Think of evolution as like a bush rather than a tree that has a central trunk or a chain.
Like all other organisms our ancestors diversified and branched into a variety of different species, all of which are extinct save for ours,
Homo sapiens.
A. ramidus is not being suggested as the last common ancestor of chimpanzees and humans (the species from whom both the chimpanzee and human species evolved).
It is a challenge for scientists to determine whether a fossil represents a member of our ancestral lineage or of a lineage that branched from ours.
Scientists use:
Homologies, which are traits that are shared between species (the wings of parrots and other birds).
Analogies, which are traits that are similar but evolved independently (the wings of birds and bats).
We share many traits with Chimpanzees (called primitive traits as they evolved earlier in time) but we also have derived traits such as habitual bipedalism.
It would be very unlikely that the common ancestor of humans and chimps would walk habitually as that would mean the trait would have had to disappear in the chimpanzee lineage after it split from ours.
When we look at a group of organisms we can often assume that those with more homologous traits (traits they have in common) are more closely related.
We have to be aware of analogies though. For example sometimes a trait does evolved independently in two separate lineages (such as the wings of birds and bats, which could mistakenly lead to concluding these types of organisms are more closely related than they really are.
When it comes to trying to figure out if a fossil was a habitual biped scientists have a large number of anatomical traits to work with.
Some things they look for are:
The positioning of the hole under the skull where the spinal cord attaches.
The length of the legs.
Shape of the hips (are they shaped for carrying weight in the upright position).
Whether there is thickening of the bones where weight will be born in bipedal locomotion.
Often news articles are not very accurate at all with what they say and there is often much more debate and doubt concerning the evidence than they report.
Here is the link to the table of contents for the special edition of Science where the original research was published:
http://www.sciencemag.org/content/vol326/issue5949/index.dtlHere is the link to their introductory article:
http://www.sciencemag.org/cgi/content/full/326/5949/36 
Topic: Fossils may turn 'evolution on its head'