to be confused with pes planus, though other afarensis individuals appear to have had arched feet.[18]
The cranial evidence recovered from Lucy is far less derived than her postcranium. Her neurocranium is small and primitive, while she possesses more spatulate canines than apes. The cranial capacity was about 375 to 500 cc.
Australopithecus afarensis also seems to have had the same conical rib-cage found in Apes like the Chimpanzee and Gorilla, indicating a longer intestine necessary for digesting plant matter. Fully 60% of the blood supply of Apes is used in the digestion process, greatly limiting the brain functions (which consume about 10% of the circulation). Heavier maxilliary masticatory musculature of the jaws also would impede the enlargement of the skull. These muscles seem to have weakened in the evolution of humans with the loss of the myosin gene MYH16, a two base pair deletion, which occurred only about 2.4 million years ago.
A study of the mandibular structure of a number of specimens of Au. Afarensis indicated that Lucy's jaw was rather unlike other hominins, having a more gorilla-like appearance.[19] Rak et al. consider that this mandible structure arose "independently in gorillas and hominins", but that Au. Afarensis is therefore "too derived to occupy a position as a common ancestor of both the Homo and robust australopith clades".[4]
Lucy was not an ape and not a human, she was somewhere inbetween.