Lucy fell out a tree

The following paper was recently published in Nature: "Perimortem fractures in Lucy suggest mortality from fall out of tall tree" (Nature 2016 Vol 537 p503), by John Kappelman, Richard A. Ketcham, Stephen Pearce, Lawrence Todd, Wiley Akins, Matthew W. Colbert, Mulugeta Feseha, Jessica A. Maisano & Adrienne Witzel

Here we propose, on the basis of close study of her skeleton, that her cause of death was a vertical deceleration event or impact following a fall from considerable height that produced compressive and hinge (greenstick) fractures in multiple skeletal elements. Impacts that are so severe as to cause concomitant fractures usually also damage internal organs; together, these injuries are hypothesized to have caused her death. Lucy has been at the centre of a vigorous debate about the role, if any, of arboreal locomotion in early human evolution. It is therefore ironic that her death can be attributed to injuries resulting from a fall, probably out of a tall tree, thus offering unusual evidence for the presence of arborealism in this species.
From the Discussion
Although most hominin fossils are fragmentary and broken because of a complex post-mortem history, skeletal elements sometimes preserve evidence of antemortem or perimortem fractures and injuries. When examining fossil taxa, such as Australopithecus afarensis, that appear to have practiced both terrestrial and arboreal locomotion, we suggest that the adaptations that facilitated bipedal terrestrial locomotion compromised the ability of individuals to climb safely and efficiently in the trees; this combination of features may have predisposed these taxa to more frequent falls from height.

Given that natural selection eliminates those who are less fit, this paper seems to raise more questions than it answers.
These observations show that Lucy is not from a successful species, since falling from trees is not going to increase fitness and the species is unlikely to survive.  Which is what we observe today, since there are no living representatives of her species.
If this is true, then Lucy cannot be a member of the species which leads to modern humans.  The ancestor of modern humans, must have been adapted for walking on two legs.   Lucy was not such a creature since it was not fully bipedal and there is evidence of knuckle walking.
It seems that there is no way natural selection can select a species that is less adapted to living in trees.  Any species of tree dwelling creatures who become more adapted to bipedalism would be less adapted to tree dwelling and would be less likely to survive to produce another generation which is better adapted to walking on the ground.  Such creatures would be eliminated by natural selection.  
There seems to be no evolutionary pathway from a tree living ape to humans who are well designed to walk on the ground.  This problem becomes greater when we consider that humans are extremely well adapted to long distance running, which they have to be to survive on the ground.