Afarensis is a 3.5-2.8 million year old hominin from the Kada Hadar member of the Hadar formation in the Middle Awash, Ethiopia. He is approximately 41 inches tall, weighs approximately 60 pounds and has a cranial capacity of a whopping 410 cc (approximately). Afarensis is currently considered to be transitional between apes and humans and displays some traits of both. Since he spends a lot of time on the couch watching monster movies, some observers question whether he is an obligate biped (although no one has observed him climbing a tree). He also has a blog called 
Relationships Among Extant and Extinct Great Apes And Humans, by Lawrence Martin, is actually a chapter written for inclusion in Major Topics in Primate and Human Evolution (which was edited by Bernard Wood, Lawrence Martin, and Peter Andrews) and was published in 1986. Since the subject is the phylogenetic relationships between I thought it would be a great piece to start the new series. However, if you wish to be a purist and start at the real beginning you can read this.
The short version of the paper is that Martin takes 123 traits drawn from morphological analysis and biochemical data and tries to work out the phylogenetic relationships among extant great apes and humans (he also looks at what the data says about the ancestral condition of these traits would be in the ape/human clade). Following that he looks at various fossil species and tries to determine where they fit. There is a little more to it than that, though.
From the 1920'2 to the early 1960's it was though that great apes and humans were related thus:
Phylogeny 1
Beginning in the 1960's - mainly due to biochemical evidence - the pictured changed and the picture below became the standard interpretation of great ape/human relationships:
Phylogeny 2
Martin's analysis supports this. In his discussion of the results Martin says:
There are some derived characters which are shared among all three living great ape genera as Kluge (1983) [This refers to a paper called Cladistics and Classification of the Great Apes - afarensis] has pointed out. Many of these appear to be functionally correlated with large body size and closed hand walking and do not, in my view, outweigh the combined morphological and molecular evidence for an African ape/human clade... Withing the African ape/human clade there is only one character (a spatulate upper lateral incisor) supporting a chimp/human clade, and no evidence for a gorilla/human clade. The shared derived possession of secondarily reduced enamel thickness, in addition to the shared specializations relating to knuckle-walking, is convincing evidence for an African ape clade, and this is the position adopted here...
When I was in college in the early to mid 90's this was still the subject of some debate, especially because there is another alternative:
Phylogeny 3
Martin mentions this possibility (see above quote) but says the evidence isn't very robust. He also mentions a contribution by Andrews, in the same book, that also says the evidence for a chimp/human clade isn't that robust. In the 23 years since, the molecular evidence has gotten quite a bit more robust and this is the current view on the subject. Richmond and Strait's paper in Nature, Evidence that humans evolved from a knuckle-walking ancestor helped as well.
One of the more interesting comments in the paper is this:
It is interesting, and probably significant, that the two clades within the great ape and human clade which are best defined by derived characters, i.e. the homo clade and the Pongo clade, are also the two to which we are currently able to attach fossil relatives, Australopithecus and Paranthropus; and Sivapithecus respectively.
Unfortunately, we still don't have that much fossil evidence relating to chimps and gorillas now than we did then.
Humanities & Social Science
Comments
Could be that the australopithecines split off from the bonobos first, and then the chimps split from the bonobos.
Posted by: Alan Kellogg | February 22, 2009 10:59 PM
No, bonobos came on the scene about 2-3 MYA - although I have heard a figure as low as about 1.8 MYA.
Posted by: afarensis, FCD | February 22, 2009 11:55 PM
The lack of fossil evidence relating to chimps and gorillas reminds me of a question I asked you a long time ago that I still think is an interesting topic for a post: how many extant species have fossil evidence and what are the reasons?
I think it's interesting because many creationists point to a lack of transitional fossils (in spite of how many there actually are) as though every single transitional should show up and the fact that some of them don't means they never existed. How many mountain goats show up in the fossil record? How many should we expect to see?
Posted by: pough | February 24, 2009 11:58 PM
That is a good question and would take more than one post to answer...
Posted by: afarensis, FCD | February 25, 2009 12:44 PM