The Ileret Skulls: My Two Cents

The internet is buzzing with the news of the two fossils found at Ileret. Hawks has an interesting take on the subject and promises more. Laelaps, Kambiz (who is skeptical of the paper), PZ, The Questionable Authority, Jason and Razib have all weighed in. Various news outlets have done their usual horrible job reporting on human evolution and, of course, the Discovery Institute has fired up the Ford Pinto (see below). There are a number of interesting points to be made about the Nature paper – which are below the fold (Sorry Joe).


The story, for those few who haven’t heard about it is as follows. A fairly complete skull dating to about 1.55 million years ago was discovered in Ileret, Kenya. This skull has been attributed to Homo erectus on the basis of the following traits:

…frontal and parietal keeling, the mediolaterally narrow temporomandibular joint, the distinct coronal and sagittal orientation of the tympanic and petrous elements, respectively, and a posterior midsagittal profile with a low occipital upper scale and opisthocranion positioned close to lambda (Supplementary Note 1.3). A multivariate analysis of calvarial dimension confirms the affinities of KNM-ER 42700 with H. erectus…

Also discovered is a partial maxilla dating to about 1.44 million years ago. This partial maxilla was assigned to H. habilis on the basis of a multivariate analysis of the post-canine dentition (although the authors don’t specifically mention it, the wide and shallow palate are traits shared with H. habilis as well).
Based on this, the authors of the paper argue that since both species overlapped in time straight line evolution from H. habilis to H. erectus to H. sapiens. They also argue that the morphology of the H. erectus skull, having as it does some traits similar to Asian forms undermines the erectus/ergaster distinction and that H. erectus is characterized by greater sexual dimorphism than humans and chimps but less than gorillas (among other things, it’s a busy little paper).
Media coverage has been horrible. Consider this from the St. Louis Post-Dispatch (itself lifted from the Associated Press):

And it also discredits that iconic illustration of human evolution that begins with a knuckle-dragging ape and ends with a briefcase-carrying man.

Feh! Apes don’t drag their knuckles – but two species do walk on them. Is it too much to ask to get the facts straight (and maybe phrase it in a less sexist fashion)? The Post article continues:

The old theory is that the first and oldest species in our family tree, Homo habilis, evolved into Homo erectus, which then became human, Homo sapiens. But Leakey’s find suggests those two earlier species lived side by side about 1.5 million years ago in parts of Kenya for at least half a million years. She and her research colleagues report the discovery in a paper published in today’s edition of the journal Nature.

This is refering to anagenesis (known as phyletic gradualism in some circles). The problem is paleoanthropologists have been debating the respective roles of anagenesis and cladogenesis in human evolution for years. This is something me and my fellow anthro students debated in college back in the mid 90’s for crying out loud. Stanley, in Macroevolution, used the human fossil record as an example of cladogenesis – as did Gould. Both used it the human fossil record to support punk eq. The authors of the Nature paper make a big deal out of the anagenesis issue and I was slightly perplexed, but then I thought about the recent find in Ethiopia (presented in an anagenetic context) as well as the paper from White last year that puts Ardipithicus ramidus, Australopithecus anamensis and A. afarensis into an anagenetic context. Kimbel, Johanson, and Rak also wrote a paper, in 1997, about a 2.33 million year old partial maxilla (A. L. 666-1) attributed to H. habilis – which falls squarely in the fossil gap around 3.0-2.0 million years ago that Spoor et al mention alot. So, I have to wonder if this isn’t, at least in part, a response to that (incidentally, the Kimbel et al paper was cited by Spoor et al). At any rate, does this look like anagenesis to you:
CE270300FG0010.gif
The reality is that paleoanthropologists have long considered both processes to play a role.
Which brings us to Casey Luskin’s response. Casey says:
In other words, habilis can no longer be considered the ancestor to the rest of the genus Homo.

Indeed, this has been a growing trend in paleoanthropology. A 1999 paper published in Science by two leaders in the field explained that “Homo” habilis should not even be considered a member of Homo, but is rather an australopithecine due to its ape-like skeletal structure (see B. Wood & M. Collard, “The Human Genus,” Science, Vol. 284:65-71, April 2, 1999).

Three points need to be made here. First, Wood and Collard do not argue that H. habilis is not an ancestor to the genus Homo and removing it from Homo would not change its ancestral status, otherwise how can, say A. afarensis or Ar. ramidus be considered ancestral to humans?. Second, outside of that particular paper very few people bought into the idea of removing H. habilis from the genus Homo (Wolpoff, Conroy and Hawks being among the few that agree with the idea). Third, Wood and Collard go on to say that putting H. habilis into Australopithecus would make Australopithecus paraphyletic which is a caditic no-no.
From there Casey proceeds to misunderstand Dennell and Roebroeks Nature article. In that article Dennell and Roebroeks argued that Asia was populated much earlier than previously thought and that H. ergaster may have migrated to Africa from Asia. However, the Spoor et al paper makes the quote Luskin provides moot. Luskin more or less quotes this bit from Dennell and Roebroeks:

The identification of east Africa as the ‘core’ area for the genus Homo (including H. ergaster) as well as tool-making seems secure to most palaeoanthropologists, and the most recent attempts at modelling early hominin dispersals start implicitly from the assumption that H. ergaster originated in east Africa and then dispersed across Asia… In fact, the evidence that H. ergaster originated in east Africa is less convincing than it seems. H. ergaster marks such a radical departure from previous forms of Homo (such as H. habilis) in its height, reduced sexual dimorphism, long limbs and modern body proportions… that it is hard at present to identify its immediate ancestry in east Africa… Not for nothing has it been described as a hominin “without an ancestor, without a clear past”…[emphasis mine, indicates Luskin quote, more or less – afarensis

Spoor et al say:

The small size of KNM-ER 42700 indicates that H. erectus showed substantial size variation throughout the early Pleistocene.

Which somewhat undermines the argument Dennell and Roebroeks was trying to make. Spoor et al go on to point out that the small size of KNM-ER 42700, a female, also implies that H. erectus was highly sexually dimorphic, further undermining the above argument. A little later Spoor et al say:

The presence of supposedly distinctive ‘Asian’ characters…, such as cranial vault keeling and a well separated petrous crest and mastoid process in KNM-ER 42700, underscores the difficulty in separating the African and Asian hypodigms of H. erectus… This difficulty is further accentuated by the observation that the more angulated occipitals and the thicker vaults and supraorbital tori seen in Asian H. erectus are allometric consequences of an increase in cranial size, rather than independent characters (Supplementary Note 2.1.2).

So, between the lack of specifically Asian characters, the small size of KNM-ER 42700, and the amount of sexual dimorphism in H. erectus there is not much left of Dunnell and Roebroeks’ quote that would support the burden Luskin is trying to place on it. It seems to me, and I could be wrong, that H. erectus sounds slightly more ape-like and slightly less human-like in Spoor et al. Then in fitting Luskin style he tries to draw support from an article authored by Hawks by linking to this dreck which I have fisked here. Strange that he didn’t read what Hawks has to say on the new finds
The end result is that none of the papers Luskin links to support his claim and all he has left to support his argument is, well, nothing…
Update 1: Several people have asked where I found the phylogeny pictured above. Strangely enough, it came from here. There are a bunch of others available on the net.
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6 Responses

  1. Outside of saying that the media did a bad job, I don’t think I’ve really “weighed in” yet (and I’d be a featherweight if I did!). I’m away from home and my stacks of books so I decided to wait until I can get back to get some references/quotes dealing with various ideas about human evolution to put things in a historical context a bit, many other bloggers (like you, John Hawks, and PZ) already having done a great job of explaining what’s going on.

  2. Not to mention that the tree you’ve shown leaves out other possible (if more debatable) hominins such as Sahelanthropus, Orrorin, Kenyanthropus, Homo antecessor, as well as significantly simplifying ape relationships… (Where did the tree come from, offhand? It’s quite a good one).

  3. Afarensis,
    The linking of the maxilla to H. habilis based on dental morphometrics is, to me, a little confusing. Loring Brace told me that the dentition of H. habilis speciments are all over the board, with some speciments sporting measurements that are within the autralopithecene range and others that fall into the H. erectus range. I realize that H. habilus is a solid taxon with a type skeleton, but it seems to my imperfect understanding that at least some of the material assigned to H. habilis is fragmentary and anatomically broad. Are they assigning this present speciment to H. habilis based on similarlity to the type skeleton, OH7? Or are there other features that are more important in such an assignment?
    MB

  4. Michael – They included a number of specimens from H. habilis and H. erectus. I think the assignment based solely on the MD and BL of the molars is, in my opinion, dicey. If you look at the graph presented in the Nature paper quite a few H. erectus fall within the 95% confidence interval for H. habilis.

  5. Afar,
    Is it just me or are the teeth in the alleged H. habilis specimen pretty banged up? How can they good, believable multivariate analyses with such mashed teeth?
    MB

  6. Yes they are, and Spoor et al admit that and say that the values they obtain are minimum values only. I’m skeptical, but in fairness to them teeth are not measured at cusp level.

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