In addition to the “missing link” trope that is being dished out about the new primate fossil, is another one, more subtle and insidious: it’s the ancestor of all primates. How do they know that? Consider a biologically realistic scenario: at the time there were probably hundreds of species of small bodied mammals with tails and feet like that. One of these species may be the ancestor of all primates, but what are the odds that a specimen from that species is the one that was preserved? Just as all primates now look remarkably similar overall, but one may be the common ancestor of a group in 50 million years or so without being the one that is fossilised, the characters of this species may in fact be shared primitive (in the sense of “came first”) traits of its group. So-called plesiomorphic traits, or underived traits, are no indicator that the specimen is a member of an ancestral species, only that it is a member of a group of species, one of which was the ancestor. We don’t even know what extant species is the ancestor of Darwin’s finches, and we have access to their biogeography, molecular properties, development, behaviour and mating systems. How can we be sure this was “the” concestor (Dawkins’ term for common ancestor) of all primates?
We’ve been bitten by this mistake many times before. Archaeopteryx was supposed to be the ancestor of birds. Neanderthals (now spelled without the “h”) were supposed to be “primitive” (i.e., came first) humans. Both are regarded as side branches of the lineage leading to birds and humans, but they show many traits that would have been shared with other species of their group at the time. And we rarely have reason to think we have a sufficient record of all species, as the Hobbit shows (it is regarded as not even a descendent of the H erectus hominids we know were in Asia around the right time).
History loses information. To make claims about history one needs positive evidence, ruling out, or at the least making extremely unlikely, alternative hsitories. Phylogenetics does not rule out all alternative histories, just some subsets. Phylogeny can rule out that a species is an ancestor, but it cannot rule it in. “Ida” may be our greatn parent, but equally it may just be a long lost cousin.