I wrote yesterday about the “Lucy” Google Doodle, which looked like this:
Some of my colleagues didn’t like that Doodle, and fixed it. They couldn’t help it . . .:
Actually, I’m not sure this is scientifically accurate, as it shows Lucy (middle figure; A. afarensis) as a lineage that split off from modern humans rather than being one of our direct ancestors. We don’t know that, as Lucy’s species could have continued evolving into modern H. sapiens. The leftmost ape, if it’s a modern chimp, is correct, as they certainly branched off before hominins. But I’m not even sure, nor are the people at the Beacon Center, whether it’s a modern chimp or some ancestor of modern humans.
I therefore asked Greg Mayer, who knows more about this stuff than I do, to tell me if I was right in what I just said. He responded at some length, and even made a figure (upshot: I was right, and readers not conversant with taxonomy needn’t read on). Greg’s comments:
Yes, afarensis may have living descendants, and thus to show it as a terminal lineage isn’t quite right (or, at least, it involves additional assumptions). Some conventions for depicting fossil species in a tree with living species are needed, and it could be plausibly argued that they should be shown as terminal taxa (i.e at the tip of a branch, as in the corrected Google doodle Figure A).
But when a fossil species may be ancestral to a living species, we would consider the branch leading to the fossil species to have 0 length (i.e. it is at the node: Figure B).
Fossil species are usually depicted as terminal taxa, on the reasonable notion that of all the many species in an ancestral group, the probability of finding the ancestor is small. (For example, we know that early synapsids are ancestral to mammals, but the chance that Dimetrodon grandis is the ancestor of all mammals is small, so it probably is accurate to show it as a terminal taxon.) But when dealing with recent events with a rich fossil record in a geographically proscribed area, the probability of finding the ancestor does go up to the point where it’s no longer safe to make the assumption that it won’t be found.
For molecular and chromosomal data, where a complete description of the phenotype/genotype (i.e., nucleotide or banding sequence) is possible, actual ancestors can often be identified, and are often shown in trees as lying along the branches or at nodes in the tree.
Based on what we know about how speciation occurs, most ancestral species are probably paraphyletic relative to their descendants (Figure C, with afarensis zoomed in on and shown as ancestral to the lineage leading to Homo), and thus some parts of afarensis are terminal relative to Homo. If Lucy were in one of these terminal bits (the X in Figure C), then the corrected Google Doodle could be construed as accurate.
(I made the figure before checking the original Google doodle, and now realize that the leftmost ape was not identified as a chimp. If it was intended to be a chimp, then all of the above holds. However, if Google intended it to be a fossil ancestral species– which I think most likely– then it should fall along the branch, and not be a terminal taxon.)