Two things can be said about this claim. Firstly, the ability to exist independently of another is not an essential property of human beings. Secondly, the inability to exist independently of another is in fact present in both fetuses and infants and so does not provide a reason for distinguishing between feticide and infanticide.
Viability is not an essential property of human beings. Fetal viability is contingent upon the medical technology of a given culture. A fetus that is not viable in Chad is viable in Los Angeles. If viability is necessary for something to be a human then a woman pregnant with a viable fetus in Los Angeles who flies from Los Angeles to Chad carries a human being when she leaves but this human being ceases to exist when she arrives in India and yet becomes human again when she returns.
A similarly-strange implication of the viability criterion is that it implies that Siamese twins are not humans either. Consider Siamese twins Bob and Scott. If Bob is a human being, then since Scott cannot live independently of Bob, Scott must not be a human person. However, it is difficult to see what property Bob has that Scott lacks which would justify considering one a human and the other not. It appears then that one would be forced to conclude that they both are, and are not, human. However, both Bob and Scott are human and killing one or both of them would be homicide despite this entailing that they are both human beings even though one cannot live independently of the other.
Not only does making viability the demarcer of humanity entail numerous absurdities but the property Sherwin points to to justify its doing so , dependence, is not something that ends at birth.
Oderberg puts the point well.
A born baby is also totally dependent on its mother, only instead of being fed and sheltered by the mother’s automatic internal processes, it is fed and sheltered by the mother’s consciously controlled external, behaviour. How can that make a difference to whether or not a foetus is a human being? 
These examples preclude an objection often raised against this type of criticism. Some critics have tried to argue that the position of a new-born infant in terms of dependence is different to non-viable fetuses in that after birth or viability other options are available. The dependence for survival can be handed on to someone else. However, before viability this is not the case. Hence, while infanticide is wrong, abortion prior to viability is not. This feature is absent in the cases of the hiker, the elderly mother and the infant.
In fact, Sherwin notes;
It is doubtful, however, that adoptions are possible for every child whose mother cannot care for it. The world abounds with homeless orphans; even in the industrialised West, where there is a waiting list for adoption of healthy (white) babies, suitable homes cannot always be found for troubled adolescents; inner city, AIDS babies, or many of the multiply handicapped children whose parents have tried to care for them but whose marriages broke under the strain.
 Susan Sherwin, “Abortion a Feminist Perspective,” in Ethical Issues in Modern Medicine, 5th ed., ed. Bonnie Steinbock & John D. Arras (Mountain View CA: Mayfield Publishing Co, 1999), 364..
 David Oderberg, Applied Ethics: A Non-Consequentialist Approach (Oxford: Blackwell Publishers Co, 2000), 5.
 Sherwin, “Abortion a Feminist Perspective,” 366.